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Title: A review of approaches for evapotranspiration partitioning

item KOOL, DILIA - Ben Gurion University Of Negev
item AGAM, NURIT - Ben Gurion University Of Negev
item LAZAROVITCH, NAFTALI - Ben Gurion University Of Negev
item HEITMAN, JOSHUA - North Carolina State University
item Sauer, Thomas
item BEN-GAL, ALON - Gilat Research Center

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/9/2013
Publication Date: 10/5/2013
Citation: Kool, D., Agam, N., Lazarovitch, N., Heitman, J.L., Sauer, T.J., Ben-Gal, A. 2013. A review of approaches for evapotranspiration partitioning. Agricultural and Forest Meteorology. 184:56-70.

Interpretive Summary: Evaporation from a land surface can come from the plants (transpiration) or from the soil (soil water evaporation). In order to use available water efficiently, growers often want to maximize transpiration, especially in arid environments where water is supplied through irrigation. There are ways to measure evapotranspiration (evaporation from soil and plant transpiration together) or soil water evaporation and transpiration separately. Partitioning evapotranspiration into its two components is helpful in understanding water use and irrigation efficiency. In this paper we review all of the techniques available for partitioning evapotranspiration. In some cases, evapotranspiration is measured with either soil water evaporation or plant transpiration and the difference is used to calculate the missing term. The best approach is to measure all three components separately as then the sum of soil water evaporation and transpiration can be compared to the evapotranspiration as a check on the accuracy of the techniques used. Various computer modeling approaches can also be used. It was concluded that it was sometimes possible to get evaporation plus transpiration to within 10% of evapotranspiration. Some of the issues in selection of the measurement method include size of the measurement area and frequency of measurements. This research is important to landowners and policymakers interested in verifying and improving water-use efficiency.

Technical Abstract: Partitioning of evapotranspiration (ET) into evaporation from the soil surface (E) and transpiration (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. E is expected to be quite significant in sparsely vegetated systems, particularly in semi-arid and arid areas or in very wet systems such as flood 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 summarize and evaluate 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 discussed briefly. To compare approaches, 51 ET partitioning studies were evaluated 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, suggesting that reduction of E could increase water-use efficiency. Only 19 studies were found to estimate E and T as well as ET, and had varied results. A number of studies succeeded to estimate E + T to within 10% of ET. Future challenges include development of continuous methods to measure E and validation of models simulating the E and T components separately.