Abstract:
Field evaluations of irrigation systems play a fundamental role in improving surface
irrigation design and management. This study evaluated performance of large scale
irrigation scheme, Metahara sugar estate, in Awash River basin, Ethiopia. Field
evaluations of on-farm irrigation systems under operation were extensively performed,
aiming at improving both systems operation and irrigation scheduling.
Field measurements were conducted during selected irrigation events. Soil, crop, and
climatic data have been collected and analyzed. Irrigation scheduling and field
application performances have been evaluated followed by optimization to improve
irrigation scheduling and system operation. Microsoft excel optimizer 'solver', CROP
WAT 8 , and SIRMOD III model were used to optimize infiltration parameters,
calculate potential evapotranspiration and optimize field application performance
respectively.
The result showed that most of the irrigation events (about 80%) were late in timing that
means they were made after management allowable deficit was reached. The design net
depth of application of 75mm is lower than readily available water holding capacities of
major soils in the root zone. Average gross applied depth of irrigation were 136mm in
lOOm furrows and 207mm in 200m furrows which are by far bigger than the design
gross application of 103mm. The field application performance evaluated against revised
target depth of application showed that application efficiency, requirement efficiencies,
and distribution uniformities were generally higher in lOOm furrows and lower in 200m
furrows. It was also found that the performances were affected by irrigation timing.
To improve the irrigation scheduling different net depth of application for major soils are
recommended. These are llOmm in heavy clays and clays, 104mm in clay over loamy
soils and 87mm in loam soils. Irrigation interval varying across soil type, season, and
crop stages were determined. The result showed that determined irrigation intervals
generally are shorter than the existing "theoretical irrigation intervals" .To improve
operation management cut off time determined using SIRMOD III simulation for selected inflow rates are recommended.
