Guidelines for Designing and Evaluating Surface Irrigation Systems

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Title: Guidelines for designing and evaluatin surface irrigation systems...

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Guidelines for designing and evaluating surface

irrigation systems

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FAO IRRIGATION AND DRAINAGE PAPER 45 by W.R. Walker

Professor and Head

Department of Agricultural and Irrigation Engineering Utah State University

Logan, Utah, USA (Consultant to FAO) FAO

FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS Rome, 1989

The designations employed and the presentation of material in this publication do not imply the

expression of any opinion whatsoever on the part of the Food and Agriculture Organization of the United Nations concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries.

M-56

ISBN 92-5-102879-6

otherwise, without the prior permission of the copyright owner. Applications for such permission, with a statement of the purpose and extent of the reproduction, should be addressed to the Director, Publications Division, Food and Agriculture Organization of the United Nations, Via delle Terme di Caracalla, 00100 Rome, Italy.

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This electronic document has been scanned using optical character recognition (OCR) software and careful manual recorrection. Even if the quality of digitalisation is high, the FAO declines all responsibility for any discrepancies that may exist between the present document and its original printed version.

Preface

This guide is intended to serve the needs of the irrigation technician for the evaluation of surface irrigation systems. The scope is focussed at the farm level. A limited series of graphical and tabular aids is given to relieve the user of some burden of computation. Unfortunately, the number of variables associated with surface irrigation prevents this from being completely practical. There are also two matters of philosophical nature that have led to the approach presented herein. First, the irrigation technician and engineer must understand the fundamental interactions characterizing surface flow in order to evaluate, improve, design and manage effectively. This suggests a mathematical presentation which briefly and

concisely portrays these interrelationships. This guide omits nearly all theoretical development and presents the most basic mathematical description. Nevertheless, the complexity of the problem still requires an extensive mathematical analysis, even at this basic level. The

expertise required of the technician is that of at least a secondary education and the engineer whose training needs to be at approximately the BSc level. The second philosophical aspect is the belief that irrigation engineering practices are moving steadily toward a computerized methodology. The interactions referred to above require large enough computational

commitments that they are only feasibly evaluated with hand-held programmable calculators or microcomputers. As a result, the procedures outlined herein have been presented so they can be applied directly via computer. A diskette copy of this program source and executable codes for IBM PC and compatible microcomputers is available from FAO.

Some of the material used to develop this paper is included in more theoretical texts of the writer's. Occasionally, direct quotes and figures have been extracted without citation in order to minimize the diversions encountered by the reader. When the work of others has been used, more careful attention to the detail of the citation has been given. Surface irrigation is a complex subject which many have investigated and written about. The purpose of this guide was not to review the technical literature exhaustively and many valuable works are not cited, but it is hoped that the essence of surface irrigation evaluation and design practice has been captured.

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Acknowledgements

This work has been Undertaken under the supervision of Dr. Abdullah Arar, Senior Regional Officer, Land and Water Development Division, FAO. His continual support and careful attention to the details involved in producing a document such as this are very much

appreciated. Numerous other staff of the FAO have also contributed to this work through their reviews, editorial oversight, and publication.

In the last decade or so, the methodology of surface irrigation engineering has moved from the empirical to the quantitative. This has been accomplished by the concerted efforts of

numerous researchers and practitioners, some of whom are acknowledged in the REFERENCES. However, many others have made substantial contributions. Of these,

perhaps the graduate students at the universities where surface irrigation technology has been extended have been the most unheralded. To those who have worked with the author, special thanks.

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1. The practice of irrigation

1.1 The perspective and objectives of irrigation

1.2 Irrigation methods and their selection

1.3 Advantages and disadvantages of surface irrigation

1.1 The perspective and objectives of irrigation

A reliable and suitable irrigation water supply can result in vast improvements in agricultural production and assure the economic vitality of the region. Many civilizations have been dependent on irrigated agriculture to provide the basis of their society and enhance the security of their people. Some have estimated that as little as 15-20 percent of the worldwide total cultivated area is irrigated. Judging from irrigated and non-irrigated yields in some areas, this relatively small fraction of agriculture may be contributing as much as 3040 percent of gross agricultural output.

Effective agronomic practices are essential components of irrigated systems. Management of the soil fertility, cropping selection and rotation, and pest control may make as much

incremental difference in yield as the irrigation water itself. Irrigation implies drainage, soil reclamation, and erosion control. When any of these factors are ignored through either a lack of understanding or planning, agricultural productivity will decline. History is absolutely certain on this point.

Irrigated agriculture faces a number of difficult problems in the future. One of the major concerns is the generally poor efficiency with which water resources have been used for irrigation. A relatively safe estimate is that 40 percent or more of the water diverted for irrigation is wasted at the farm level through either deep percolation or surface runoff. These losses may not be lost when one views water use in the regional context, since return flows become part of the usable resource elsewhere. However, these losses often represent foregone opportunities for water because they delay the arrival of water at downstream

diversions and because they almost universally produce poorer quality water. One of the more evident problems in the future is the growth of alternative demands for water such as urban and industrial needs. These uses place a higher value on water resources and therefore tend to focus attention on wasteful practices. Irrigation science in the future will undoubtedly face the problem of maximizing efficiency.

Irrigation in arid areas of the world provides two essential agricultural requirements: (1) a moisture supply for plant growth which also transports essential nutrients; and (2) a flow of water to leach or dilute salts in the soil. Irrigation also benefits croplands through cooling the soil and the atmosphere to create a more favourable environment for plant growth.

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The method, frequency and duration of irrigations have significant effects on crop yield and farm productivity. For example, annual crops may not germinate when the surface is inundated causing a crust to form over the seed bed. After emergence, inadequate soil moisture can often reduce yields, particularly if the stress occurs during critical periods. Even though the most important objective of irrigation is to maintain the soil moisture reservoir, how this is accomplished is an important consideration. The technology of irrigation is more complex than many appreciate. It is important that the scope of irrigation science not be limited to diversion and conveyance systems, nor solely to the irrigated field, nor only to the drainage pathways. Irrigation is a system extending across many technical and non-technical disciplines. It only works efficiently and continually when all the components are integrated smoothly.

