Modeling response of cotton yield and water productivity to irrigation amount under mulched drip irrigation in North Xinjiang

Abstract: Xinjiang Uygur Autonomous Region is the largest cotton-production area of China. As a water saving and high yield irrigation technique, mulched drip irrigation is popular water application method for cotton production in Xinjiang Uygur Autonomous Region. Studying the response of cotton yield to irrigation amount is of great importance. In this study, a two-dimensional soil water transport and crop growth coupled model was calibrated and validated using field data of cotton under mulched drip irrigation, and then used to estimate the cotton yield under different irrigation amount. The field experiment was conducted at the experimental station located in Urumqi, Xinjiang Uygur Autonomous Region, China, during the cotton growing seasons of 2010 and 2011. In this experiment, 3 irrigation levels of 50%, 75% and 100% of full irrigation were adopted. For the full irrigation treatment, irrigation was applied when the averaged soil moisture within the root zone (40 cm for the squaring stage and 60 cm for the bloom stage) was depleted to 60% and 70% of the field capacity for the squaring and bloom stages, respectively. The irrigation was applied until the soil water content reached to 85 % and 95% of the field capacity for the squaring and bloom stage, respectively. Soil water content was measured weekly by a Trime-FM probe to 100 cm depth to determine irrigation schedule. Leaf area index and aboveground biomass of cotton plant were observed at squaring, bloom, and boll-forming stages. At the end of each growing season, the seed cotton was harvested by hand. Considering the features of water transport from emitters into soil, a process-based two-dimensional soil water transport and crop growth simulation tools would be preferred to modeling the response of crop yield to irrigation amount under mulched drip irrigation. The coupled model was coded in program subroutines and functions integrated with CHAIN_2D and the crop growth model of EPIC. This model was written in FORTRAN 90 for Windows system. In the coupled model, the root water uptake model of Vrugt was coupled with the root depth growth model in order to consider the interaction between root water uptake and crop growth. To study the response of cotton yield and water productivity to irrigation amount, the two-dimensional soil water transport and crop growth coupled model was calibrated and validated by soil water content dynamic, crop growth indexes and seed cotton yield obtained from the field experiments. The calibration and validation results indicated that the coupled model performed well in predicting the soil moisture, above ground biomass, seed cotton yield and total water use. The values of normalized root mean square error (nRMSE) and index of agreement between observed and simulated soil water contents was 4.6%-23.4%, and 0.677-0.974, respectively. The nRMSE values of leaf area index and aboveground biomass was 6.3%-15.7% and 7.2%-14.1%, respectively. The differences between simulated and measured seed cotton yields and water uses were ranged from 1.1% to 6.7% and from 0.3% to 9.2%, respectively. Furthermore, the calibrated two-dimensional soil water transport and crop growth coupled model was used to simulate seed cotton yield and water productivity under 10 irrigation scenarios, i.e. 40%-130% of full irrigation amount with the increment of 10%. The results showed that the seed cotton yield and water productivity were increased and decreased through quadratic functions as the irrigation amount increased, respectively. Therefore, this study suggested that the appropriate mulched drip irrigation amounts for cotton in the North Xinjiang region ranges from 280 to 307 mm considering the yield and water productivity.