**2.1. Evapotranspiration**

Evaporation and transpiration are two important processes involved in the removal of water from soil and plants into the atmosphere. These processes occur simultaneously and are inherently connected to each other [12]. While transpiration and evaporation occur simultaneously, evaporation is based on the availability of water in topsoil and the amount of solar radiation reaching the soil surface [13]. Transpiration is a function of crop canopy density and soil water status. Evaporation accounts for the majority of crop evapotranspiration (ET<sup>c</sup> ) during early stages of crop growth in bare-ground plantings, while transpiration contributes to nearly 90% of the ETc for a mature crop [14].

Evapotranspiration can be separated into ETo and ETc . Crop evapotranspiration is calculated from ETo of a given area and the crop coefficient (K<sup>c</sup> ) of the crop being measured. Factors affecting ET<sup>c</sup> include extent of ground cover, crop canopy properties, and aerodynamic resistance [12]. Reference ETo is the amount of water exiting the soil at any time from a reference surface covered by grass at a 0.12 m height that is adequately watered, actively growing, and with a fixed surface resistance [14]. Weather conditions are also important to quantify as they affect the amount of energy available for ET<sup>o</sup> to occur. The four most important conditions to measure are solar radiation, wind speed, temperature, and humidity, with the most important factor being solar radiation [15].

Crop coefficients are an adjustable constant that define the amount of transpiration occurring within a plant at a given stage of development. Crop coefficients are computed as the ratio ETo :ET<sup>c</sup> . Environmental and physiological factors affecting K<sup>c</sup> include crop type, crop growth stage, climate, and soil type [14]. Plant developmental stage encompasses the relative activity of the plant. Plant size is also impacted by the crop development stage, thus affecting leaf area and canopy density, which in turn impacts transpiration. Accounting for environmental and management factors that influence the rate of canopy development is also important in calculating K<sup>c</sup> . Climatic factors that significantly affect K<sup>c</sup> are rainfall frequency, wind speed, temperature, and photoperiod [14]. Soil profile characteristics that affect K<sup>c</sup> development are water table depth and soil porosity. Therefore, regional K<sup>c</sup> estimates from several seasons are important to account for the variability in weather, irrigation, drainage, and runoff [16, 17].

Several WB-based methods exist to calculate ET<sup>o</sup> rate, such as the Priestley-Taylor method and Hargreaves method. The Priestly-Taylor equation is a modification of the Penman-Monteith equation that approximates parameters established by the Penman-Monteith, using solar radiation to determine ETo . However, calculations at a research site in the humid Southeastern USA found that Priestley-Taylor could overestimate ET<sup>o</sup> for the region [18]. Priestly-Taylor has also been reported to overestimate the cumulative ETo for the Georgian Coastal Plain area during months with significant rainfall, corresponding to peak early summer vegetable production [18]. Another method that has been used to estimate ET<sup>o</sup> has been the Hargreaves method. This equation is an empirical model that considers incoming solar energy, evaporation, monthly maximum and minimum temperature, and a temperature coefficient [19]. This method has a high correlation with the Penman-Monteith model for estimates of average weekly ETo in humid regions [19]. These methods of calculating evapotranspiration are easier to use than the Penman-Monteith method; however, this can also result in reduced precision over the course of a season.
