| Summary: | Riparian evapotranspiration (ET) is the most challenging component to quantify when determining water budgets in semiarid basins. Primary interest has focused upon water use by riparian overstory vegetation; however understory vegetation has been ignored. Whole plant transpiration of the dominant understory shrub, seep willow (Baccharis salicifolia), was measured along a perennial reach of the San Pedro River for 133 days using the heat balance sap flow technique while key meteorological driving variables were measured from an adjacent meteorological tower. Transpiration was compared under two environmental conditions: a more open environment with decreased overstory canopy cover, and a more closed environment underneath a cottonwood (Populus fremontii) forest canopy. Transpiration at both sites was similar, indicating that transpiration was rarely demand-limited. Shrub growing season transpiration was greater than precipitation and of comparable magnitude to the overstory cottonwood. These results suggest that understory vegetation water use can be an important component of a semiarid riparian water budget.
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