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


Actual evapotranspiration (ETa) 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 ETa (about 66% of long-term annual precipitation at the study site) indicates the importance of accurate ETa estimates for water resources planning. The time and cost associated with direct measurements of ETa and the rarity of historical measurements of ETa 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 (Ep)) were related to measured ETa using regression methods to estimate PM bulk surface conductance, PT α, ET0 vegetation coefficient, and Ep pan coefficient. The PT method, where the PT α is a function of green-leaf area index (LAI) and solar radiation, provided the best relation with ETa (standard error (SE) for daily ETa 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 Ep method (SE=0.40 mm) were found to be a function of LAI and Ep. 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 ETa in the absence of direct measurements of ETa. Additionally, relations among PM, PT, and ET0 methods and ETa can provide estimates of ETa in other, environmentally similar, pasture settings for which meteorological and LAI data can be obtained or estimated.


Earth Systems Research Center

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Journal of Hydrology



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© 2004 Elsevier B.V. All rights reserved.