Estimation of crop water requirements in arid region using Penman–Monteith equation with derived crop coefficients: a case study on Acala cotton in Sudan Gezira irrigated scheme

The recommended Penman–Monteith reference crop evapotranspiration (ET0) with derived crop coefficients (Kc) from the phenomenological stages of Acala cotton is used to estimate the crop water requirements (CWRs) of Acala cotton in the Gezira area of Sudan. The published basal crop factors of Acala cotton were used with Penman–Monteith equation as well to estimate ET. The results were compared with the current practice that uses Penman evaporation (E0) from free water surface and crop factors (Kf) derived by Farbrother [Farbrother, H.G., 1970. Irrigation practices on Gezira clay-rates and intervals. Gezira miscellaneous paper no. 94. Gezira Research Station, Wad Medani, Sudan] and still in use in Sudan. The two methods were compared with the actual ET of Acala cotton measured by Fadl [Fadl, O.A., 1987. Water use of Acala cotton. Annual report 1978–1979. Gezira Research Station, Wad Medani, Sudan, pp. 143–147]. Penman–Monteith equation was found to be better than Farbrother method in terms of the total predicted CWR, coefficient of determination (r2), the slope of the linear regression line and the standard error of estimate with both basal and derived (Kc) values. The trends of weather examined for the period 1966–1993 showed an increasing ET0 during the rainy season due to the recent drought conditions that prevailed in the region. Care must be taken when predicting CWR during such period.