Crop Water Stress Index and Yield of Water‐Deficit‐Stressed Alfalfa

Abstract

Abstract The yield relationship of water‐deficit‐stressed alfalfa ( Medicago sativa L.) with the Crop Water Stress Index (CWSI) may be dynamic over the alfalfa harvest period. This investigation was conducted to define the alfalfa yield‐CWSI relationship(s), to test possible advantages of using the CWSI over the canopy‐minus‐air temperature (Tc−Ta) differential, and to combine the yield‐CWSI relationship with a growth function. ‘Apollo II’ alfalfa was grown for 2 yr in 100‐L containers set into the ground and protected from rain by a movable shelter. The soil was a Nicollet silt loam topsoil (fine‐loamy, mixed, mesic Aquic Hapludoll). Treatments were irrigation levels of 112, 100, 88, 77, and 65% field capacity. Five harvests were taken at 7‐d intervals beginning 21 d after an initial clipping. Canopy temperatures were taken daily with an infrared thermometer. A series of exponential yield‐CWSI curves resulted from the sequential harvests. Yield reductions of about 10 and 20% resulted from mean CWSI of 0.05 and 0.10, respectively, over 42‐d periods. The Gompertz growth function was modified for water‐deficit‐stress by combining it with the exponential yield‐CWSI relationship. Vapor‐pressure deficit (VPD) effectively normalized the Tc−Ta data, allowing yield data of the 2 yr to be combined in the yield‐mean CWSI relationship. Yield and Tc−Ta data not normalized for VPD showed clear separation by year for slight VPD differences. Alfalfa yield response to water‐deficit‐stress over time appeared to be adequately described by the combined Gompertz function and the yield‐CWSI (Tc−Ta normalized for VPD) curves.