Evaluation of Resistances for Bermudagrass Turf Crop Water Stress Index Models

AbstractEvaluation of the Penman‐Monteith‐based crop water stress index (CWSI), as a potential indicator of turf irrigation timing, requires reliable estimates of potential (minimum) canopy resistance, rcp, and aerodynamic resistance, ra. This paper compares various methods of estimating rcp and ra for evaluation of water stress of bermudagrass turfgrass (Cynodon dactylon cv. Midiron) in the desert Southwest of the USA. First, two empirical CWSI models were developed using field data collected from well‐watered and severely water‐stressed turf plots during the 1986 growing season in Tucson, AZ. Second, the regression constants in the empirical lower limits of CWSI (canopy temperature minus air temperature, Tc − Ta, of nonwater‐stressed turf) and the parameters associated with similar terms (net radiation and vapor pressure deficit) in the Penman‐Monteith equation were equated, thus solving for rcp and ra. This yielded mean rcp and ra values of 79 and 13 s m−1, respectively. Using these estimates, the predicted lower limits of Tc − Ta from the Penman‐Monteith equation were in good agreement with values measured over well‐watered turf. However, the predicted Tc − Ta for severely water‐stressed turf agreed very poorly with the measured values. The use of the log‐law equation yielded independent estimates of ra generally greater than 45 s m−1 and resulted in erroneous estimates of Tc − Ta by the Penman‐Monteith equation. Using a crop‐based method and data from the turf plots, estimates of the resistances were made in which constant rcp and ra values of 62.5 and 20 s m−1, respectively, were found. The crop‐based method was found to be not only simple to use, but also performed satisfactorily in predicting Tc − Ta. A well‐defined curvilinear relationship between estimates of midday canopy resistance (rc) and CWSI was determined for a wide range of turf water stress, implying that these two estimates nearly equally reflect the state of turf water stress. Our findings also suggest threshold midday CWSI and rc values for short bermudagrass turf of 0.16 and 125 s m−1, respectively, to indicate the need for irrigation.