Calibration and Validation of the FAO Aquacrop Model for Rainfed Cereals in Nigeria, West Africa.

The objectives of this study were to evaluate AquaCrop’s ability to simulate the cumulative grain yield of rainfed maize for different soil fertility levels in the northern Guinea Savanna zone of Nigeria. Seven years (2007-2013) of field experimental data on maize grown under rainfed condition at the Institute for Agricultural Research were used to calibrate (2007 data set) and validate (2008-2013 data set) AquaCrop. We assessed the agreement between model simulated and actual maize yields using correlation coefficient, R2, and the index of agreement, d. R2 values ranged from 0.86 to 0.95 while values of d ranged from 0.878 to 0.989, indicating very strong to excellent agreement. Other statistical indicators of fit, including the root mean squared error and mean absolute error all yielded satisfactory results. In percentage terms, the median difference between observed and simulated yields is respectively 13%, 13%, 15%, and 8% for poor, about half, moderate, and near optimal nutrient status. In two thirds of the treatments analyzed, the differences are within 15% of the actual yields, indicating acceptable results. Despite encouraging results, simulated yields systematically over-estimates observed yields a not uncommon result that suggests the need for additional calibration. The grain yield evaluation results were consistent with other validation studies of the model. Keywords: AquaCrop, Calibration, Validation, crop Yield, Crop Model, Maize, Rainfed, Nigeria.