1.2 Irrigation methods and their selection

1.2.1 Compatibility 1.2.2 Economics 1.2.3 Topographical characteristics 1.2.4 Soils 1.2.5 Water supply 1.2.6 Crops 1.2.7 Social influences 1.2.8 External influences 1.2.9 Summary

There are three broad classes of irrigation systems: (1) pressurized distribution; (2) gravity flow distribution; and (3) drainage flow distribution. The pressurized systems include sprinkler, trickle, and the array of similar systems in which water is conveyed to and distributed over the farmland through pressurized pipe networks. There are many individual system configurations identified by unique features (centre-pivot sprinkler systems). Gravity flow systems convey and distribute water at the field level by a free surface, overland flow regime. These surface irrigation methods are also subdivided according to configuration and operational

characteristics. Irrigation by control of the drainage system, subirrigation, is not common but is interesting conceptually. Relatively large volumes of applied irrigation water percolate through the root zone and become a drainage or groundwater flow. By controlling the flow at critical points, it is possible to raise the level of the groundwater to within reach of the crop roots. These individual irrigation systems have a variety of advantages and particular applications which are beyond the scope of this paper. Suffice it to say that one should be familiar with each in order to satisfy best the needs of irrigation projects likely to be of interest during their formulation.

Irrigation systems are often designed to maximize efficiencies and minimize labour and capital requirements. The most effective management practices are dependent on the type of

irrigation system and its design. For example, management can be influenced by the use of automation, the control of or the capture and reuse of runoff, field soil and topographical variations and the existence and location of flow measurement and water control structures. Questions that are common to all irrigation systems are when to irrigate, how much to apply, and can the efficiency be improved. A large number of considerations must be taken into account in the selection of an irrigation system. These will vary from location to location, crop to crop, year to year, and farmer to farmer. In general these considerations will include the compatibility of the system with other farm operations, economic feasibility, topographic and soil properties, crop characteristics, and social constraints (Walker and Skogerboe, 1987).

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The irrigation system for a field or a farm must function alongside other farm operations such as land preparation, cultivation, and harvesting. The use of the large mechanized equipment requires longer and wider fields. The irrigation systems must not interfere with these

operations and may need to be portable or function primarily outside the crop boundaries (i.e. surface irrigation systems). Smaller equipment or animal-powered cultivating equipment is more suitable for small fields and more permanent irrigation facilities.

1.2.2 Economics

The type of irrigation system selected is an important economic decision. Some types of pressurized systems have high capital and operating costs but may utilize minimal labour and conserve water. Their use tends toward high value cropping patterns. Other systems are relatively less expensive to construct and operate but have high labour requirements. Some systems are limited by the type of soil or the topography found on a field. The costs of maintenance and expected life of the rehabilitation along with an array of annual costs like energy, water, depreciation, land preparation, maintenance, labour and taxes should be included in the selection of an irrigation system.

1.2.3 Topographical characteristics

Topography is a major factor affecting irrigation, particularly surface irrigation. Of general concern are the location and elevation of the water supply relative to the field boundaries, the area and configuration of the fields, and access by roads, utility lines (gas, electricity, water, etc.), and migrating herds whether wild or domestic. Field slope and its uniformity are two of the most important topographical factors. Surface systems, for instance, require uniform grades in the 0-5 percent range.

1.2.4 Soils

The soil's moisture-holding capacity, intake rate and depth are the principal criteria affecting the type of system selected. Sandy soils typically have high intake rates and low soil moisture storage capacities and may require an entirely different irrigation strategy than the deep clay soil with low infiltration rates but high moisture-storage capacities. Sandy soil requires more frequent, smaller applications of water whereas clay soils can be irrigated less frequently and to a larger depth. Other important soil properties influence the type of irrigation system to use. The physical, biological and chemical interactions of soil and water influence the hydraulic characteristics and filth. The mix of silt in a soil influences crusting and erodibility and should be considered in each design. The soil influences crusting and erodibility and should be considered in each design. The distribution of soils may vary widely over a field and may be an important limitation on some methods of applying irrigation water.

1.2.5 Water supply

The quality and quantity of the source of water can have a significant impact on the irrigation practices. Crop water demands are continuous during the growing season. The soil moisture reservoir transforms this continuous demand into a periodic one which the irrigation system can service. A water supply with a relatively small discharge is best utilized in an irrigation system which incorporates frequent applications. The depths applied per irrigation would tend to be smaller under these systems than under systems having a large discharge which is available less frequently. The quality of water affects decisions similarly. Salinity is generally the most significant problem but other elements like boron or selenium can be important. A poor quality water supply must be utilized more frequently and in larger amounts than one of good quality.

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1.2.6 Crops

The yields of many crops may be as much affected by how water is applied as the quantity delivered. Irrigation systems create different environmental conditions such as humidity, temperature, and soil aeration. They affect the plant differently by wetting different parts of the plant thereby introducing various undesirable consequences like leaf burn, fruit spotting and deformation, crown rot, etc. Rice, on the other hand, thrives under ponded conditions. Some crops have high economic value and allow the application of more capital-intensive practices. Deep-rooted crops are more amenable to low-frequency, high-application rate systems than shallow-rooted crops.

1.2.7 Social influences

Beyond the confines of the individual field, irrigation is a community enterprise. Individuals, groups of individuals, and often the state must join together to construct, operate and maintain the irrigation system as a whole. Within a typical irrigation system there are three levels of community organization. There is the individual or small informal group of individuals

participating in the system at the field and tertiary level of conveyance and distribution. There are the farmer collectives which form in structures as simple as informal organizations or as complex as irrigation districts. These assume, in addition to operation and maintenance, responsibility for allocation and conflict resolution. And then there is the state organization responsible for the water distribution and use at the project level.

Irrigation system designers should be aware that perhaps the most important goal of the irrigation community at all levels is the assurance of equity among its members. Thus the operation, if not always the structure, of the irrigation system will tend to mirror the community view of sharing and allocation.

Irrigation often means a technological intervention in the agricultural system even if irrigation has been practiced locally for generations. New technologies mean new operation and maintenance practices. If the community is not sufficiently adaptable to change, some irrigation systems will not succeed.

