The water use of two dominant vegetation communities in a semiarid riparian ecosystem

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 grou ndwater studies the amount of water used by phreatophytes is estimated by e mpirical formulae and extrapolation of measurements taken elsewhere. These approaches are problematic due to the uncertainties regarding the vegetatio n's water source (e.g., groundwater or recent precipitation) and its magnit ude. 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 southeaster n 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 estim ates from previous studies. Additionally, measurements of soil water conten t and water table levels are used to infer the dominant sources of the evap orated water. The results indicate that the grassland relied primarily on r ecent precipitation, while the mesquite obtained water from deeper in the s oil profile. Neither appears to be strongly phreatophytic, which suggests t hat the dominant, natural groundwater withdrawals in the Upper San Pedro Ba sin are mainly confined to the narrow cottonwood/willow gallery that lines the river. (C) 2000 Elsevier Science B.V. All rights reserved.