A modified SWAP model for soil water and heat dynamics and seed–maize growth under film mulching

Soil–Water–Atmosphere–Plant (SWAP) model fails to consider the impacts of plastic film mulching on soil moisture and heat regimes, and crop growth and yield. In this study, SWAP model was modified in the modules of precipitation interception, soil evaporation, soil temperature, and crop growth to accommodate the changes of soil moisture and soil temperature caused by film mulching and the consequent variation of crop growth. The modified SWAP model was compared with original SWAP model. Here, the original model showed the case where there was no film mulching effect under the same input conditions as the modified model. The models were calibrated and validated by a field experiment of seed–maize conducted in the Shiyang River Basin of Northwest China in 2017, 2018 and 2019. The experiment included three drip irrigation treatments under film mulching conditions, i.e., WF (full irrigation), WM (medium irrigation, 70%WF), WL (low irrigation, 40%WF). Results showed the normalized root mean square errors (NRMSEs) for soil water storage (SWS), soil temperature at 0, 5, 10, 20 cm soil depths, leaf area index (LAI), aboveground dry biomass (ADB) and yield under different irrigation treatments for the three seasons by the modified model were 14.8%, 43.5% (an averaged value for the 4 soil depths), 70.5%, 56.4% and 82.1% lower than those of the original model, respectively, which demonstrated higher simulation accuracy of the modified model for mulched field. The modified model could more accurately depict the changes of the SWS and LAI caused by the film mulching at various growth stages. It could also reveal the enhanced soil temperature by film mulching especially at the early stage (0–60 day after sowing (DAS)). We analyzed the film mulching effects by comparing the simulation results of the modified model and the original model which could represent the difference between film mulching and no mulching. Results showed that soil evaporation and evapotranspiration under film mulching conditions were 60.7% and 10.1% lower while crop transpiration was 20.2% higher compared with no mulching. The yield and water use efficiency (WUE) under film mulching conditions improved by 38.9% and 54.3% compared with no mulching. Film mulching had more significant influence on the SWS, LAI, ADB, yield, crop transpiration and WUE under WL treatment than under WF and WM treatments.