Utility of Penman-Monteith, Priestley-Taylor, reference evapotranspiration, and pan evaporation methods to estimate pasture evapotranspiration

Actual evapotranspiration (ET(a)) was measured at 30-min resolution over a 19-month period (September 28, 2000-April 23, 2002) from a nonirrigated pasture site in Florida, USA, using eddy correlation methods. The relative magnitude of measured ET(a) (about 66% of long-term annual precipitation at the study site) indicates the importance of accurate ET(a) estimates for water resources planning. The time and cost associated with direct measurements of ET(a) and the rarity of historical measurements of ET(a) make the use of methods relying on more easily obtainable data desirable. Several such methods (Penman-Monteith (PM), modified Priestley-Taylor (PT), reference evapotranspiration (ET0), and pan evaporation (E(p))) were related to measured ET(a) using regression methods to estimate PM bulk surface conductance, PT alpha, ET0 vegetation coefficient, and E(p) pan coefficient. The PT method, where the PT alpha is a function of green-leaf area index (LAI) and solar radiation, provided the best relation with ET(a) (standard error (SE) for daily ET(a) of 0.11 mm). The PM method, in which the bulk surface conductance was a function of net radiation and vapor-pressure deficit, was slightly less effective (SE=0.15 mm) than the PT method. Vegetation coefficients for the ET0 method (SE=0.29 mm) were found to be a simple function of LAI. Pan coefficients for the E(p) method (SE=0.40 mm) were found to be a function of LAI and E(p). Historical or future meteorological, LAI, and pan evaporation data from the study site could be used, along with the relations developed within this study, to provide estimates of ET(a) in the absence of direct measurements of ET(a). Additionally, relations among PM, PT, and ET0 methods and ET(a) can provide estimates of ET(a) in other, environmentally similar, pasture settings for which meteorological and LAI data can be obtained or estimated.