Different methods are used to determine when to irrigate. They may be grouped into the following.
- Plant indicator - Soil indicator
- Water budget techniques.
3.2.1 Plant Indicators
Monitoring crop or plants is the most direct method of determining when to irrigate. Since the primary objective of irrigation is to supply plant with the water they need when it is required. Normally, it is necessary to relate plant parameter to soil water content in order to determine the amount of irrigation.
a) Appearance and Growth
Visual indicator (appearance) of the need for water includes leaf wilting, shoot wilting and leaf colour. Measurement of stern diameter and height is monitored regularly to determine growth rate. The need to irrigate is indicated by a low growth rate. Appearance and growth are not after effective parameters for scheduling full irrigation since plant have been short of water for long time enough to adversely affect production when changes in appearance and for growth can be dictated. Crop appearance most be carefully interpreted since diseases and improper nutrient levels may produce changes in appearance similar to those associated with water stress. The primary advantage of appearance and growth as indicator of when to irrigate is simplicity.
b) Leaf Temperature
Rises in leaf temperature in addition to those related to increases in air temperature are associated with reduced transpiration rate resulting to partial or total stomata closure. Air and leaf temperature may be remotely sensed from the ground, aircraft and possibly satellite. One popular method to use of hand held infra red thermometer to measure the different between plant canopy or leave and ambient air temperature for each day (usually 1-1½ hours after noon). The numbers of degree by
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which company temperatures exceed air temperature for each day are accumulated until a certain critical level is reached. When this level which depends on the crop and soil is reached, it is time to irrigate. Days when company temperature is less than the air temperature are neglected.
c) Leaf Water Potential
The measurement of leaf water potential is another indicator of the plants need for water. Lower potentials indicate a greater need for water.
Leaf water potential measurement is a destructive procedure that involves the removing a leaf and placing it in a pressurized chamber.
The pressure in the chamber is slowly increased until fluid is forced from the leaf stem. The pressure required is a measure of the leaf moisture potential. Care must be taking in carrying out this measurement because leaf age, leaf exposure to solar radiation and time of the day can significantly affect the result. Usually, mature leaves are selected from a specific standardized location on the plants and measurements are made of a particular time of the day. Although commercial instruments for measuring leaf water potential by irrigators is not wide spread. Since considerable time, care and training are required to obtain reliable results.
d) Stomata Resistance
Stomata resistance is an index to the need for water since it is related to the degree of stomatal opening and the rate of transpiration. In general, high resistances indicate significant stomata closure, reduced transpiration rate and the need for water. Commercially available leaf or diffusion promoters are used to measure stomatal resistance. The large amount of time and the increase level of skill required to make and interprete stomatal resistance measurement limit their use to research purpose.
3.2.2 Soil Indicators
Soil based irrigation scheduling involves defer mining the current water content of the soil comparing it with a predetermined minimum water content and irrigating
a) Appearance and Feel Method
With experience, one can judge soil water content by the appearance and feel of the soil. The soil probe (anger) is needed to obtain soil sample from lower portion from the root zone for examination (Tables present guides for judging how much available water has been removed from the soil).
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b) Gravimetric Sampling
This is a direct method of measuring the water content of soil samples taken from a field. Samples are weight, dried at 105-110oc and re-weight after drying. Usually 24 hours is required to dry samples to a constant weight. Equation below is used to compute the percentage water content on a dry weight bases.
%MC = Mwet – Ddry x 100 Mdry
On a volume bases it is computed using the following equation
% Mc by volume = Volume of water in soil (vw) x 100
Total volume of soil and soil and voids (vt) Example = Given the following data calculate the soil water conContent
Weight of wet soil 100cm3 soil = 131g Weight of dry soil 100cm3 soil = 121g Solution = % Mc = Mwet – Mdry x 100
Mdry
= 131 – 121 x 100 121
= 10 x 100 121
= 8.26%
Although the gravimetric method is simple and reliable, the sample is distractive and techniques time consuming. In addition, data are least one day old when they became available for scheduling.
c) Tensiometer
Tensiometer is a ceramic cup filled with water and connected through a wetted filled tube to either a vacuum gange or a mercury monometer.
Water moves in and out of the cup in response to changes in soil water conContent. Tensiometer produces measurement of soil water potentials that are related to soil. Tensiometer may interfere with cultivation and can require considerable time for installation and maintenance. In addition, they have a relatively limited range of operation (0- 80 centibars (Pf).
3.2.3 Water Budgeting Techniques
Here you use the pan evaporation data of reference crop evapotranspiration to compute when to irrigate it is similar to soil indicator method. Instead of measuring soil water content, it is computed using the following equation.
0i = 0 (I – 1) – 100 (ET – Pe) I Drz
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Where 0i = soil water in percentage by volume at the end of the I and the i-1 respectively
Pe = Effective precipitation ET = Evapotranspiration Drz = Depth of root zone.
Other methods used in determining irrigation scheduling include farmers experience which farmers usually rely on, neuton probe, porous blocks and feeling the soil with hard.
SELF-ASSESSMENT EXERCISE
i. Define irrigation scheduling
ii. What are the benefit of irrigation scheduling
iii. List the plant indicator methods used in determining when to irrigate.