Evaluation of criteria for use in determining growing season Onset dates in the Kenyan Lake victoria Basin

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