Simulation of Silage Maize Dry Matter and Mean 0–30 cm Soil Moisture at Maturity Using the Root Zone Water Quality Model 2 (RZWQM2) for Irrigation–Nitrogen Management in the Hexi Irrigation District

This study addresses how limited field observations can be extended into a defensible irrigation–nitrogen management range for plastic-mulched drip-fertigated silage maize in the Hexi Irrigation District. The Root Zone Water Quality Model 2 (RZWQM2) was calibrated using six fertilized treatments in 2022 and independently validated using the corresponding treatments in 2023. The calibrated model was then used to compare 25 irrigation–nitrogen scenarios based on dry matter at maturity, mean 0–30 cm soil moisture at maturity, interannual stability, and proximity to the field reference treatment. RZWQM2 reproduced both variables with acceptable accuracy. For dry matter, RMSE values were 0.40 and 0.80 t·hm−2 in 2022 and 2023, with corresponding nRMSE values of 1.61% and 3.18%, respectively. For soil moisture, based on 18 layer-specific observations per year, RMSE values were 0.0057 and 0.0064 m3·m−3, and nRMSE values were 3.65% and 4.14%, respectively. Dry matter reached its maximum under I1N4 in both years, but its advantage over the baseline scenario I3N3 was limited. When dry matter performance, soil-moisture deviation from the baseline scenario, interannual stability, and proximity to the baseline management level were considered jointly, I2N3, I2N4, and I3N4 were identified as priority scenarios. These scenarios corresponded to 4981.5–5535.0 m3·hm−2 irrigation and 280–308 kg N·hm−2 additional urea-N input. The results indicate that a slight irrigation reduction to baseline irrigation combined with baseline to moderately increased urea-N input represents a suitable local management range for plastic-mulched drip-fertigated silage maize under the tested soil–climate–management conditions.