Models for Predicting the Lower Limit of the Canopy‐Air Temperature Difference of Two Cool Season Grasses

Estimation of the lower limit of the canopy‐air temperature differential, (T c −T a ) LL , is required for calculation of an empirically‐based crop water stress, index. This research determined the complexity of model needed for accurate estimation of (T c −T a ) LL for several field grown cultivars of Kentucky bluegrass ( Poa pratensis L.) and for creeping bentgrass ( Agrostis stolonifera L. var. palustris (Huds.) Farw.). Regression models using vapor pressure deficit of the air (VRD), net radiation load ( R n ), and wind speed (WS) were developed for predicting (T c −T a ) LL . The best one to three‐variable regression models for predicting (T c −T a ) LL used variable groups of VPD ( r 2 = 0.47); VPD and R n ( R 2 = 0.66); and VPD, R n , and WS ( R 2 = 0.82). Models developed for predicting (T c −T a ) LL on individual Kentucky bluegrass cultivars, across Kentucky bluegrass, and across both species were tested on a validation data set. Models using only VPD accounted for <2% of variation in actual (T c −T a ) LL of nonwater‐stressed turf. Models using VPD and R n developed from pooled Kentucky bluegrass data, individual Kentucky bluegrass cultivars, or Kentucky bluegrass and creeping bentgrass data accounted for an average of 15, 13, and 14% of variation in actual (T c −T a ) LL , while models using VPD, R n , and WS accounted for an average of 62, 62, and 64%, respectively. On creeping bentgrass, the Kentucky bluegrass model and dual species model introduced a large amount of bias to predicted (T c −T a ) LL . At sites where environmental conditions are highly variable, the effects of VPD, R n , and WS must be taken into account to accurately predict (T c −T a ) LL of turfgrass. A single model appears appropriate for prediction of (T c −T a ) LL across Kentucky bluegrass cultivars; a separate model for creeping bentgrass is required.