Estimation of Actual Irrigation Requirement by the Water Balance Method

Actual irrigation situation in a typical upland irrigation district was estimated by the water balance method during the 1993 rainy and the 1994 dry year periods. Soil moisture conditions were analysed for cases where daily district water use was supplied to all the cropped upland farms, and to reduced irrigation area percentages. Consumptive use was obtained through monthly weighted crop coefficients used to reduce penman's potential evapotranspiration. The moisture budget was examined at three levels of total readily available moisture, TRAM, i.e., 20,30 and 40 mm; the effect of upward capillary rise was also considered. The results of this study indicated that the actual moisture conditions in upland irrigated farms is influenced by irrigation area, cropping pattern, stage of crop growth and upward capillary rise in the farms. Actual irrigated area was within the range of about 44 to 100% of cropped area while upward capillary rise was between 0.1 and 1 mm day-1, TRAM level of 30 to 40 mm was considered most appropriate for the district. Monthly effective rainfall percentages from the water balance method were fairly close to that determined by the basic irrigation requirement method for some months. Basic irrigation requirement may therefore be considered to incorporate capillary water contribution