Primary and Secondary Data Collection

The study was conducted during the dry season when the crops are being cultivated under irrigation. The research work was started in the month of November 2012 and continued until March 30 2013. During the study period, regular visits and observations were made to assess the method of water applications and practices related to water management at the study sites. Data collected were including primary and secondary data which would help to achieve the research objectives.

3.3.1. Secondary data collection

Secondary data were collected from different sources, published and unpublished documents of respective offices and departments, related journals and books. Climatic data of the nearest station (Gelemso station) were collected from the National Meteorological Station and other relevant data from West Hararge Zone and Woreda Offices of Agriculture and Water resource and Energy.

27 3.3.2. Primary data collection

The primary data were obtained through field measurements and/or observations and laboratory analyses which include field topography and configurations, soil data, irrigation delivery and structures, irrigation phases and field irrigation method. To locate the boundary of the command area, actual canals network and location of canal structures, transverse survey was made with GPS (global positioning system). This was done by walking around the boundary of the command area and along canals and taking point data. This point data was transferred to map source then be downloaded to global mapper and GIS software, and then digitized to locate the command area with irrigation canal net work and layout within the boundary on Arc GIS.

3.3.3. Field survey

On the selected farmers' fields, topographic survey was made by placing stakes on the four corners of the plots and taking the elevation of the points using engineers’ level. The stakes were also placed 5 meters apart in the flow direction along the furrow. The distance of each point from the field inlet as well as the field dimensions (length of the field in the primary direction of water movement as well as field width) was determined. From this survey, the field slope and its uniformity in the direction of flow and normal to it and area of the field were determined.

3.3.3.1. Soil characterization

To know the particle size distribution, the organic matter content and pH, disturbed soil samples were taken from different points at depths of 0-30 cm and 30-60 cm in a zigzag fashion. After sampling the whole area, samples were mixed thoroughly. These composite samples were transferred to the sample box and were taken to the soil laboratory of Haramaya University for the intended analysis. To investigate the moisture content of the soil at field capacity (FC), permanent wilting point (PWP) and the bulk density undisturbed soil samples were collected with core sampler from the profiles pit excavated at three locations (upper,

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middle, and tail end of the scheme) up to a depth of 60 cm based on effective root depth of the crop (shallot) at 30 cm depth intervals and the samples were taken to the soil laboratory of Water Works Design and Supervision Enterprise for the analysis.

(i) Texture

The textural analysis was done by using hydrometric method. Then the soil textural classes were determined using USDA soil textural triangle (Bouyoucos, 1951 and Asadu, 1996).

(ii) Organic matter content

Titration method, which is oxidation under standardized condition with potassium dichromate in sulpheric acid, was followed for organic carbon determination. Finally, conversion of organic carbon to organic matter, therefore, was obtained by multiplying percentage organic carbon by 1.724 (Staney and Bernard, 1992).

(iii) pH measurement

pH of the soil was measured potentiometrically in the supernatant suspension of 1:1.5, soil: liquid mixture (Staney and Bernard, 1992).

(iv) Field capacity, permanent wilting point and moisture content determination

In the laboratory, soil samples were analyzed for field capacity (FC) and permanent wilting point (PWP) using pressure plate apparatus at 1/3 and 15 bar, respectively. The soil moisture content measurements before and two days after irrigation were made by gravimetric method which involves collecting soil samples with auger, weighing the wet soil samples, removing the water by drying in an oven at 1050 C and re-weighing the sample to determine the amount of water removed. Soil samples were taken to the laboratory six times at three growth stages of the shallot (initial stage, developmental stage and maturity stage) per plot. For every nine plots, soil samples were taken before irrigation and two days after irrigation from 0 - 30 cm

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and 30 – 60 cm depths per test pit. Soil moisture content in each sample was determined on weight basis using the equation.

Wds Wds Wws θdw  *100 (3.1) where,

θdw is the soil moisture content on weight basis (%) Wws is the weight of the wet soil sample (g),

Wds is the weight of the soil sample after oven drying (g) and then the moisture content of soil samples were converted to the volumetric water content ( ) by multiplying with bulk density (ρb) as:

ρb * θdw

θv  (3.2)

Soil moisture content was also expressed in terms of equivalent ( ) depth as:

Equivalent depth ( )10*θv(%) (3.3)

Total available water (TAW) which is an estimate of the amount of water a crop can use from the soil for the selected fields was computed from the moisture content in volume percent at field capacity and permanent wilting point (Allen et al., 1998):

TAW (mm) = 1000 * ( FC - PWP) * Zr (3.4)

where TAW is total available water in the root zone (mm), θFC is moisture content at field capacity (m3/m3), PWP is the moisture content at permanent wilting capacity (m3/m3) and Zr is the root depth (m)

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The potential soil moisture storage depth which is equal to the actual allowable depletion depth in the selected fields just before at the time of irrigation was computed (Walker and Skoerboe, 1987) as follows:

SMD (mm/m) = 10*[ FC - BI] (3.5)

Or SMD = [θ θ ]

[θ θ ] (3.6)

where SMD is the actual soil moisture depletion at the time of irrigation and it is the maximum amount of water which can be stored in the root zone at the moment of irrigation without deep percolation loss, FC and PWP are soil moisture contents in percent volume at FC, PWP and BI is soil moisture contents in percent volume before irrigation. The actual soil moisture depletion fraction at time of irrigation was computed using equation (3.6) and compared with the literature value depletion fraction (p) for the crop growth stages which was computed by numerical approximation for adjusting ptable value for (ETc ≠ 5 mm/day) using (Allen et al., 1998) equation given as:

P = Ptable + 0.04(5 - ETc) (3.7)

where, Ptable is the recommended P value equal to 0.30 for onion (ETc = 5mm/day) (Allen et

al., 1998). The actual moisture storage or retention (AMS) after irrigation in mm/m was

computed as:

AMS = 10*[θAI – θBI] (3.8)

where, θAI = the actual moisture storage after irrigation in volume percent, θBI = moisture content of the soil before irrigation in volume percent

31 (v) Bulk density

The core soil samples were dried at 105 °C for 24 hours and the bulk density was then calculated using the following equation.

Vc Ms

ρb  (3.9)

where, ρb = soil bulk density (g/cm3),

Ms = weight of dry soil (g), and

Vc = volume of core sampler (cm3)

3.3.3.2. Infiltration characterization

To determine infiltration characteristics of the soil in the scheme, double ring infiltrometer of 30 cm and 60 cm inner and outer ring diameters respectively were used at the selected representative three fields (from head, middle, and tail ends of the scheme). Double ring infiltrometer was driven to the depth of 15 cm into the soil by hammer. A minimum head of 10 cm was maintained in both rings during measurement. Depths to water levels were measured at increasing time intervals from the datum established on the edge of each cylinder.