Non-Water-Stressed Baselines for Calculating Crop Water Stress Index (Cwsi) for Alfalfa and Tall Fescue Grass

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