Improving the soil water module of the Decision Support System for Agrotechnology Transfer cropping system model for subsurface irrigation

Ensuring that crops use water and nutrients efficiently is an important strategy for increasing the profitability of farming and reducing the environmental load from agriculture. Subsurface irrigation can be an alternative to surface irrigation as a means of losing less irrigation water, but the application timing and amount are often difficult to determine. Well-defined soil and crop models are useful for assisting decision support, but most of the models developed to date have been for surface irrigation. The present study examines whether the Decision Support System for Agrotechnology Transfer (DSSAT, version 4.5) cropping system model is applicable for the production of processing tomatoes with subsurface irrigation, and it revises the soil module to simulate irrigation schemes with subsurface irrigation. Five farmed fields in California, USA, are used to test the performance of the model. The original DSSAT model fails to produce fruit yield by overestimating the water deficiency. The soil water module is then revised by introducing the movement of soil moisture due to a vertical soil moisture gradient. Moreover, an external parameter optimization system is constructed to minimize the error between the simulation and observations. The revised module reduces the errors in the soil moisture profile at each field compared to those by the original DSSAT model. The average soil moisture error decreases from 0.065m^3/m^3 to 0.029m^3/m^3. The yields estimated by the modified model are in a reasonable range from 80 to 150 ton/ha, which is commonly observed under well-managed conditions. The present results show that although further testing is required for yield prediction, the present modification to the original DSSAT model improves the precision of the soil moisture profile under subsurface irrigation and can be used for decision support for efficient producting of processing tomatoes.