1.2.8 External influences

Conditions outside the sphere of agriculture affect and even dictate the type of system selected. For example, national policies regarding foreign exchange, strengthening specific sectors of the local economy, or sufficiency in particular industries may lead to specific

irrigation systems being utilized. Key components in the manufacture or importation of system elements may not be available or cannot be efficiently serviced. Since many irrigation projects are financed by outside donors and lenders, specific system configurations may be precluded because of international policies and attitudes.

1.2.9 Summary

The preceding discussion of factors affecting the choice of irrigation systems at the farm level is not meant to be exhaustive. The designer, evaluator, or manager of irrigation systems should be aware of the broader setting in which irrigated agriculture functions. Ignorance has led to many more failures or inadequacies than has poor judgement or poor training.

As the remainder of this guide deals with specific surface irrigation issues, one needs to be reminded that much of the engineering practice is art rather than science. Experience is often a more valuable resource than computational skill, but both are needed. It is a poor

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1.3 Advantages and disadvantages of surface irrigation

1.3.1 Advantages 1.3.2 Disadvantages

The term 'surface irrigation' refers to a broad class of irrigation methods in which water is distributed over the field by overland flow. A flow is introduced at one edge of the field and covers the field gradually. The rate of coverage (advance) is dependent almost entirely on the differences between the discharge onto the field and the accumulating infiltration into the soil. Secondary factors include field slope, surface roughness, and the geometry or shape of the flow cross-section.

The practice of surface irrigation is thousands of years old. It collectively represents perhaps as much as 95 percent of common irrigation activity today. The first water supplies were developed from stream or river flows onto the adjacent flood plain through simple check-dams and a canal to distribute water to various locations where farmers could then allocate a portion of the flow to their fields. The low-lying soils served by these diversions were typically high in clay and silt content and tended to be most fertile. The land slope was normally small because of the structure of the flood plain itself.

With the advent of modern equipment for moving earth and pumping water, surface irrigation systems were extended to upland areas and lands quite separate from the flood plain of local rivers and streams. These lands tend to have more variable soils and topographies, are usually better drained, and may be naturally less fertile. Thus, these lands usually require greater attention to design and operation.

1.3.1 Advantages

Surface irrigation offers a number of important advantages at both the farm and project level. Because it is so widely utilized, local irrigators generally have at least minimal understanding of how to operate and maintain the system. In addition, surface systems are often more

acceptable to agriculturalists who appreciate the effects of water shortage on crop yields since it appears easier to apply the depths required to refill the root zone.

The second advantage of surface irrigation is that these systems can be developed at the farm level with minimal capital investment. The control and regulation structures are simple, durable and easily constructed with inexpensive and readily-available materials like wood, concrete, brick and mortar, etc. Further, the essential structural elements are located at the edges of the fields which facilitates operation and maintenance activities. The major capital expense of the surface system is generally associated with land grading, but if the topography is not too undulating, these costs are not great. Recent developments in surface irrigation technology have largely overcome the irrigation efficiency advantage of sprinkler and trickle systems. An array of automating devices roughly equates labour requirements. The major trade-off between surface and pressurized methods lies in the relative costs of land levelling for effective gravity distribution and energy for pressurization. Energy requirements for surface irrigation systems come from gravity. This is a significant advantage in today's economy. Another advantage of surface systems is that they are less affected by climatic and water quality characteristics. Even moderate winds can seriously reduce the effectiveness of

sprinkler systems. Sediments and other debris reduce the effectiveness of trickle systems but may actually aid the performance of the surface systems. Salinity is less of a problem under surface irrigation than either of these pressurized systems.

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There are other advantages specific to individual regions that might be mentioned. Surface systems are better able to utilize water supplies that are available less frequently, more uncertain, and more variable in rate and duration. The gravity flow system is a highly flexible, relatively easily-managed method of irrigation.

1.3.2 Disadvantages

There is one disadvantage of surface irrigation that confronts every designer and irrigator. The soil which must be used to convey the water over the field has properties that are highly varied both spatially and temporally. They become almost undefinable except immediately preceding the watering or during it. This creates an engineering problem in which at least two of the primary design variables, discharge and time of application, must be estimated not only at the field layout stage but also judged by the irrigator prior to the initiation of every surface irrigation event. Thus while it is possible for the new generation of surface irrigation methods to be attractive alternatives to sprinkler and trickle systems, their associated design and

management practices are much more difficult to define and implement.

Although they need not be, surface irrigation systems are typically less efficient in applying water than either sprinkler or trickle systems. Many are situated on lower lands with heavier soils and, therefore, tend to be more affected by waterlogging and soil salinity if adequate drainage is not provided. The need to use the field surface as a conveyance and distribution facility requires that fields be well graded if possible. Land levelling costs can be high so the surface irrigation practice tends to be limited to land already having small, even slopes. Surface systems tend to be labour-intensive. This labour need not be overly skilled. But to achieve high efficiencies the irrigation practices imposed by the irrigator must be carefully implemented. The progress of the water over the field must be monitored in larger fields and good judgement is required to terminate the inflow at the appropriate time. A consequence of poor judgement or design is poor efficiency.

One sometimes important disadvantage of surface irrigation methods is the difficulty in applying light, frequent irrigations early and late in the growing season of several crops. For example, in heavy calcareous soils where crust formation after the first irrigation and prior to the germination of crops, a light irrigation to soften the crust would improve yields

substantially. Under surface irrigation systems this may be unfeasible or impractical as either the supply to the field is not readily available or the minimum depths applied would be too great.

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2. Surface irrigation systems

2.1 Introduction to surface irrigation

2.2 Surface irrigation methods

2.3 Requirements for optimal performance 2.4 Surface irrigation structures

2.1 Introduction to surface irrigation

2.1.1 Definition

Surface irrigation has evolved into an extensive array of configurations which can be broadly classified as: (1) basin irrigation; (2) border irrigation; (3) furrow irrigation; and (4) uncontrolled flooding. As noted previously, there are two features that distinguish a surface irrigation

system: (a) the flow has a free surface responding to the gravitational gradient; and (b) the on-field means of conveyance and distribution is the on-field surface itself.

