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摘要
The evapotranspiration crop coefficients (K-c) that are used to estimate actual evapotranspiration (ETa) using a two-step approach (i.e., K-c x reference ET) for limited irrigated and rainfed maize are extremely rare. The effects of full and limited irrigation and rainfed practices on maize ETa and soil water dynamics were quantified. Grass-and alfalfa-based crop coefficients (K-co and K-cr) were developed for full-and limited-irrigation settings as well as for rainfed conditions. Four irrigation regimes [fully irrigated treatment (FIT), 75% FIT, 60% FIT, 50% FIT] and rainfed treatments were implemented. In general, depletion in available soil water increased with the decrease in irrigation regime from FIT to rainfed treatment. Maize ETa increased with irrigation amounts and ranged from 481 mm for the rainfed treatment to 620 mm for the FIT in 2009 and from 579 to 634 mm for the same treatments, respectively, in 2010. Reduction in seasonal ETa relative to FIT were 22, 9, 7, and 2% for the rainfed, 50% FIT, 60% FIT, and 75% FIT, respectively, in 2009. The reductions were 9, 4, 2, and 1% in 2010 for the same treatments. On average, a two-step approach overestimated maize ETa by approximately 26% in 2009 and 17% in 2010. The difference between the two methods ranged from 18% for the FIT to 38% for the rainfed treatment in 2009. The difference in 2010 was smaller owing to a greater amount of precipitation (563 mm in 2010 versus 426 mm in 2009), ranging from 21% for FIT to 12% for the rainfed treatment. One method was not able to effectively account for the impact of water-limiting and rainfed conditions on K-c when estimating ETa, resulting in considerable error in 2009. A new set of maize K-co and K-cr values under rainfed, limited, and fully irrigated settings were developed as a function of the thermal unit and days after emergence. The midseason average K-co values were 1.26, 1.20, 1.11, 1.08, and 1.10 for the FIT, 75% FIT, 60% FIT, 50% FIT, and rainfed treatments, respectively. The midseason average K-cr values were 1.05, 1.00, 0.97, 0.95, and 0.92 for the same treatments, respectively. In general, more water limitation and crop water stress resulted in lower midseason K-c values, and there was a gradual decrease in the peak K-co and K-cr values from FIT toward the rainfed treatment. The new set of K-co and K-cr equations can be beneficial for estimating maize water use for in-season irrigation management under full-and limited-irrigation settings as well as estimating evaporative losses under rainfed conditions for the locations that have similar soil, crop, climate, and management conditions of this study.
- 出版日期2013-6
