Sensitivity of the Food and Agriculture Organization Penman–Monteith Evapotranspiration Estimates to Alternative Procedures for Estimation of Parameters
Publication: Journal of Irrigation and Drainage Engineering
Volume 131, Issue 3
Abstract
Reference crop evapotranspiration is a key variable in procedures established for estimating evapotranspiration rates of agricultural crops. As per internationally accepted procedures outlined in the United Nations Food and Agriculture Organization's Irrigation and Drainage Paper No. 56 (FAO-56), using the Penman–Monteith (PM) combination equation is the recommended approach to computing from ground-based climatological observations. Applying of the PM equation requires converting input climate and site data into a number of parameters, and FAO-56 recommends exact procedures for estimating these parameters. However, a plethora of alternative procedures for estimating parameters exist in literature. As a consequence, it is likely that ambiguous results may be obtained from the FAO-56 PM equation because of the adoption of such alternative (nonrecommended) supporting equations. The purpose of the present study is to evaluate differences that could arise in FAO-56 estimates if nonrecommended equations are used to compute the parameters. Using historical climate records from 1973 to 1992 of a station located in the humid tropical region of Karnataka State, India, monthly estimates computed by FAO-56 recommended procedures were statistically compared with those obtained by introducing alternative procedures for estimating parameters. In all, 13 alternative algorithms for estimation were formulated, involving modified procedures for parameters associated with weighting factors, net radiation, and vapor-pressure-deficit terms of the PM equation. For the period considered, nine of these algorithms yielded estimates that were in close correspondence with FAO-56 estimates as indicated by mean absolute relative difference (AMEAN) values within 1% and maximum absolute relative difference (MAXE) values within 2%. The remaining four algorithms, involving nonrecommended procedures for the vapor-pressure-deficit and net-radiation parameters, yielded considerably different estimates, giving rise to AMEAN values in the range of 2 to 8% and MAXE values ranging between 8 and 28%. The results of this study highlight the need for strict adherence to recommended procedures, especially for estimating of vapor-pressure-deficit and net-radiation parameters if consistent results are to be obtained by the FAO-56 approach.
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Acknowledgment
Research reported in this paper is part of an ongoing project titled “Integrating Remote Sensing Data, GIS and Hydrological Modeling for Assessment of Rural Water Supplies,” sponsored by the Indian Space Research Organisation (ISRO) under RESPOND (Project No. 363). Funding received is gratefully acknowledged. We thank Prof. Earl C. Stegman and two other anonymous reviewers for their useful comments and suggestions.
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Published In
Journal of Irrigation and Drainage Engineering
Volume 131 • Issue 3 • June 2005
Pages: 238 - 248
Copyright
© 2005 ASCE.
History
Received: Nov 19, 2003
Accepted: Mar 23, 2004
Published online: Jun 1, 2005
Published in print: Jun 2005
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