Determination of evapotranspiration and crop coefficients for alfalfa and fescue using lysimetry

Climate change is causing more frequent droughts, even in regions like Uruguay, which has increased the use of irrigation in pastures. Determining the crop coefficient (Kc) is key to optimizing irrigation, improving water use efficiency, and promoting more sustainable production systems. The objective of this study was to determine crop evapotranspiration (ETc) and Kc values ​​in tall fescue (Festuca arundinacea Schreb. cv. Aurora) and Medicago sativa L. cv. Chaná under different irrigation strategies. ETc was determined using drainage lysimeters, both on a cycle-between-cutting and seasonal scale. Two treatments were evaluated for each species. In tall fescue, frequent irrigation (T1), with a depletion threshold of 32% of available soil water, was compared to spaced irrigation (T2), with a threshold of 52%. Cuttings were performed when the crop reached the 2.5-leaf stage. In alfalfa, the treatments consisted of replenishing 40% (T1) and 55% (T2) of the available water, with cuttings performed at the nine-node stage. The crop coefficient (Kc) was estimated for T1 and the adjusted coefficient (Kc aj) for T2. Their relationship with the relative growth cycle and crop variables was analyzed using a linear-plateau model. In fescue, the accumulated ETc in T1 was 273 mm in spring and 464 mm in summer, while in T2 it was 247 mm and 416 mm. The average Kc and Kc aj values ​​between cuttings differed significantly (0.89 and 0.78), as did the maximum values ​​(0.94 and 0.81) and dry matter production. In alfalfa, the accumulated ETc in summer was 430 mm in T1 and 379 mm in T2. The maximum Kc values ​​(0.91) and adjusted Kc (0.78) differed significantly, while dry matter production showed no differences. The average Kc ranged from 0.71 to 0.79. These results demonstrate that adjusting Kc according to the irrigation strategy allows for improved water use efficiency without compromising productivity.