Issues, requirements and challenges in selecting and specifying a standardized ET equation

摘要

The ASCE Evapotranspiration in Irrigation and Hydrology Committee in cooperation with the Water Management Committee of the Irrigation Association has defined and established a standardized reference evapotranspiration (ET) equation. The purpose ofthe equation and standardized calculation of parameters is to bring a commonality to the methodology of reference ET and to the basis used to determine crop coefficients for both agricultural and landscape use. Issues and requirements in the selection and definition of the standardized procedure are that the procedure be understandable, defensible, accepted by science/engineering communities, relatively simple, and use existing and historical data and technology. The primary challenge was to select oneor more equations that would satisfy the selection criteria. There have been traditionally two types of reference crops (grass and alfalfa) and six major reference equation types (Kimberly Penman, CIMIS~2 Penman, FAO Penman, ASCE Penman-Monteith, FAO-56Penman-Monteith and NRCS Penman-Monteith) have evolved over the past thirty years, each having large groups of followers and related crop coefficients. In addition, the reference crop is a living crop and therefore changes in its height, leaf area, stageof growth, age, variety, stomatal feedback, and irrigation frequency impact its ET rate. Other challenges include a myriad of procedures used to predict net radiation, vapor pressure deficit and other equation parameters. Hourly and daily computation are both in common usage, therefore, it is important that hourly calculations, when summed for 24 hours agree relatively closely with calculations made on daily time steps.