A surface irrigation event is composed of four phases as illustrated graphically in Figure 1. When water is applied to the field, it 'advances' across the surface until the water extends over the entire area. It may or may not directly wet the entire surface, but all of the flow paths have been completed. Then the irrigation water either runs off the field or begins to pond on its surface. The interval between the end of the advance and when the inflow is cut off is called the wetting or ponding phase. The volume of water on the surface begins to decline after the water is no longer being applied. It either drains from the surface (runoff) or infiltrates into the soil. For the purposes of describing the hydraulics of the surface flows, the drainage period is segregated into the depletion phase (vertical recession) and the recession phase (horizontal recession). Depletion is the interval between cut off and the appearance of the first bare soil under the water. Recession begins at that point and continues until the surface is drained.

Figure 1. Time-space trajectory of water during a surface irrigation showing its advance, wetting, depletion and recession phases.

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The time and space references shown in Figure 1 are relatively standard. Time is cumulative since the beginning of the irrigation, distance is referenced to the point water enters the field. The advance and recession curves are therefore trajectories of the leading and receding edges of the surface flows and the period defined between the two curves at any distance is the time water is on the surface and therefore also the time water is infiltrating into the soil. It is useful to note here that in observing surface irrigation one may not always observe a ponding, depletion or recession phase. In basins, for example, the post-cut off period may only involve a depletion phase as the water infiltrates vertically over the entire field. Likewise, in the irrigation of paddy rice, an irrigation very often adds to the ponded water in the basin so there is neither advance nor recession - only wetting or ponding phase and part of the depletion phase. In furrow systems, the volume of water in the furrow is very often a small part of the total supply for the field and it drains rapidly. For practical purposes, there may not be a depletion phase and recession can be ignored. Thus, surface irrigation may appear in several configurations and operate under several regimes.

2.1.2 Scope of the guide

The surface irrigation system is one component of a much larger network of facilities diverting and delivering water to farmlands. Figure 2 illustrates the 'irrigation system' and some of its features. It may be divided into the following four component systems: (1) water supply; (2) water conveyance or delivery; (3) water use; and (4) drainage. For the complete system to work well, each must work conjunctively toward the common goal of promoting maximum on-farm production. Historically, the elements of an irrigation system have not functioned well as a system and the result has too often been very low project irrigation efficiencies.

The focus of surface irrigation engineering is at the water use level, the individual irrigated field. For design and evaluation purposes, these guidelines will note elements of the

conveyance and distribution system, especially those near the field such as flow measurement and control, but will leave detailed treatment to other technical sources.

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Figure 2. Typical irrigation system components (redrafted from USDA-SCS, 1967)

2.1.3 Evolution of the practice

Although surface irrigation is thousands of years old, the most significant advances have been made within the last decade. In the developed and industrialized countries, land holdings have become as much as 10-20 times as large, and the number of farm families has dropped

sharply. Very large mechanized farming equipment has replaced animal-powered planting, cultivating and harvesting operations. The precision of preparing the field for planting has improved by an order of magnitude with the advent of the laser-controlled land grading equipment. Similarly, the irrigation works themselves are better constructed because of the application of high technology equipment.

The changes in the lesser-developed and developing countries are less dramatic. In the lesser-developed countries, trends toward land consolidation, mechanization, and more elaborate system design and operation are much less apparent. Most of these farmers own and operate farms of 1-10 hectares, irrigate with 20-40 litres per second and rely on either small mechanized equipment or animal-powered farming implements.

Probably the most interesting evolution in surface irrigation so far as this guide is concerned is the development and application of microcomputers and programmable calculators to the design and operation of surface irrigation systems. In the late 1970s, a high-speed

microcomputer technology began to emerge that could solve the basic equations describing the overland flow of water quickly and inexpensively. At about the same time, researchers like Strelkoff and Katapodes (1977) made major contributions with efficient and accurate

numerical solutions to these equations. Today in the graduate and undergraduate study of surface irrigation engineering, microcomputer and programmable calculator utilization is, or should be, common practice.

Microcomputers and programmable calculators provide several features for today's irrigation engineers and technicians. They allow a much more comprehensive treatment of the vital hydraulic processes occurring both on the surface and beneath it. One can find optimal designs and management practices for a multitude of conditions because designs historically requiring days of effort are now made in seconds. The effectiveness of existing practices or proposed ones can be predicted, even to the extent that control systems operating, sensing and adjusting on a real-time basis are possible.

2.2 Surface irrigation methods

2.2.1 Basin irrigation 2.2.2 Border irrigation 2.2.3 Furrow irrigation 2.2.4 Uncontrolled flooding

The classification of surface methods is perhaps somewhat arbitrary in technical literature. This has been compounded by the fact that a single method is often referred to with different names. In this guide, surface methods are classified by the slope, the size and shape of the field, the end conditions, and how water flows into and over the field.

Each surface system has unique advantages and disadvantages depending on such factors as were listed earlier like: (1) initial cost; (2) size and shape of fields; (3) soil characteristics;

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(4) nature and availability of the water supply; (5) climate; (6) cropping patterns; (7) social preferences and structures; (8) historical experiences; and (9) influences external to the surface irrigation system.

2.2.1 Basin irrigation

Basin irrigation is the most common form of surface irrigation, particularly in regions with layouts of small fields. If a field is level in all directions, is encompassed by a dyke to prevent runoff, and provides an undirected flow of water onto the field, it is herein called a basin. A basin is typically square in shape but exists in all sorts of irregular and rectangular

configurations. It may be furrowed or corrugated, have raised beds for the benefit of certain crops, but as long as the inflow is undirected and uncontrolled into these field modifications, it remains a basin. Two typical examples are shown in Figure 3, which illustrate the most common basin irrigation concept: water is added to the basin through a gap in the perimeter dyke or adjacent ditch.

Figure 3. Typical irrigated basins (from Walker and Skogerboe, 1987)

a. large basin in the USA b. paddy basin in Asia

There are few crops and soils not amenable to basin irrigation, but it is generally favoured by moderate to slow intake soils, deep-rooted and closely spaced crops. Crops which are sensitive to flooding and soils which form a hard crust following an irrigation can be basin irrigated by adding furrowing or using raised bed planting. Reclamation of salt-affected soils is easily accomplished with basin irrigation and provision for drainage of surface runoff is

unnecessary. Of course it is always possible to encounter a heavy rainfall or mistake the cut-off time thereby having too much water in the basin. Consequently, some means of

emergency surface drainage is good design practice. Basins can be served with less command area and field watercourses than can border and furrow systems because their level nature allows water applications from anywhere along the basin perimeter. Automation is easily applied.

