Use of CROPWAT model to predict water use in irrigated tomato production at Kabete, Kenya

rainfed agriculture and simulated irrigation requirement for tomato production in the Kabete Field Station. The model predicted increased irrigation requirement for a tomato crop of 33.1, 28.1 and 36.6 mm of water, in the 1st, 2nd and 3rd 10-day periods of development stage, respectively. The crop evapotranspiration (ETc) requirements by tomato crop were predicted as 456.5 mm for the short rainy season while actual evapotranspiration (ETa) was 232.1 mm for the short rains giving a yield response factor of 0.49. The model suggested an addition of 253.7 mm of irrigation water in order to realize optimal tomato yields as the crop experienced an irrigation deficiency of 48.8%. The moisture deficit at harvest was 63.6 mm of water which resulted in total yield reduction of 51.3%. In relation to actual yields calculated, the mean potential optimal tomato yields in the study area were 23.3 Mg/ha with proper soil management and adequate water supply. The suggested supply system was at 10 days irrigation interval/stage where the soils were irrigated just below or above field capacity. Rainfall losses and irrigation requirements would be reduced to 41.9 and 267.7 mm, with minimum water deficit at harvest of 15.5 mm and an irrigation efficiency of 100%. At this point, ETa would equal ETm and optimal tomato yield would be obtained with yield losses predicted at 0.1%. Yield gap analysis revealed that radiation, sunshine and temperature are favourable for crop production, but the heavy dependence on rainfall makes the area very vulnerable to drought