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This paper presents the performance evaluation of four selected crop water production functions (CWPF). The general objective of the study was to test the capability and suitability of the models in predicting grain yield of a maize crop for a study area giving the models� input parameters. The four functions evaluated were Jensen [Jensen, M.E., 1968. Water consumption by agricultural plants. In: Kozlowski, T.T. (Ed.), Water Deficits in Plant Growth, vol. 1. Academic Press, New York, pp. 1�22], Minhas et al. [Minhas, B.S., Parkhand, K.S., Srinivasan, T.N., 1974. Towards the structure of a production function for wheat yields with dated input of irrigation water. Water Resour. Res. 10, 383�386], modified Stewart et al. [Stewart, J.L., Danielson, R.E., Hanks, R.J., Jackson, E.B., Hagon, R.M., Pruit, W.O., Franklin, W.T., Riley, J.P., 1977. Optimizing Crop Production through Control of Water and Salinity Levels in the Soil. Utah Water Res. Lab. PR. 151-1, Logan, UT, 191 pp.], and modified Bras and Corodova [Bras, R.L., Corodova, J.R., 1981. Intra-seasonal water allocation in deficit irrigation. Water Resour. Res. 17 (4), 886�874] models. Field experiments were carried out during the year 2004 and 2005 irrigation seasons of June�October at Igurusi ya Zamani irrigation scheme in the Mkoji sub-catchment of the Great Ruaha River basin in Tanzania. The moisture stress sensitivity indices of the crop water production models were determined using field data from the 2004 season. The grain yields simulated by the crop water production functions were compared with the field-measured data for the 2005 season. The base moisture stress sensitivity indices obtained for the vegetative, flowering and grain-filling growth stages of the maize crop were 0.29, 1.07, and 0.54, respectively, for the Jensen model, and 1.24, 3.36, and 1.69, respectively, for the Minhas et al. model. The moisture stress sensitivity indices for the vegetative, flowering, and grain-filling growth stages for the modified Bras and Cordova model were 0.21, 0.86, and 0.49, respectively; and 0.21, 0.86, and 0.49, respectively, for the modified Stewart et al. model. The indices for the different growth stages were considered as appropriate weights of the moisture sensitivity of the different growth stages of the maize crop. The Jensen model over predicted relative yield by 10%, the modified Bras and Cordova, Stewart et al., and Minhas et al. over predicted relative yield by 6, 15, and 18%, respectively. Although these performances of the models were considered fairly adequate, bearing in mind that empirical models rarely perfectly simulate field data due to some inherent variability in field data that models may not capture, the performance of the Jensen and modified Bras�Cordova models were considered better compared to the Minhas et al. and modified Stewart et al. models. The Jensen [Jensen, M.E., 1968. Water consumption by agricultural plants. In: Kozlowski, T.T. (Ed.), Water Deficits Plant Growth, vol. 1. Academic Press, New York, pp. 1�22] and the modified Bras�Cordova models with their moisture stress indices obtained in this study are recommended as the multiplicative and additive CWPF for the maize crop in the study area.