Basin irrigation has a number of limitations, two of which, already mentioned, are associated with soil crusting and crops that cannot accommodate inundation. Precision land levelling is very important to achieving high uniformities and efficiencies. Many basins are so small that precision equipment cannot work effectively. The perimeter dykes need to be well maintained to eliminate breaching and waste, and must be higher for basins than other surface irrigation methods. To reach maximum levels of efficiency, the flow per unit width must be as high as possible without causing erosion of the soil. When an irrigation project has been designed for either small basins or furrows and borders, the capacity of control and outlet structures may not be large enough to improve basins.

2.2.2 Border irrigation

Border irrigation can be viewed as an extension of basin irrigation to sloping, long rectangular or contoured field shapes, with free draining conditions at the lower end. Figure 4 illustrates a typical border configuration in which a field is divided into sloping borders. Water is applied to individual borders from small hand-dug checks from the field head ditch. When the water is shut off, it recedes from the upper end to the lower end. Sloping borders are suitable for nearly any crop except those that require prolonged ponding. Soils can be efficiently irrigated which have moderately low to moderately high intake rates but, as with basins, should not form dense crusts unless provisions are made to furrow or construct raised borders for the crops. The stream size per unit width must be large, particularly following a major tillage operation,

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although not so large for basins owing to the effects of slope. The precision of the field topography is also critical, but the extended lengths permit better levelling through the use of farm machinery.

Figure 4. Typical border irrigated field

2.2.3 Furrow irrigation

Furrow irrigation avoids flooding the entire field surface by channelling the flow along the primary direction of the field using 'furrows,' 'creases,' or 'corrugations'. Water infiltrates through the wetted perimeter and spreads vertically and horizontally to refill the soil reservoir. Furrows are often employed in basins and borders to reduce the effects of topographical variation and crusting. The distinctive feature of furrow irrigation is that the flow into each furrow is independently set and controlled as opposed to furrowed borders and basins where the flow is set and controlled on a border by border or basin by basin basis.

Furrows provide better on-farm water management flexibility under many surface irrigation conditions. The discharge per unit width of the field is substantially reduced and topographical variations can be more severe. A smaller wetted area reduces evaporation losses. Furrows provide the irrigator more opportunity to manage irrigations toward higher efficiencies as field conditions change for each irrigation throughout a season. This is not to say, however, that furrow irrigation enjoys higher application efficiencies than borders and basins.

There are several disadvantages with furrow irrigation. These may include: (1) an

accumulation of salinity between furrows; (2) an increased level of tailwater losses; (3) the difficulty of moving farm equipment across the furrows; (4) the added expense and time to make extra tillage practice (furrow construction); (5) an increase in the erosive potential of the flow; (6) a higher commitment of labour to operate efficiently; and (7) generally furrow systems are more difficult to automate, particularly with regard to regulating an equal discharge in each furrow. Figure 5 shows two typical furrow irrigated conditions.

Figure 5. Furrow irrigation configurations (after USDA-SCS, 1967) (a) graded furrow irrigation system

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2.2.4 Uncontrolled flooding

There are many cases where croplands are irrigated without regard to efficiency or uniformity. These are generally situations where the value of the crop is very small or the field is used for grazing or recreation purposes. Small land holdings are generally not subject to the array of surface irrigation practices of the large commercial farming systems. Also in this category are the surface irrigation systems like check-basins which irrigate individual trees in an orchard, for example. While these systems represent significant percentages in some areas, they will not be discussed in detail in this paper. The evaluation methods can be applied if desired, but the design techniques are not generally applicable nor need they be since the irrigation practices tend to be minimally managed.

2.3 Requirements for optimal performance

2.3.1 Inlet discharge control

2.3.2 Wastewater recovery and reuse

There is substantial field evidence that surface irrigation systems can apply water to croplands uniformly and efficiently, but it is the general observation that most such systems operate well below their potential. A very large number of causes of poor surface irrigation performance have been outlined in the technical literature. They range from inadequate design and

management at the farm level to inadequate operation of the upstream water supply facilities. However, in looking for a root cause, one most often retreats to the fact that infiltration

changes a great deal from irrigation to irrigation, from soil to soil, and is neither predictable nor effectively manageable. The infiltration rates are an unknown variable in irrigation practice. In those cases where high levels of uniformity and efficiency are being achieved, irrigators utilize one or more of the following practices: (1) precise and careful field preparation; (2) irrigation scheduling; (3) regulation of inflow discharges; and (4) tailwater runoff restrictions, reduction, or reuse. Land preparation is largely a land grading problem which will be discussed in Section 5. Irrigation scheduling is a theme covered separately by several publications such as the FAO Irrigation and Drainage Paper 24 (Rev) by Doorenbos and Pruitt (FAO, 1977). The attention here then is focused on inflow regulation and tailwater control.

2.3.1 Inlet discharge control

Surface irrigation systems have two principal sources of inefficiency, deep percolation and surface runoff or tailwater The remedies are competitive. To minimize deep percolation the advance phase should be completed as quickly as possible so that the intake opportunity time over the field will be uniform and then cut the inflow off when enough water has been added to refill the root zone. This can be accomplished with a high, but non-erosive, discharge onto the field. However, this practice increases the tailwater problem because the flow at the

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downstream end must be maintained until a sufficient depth has infiltrated. The higher inflow reaches the end of the field sooner but it increases both the duration and the magnitude of the runoff.

There are three options available to solve this problem, at least partially: (1) dyke the downstream end to prevent runoff as in basin irrigation; (2) reduce the inflow discharge to a rate more closely approximating the cumulative infiltration along the field following the advance phase, a practice termed 'cutback'; or (3) select a discharge which minimizes the sum of deep percolation and tailwater losses, i.e., optimize the field inflow regime. Examples of these alternative practices are discussed and illustrated in Section 5. In this configuration, the head ditch is divided into a series of level bays which are differentiated by a small change in elevation. Water levels are regulated in two bays simultaneously so that the lower bay has sufficient head to produce an advance phase flow in the furrows while in the upper bay the head is only sufficient to produce the cutback flow. Thus, the system operates by moving the check-dam from bay to bay along the upper end of the field.

