Water use by eight annual crops at Kabete. Kenya 1978/79
Investigations involving mainly food crops adapted to the Kabete area (the complete range of crops investigated being maize, field beans, Irish potatoes, sweet potatoes, wheat, sunflower, soyabeans and linseed) sought to obtain data on particular moisture aspects. These aspects included: (1) seasonal water use at different stages of growth by the different crops in relation to the natural rainfall, in both the short and long rainsf (2) the relationship between open free water evaporation, as calculated by the modified Penman method and Pan A data, and the actual water use of the crops; and (3) the yield of the crops studied during the time of the experiment a8 affected by water and other factors. Two methods were employed, namely the neutron probe and gravimetric methods for determining moisture content of the soil to a depth of 180 centimetres. Calibration of the probe machine showed that this method may not be very accurate for predicting volumetric moisture content in the Kabete soil, due to lack of precision at the higher moisture range (above 37%) and greater soil depths (90 + cm depth) due to high clay content. However, it was shown that volumetric moisture content and probe count ratio are linearly related (r= 0.8675 for a 0 - 30 centimetre depth and r= 0.7432 for 30 - 180 cm depth). Because of the difficulties encountered in obtaining adequate calibration curves, water use calculations for the crops were based on gravimetric samplings. The available moisture for the Kabete soil in a profile of 180 centimetres was about 161.5 millimetres which was in close agreement with what Pereira (1957) found for similar soils. Periodic and seasonal total water use by crops was found to be affected by the length of the crop's growing season, amount and frequency of the rainfall, soil dryness (mainly soil water availability)-and crop development. Irish potatoes used the least amount of water (266.1 millimetres (mm) in the short rains and 477.5 mm in the long rains). These were followed by field beans (299.5 mm (SR) and 478.9 mm (LR», ~-wheat (324.8 mm) linseed (400.1 mm), soyabeans (478.8 mm) sunflower (516.6 mm), maize (518.4 mm (SR) and 619.2 mm (LR» and sweet potatoes (629.3 mm (SR) and 641.4 mm (LR», respectively. Irish potatoes and field beans had short effective rooting depths of 50 - 70 cm range, followed by linseed which had 90 - 120 cm range of effective rooting depth. Wheat, maize, sweet potatoes, soyabeans and sunflower had effective rooting depth of 150 - 180 cm. Open free water evaporation values obtained from Pan A data were found to be consistently lower than those calculated from the modified Penman method (McCulloch, 1965). Thus the crop coefficients (Et/EO Penman) were consistently lower during the 'twoseasons, for all crops, than the pan coefficients (Et/E o Pan A). These coefficients were found to be influenced by the frequency of wetting and amount of rainfall as found by Kowal and Andrews (1973) and Wangati (1972), in addition to being influenced by the crops development and phenology. They were alao found to be'closely related to water consumption. The most efficient crap in terms of water use waa Irish potatoes, 36.42 kg/ha/mm of water use and 20.50 kg/ha/mm of water use were realised during the short and long rains respectively. Maize was the next most efficient crop, wlth\9.50 kg/ha/mm and ?22 kg/ha/mm)of water use during the short and long rains respectively. These were followed by sweet potatoes which had 6.93kg/ha/mm and 5.13 kg/ha/mm, field beans (2.42 kg/ha/mm and 2.80 kg/ha/mm) during the short and long rains respectively. Wheat, sunflower, soyabeans and linseed had 2.10, 1.24, 1.03 and 0.23 kg/ha/mm of water use during the short rains, respectively. However, water use efficiency reflected total yields and therefore moisture availability, in addition to other factors, such as pests and diseases, influencing yields.