Simple Equation to Estimate Reference Evapotranspiration from Evaporation Pans Surrounded by Fallow Soil

摘要

Reliable estimates of reference evapotranspiration (ET_0) are key elements for efficient water resource management, and estimating ET_0, based on "Class 'A' pan evaporation" data is common in arid climates. A pan coefficient (K_p), which depends on the distance (or fetch) of green vegetation or fallow soil around the pan (F), wind run (U), and relative humidity (RH), is used to convert from pan evaporation to ET_0. Several researchers have developed models for estimating K_p values for pans surrounded by green vegetated fetch, but there is only one equation to estimate K_p values for dry fetch conditions. The equation is complex, so the objective of this research was to develop a new simple equation to estimate K_p under fallow soil fetch conditions. The new K_p equation and the more complex equation were compared with tabular values published by the United Nations Food and Agriculture Organization. The new equation performed slightly better at matching the tabular K_p values than the complex equation. The equation derivation and evaluation are presented.