Evapotranspiration from a GreasewoodCheatgrass community

Groundwater elevation, soil moisture, and precipitation were monitored to evaluate the components of water loss from two greasewood-cheatgrass (Sarcobatus vermiculatusBromus tectorum) communities in south central Washington. Annual evapotranspiration was 21-25 cm, 18-31% of which was the transpiration of groundwater. The greatest loss from the system was the evapotranspiration of stored soil moisture, but this moisture was unavailable to greasewood. This study confirms that water use rates are a function of depth to water up to 2.3 meters, but indicates a more complicated mechanism at depths of up to 13 meters. Shrub height, canopy coverage, and total leaf surface area are inversely related to depth to water, and the rate of water use is in turn directly related to these plant-associated features. Semiarid lands of the Pacific Northwest are characterized by low annual precipitation that occurs principally between October and March when evapotranspiration demands are the least. This timing results in the improved efficiency of precipitation in terms of water availability to plants. Most wild plants of the region exhibit life cycles in phase with the relatively moist and cool period of the year. A major exception involves phreatophyte communities and the continuous availability of water to them. This report describes the water use of a plant community dominated by the greasewood shrub (Sarcobatus vermiculatus). During the dry season water is available to greasewood from a groundwater supply perched near the soil surface. The community studied is similar to those that cover thousands of square kilometers of saline areas in the Great Basin [Robinson, 1958]. The soil differs somewhat in that salinity is not the result of impeded drainage but rather is a result of the geologic past.. The groundwater is not saline although the surface soil is rich in sodium and other cations, especially immediately beneath greasewood shrubs where localized recycling occurs [Rickard, 1964; Rickard and Keough, 1968]. The relationship between groundwater drawdown and phreatophyte transpiration was rec* Now at School of Forestry, Oregon State University, Corvallis, Oregon 97331. ognized as early as 1916 [White, 1932]. This relationship has been used in numerous water balance investigations such as the studies of riparian vegetation by Qashu and Evans [1967] and the salt cedar (Tamarix sp.) tank experiments of van Hylckama [1970]. It has also been applied to greasewood in several tank studies [White, 1932; Robinson, 1970], but our study estimates the water use by naturally occurring undisturbed greasewood plants.