Optimization of irrigation scheduling under deficit irrigation with saline water for spring wheat based on SWAP model

Overexpansion of irrigation in the Shiyang River basin (102°52'E, 37°52'N) has produced negative effects such as decrease of fresh water resources, deterioration of water quality and increase in soil salinization. With the limit of the fresh water resource, saline ground water has to be widely used for wheat production, but at the same time, soil salinization should be avoided. In order to investigate the most applicable deficit irrigation schedule with saline water for spring wheat in the Shiyang River Basin, the agro-hydrological Soil-Water-Atmosphere-Plant (SWAP) model was used to evaluate the effect of various irrigation schedule on water and salt balance in soil, crop yield and water use efficiency for the different hydrological years. An irrigation experiment was set up with water quantity of 375, 300 and 225 mm in combination with irrigation water salt concentrations of 0.7, 3, and 6 g /L in 2008 and 2009. In total, there were nine irrigation treatments. Experimental data in 2008 was used for SWAP model calibration while data in 2009 was used for the model validation. The validated SWAP model was then used to simulate soil water content, soil salt concentration and crop relative yield. Based on the meteorological data of 1951 to 2010, sufficient and deficit irrigation schedule for 25%, 50% and 75% hydrologic year was set up and simulated by use of the SWAP model. The simulated results indicated that appropriate deficit irrigation was beneficial for increasing crop yield, water use efficiency and decreasing soil salinity when saline water was used. However, the maximum yield and the corresponding irrigation water quantity in saline irrigation decreased with the increased of water salinity. The simulated results also indicated that the optimal irrigation schedule of spring wheat was 1) three times of irrigation were needed with the total irrigation water of 332 mm for three water qualities of 0.7, 3 and 6 g salt/L at the hydrologic years of 25%; 2) three times of irrigation were needed with the total amount of 328, 287 and 246 mm water for water salt concentrations of 0.7, 3 and 6 g /L, respectively at the hydrologic year of 50%; and 3) four times of irrigation were needed with the total amount of 440 and 396 mm water for 0.7 and 3 g salt/L water quality, and three times of irrigation with 352 mm water for water quality of 6 g salt/L at the hydrologic year of 75%. Winter irrigation was suggested from this study for water storage and salt leaching in this area.