Abstract:
This paper presents evaluation of criteria to be used in advising farmers on the planting
dates in Lake Victoria Basin. The criteria were evaluated using relative yield over a 30
days period following sowing computed by means of soil water balance technique. Crop
failure or a false start to the season is indicated by a relative yield rate of less than 35%.
Due to the high ratio of computed relative yields in the region, the threshold values were
varied from 35% to 75% in order to capture the failure rates. Using this method, onset
dates were determined for 26 stations spatially distributed in the basin. Daily rainfall and
mean daily evaporation data for a period of 20-30 years starting from 1970 were used.
The criteria for accumulated rainfall depth over a specified period were evaluated. The
tested criteria using RAIN software included; the accumulation of 20 mm of rainfall in 3
days, accumulation of 40 mm in 4 days and accumulation of 60 mm in 6 days.
Comparison made between the accumulated depth criteria and those based on root zone
depletion equal to readily available water (RAW) in 4 days and the soil moisture content
at field capacity (SMC) in 4 days showed that the accumulation of 40mm in 4 days
criterion compared well with (RAW) criterion but the SMC criterion gave delayed onsets
in most cases. Results reveal that the accumulated depth criterion of 40mm in 4 days can
be used as an operational criterion
