1. Introduction

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, conserving the remaining water is paramount.

The Texas High Plains lies in the Southern Great Plains near the southern end of the Ogallala Aquifer (see Figure 1). Agriculture is the predominant land use and irrigated land accounts for the majority of the agricultural production in this region. In the state of Texas, irrigation accounts for 60% of total water use; however, in the Texas High Plains, irrigation accounts for 89% of the total water use [2]. The Texas High Plains is a major corn-, cotton-, wheat-, and sorghum-producing region with much of the agricultural production under irrigation. The vast majority of irrigation water is withdrawn from the Ogallala Aquifer. With limited and sporadic rainfall, the Ogallala Aquifer receives little to no recharge in this region and is essentially being mined; therefore, conservation is an integral part of the regional water plan [3]. The northern and southern parts of the Texas High Plains are similar in size; however, the northern Texas High Plains irrigates over 1.1 million ha, while the southern Texas High Plains irrigates over 760,000 ha [4]. In both the northern and southern regions, irrigated crop yields are at least double that of dryland yields (on average).

In the southern Texas High Plains (see Figure 1), cotton is the major crop comprising 65% of the total irrigated area [4]. The popularity of cotton in this area is a reflection of the water resource limitations where the saturated thickness of the Ogallala Aquifer decreases near the southern boundary. Cotton only requires an annual average of 170 mm (6.7 in.) of irrigation in the Texas High Plains [3]. Peanuts are the second most grown crop in the southern region with about 9% of total irrigated area. Grain corn only accounts for 3% of irrigated area with winter

The decline in the saturated thickness of the Ogallala Aquifer has caused some local groundwater conservation districts to begin regulating annual water withdrawals. In Texas, groundwater conservation districts have been granted the authority to regulate water withdrawals to extend the life of the Ogallala Aquifer and meet the goals of regional water plans approved by the state. As part of the Texas State Water Plan, the Panhandle Water Regional Planning Group set the goal of nominally, on average, retaining 50% of current available water in 50 years [5]. Currently, regional irrigation demand is determined by advanced models such as MODFLOW [6] and the Texas A&M-Amarillo [3] model. MODFLOW is a complex model that assesses groundwater resources, which requires ET as an input. In 1999, the Texas A&M-Amarillo (TAMA) model was developed as a new estimation methodology for the region [5]. It was used to accurately estimate irrigation

demand in the northern Texas High Plains. The TAMA model estimates the seasonal irrigation demand per crop per county for 21 counties in the northern region of the Texas High Plains. The TAMA model requires inputs of ET, precipitation, and soil characteristics. Accurate ET data and local acreage knowledge beyond USDA-Farm

Since modeling is one of the main ways regional water plans are developed and assessed, accurate model outputs are highly desired. Many of the models use ET as an input, and the outputs are heavily affected by the accuracy of the inputs. High levels of accuracy are beneficial in regional water planning so that the best decisions are made regarding water allotment and water availability. This creates the need for

Measuring or estimating ET can be difficult but numerous instruments and methods do exist. A common (and relatively simple) method of estimating ET is using reference ET (ETref [7]) which uses meteorological data to estimate the water demand of a reference crop, usually a short, clipped grass or alfalfa. To get ET for a specific crop from the reference ET, a crop coefficient (Kc) can be applied to yield potential crop ET or ETc [7]. When measured ETc data are available, the Kc values can be obtained by dividing ETc by ETref. This approach requires accurate data for ETc to obtain the best results. Kc values for a wide variety of crops are available

Single and dual crop coefficient methods are available. For the single crop coefficient approach, water loss through transpiration is combined with soil evaporation, and a single Kc value is used. In the dual crop coefficient approach, the transpiration and evaporation components are split into a basal crop coefficient (Kcb) for transpiration and a soil evaporation (Ke) component [7]. The ETc from the Kc approach provides the amount of water that would be used by the crop if there is no water limitation. In most cases, ET can be lower than the potential rate due to stresses from water, nutrients, pests, etc. A stress coefficient (Ks) can be applied to the Kc to account for water stress when using ETref [7]. To account for reduced ET due to stresses, the term actual ET (ETa) is used. ETa corresponds to the actual

Service Agency values are essential for model accuracy.

high levels of accuracy in ET estimation.

1.1 Evapotranspiration

throughout the literature [7–9].

5

wheat and grain sorghum at 7% each [4].

Field-Scale Estimation of Evapotranspiration DOI: http://dx.doi.org/10.5772/intechopen.80945

In the northern Texas High Plains (see Figure 1), about 55% of the cropland is irrigated and uses about 1.76 billion m<sup>3</sup> (1.43 million ac-ft) of water annually for irrigation [3]. Irrigated winter wheat, grain corn, cotton, and grain sorghum are the predominant crops, comprising 30, 26, 23, and 10% of the total irrigated area, respectively [4]. Corn is a relatively large water use crop, requiring an annual average of over 480 mm (19 in.) of irrigation [3], and all of the corn area in this region requires irrigation. Currently, silage and forage crops are minor crops in the region but are increasing dramatically to meet the demands of new dairy operations that continue to expand into the area.

Figure 1. Ogallala Aquifer and Texas High Plains regions.
