Temperature corrections in the Priestley-Taylor equation of evaporation

- The Priestley-Taylor equation expects the typically unknown wet-environment air temperature.
- Wet environment air temperature is close to wet-environment surface temperature.
- We estimate the latter by two different approaches yielding close results.
- We validate the estimates by surface temperature and evaporation data.
- Estimated wet-environment surface temperatures improve the Priestley-Taylor equation.

SummaryThe Priestley-Taylor equation (PTE) is frequently applied in actual areal evapotranspiration (ET) estimation methods for obtaining the maximum daily rate of evaporation with data from sub-humid conditions. Since PTE was parameterized under humid conditions, a temperature correction is necessary to avoid overestimation of the maximum rate of ET. Wet-environment surface temperature (Tws), a proxy of the wet-environment air temperature (Twa), is estimated by the Szilagyi-Jozsa (SJ) approach as well as by a re-parameterized version of Monteith. The latter yields higher values but typically within 1 °C of the former. Tested by daily FLUXNET data, the estimates are only mildly sensitive to the mean daily wind velocity which thus can be replaced by a region-representative monthly average. From long-term simplified water-balances - plus monthly Moderate Resolution Imaging Spectroradiometer (MODIS) and ERA-Interim re-analysis data - the re-parameterized Monteith method appears to yield more accurate Tws estimates, while the PTE performs better with the SJ provided Tws values since they are closer to Twa, the PTE expects. Both methods require net radiation, air temperature, humidity and monthly mean wind velocity values plus ground heat fluxes when employed on a daily basis.