Model Validation, Crop Coefficients and Yield Response Factors for Maize Irrigation Scheduling based on Long-term Experiments

This paper reports on using independent historic datasets relative to 19 years (1972–1990) experiments with maize for the calibration, validation and further testing of the irrigation scheduling simulation model ISAREG, and to derive appropriate crop parameters. The experiments concern diverse irrigation and rainfed treatments in a Vertisol at Pustren experimental site (Stara Zagora). The calibration of the model consists of deriving crop coefficients Kc and depletion fractions for no stress relative to the main crop growth stages, thus adapted to local climate and soil conditions, through searching the minimal differences between observed and model-simulated soil water content. The calibration was performed using data for 1976, while data for 1987 was used for validation. The resulting average errors of the estimate (AEE) for the soil water content are respectively 0·012 and 0·023 cm3 cm−3, thus indicating appropriate model parameterisation. The accuracy of the calibrated model was tested against experimental long-term data (1972–1990) from all irrigation and rainfed treatments by comparing the computed versus observed seasonal evapotranspiration. Results show a regression slope b close to the 1:1 line and a relatively small value for AEE, 49·75 mm season−1, which is less than 10% of the average crop evapotranspiration observed. The yield response factor Ky was also derived from the observed data sets for the two hybrid varieties used, resulting in a value for Ky of 1·0 for the hybrid Kn606 adopted during the first decade and a value of 1·5 for the hybrid H708 used during the second one. A statistical test was also performed to compare the model predicted versus observed relative yield decrease due to water stress for both varieties yielding a regression slope b of 0·952 and an error AEE of 0·048 for the hybrid Kn606 and b of 0·958 and AEE of 0·067 for the H708, which show that model predictions are appropriate. The results obtained support model use for developing water saving and environmentally oriented irrigation practices in the Trace region, South Bulgaria.