Two very recent additions to the efforts to control surface irrigation systems more effectively are the 'Surge Flow' system (Figure 6) developed at Utah State University, USA and the 'Cablegation' system developed at the US Department of Agriculture's Snake River Water Conservation Research Center in Kimberly, Idaho, USA. These systems will be dealt with in more detail in a later section.

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2.3.2 Wastewater recovery and reuse

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runoff to improve surface irrigation performance. Reuse systems have not been widely

employed historically because water and energy have been inexpensive. Even today it is often more economical to regulate the inflow rather than to collect and pump the runoff back to the head of the field or to another field, tailwater reuse systems are more cost-effective when the water can be added to the flow serving lower fields and thereby saving the cost of pumping.

2.4 Surface irrigation structures

2.4.1 Diversion structures

2.4.2 Conveyance, distribution and management structures 2.4.3 Field distribution systems

Surface irrigation systems are supported by a number of on- and off-farm structures which control and manage the flow and its energy. In order to facilitate efficient surface irrigation, these structures should be easily and cheaply constructed as well as easy to manage and maintain. Each should be standardized for mass production and fabrication in the field by farmers and technicians.

It is not the intent of this guide to be comprehensive with regard to the selection and design of these structures since other sources are available, but it is worthwhile to note some of these structures by way of presenting a larger view of surface irrigation.

The structural elements of a surface system perform several important functions which

include: (1) turning the flow to a field on and off; (2) conveying and distributing the flow among fields; (3) water measurement, sediment and debris removal, water level stabilization; and (4) distribution of water onto the field.

2.4.1 Diversion structures

Most surface irrigation systems derive their water supplies from canal systems operated by public or semi-public irrigation departments, districts, or companies. Some irrigation water is supplied in piped delivery systems and some directly pumped from groundwater. Diversion structures perform several tasks including (1) on-off water control which allows the supply agency to allocate its supply and protects the fields below the diversion from untimely flooding; (2) regulation and stabilization of the discharge to the requirements of field channels and watercourse distribution systems; (3) measurement of flow at the turnout in order to establish and protect water entitlements; and (4) protection of downstream structures by controlling sediments and debris as well as dissipating excess kinetic energy in the flow. A typical turnout structure is shown in Figure 7.

Figure 7. Typical turnout from a canal or lateral (from walker end Skogerboe, 1987)

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2.4.2 Conveyance, distribution and management structures

Conveying water to the field requires similar structures to those found in major canal networks. The conveyance itself can be an earthen ditch or lateral, a buried pipe, or a lined ditch. Lined sections can be elevated as shown in Figure 8, or constructed at surface level. Pipe materials are usually plastic, steel, concrete, clay, or asbestos cement, or they may be as simple as a wooden or bamboo construction. Lining materials include slip-form cast-in-place, or

prefabricated concrete (Figure 9), shotcrete or gunite, asphalt, surface and buried plastic or rubber membranes, and compacted earth.

Figure 8. Elevated concrete channel in Iran Figure 9. Slip-form concrete lining in the USA

The management of water in the field channels involves flow measurement, sediment and debris removal, divisions, checks, drop-energy dissipators, and water level regulators. Some of the more common flow control structures for open channels are shown in Figure 10.

Associated with these are various flow measuring devices like weirs, flumes, and orifices. The designs of these structures have been standardized since they are small in size and capacity. Designs for flow measurement and drop-energy dissipator structures need more attention and construction must be more precise since their hydraulic responses are quite sensitive to their dimensions.

Figure 10. On-farm water management structures (from Skogerboe et al., 1971)

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b. a typical check-divider

2.4.3 Field distribution systems

After the water reaches the field ready to be irrigated, it is distributed onto the field by a variety of means, both simple and elaborately constructed. Most fields have a head ditch or pipeline

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running along the upper side of the field from which the flow is distributed onto the field. In a field irrigated from a head ditch, the spreading of water over the field depends somewhat on the method of surface irrigation. For borders and basins, open or piped cutlets as illustrated in Figure 11 are generally used. Furrow systems use outlets which can be directed to each furrow.

Figure 11. Head ditch outlets for borders and basins (after Kraatz and Mahajan, FAO, 1975)

Figure 12 shows a system in which siphon tubes are used as a means of serving each furrow. Field distribution and spreading can also be through portable pipelines running along the surfaces or permanent pipelines running underground. Basins and borders usually receive water through buried pipes serving one or more gated risers within each basin or border. A typical riser outlet, known as an alfalfa valve, is shown in Figure 13. The most common piped method of furrow irrigation uses plastic or aluminium gated pipe like that shown in Figure 14. The gated pipe may be connected to the main water supply via a piped distribution network with a riser assembly like the one shown in Figure 13, directly to a canal turnout, or through an open channel to a piped transition.

Figure 12. Siphons for furrow irrigation

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More details

3. Field measurements

3.1 Field topography and configuration

3.2 Determining water requirements 3.3 Infiltration

3.4 Flow measurement 3.5 Field evaluation

The evaluation of surface irrigation at the field level is an important aspect of both

management and design. Field measurements are necessary to characterize the irrigation system in terms of its most important parameters, to identify problems in its function, and to develop alternative means for improving the system.

System characterization necessitates a series of basic field measurements before, during, and after the irrigation. The objectives of the evaluation will dictate whether the field measurements are comprehensive or are simplified for special purposes. In some cases, there are alternative methodologies and equipment for accomplishing the same ends. The selection provided herein is based on a limited selection found to be most useful during numerous field evaluations and, in some measure, the practicality in the international sense.

Five classes of field measurements are presented: (1) field topography and configuration; (2) water requirements; (3) infiltration; (4) flow measurement; and (5) irrigation phases.

3.1 Field topography and configuration

All field evaluations should include a relatively simple assessment of the field topography and layout. These measurements are well enough known that only their brief mention is required. There is first of all the field's primary elevations. This information requires that a surveying instrument be used to measure elevations of the principal field boundaries (including dykes if present), the elevation of the water supply inlet (an invert and likely maximum water surface elevation), and the elevations of the surface and subsurface drainage system if possible. These measurements need not be comprehensive nor as formalized as one would expect for a land levelling project.

