Determining water use and crop coefficients of drip-irrigated cotton in south Xinjiang of China under various irrigation amounts

Crop evapotranspiration (ETc) and crop coefficient (Kc) can vary among various geographical locations and cultivated species. Therefore, determining regional- and species-specific crop coefficients is fundamental for rational irrigation water management. A two-year field study was conducted in south Xinjiang of northwest China to estimate periodic ETc and reference evapotranspiration (ET₀), and thus develop the regional crop coefficient (Kc) at various growth stages of fertigated cotton (Gossypium hirstum L. Xinluzhong No 67) under various irrigation amounts. ETc was determined by the soil water balance method, while ET₀ was obtained using the FAO (Food and Agriculture Organization of the United Nations) Penman-Monteith model. The results indicated that increasing irrigation amounts resulted in increased seed cotton yields, but reduced crop water productivity (WP). The water consumption at flowering and boll setting stages accounted approximately for 60% of the seasonal water use of cotton under various irrigation amounts. The average locally developed Kc (Kc₋ₗₒcₐₗ) values over two seasons were 0.38, 1.30 and 1.09 for initial, mid-season, and end-season stages, respectively. The locally developed initial Kc (Kc₋ₗₒcₐₗ₋ᵢₙᵢ) (0.38) was higher than the optimized initial-stage Kc (Kc₋ₒₚₜ₋ᵢₙᵢ) (0.23) and similar to the FAO recommended Kc (Kc₋FAO) (0.35). The optimized Kc (Kc₋ₒₚₜ) did not significantly differ from Kc₋FAO at mid-season and end-season stages. The locally developed middle-stage Kc (Kc₋ₗₒcₐₗ₋ₘᵢd) was 10% higher than the optimized middle-stage Kc (Kc₋ₒₚₜ₋ₘᵢd) and the FAO recommended middle-stage Kc (Kc₋FAO₋ₘᵢd), and the locally developed end-stage Kc (Kc₋ₗₒcₐₗ₋ₑₙd) was approximately 82% higher than the optimized end-stage Kc (Kc₋ₒₚₜ₋ₑₙd) and the FAO recommended end-stage Kc (Kc₋FAO₋ₑₙd). The difference between Kc₋ₗₒcₐₗ, Kc₋ₒₚₜ and Kc₋FAO was attributed to the variations of climate condition, cultivated variety and field management. Kc₋ₗₒcₐₗ determined by the soil water balance method in the present study fully considered the regional conditions including weather, species and management practices, which can be adopted to guide the irrigation management in this region. This study provided credible cotton periodic Kc in the arid and semiarid regions of northwest China. A second-order polynomial relationship between cotton Kc and t/T (the days after emergence/the days of total cotton growth period) was established, which can provide a reference for the determination of local cotton Kc and irrigation management.