ANALYSIS OF WATER BUDGETS IN SEMIARID LANDS FROM SOIL-WATER RECORDS

Four components of the water budget for a growing season, namely stora ge, drainage, transpiration and direct evaporation from the soil surfa ce, were estimated using a suite of techniques. The only data requirem ents were rainfall, neutron probe measurements of soil water content a nd microlysimeter measurements of evaporation from the soil. Data from four growing seasons at Kiboko, Kenya between 1990 and 1992 were used to provide examples of the estimations. Drainage was significant (abo ut 10% of rainfall) in one season only; in the other seasons, total ev aporation comprised at least 95% of the seasonal rainfall. Drainage wa s determined using a relationship between unsaturated hydraulic conduc tivity and soil water content that was determined during the early par t of the rainy season when water was penetrating to depth. This analys is made it possible to identify a critical water content at the base o f the soil profile, above which there would be significant drainage. H owever, there are large errors associated with estimation of drainage if significant drainage occurs. Estimates of direct evaporation from t he soil surface were used as the basis of distinguishing transpiration from total evaporation. Microlysimetry was used to develop a model of evaporation from these sandy soils, which was based on the assumption that the evaporation from the soil surface following heavy rainfall i s a unique function of time from rainfall, and little influenced by th e presence of a sparse crop. This method showed that direct evaporatio n from the soil accounted for between 70 and 85% of total evaporation in seasons when total evaporation estimates ranged from 150 to 325 mm.