Estimation of drip irrigated summer maize soil water content and evapotranspiration based on SIMDualKc model

Abstract: Accurate estimation of soil evaporation, crop transpiration, evapotranspiration (ET), and the ratio of soil evaporation to evapotranspiration are critical for the precise water management in areas with scarce water resources. This study aimed to evaluate the SIMDualKc model for ET estimation and partition in terms of accuracy in summer maize under drip irrigation. A 2-year controlled drip irrigation experiment was conducted in the summer maize field in semi-arid regions of northwest China. This drip irrigation experiment included 3 water-supply levels (i.e., normal irrigation, W1; moderate deficit, W2; and medium deficit, W3). The measured soil water content for W2 treatment was selected for parameter calibration in SIMDualKc model, while the measured soil water content for W1 and W3 treatments were used for model validation. Based on those, the parameters such as transpiration, soil evaporation, ET and soil water stress coefficient for each treatment were simulated and analyzed. The results showed a good agreement in the measured soil water content and the simulated values from SIMDualKc model, with R2> 0.88 and normalized root mean square error smaller than 5%, which indicated that the SIMDualKc model was suitable for describing the dynamic changes of soil water content in this experiment. The values of basal crop coefficient for summer maize at the initial-, mid-, and late-season growth stages was 0.15, 1.13, and 0.2, respectively. Furthermore, the SIMDualKc model exhibited a high accuracy in estimating soil water stress coefficient, transpiration and soil evaporation during the whole growth stage of summer maize for all the treatments, but it overestimated soil water stress coefficient before the seed emergence. Soil evaporation mainly occurred in the early growth stage. While for transpiration, it was mainly in the rapid growth period and middle growth period. It increased and then decreased in the whole growth, peaking at the development and mid-season stages. Evapotranspiration varied with changes in the soil evaporation and crop transpiration, which was mainly affected by soil evaporation at the initial stage, and by crop transpiration at the development, mid-season and later stages. Specifically, values of soil evaporation, transpiration, ET, and the ratio of soil evaporation to ET for W1-W3 were 78.1-100.2 mm, 221.8-293.3 mm, 299.3-383.0 mm, and 24.1%-28.7%, respectively. Besides, values of soil evaporation, transpiration and ET had a downward trend with the decrease in water supply amount. Compared with the W1, the W2 and W3 declined on average by 3.74%-21.7%, while the ratio of soil evaporation to ET increased with the decrease in water supply amount. The W2 and W3 treatments increased by 11.1% and 3.8% as compared to W1 during the 2 growing seasons. This study can provide a basis for the establishment of reasonable drip irrigation scheduling and irrigation decision-making for summer maize in the semi-arid regions of northwest China.