Evapotranspiration from Natural Vegetation in the Central Valley of California: Monthly Grass Reference-Based Vegetation Coefficients and the Dual Crop Coefficient Approach

Abstract

Restoration activities in the Central Valley of California and elsewhere require accurate evapotranspiration information, which can then be used for a wide variety of surface and subsurface hydrologic evaluations. However, directly measuring evapotranspiration can be difficult or impossible depending on the evaluation's time frame. Transferability of measured evapotranspiration in time and space is also necessary but typically requires a weather-based reference. For nonagricultural vegetation, there is at present time no standard reference, which makes the evaluation of a variety of vegetation types from different sources difficult and time-consuming. This paper examines several methods used to estimate evapotranspiration from native vegetation, including the use of vegetation coefficients (Kv). Vegetation coefficients are based on a standardized reference and are computed as the ratio of vegetation evapotranspiration (ETv) to the grass reference evapotranspiration (ETo). These monthly Kv values are used to compute the long-term (for this study, 1922-2009) average ETv for vegetation types documented to exist in California's Central Valley prior to the arrival of the first European settlers in the mid-18th century. For vegetation that relies on precipitation and soil moisture storage, a calibrated daily soil-water balance with a dual crop coefficient approach was used to compute evapotranspiration regionally over the time frame.