Alfalfa (Medicago sativa L.) was grown in a limited-rooting volume field-plot experiment in 1983 and 1984. An automatic rain shelter allowed water-deficit-stress to be imposed on eight sub- plot treatments of a split-plot experimental design. There were five replicates of the experiment, with five weekly harvests as whole-plot treatments;Midday canopy temperature, air temperature, and vapor pressure deficit were used to calculate the Crop Water Stress Index (CWSI) of Idso et al. (1981). The CWSI was used to develop exponential yield-CWSI relationships for a range of water-deficit-stressed and well-watered alfalfa during a growth cycle. The exponential curves became more curvilinear with time. The CWSI allowed the 1983 and 1984 yield data to be combined for the yield-CWSI curves;A response surface of yield, the CWSI, and time was developed by combining the exponential yield-CWSI curves with double expo- nential Gompertz curves of yield regressed on time. The response surface equation had an R-squared of 0.65 (n = 395);Analysis of windspeed effects on canopy minus air temperature and the CWSI indicated that highly water-deficit-stressed alfalfa showed stronger responses to increased windspeed than well-watered alfalfa. The CWSI was better kept within its theoretical boundaries of 0 and 1 by the incorporation of windspeed into its upper and lower limits. When row cover is incomplete, as it was for this experiment, the lower limit may be more adequately character- ized by the addition of more meteorological variables;Growth analyses of leaf area, total dry weight, and leaf dry weight showed that highly water-deficit-stressed alfalfa had more leaf dry weight and leaf area per unit total dry weight than well-watered alfalfa, but low leaf area to leaf dry weight ratios, meaning that water-deficit-stressed alfalfa had thicker leaves than well-watered alfalfa;Well-watered alfalfa had more total dry weight, leaf area, and leaf dry weight than water-deficit-stressed alfalfa; Reference;Idso, S. B., R. D. Jackson, P. J. Pinter, Jr., R. J. Reginato, and J. L. Hatfield. 1981. Normalizing the stress-degree-day parameter for environmental variability. Agric. Meteorol. 24: 45-55.