TN-45 : Estimation of evapotranspiration

Abstract

Next to rainfall, evapotranspiration is the most important term in the water balance of catchment areas. The subject of evapotranspiration, which includes evaporation of water from land and water surfaces and transpiration by vegetation, is becoming increasingly more significant. Therefore, it is of interest to develop and test methods for estimating evapotranspiration.

A knowledge of evapotranspiration is necessary in planning and operating water resources development. Evapotranspiration data are essential for estimating water requirements for irrigation, and are useful for estimating municipal and industrial water needs, rainfall disposition, safe yields of ground water basins, water yields from mountain watersheds, and stream-flow depletions in river basins. Actual measurements of evapotranspiration under each of the various physical and climatic conditions of any large area are time-consuming and expensive. Thus, rapid and reliable methods are needed by hydrologists for estimating evapotranspiration.

To make a fair estimate of the evapotranspiration losses, one must conduct the extensive field surveys, and use one or more of the appropriate calculation methods. A common approach to predict evapotranspiration is to estimate the potential atmospheric energy available at the plant and soil surfaces, then determine the proportion of this energy utilized for conversion of liquid water to vapour depending on the water availability or rate of transmission from within the soil profile. Several methods to estimate the potential evapotranspiration have been developed and tested. They contain one or more atmospheric variables, or an indirect measurement of them, which are often combined with a representation of the surface conditions and interactions. Some are based on the physics of combining the vertical radiation budget with turbulent boundary flow over the land surface. More empirical methods based. on solar radiation, air temperature, air humidity, pan evaporation, or some combination of these have proven to be practical. Several models of the complete dynamic evapotranspiration process have been developed in recent years which vary considerably in complexity from single equations with empirical coefficients to very detailed physical representations. The utility of each method depends upon its requirements for input data, location calibration, and expected accuracy.

The present report deals with the major system processes which determine evapotranspiration. The report provides a summary of : theory of evaporation and evapotranspiration; measuring techniques for evapotranspiration; estimating evapotranspiration from meteorological data; and the recent numerical model studies.