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ITRC Paper No. P 14-003.  Daniel J. Howes and Mariana Pasquet

Grass Referenced Based Vegetation Coefficients for Estimating Evapotranspiration for a Variety of Natural Vegetation

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In arid and semi-arid regions, evapotranspiration from vegetation results in the significant utilization of available water. Accurate estimates of evapotranspiration are required for surface and subsurface hydrologic evaluations as well as irrigation district water balance studies. A significant amount of transferable information exists for irrigated agricultural crops through past and current research in the form of grass or alfalfa reference based crop coefficients (Kc) and basal crop coefficients (Kcb). However, transferable evapotranspiration information on natural vegetation is limited. Much of the work was conducted in the early to mid-1900’s and is presented as actual evapotranspiration from the vegetation at the research site either as annual or monthly values. In some cases, the data may have been referenced to evaporation pan measurements (typically Class A type pans) with unknown site conditions. An intensive literature review was conducted to extract monthly measured evapotranspiration information for natural vegetation types under various conditions. Monthly vegetation coefficients (Kv) for standardized grass reference based evapotranspiration (ETo) were computed using long-term average grass reference evapotranspiration information computed with data from nearby weather stations. Comparisons of the Kv values for similar vegetation indicate higher variability during the non-summer months but results from most of the studies examined are in good agreement. These Kv values provide some level of transferability so that it is possible to compute an accurate estimate of vegetative evapotranspiration with daily or monthly standardized grass reference evapotranspiration values in areas away from the original study.



Howes, D.J., and M. Pasquet. 2014. Grass Referenced Based Vegetation Coefficients for Estimating Evapotranspiration for a Variety of Natural Vegetation. Presented at USCID Conference. Denver, Colorado. October 22-25, 2013. United States Committee on Irrigation and Drainage. 14 pp.


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