Actual evapotranspiration and variable crop coefficients for scheduling turfgrass irrigation

Abstract Turfgrass irrigation based on evaporative requirements strengthens water conservation efforts. A study was conducted from 1998 to 2000 to determine actual evapotranspiration (ET a ) of warm and cool‐season turfgrasses and to develop crop coefficient ( K C ) values normalized for growing degree days. Predicted values of maximum ET a and K C were calculated, and data from a second study were used to validate the fitted polynomial functions. Estimated ET a differed in 1998 and 1999 and ranged from 5.42 mm day −1 ( Poa pratensis L. ‘Adelphi’ in 1998) to 6.69 mm day −1 ( Lolium perenne L. ‘Seville’ in 1999) for cool‐season turfgrasses (CS) and from 4.54 mm day −1 [ Buchloe dactyloides (Nutt.) Engelm. ‘Bison’ in 1999] to 5.15 mm day −1 [ Cynodon dactylon (L.) Pers. ‘Guymon’ in 1998] for warm‐season turfgrasses (WS). For CS, between‐year variation was greater than differences within years, but for WS, within‐year differences were greater than between years. A quadratic function was used to model the trend in K C . For CS, K C differed among years, with similar trends in 1998 and 1999. Generally, values for CS ranged from 0.76 to 0.95 and for WS from 0.68 to 0.76. We were unable to establish a clear trend that would group both CS and WS into high water use versus low water use. If a variable K C based on our models had been applied during the 3‐year period, irrigation amounts would have been reduced by approximately 10% for CS and by 15% for WS when compared to a constant K C .