The field topography and geometry should be measured. This requires placing a simple reference grid on the field, usually by staking, and then surveying the elevations of the field surface at the grid points to establish slope and slope variations. Usually one to three lines of stakes placed 20-30 metres apart or such that 5-10 points are measured along the expected flow line will be sufficient. For example, a border or basin would require at most three stake lines, a furrow system as little as one, depending on the uniformity of the topography. The survey should establish the distance of each grid point from the field inlet as well as the field

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dimensions (length of the field in the primary direction of water movement as well as field width). There are important items of information that should be available from the survey: (1) the field slope and its uniformity in the direction of flow and normal to it; (2) the slope and area of the field; and (3) a reference system in the field establishing distance and elevation

changes.

It is also worthwhile at this stage of the evaluation to record the location and extent of major soil types (this may require sampling and some laboratory analyses). The cropping pattern should be determined and, if a crop is on the field at the time of the evaluation, any obvious differences in growth and vigour should be noted. Similarly, the cultivation practices should be recorded.

3.2 Determining water requirements

3.2.1 Evapotranspiration and drainage requirements 3.2.2 Soil moisture principles

3.2.3 Soil moisture measurements

3.2.4 An example problem on soil moisture

The irrigation system may not be designed to supply the total amount of moisture required for crop growth. In some cases, precipitation or upward flow from a water table may contribute substantially towards fulfilling crop water requirements. It is also unrealistic to expect that irrigation can be practiced without losses due to deep percolation, or tailwater runoff. The fraction of the water that is used should be maximized, but this fraction cannot be 100 percent without other serious problems developing such as a salt build-up in the crop root zone. The dependency on irrigation in an area requires some analyses of the water balance. Water balance may have three perspectives. The first is the balance of agricultural demands within a watershed as depicted in Figure 15. The outcome of such an analysis establishes the safe yield of water from various sources and thereby indicates the area of a project, the priorities among projects, and the configuration of the large systemic components of the project. An evaluation at the field level presumes that this information is available, and it should be generally understood in as much as the limits of on-farm irrigation may be dictated by the magnitude and distribution of the total water supply.

Figure 15. The perspective of water balance at the river basin level (from Walker, 1978)

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The second water balance perspective, illustrated in Figure 16, is the water balance within the farm or command area. An individual field is generally irrigated in concert with others in the command or farm through sharing the water delivered through a canal turnout or a well. Fields also typically share drainage channels. Water balance at the farm or command area level is established on a field's access to water, its priority, timing and duration. Again, a field

evaluation presumes that these factors have been formulated and can be determined. Figure 17 illustrates the perspective of water balance at the field level.

Figure 16. A perspective of the on-farm water balance

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The water balance within the confines of a field is a useful concept for characterizing,

evaluating or monitoring any surface irrigation system. In using this aspect of water balance, an important consideration is the time frame in which the computations are made, i.e. whether the balance will use annual data, seasonal data, or data describing a single irrigation event. If a mean annual water balance is computed, then it becomes reasonable that the change in

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root zone soil moisture storage could be assumed as zero. In some irrigated areas,

precipitation events are so light that the net rainfall can be reasonably assumed to equal the measured precipitation. Under other circumstances, various other terms can be neglected. In fact, the time base and field conditions are often selected to eliminate as many of the

parameters as possible in order to study the behaviour of single parameters.

One of the more important is crop evapotranspiration. The upward movement of groundwater to the root zone can usually be ignored if the water table is at least a metre below the root zone. Then if the soil moisture is measured before and after a period when there is no precipitation or irrigation, the depletion from the root zone is a viable estimate of crop water use.

There are two particularly important components in the field water balance which impact design and evaluation. The first is the irrigation requirement of the crop, or its

evapotranspiration and leaching needs. This is a design parameter and will be briefly

described here, but a detailed treatment is left to the FAO Irrigation and Drainage Paper 24, Crop Water Requirements, by Doorenbos and Pruitt (FAO, 1977). The second important component deals with field evaluation and concerns the nature of moisture content changes in the soil profile.

3.2.1 Evapotranspiration and drainage requirements

Evapotranspiration, ET, is dependent upon climatic conditions, crop variety and stage of growth, soil moisture depletion, and various physical and chemical properties of the soil. A two step procedure is generally followed in estimating ET: (1) the seasonal distribution of

reference crop "potential evapotranspiration", E_tp, which can be computed with standard formulae; and (2) the E_tp is adjusted for crop variety and stage of growth. Other factors like moisture stress can be ignored for the purposes of design computations.

There are perhaps twenty commonly used methods for calculating evapotranspiration, ranging in complexity from the Blaney-Criddle Method using primarily mean monthly temperature to more complete equations such as the Penman Method requiring radiation, temperature, wind velocity, humidity and other factors comprising the net energy balance at the crop canopy. The actual crop water demand depends on its stage of development and variety. Generally it is estimated by multiplying E_tp by a crop growth stage coefficient, k_CO. Values of k_CO have been published by Jensen (1973), Kincaid and Heermann (1974) and Doorenbos and Pruitt (FAO, 1977) for a wide range of crops grown worldwide.

Some irrigation water should be applied in excess of the storage capacity of the soil to leach salts from the rooting region, although this does not have to be achieved during each irrigation event. It can usually be applied on an annual basis. As a matter of practicality, the normally occurring deep percolation under most surface irrigation systems exceeds the leaching

fraction necessary for salt balance, particularly for the first and second irrigations each season when deep percolation losses are typically greatest. In addition, precipitation helps leach salts throughout the year. Nevertheless some irrigated areas maintain a salt balance in the root zone with excess leaching during only years of plentiful water supplies, which may occur as infrequently as every three to eight years.

3.2.2 Soil moisture principles

Important soil characteristics in irrigated agriculture include: (1) the water-holding or storage capacity of the soil; (2) the permeability of the soil to the flow of water and air; (3) the physical features of the soil like the organic matter content, depth, texture and structure; and (4) the

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soil's chemical properties such as the concentration of soluble salts, nutrients and trace elements.

The total available water, TAW, for plant use in the root zone is commonly defined as the range of soil moisture held at a negative apparent pressure of 0.1 to 0.33 bar (a soil moisture level called 'field capacity') and 15 bars (called the 'permanent wilting point'). The TAW will vary from 25 cm/m for silty loams to as low as 6 cm/m for sandy soils. Some typical values of TAW, field capacity, permanent wetting point and miscellaneous features have been given in various texts. A typical summary is shown in Figure 18.

