MODIFICATIONS TO THE DSSAT VERTICAL DRAINAGE MODEL FOR MORE ACCURATE SOIL WATER DYNAMICS ESTIMATION

Abstract

Accurate modeling of soil water dynamics during vertical drainage is needed for reasonable prediction of crop yield and agrochemical leach- ing. This study was carried out to improve the soil water drainage model in the DSSAT (Decision Support System for Agrotechnology Transfer) family of crop models. In the existing model, the daily change in soil wa- ter content (� ) from a soil layer is calculated by multiplying the drainable soil water (initial � - drained upper limit � (� dul )) by a parameter, C, that represents the soil hydraulic properties. In this model, one value for C is assumed for the whole soil profile, and C is lacking theoretical basis. Two changes have been introduced to this model: a relationship between C anddul was developed and a new coefficient, F, which accounts for the incoming soil water flow into a particular layer of the profile (Q i ), was added. By stepping through the texture triangle in increments of 5% clay and 5% sand, theoretical values of C for all possible soil texture combi- nations were calculated using numeric solutions. A quadratic relationship between these C values and the correspondingdul was developed. A pub- lished international data set of drainage cycles of more than 50 soil pro- files was used to examine this relationship. The root mean square differ- ence (RMSD) between estimated and numerical C for the international soils was 0.136 days � 1 . The performance of the DSSAT and of the mod- ified DSSAT models as a whole was evaluated for two soils in the sum- mer of 1997. The modified DSSAT estimated the dailyreasonably well at the different depths throughout the drainage cycle (maximum RMSD � 0.013 m 3 m � 3 ), outperforming the original DSSAT by reducing the RMSD 2-4 fold at most of the soil depths (Soil Science 2004;169:000-000)