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Notes & Unique Phenomena

Explicit and Recursive Calculation of Potential and Actual Evapotranspiration

^a Texas A&M Univ. Res. and Ext. Center, 3810 4th Street, Lubbock, TX 79415
^b Soil & Crop Sciences Dep., Texas A&M Univ., 245 Pecan Valley Rd., Center Point, TX 78010

^* Corresponding author (r-lascano{at}tamu.edu)

Received for publication May 22, 2006. The explicit combination method (ECM; Penman, 1948) to calculate potential evapotranspiration (ET_p) is a physically based model using standard climatological data. It is based on an assumption regarding the temperature and humidity at the evaporating surface that is not made in a recursive combination method (RCM; Budyko, 1958). Our objective was to compare the two methods by calculating values of ET_p and of actual evapotranspiration (ET_a) using hourly weather data collected on 45 d during the warm season in Lubbock, TX. Results show that on hot summer days ECM underestimated the daily value of ET_p and of ET_a by as much as 25% compared with RCM. The proposed RCM procedure is based on the same physical principles as ECM, but uses iteration to find an accurate answer. It can easily be used with commercially available mathematical software that has proven to be stable. The RCM needs experimental verification before implementation for crop irrigation.

Abbreviations: ECM, explicit combination method • ET, evapotranspiration (kg m^–2 d^–1 or mm d^–1) • ET_a, actual crop evapotranspiration (kg m^–2 s^–1 or mm d^–1) • ET_p, potential evapotranspiration (kg m^–2 s^–1 or mm d^–1) • r_c, canopy resistance (s m^–1) • RCM, recursive combination method • T_s, surface temperature (°C)