Figure 18. Relationships between soil types and total available soil moisture holding capacity, field capacity and wilting point (from Walker and

Skogerboe, 1987)

Other important soil parameters include its porosity, f , its volumetric moisture content, q ; its saturation, S; its dry weight moisture fraction, W; its bulk density, g _b; and its specific weight, g

s. The relationships among these parameters are as follows.

The porosity, f , of the soil is the ratio of the total volume of void or pore space, V_p, to the total soil volume V:

f = V_p/V (1)

The volumetric water content, q , is the ratio of water volume in the soil, V_W, to the total volume, V:

q = V_b/V (2)

The saturation, S, is the portion of the pore space filled with water: S = V_W/V_p (3)

These terms are further related as follows: q = S * f (4)

When a sample of field soil is collected and oven-dried, the soil moisture is reported as a dry weight fraction, W:

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To convert a dry weight soil moisture fraction into volumetric moisture content, the dry weight fraction is multiplied by the bulk density, g _b; and divided by specific weight of water, g _w which can be assumed to have a value of unity. Thus:

q = g _bW/g _w (6)

The g _b is defined as the specific weight of the soil particles, g _s, multiplied by the particle volume or one-minus the porosity:

g _b = g _b * (1 - f ) (7)

The volumetric moisture contents at field capacity, q fc, and permanent wilting point, q wp, then

are defined as follows: q _fc = g _bW_fc/g _w (8) q _wp = g _bW_wp/g _w (9)

where Wfc and Wwp are the dry weight moisture fractions at each point.

The total available water, TAW is the difference between field capacity and wilting point moisture contents multiplied by the depth of the root zone, RD (refer to Table 1):

TAW = (q fc - q wp) RD (10)

Table 1 AVERAGE ROOTING DEPTHS FOR COMMONLY GROWN CROPS 1

Crop Root Depth (metres)

Alfalfa 1.5 Almonds 1.8 Apricots 1.8 Artichokes 1.4 Asparagus 1.5 Bananas 0.9 Beans 0.9 Beets 0.8 Broccoli 0.5 Cabbage 0.5 Cantaloupes 1.5 Carrots 0.9 Cauliflower 0.6 Celery 0.4 Cherries 2.0 Citrus 1.4 Corn (maize) 1.3 Cotton 1.2

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Cucumber 1.1

Eggplant 0.9

Figs 1.5

Grains and flax 1.2

Grapes 1.5 Groundnuts. 0.7 Ladino clover 0.6 Lettuce 0.3 Melons 1.3 Milo (Sorghum) 1.2 Mustard 1.1 Olives 1.5 Onions 0.3 Palm Trees 0.9 Peaches 1.6 Pears 1.6 Peas 0.8 Peppers 0.9 Pineapple 0.5 Potatoes 0.9 Prunes 1.5 Pumpkins 1.8 Radishes 0.5 Safflower 1.5 Soybeans 1.0 Spinach 0.6 Squash (summer) 0.9 Strawberries 0.5 Sudan grass 1.8 Tomatoes 1.5 Turnips 0.9 Walnuts 2.0 Watermelon 1.2

Summarized from Marr (1967) and Doorenbos and Pruitt (FAO, 1977)

The Soil Moisture Deficit, SMD, is a measure of soil moisture between field capacity and existing moisture content, q _i, multiplied by the root depth:

SMD = (q fc - q i) * RD (11)

A similar term expressing the moisture that is allotted for depletion between irrigations is the 'Management Allowed Deficit', MAD. This is the value of SMD where irrigation should be scheduled and represents the depth of water the irrigation system should apply. Later this will be referred to as Z_req indicating the 'required depth' of infiltration.

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3.2.3 Soil moisture measurements

The soil moisture status requires periodic measurements in the field, from which one can project when the next irrigation should occur and what depth of water should be applied. Conversely, such data can indicate how much has been applied and its uniformity over the field. As noted in the previous subsections, bulk density, field capacity and the permanent wilting point are also needed.

There are numerous techniques for evaluating soil moisture. Perhaps the most useful are gravimetric sampling, the neutron probe and the touch-and-feel method.

i. Gravimetric sampling

Gravimetric sampling involves collecting a soil sample from each 15-30 cm of the soil profile to a depth at least that of the root penetration. Typical samplers are shown in Figure 19. The soil sample of approximately 100-200 grammes is placed in an air tight container of known weight (tare) and then weighed. The sample is then placed in an oven heated to 105° C for 24 hours with the container cover removed. After drying, the soil and container are again weighed and the weight of water determined as the before and after readings. The dry weight fraction of each sample can be calculated using Eq. 5. Knowing the bulk density, one can determine moisture contents from Eq. 6 and the soil moisture depletion from Eq. 11.

Figure 19. Small equipment used for collecting soil samples from the field

a. sampling auger b. sampling tube

ii. The neutron Probe

The neutron probe and scaler for making soil moisture measurements are illustrated in Figure 20. The neutron probe is inserted at various depths into an access tube and the count rate is read from the scaler. The manufacturers of neutron probe equipment furnish a calibration relating the count rate to volumetric soil moisture content. Field experience suggests that these calibrations are not always accurate under a broad range of conditions so it is advisable for the investigator to develop an individual calibration for each field or soil type. Most

calibration curves are linear, best fit lines of gravimetric data and scaler readings but may in some cases be slightly curvilinear (van Baval et al., 1963).

Figure 20. A neutron probe and scaler for soil moisture measurements (after Walker and Skogerboe, 1987)

The volume of soil actually monitored in readings by the neutron probe depends on the moisture content of the soil, increasing as the soil moisture decreases. The accuracy of soil moisture determinations near the ground surface is affected by a loss of neutrons into the atmosphere thereby influencing measurements prior to an irrigation more than afterwards. As a consequence, soil moisture measurements with a neutron probe are usually unreliable within 10-30 cm of the ground surface.

iii. Touch-and-feel

As a means of developing a rough estimate of soil moisture, the Touch-and-feel method can be used. A handful of soil is squeezed into a ball. Then the appearance of the squeezed soil can be compared subjectively to the descriptions listed in Table 2 to arrive at the estimated depletion level. Merriam (1960) has developed a similar table which gives the moisture deficiency in depth of water per unit depth of soil. Over the years various investigators have