**Estimating Potential Evapotranspiration**

Increasing population and needs for an augmented food supply give greater importance to improved procedures for estimating agricultural water requirements both for irrigation and for rain-fed agriculture. Four methods for estimating potential evapotranspiration are compared and evaluated. These are the Class A evaporation pan located in an irrigated pasture area, the Hargreaves equation, the Jensen-Haise eguation, and the Blaney-Criddle method. The evaporation pan is rated as superior to the other methods. However, the difference in reliability between the pan and the Hargreaves method are not considered to be very significant. Both the Jensen-Haise and the Hargreaves methods require either measured or estimated solar radiation. Methods are presented for estimating solar radiation from the difference between maximum and minimum temperature, from the percentage of possible sunshine, and from relative humidity. These procedures have some limitations, but provide improved reliability and make the estimates more universal.[1]

**Evapotranspiration: Concepts and Future Trends**

Past research on evapotranspiration has provided sound theoretical knowledge and practical applications that have been validated through field measurements. Many different approaches have been used; however, when primary concepts and standard definitions are accepted, it is possible to find reasonable agreement among methods. This paper reviews such approaches, from Penman to Penman-Monteith. The standard concepts of potential evaporation (EP) and equilibrium evaporation (Ee), and the introduction of the climatic resistance (re), provide a better understanding of the role of the climate together with surface and aerodynamic resistances (rs and ra). Therefore, the concept of reference evapotranspiration (ETo), particularly the new one adopted by the Food and Agricultural Organization of the United Nations, can be better understood, as well as its limitations. Crop evapotranspiration (ETc) is related to both ETo and Ee. Crop coefficients (Kc) can be shown to have two components, αo and αc, with Kc = αoαc. The αo is a function of the climatic resistance and of the aerodynamic resistances of the crop and of the reference crop. The αc is a function of both surface and aerodynamic resistances of the crop and of the reference crop. From this analysis some ideas on future developments result that are directed toward providing compatibility between the one- and two-step calculation of ETc.[2]

**A review of approaches for evapotranspiration partitioning**

Partitioning of evapotranspiration (ET) into evaporation from the soil (E) and transpiration through the stomata of plants (T) is challenging but important in order to assess biomass production and the allocation of increasingly scarce water resources. Generally, T is the desired component with the water being used to enhance plant productivity; whereas, E is considered a source of water loss or inefficiency. The magnitude of E is expected to be quite significant in sparsely vegetated systems, particularly in dry areas or in very wet systems such as surface irrigated crops and wetlands. In these cases, ET partitioning is fundamental to accurately monitor system hydrology and to improve water management practices. This paper aims to evaluate and summarize available methods currently used to separately determine E and T components. We presuppose that, to test the accuracy of ET partitioning methods (measurements and/or modeling), all three components, i.e., E, T and ET, must be estimated independently, but recognize that sometimes one of the components is taken as the residual of the other two. Models that were validated against measurements for their ability to partition between E and T are briefly discussed. To compare approaches, 52 ET partitioning studies were considered regarding estimates of the relative amount of E and for success of agreement in closing the ET = E + T equation. The E/ET ratio was found to exceed 30% in 32 of the studies, which confirms the hypothesis that E often constitutes a large fraction of ET and deserves independent consideration. Only 20 studies estimated E and T as well as ET, and had varied results. A number of studies succeeded to estimate E + T to within 10% of measured ET. Future challenges include development of models simulating the components of ET separately and advancement of methods for continuous measurement of E, T and/or the ratio between the two.[3]

**Defining and Using Reference Evapotranspiration**

Values of reference evapotranspiration (ET0) are used with crop coefficients (KC) for many aspects of irrigation and water resources planning and management. More than a score of methods are used for estimating ET0. Estimated values vary widely due to the lack of standardization of the reference. Evaporation (ET) measured by lysimeters of various grasses and/or alfalfas has been used as the standard for developing estimating equations. Due to variation in the references used, some international organizations now wish to promote the use of a single equation or method to avoid the confusion caused by the current diversity. Different versions of the Penman combination equation have been proposed. The Research Center for the European Community and the ASCE Committee on Irrigation Requirements have evaluated various equations for estimating ET0. Due to its simplicity and the accuracy of estimates, the 1985 Hargreaves et al. equation is recommended for general use. ET0 is used in irrigation planning, design, and scheduling and for other water adequacy studies. [4]

**Estimating Potential Evapotranspiration**

Simple computational procedure whereby average daily potential evapotranspiration is represented as proportional to product of day-time hours squared and to saturated water vapor concentration at mean temperature; day time factor depends on consideration of disparity between net radiation and temperature, and fact that transpiration is restricted during darkness since leaf stomata are closed; comparison with C.W.Thornthwaite’s method and other methods. (31 refs.)[5]

**Reference**

[1] Hargreaves, G.H. and Samani, Z.A., 1982. Estimating potential evapotranspiration. Journal of the irrigation and Drainage Division, 108(3), pp.225-230.

[2] Pereira, L.S., Perrier, A., Allen, R.G. and Alves, I., 1999. Evapotranspiration: concepts and future trends. Journal of irrigation and drainage engineering, 125(2), pp.45-51.

[3] Kool, D., Agam, N., Lazarovitch, N., Heitman, J.L., Sauer, T.J. and Ben-Gal, A., 2014. A review of approaches for evapotranspiration partitioning. Agricultural and forest meteorology, 184, pp.56-70.

[4] Hargreaves, G.H., 1994. Defining and using reference evapotranspiration. Journal of irrigation and drainage engineering, 120(6), pp.1132-1139.

[5] Hamon, W.R., 1961. Estimating potential evapotranspiration. Journal of the Hydraulics Division, 87(3), pp.107-120.