Determination of Water Requirement and Crop Coefficient for Sorghum (Sorghum bicolor L.) at Melkassa, Ethiopia

Abstract

Knowledge of crop evapotranspiration (ET_c), the combined process of evaporation and plant transpiration, is important in agriculture for  scheduling farm  operations and designing and managing irrigation and drainage systems. Development of crop coefficient (K_c) can enhance crop evapotranspiration (ETc) estimates in specific crop growth stages. However, locally determined K_c information is not available for many important crops in Ethiopia. This research was, therefore, conducted to determine growth stage specific Kc and crop water use for sorghum (Sorghum bicolor L.) var. Gambella-1107 at the Melkassa Agricultural Research Center which is located in a semi arid climate zone in Ethiopia. Drainage type lysimeter was used to measure Gambella-1107 crop water use under water balance system on a clay loam soil and local weather data were used to determine the reference  evapotranspiration (ETo). Crop coefficient was developed from measured ET_c and ETo calculated using weather data. The growth stages of the crop were assessed as the seasonal change of plant height (pH), leaf area (LA) and leaf area index (LAI). The yield obtained was 5.3 t ha-1 and the measured LAI were 0.2, 4.2, 4.9 and 1.6 at the initial, development, midseason and late season stages, respectively. The maximum LAI was achieved when the plants reach their maximum height at mid-season stage with high crop evapotranspiration due to leaf enlargement that increases transpiration. The measured ET_c values were 53.8, 138.5, 214.4, and 94.0 mm during the initial, development, midseason and late-season stages, respectively, and the seasonal total value was 500.7 mm. The calculated K_c values for the crop were 0.45, 0.83, 1.18 and 0.78 during the initial, development, mid-season and late-season stages, respectively. These values were greater than those reported in FAO publication for  sorghum varieties which could be a result of soil, climate and crop genetic differences. This suggests the need for developing site-specific K_c values for proper irrigation management.