Cotton Genomics and Genetics 2025, Vol.16, No.2, 57-71 http://cropscipublisher.com/index.php/cgg 64 soil water content was not less than 60%), the fiber quality could be maintained at a good level (Sheng et al., 2014). In the context of saline-alkali water irrigation, moderate irrigation and leaching are necessary, but if excessive irrigation leads to premature aging in the later stage, it will also make the fibers fragile. Therefore, high-quality and high-yield cotton requires a "double guarantee" of water supply: both to meet the formation of yield and to take into account fiber development. Practical experience shows that keeping the cotton field moderately moist before boll opening can give the fiber ample time to complete its development; and proper water control during the boll opening period to prevent premature aging can also help improve the maturity of the fiber in the later stage. Through such irrigation regulation, high-strength, long-staple, and moderately fine high-quality cotton fibers can be produced, thereby improving the economic value and textile performance of cotton. 5.3 Harvest index and economic return The harvest index is usually used to express the ratio of crop economic yield (seed cotton) to total biological yield (including cotton stalks and leaves, etc.), which can reflect the efficiency of dry matter allocation to economic yield. Irrigation strategies will change the harvest index by affecting the balance between cotton vegetative growth and reproductive growth. Under sufficient irrigation conditions, cotton vegetative bodies (stems and leaves) and reproductive bodies (cotton bolls) grow well, and the harvest index is generally maintained at around 0.3-0.4; under water stress conditions, cotton often prioritizes the growth of rhizomes to survive, and the decline in economic yield is greater than that of biomass, resulting in a decrease in the harvest index (Buttar et al., 2009). On the contrary, if the irrigation strategy promotes moderate "vigorous control" of cotton and reduces ineffective growth, more dry matter can be used for boll formation and improve the harvest index. Studies have shown that sub-film drip irrigation can increase the cotton harvest index by about 10% compared with flood irrigation due to the reduction of cotton transpiration and ineffective branch and leaf growth, which means that a higher proportion of biomass is converted into cotton lint fiber and seeds (Wang et al., 2021a). From the perspective of economic return, optimizing irrigation strategies can increase the output value of cotton per unit of water use. Although there is an initial investment in the use of advanced water-saving technologies, a higher input-output ratio can usually be achieved through water-saving and increased production. For example, after the large-scale promotion of drip irrigation in southern Shanxi, the economic output value of cotton production per cubic meter of water increased by more than 30% compared with traditional flood irrigation (Wang and Sun, 2013). In addition, precision irrigation and automated control can reduce labor and energy costs and improve economic benefits. However, it is necessary to balance the impact of water fees and equipment costs on income. In areas with high water prices, the cost savings brought about by water saving will significantly increase net profits; while in areas with low water resource costs, farmers are more concerned about increased production and income. In general, reasonable irrigation strategies can bring higher economic returns and risk resistance to cotton farmers by improving cotton harvest index and water productivity. This is also the driving force for the widespread application of water-saving irrigation technology in cotton production: only when irrigation optimization is reflected in real economic benefits, farmers will be motivated to adopt new water use strategies. Therefore, when promoting water-saving irrigation, local governments should analyze input-output and demonstrate the effect of water-saving and yield-increasing according to local economic conditions and water price policies, so as to promote a win-win situation for the economy and environment in cotton production. 6 Water Use Efficiency and Sustainability 6.1 Irrigation water productivity of cotton systems Water use efficiency is one of the core indicators to measure the effectiveness of irrigation strategies. In cotton production, there are various related concepts, such as irrigation water use efficiency (IWUE, yield per unit of irrigation water) and total water productivity (WUE including precipitation). Improving the irrigation water productivity of cotton means producing more cotton with less water. By optimizing the timing and intensity of irrigation, the water productivity of cotton fields can be significantly improved. For example, in a two-year field trial conducted on the southern edge of the Taklimakan Desert in northwestern China, cotton yield under full irrigation (100% field capacity) reached 4376 kg/ha (about 1458 kg/mu), water consumption (ETc) was 1079 mm, and IWUE was 0.48 kg/m³. Under 80% irrigation, yield only decreased by 13%, while IWUE increased. Under
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