NON-WATER-STRESSED BASELINES FOR CALCULATING CROP WATER STRESS INDEX (CWSI) FOR ALFALFA AND TALL FESCUE GRASS

Abstract

The lack of transferability of the Crop Water Stress Index (CWSI) baselines, together with the restriction of having to make required measurements close to noon and under clear-sky conditions, are major drawbacks that restrict the use of the empirical CWSI method for irrigation scheduling. The objectives of this study were to: (1) evaluate the effect of solar radiation (Rs) on the non-water-stressed baselines (NWSBs) of alfalfa (Medicago sativa L.) and tall fescue grass (Festuca arundinacea), and (2) develop empirical equations to estimate their NWSBs, which could be applied at any time during the daytime cycle and under conditions of full canopy cover. A Bowen ratio system was used to measure 20 min averages of radiometric surface temperature, air temperature (Ta), wind speed (u2), dew point, and Rs over the two crop canopies during the 1991 growing season at Kimberly, Idaho. Using this dataset, empirical NWSBs for different Rs ranges were derived, which tended to diverge from each other as vapor pressure deficit (VPD) increased, indicating that Rs considerably affected the baselines and that its effect was more pronounced as the air got drier. Multiple regression analysis was also used to develop equations to estimate the NWSBs for the entire daytime cycle and specifically for the near-noon period. For alfalfa, the equation derived for the entire daytime cycle estimated the NWSBs as a function of Rs, VPD, Ta, u2, and plant canopy height (h) (r2 = 0.89). For grass, the equation only included Rs, VPD, Ta, and u2 (r2 = 0.89). For alfalfa, the near-noon equation included Rs, VPD, Ta, u2, and h (r2 = 0.92). For grass, on the other hand, Ta and h were not statistically significant, and the near-noon equation only included VPD, Rs, and u2 (r2 = 0.94). Since all variables that significantly affected the NWSBs for these crops were included in the equations, we expect them to be transferable to other locations; however, additional testing at other locations is needed to confirm this hypothesis.