Agricultural and Forest Meteorology 105 (2000) 241–256 The water use of two dominant vegetation communities in a semiarid riparian ecosystem Russell L. Scott a,∗ , W. James Shuttleworth a , David C. Goodrich b , Thomas Maddock III a a Department of Hydrology and Water Resources, University of Arizona, Tucson, AZ 85721, USA b USDA-ARS, Southwest Watershed Research Center, Tucson, AZ 85719, USA Abstract Consumptive water use from riparian evapotranspiration is a large component of many semiarid basins’ groundwater budgets — comparable in magnitude to mountain front recharge and surface water discharge. In most long-term groundwater studies the amount of water used by phreatophytes is estimated by empirical formulae and extrapolation of measurements taken elsewhere. These approaches are problematic due to the uncertainties regarding the vegetation’s water source (e.g., groundwater or recent precipitation) and its magnitude. Using micrometeorological techniques in this study, surface energy and water fluxes were measured for an annual cycle over two dominant types of vegetation in the riparian floodplain of the San Pedro River in southeastern Arizona. The vegetation communities were a perennial, floodplain sacaton grassland (Sporobolus wrightii) and a tree/shrub grouping composed largely of mesquite (Prosopis velutina). These measurements are compared with estimates from previous studies. Additionally, measurements of soil water content and water table levels are used to infer the dominant sources of the evaporated water. The results indicate that the grassland relied primarily on recent precipitation, while the mesquite obtained water from deeper in the soil profile. Neither appears to be strongly phreatophytic, which suggests that the dominant, natural groundwater withdrawals in the Upper San Pedro Basin are mainly confined to the narrow cottonwood/willow gallery that lines the river. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Evapotranspiration; Riparian corridor; Bowen ratio; Biometeorology; Water budget; Phreatophytes; Sporobolus wrightii; Prosopis velutina 1. Introduction For many of the human settlements in the semiarid Southwest, water from regional aquifers has become the largest single source of fresh water. Without this groundwater resource, the further development and perhaps even the sustainability of these communities would not be possible. This reliance has led to a signif- ∗ Corresponding author. Present address: USDA-ARS, 2000 E. Allen Road, Tucson, AZ 85719, USA. E-mail address: russell@tucson.ars.ag.gov (R.L. Scott). icant effort to improve our understanding of the water balance of these regional groundwater systems. In the basin and range physiographic province that characterizes much of the Southwest, the main natural inlet and outlet of the underlying groundwater sys- tems are mountain front recharge and riparian zone recharge/discharge areas. Mountain front recharge is the infiltration of mountain precipitation into the “headwaters” of the aquifer. This typically occurs from streams that carry water out onto the highly per- meable sediments of the mountain pediments. Water, having thus entered the regional groundwater aquifer, flows down gradient to the center of the basin. There, 0168-1923/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII:S0168-1923(00)00181-7