Penman Monteith crop coefficients for use with desert turf systems

Irrigation scheduling systems which estimate actual evapotranspiration (ETa) by adjusting reference evapotranspiration (ETo) with crop coefficients (K_cs) have been suggested as a means of improving irrigation management of turfgrass in the desert southwest. The objective of this study was to develop turfgrass K_cs for use with ETo computed by the Penman Monteith Equation recommended by the United Nations Food and Agricultural Organization (FAO). Crop coefficients were developed for fairway quality 'Tifway' bermudagrass (Cynodon dactylon L. × C. transvaalinsis Davy) in summer and overseeded 'Frog-hair' intermediate ryegrass (Lolium perenne × L. multiflorum) in winter by relating daily measurements of ETa obtained from weighing lysimeters to ETo computed with meteorological data. Monthly and seasonal K_cs were developed by (i) computing the mean of individual daily K_cs, (ii) dividing cumulative ETa by cumulative ETo for the period, and (iii) computing the slope of least squares regression lines relating ETa to ETo. The three computation procedures did not greatly affect the resulting K_c value. Bermudagrass K_cs ranged from 0.78 in June to 0.83 in September, with monthly variation related to turf growth rate. Use of a constant K_c of 0.80 would suffice for estimating ETa in summer. Monthly K_cs for intermediate ryegrass ranged from 0.78 in January to 0.90 in April and varied in relation to mean air temperature. Increased bulk surface resistance resulting from chill-induced reductions in stomatal conductance and/or a reduction in turf growth rate and leaf area index may lower ETa and K_cs during the colder winter months, making use of a constant seasonal K_c less suitable in winter. An inverse linear relationship was obtained between the coefficient of variation of mean monthly K_c and the ratio of measured to theoretical clear sky solar radiation, indicating K_cs are less reliable during periods of cloudy weather.