Rice (Oryza sativa L.), maize (Zea mays L.) and wheat (Triticum aestivum L.) are the major cereals cultivated in the Terai region of Nepal. In the region, agriculture is mainly rainfed and fertilizer applications are low. In this study, water and soil fertility management was modeled with the FAO AquaCrop model. For the model calibration and validation, field trials were set up during two years in Rampur, a representative area of Terai at Chitwan. The statistical comparison between the simulated and observed yield of the validation fields resulted in an average RRMSE (relative root mean squared error), R2 (coefficient of determination) and EF (Nash–Sutcliffe efficiency) of 0.05, 0.94 and 0.90 for rice, 0.08, 0.90 and 0.87 for wheat and 0.08, 0.97 and 0.96 for maize, respectively. The validated model was subsequently used to simulate different scenarios with 30 years of historical climatic data to determine realistic soil fertility and water management strategies for increasing crop yields. For the monsoon crops (rice and maize) that benefit from abundant rainfall, only an improved fertility management could increase crop yields by up to 65% and 58%, for rice and maize, respectively. During the winter and spring, seasons with lesser rainfall, crops were severely under-watered in absence of irrigation. For winter wheat, application of deficit irrigation increased yield by up to 67–197% compared to the rainfed crop yield for different fertility levels. For spring maize, full irrigation with increased fertilizer application was recommended to increase the yield. However, by considering the water scarcity in the region, application of deficit irrigation resulted in good and stable yields under low to moderate fertility and always resulted in a better water productivity than fully irrigated crops. It was demonstrated that increasing soil fertility of the winter and spring crops without considering irrigation could sometimes result in crop failure, instead of a yield increase. For fertilizer applications above 50% of the national recommended fertilizer applications, full irrigations are recommended for best yield results for winter and spring crops.
