Abstract:
This study presents the use of AquaCrop model in prediction of optimal onset dates for
wheat crop grown under ASAL conditions in Laikipia County, Kenya. Modeling was
done through simulation of root zone moisture content to reduce yield losses from crop
failure. The optimal sowing date(s) for rain-fed wheat was based on AquaCrop rainfall
criterion for generation of onset dates and optimization analysis of AquaCrop simulated
grain yields. The cumulative rainfall of 10 mm in 4 successive days’ onset criteria
equivalent to the calculated Readily Available Water (RAW) required at 10 cm soil depth
was adopted. Based on the onset criteria, a total of 57 simulations (19 years’ rainfall
record and three onsets: early, normal and wet) was run using AquaCrop model and
frequency analysis applied to the simulated yields to get the yield levels expected at
varying levels of probability of exceedance. The set threshold was incremental at 0.5
ton/ha level from 0-12 ton/ha. Probability of exceedance was zero for threshold mean
yields beyond 5.5, 11.0, 11.5 ton/ha for early (SD1), normal (SD2) and late (SD3) onset
respectively. At 20%, 50% and 80% probability of exceedance, the average expected yield
was less than or equal to 4.5, 0.5 and 0.38 ton/ha for SD1, 4.5, 0.75 and 0.38 ton/ha for
SD2 and 6.0, 0.75, and 0.38 ton/ha for SD3 respectively. Results indicated that zero
tillage optimized the yield in all the sowing dates selected. In conventional tillage, early
onset had high yield advantage for the viable early onset but the risk of failure was high
representing 22.11%. It is recommended that sowing date, which is a technology
problem, requires to be given a lot of attention.
