Estimating Evapotranspiration

Chapter 1

Abstract

improve further.

1. Introduction

3

covariance, scintillometer, remote sensing

ing the remaining water is paramount.

Jerry E. Moorhead

Field-Scale Estimation of

Evapotranspiration (ET) is a major component of the water cycle, which makes it an integral part of water resources management, especially in arid and semiarid environments. ET data are used for water management, irrigation scheduling, various modeling activities, and much more. Some areas of scarce water resources place limitations on water use, which are typically determined from various modeling approaches. As many models use ET as an input, or for validation, accurate ET data is essential to ensure accurate model outputs. In addition, most water management practices are done at the field scale; ET data of a similar scale is needed. Many ET measurement or estimation methods exist and vary widely in approach, instrumentation, complexity, and purpose. A lysimeter is considered the standard for ET measurement and is the most accurate. Other, more portable options are available, such as eddy covariance, scintillometer, Bowen ratio, and remote sensing, all capable of estimating actual field ET within approximately 30% of actual values. Although other methods may not be as accurate as a lysimeter, each has benefits in certain situations. Depending on the purpose, the level of accuracy may be suitable. ET estimation methods are constantly evolving, and accuracy should continually

Keywords: irrigation scheduling, energy balance, water balance, lysimeter, eddy

Fresh water is an essential resource that is becoming increasingly limited. In some arid and semiarid regions, groundwater resources are being exhausted with little to no surface water available as an alternate source. Proper water resources management is essential for these areas. In many cases, water management strategies rely on the use of evapotranspiration (ET) to account for some of the water losses. ET is a combined term that represents water lost through evaporation from the soil or plant surface, as well as water lost through transpiration from the plant. In many regions, such as the Texas High Plains, ET is the largest water loss component in the hydrologic budget. This fact makes accurate ET estimates vital for accurately and properly managing crop water. In the Texas High Plains, and the rest of the southern Ogallala Aquifer region, groundwater recharge is very low at 11 mm yr<sup>1</sup> [1]. With such little recharge, the Ogallala Aquifer is deemed a finite resource. In order to preserve this natural resource for future generations, conserv-

Evapotranspiration
