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

• Published in: Journal of hydrology (Amsterdam) 2005-07, Vol.308 (1), p.81-104
• Main Authors: Sumner, David M., Jacobs, Jennifer M.
• Format: Article
• Language: English
• Subjects: Earth sciences; Earth, ocean, space; equations; estimation; Evapotranspiration; Exact sciences and technology; Hydrology; Hydrology. Hydrogeology; mathematics and statistics; Pan evaporation; Pasture; pastures; Penman-Monteith equation; Priestley-Taylor equation; Reference evapotranspiration
• ISSN: 0022-1694; 1879-2707
• DOI: 10.1016/j.jhydrol.2004.10.023

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 (ET 0), and pan evaporation ( E p)) were related to measured ET a using regression methods to estimate PM bulk surface conductance, PT α, ET 0 vegetation coefficient, and E p 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 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 ET 0 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 ET 0 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.