|Scott, R - UNIV. OF ARIZ.|
|Shuttleworth, W - UNIV. OF ARIZ.|
|Maddock, T - UNIV. OF ARIZ.|
Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 20, 2000
Publication Date: December 12, 2000
Interpretive Summary: In many semi-arid basins, groundwater from aquifers is the primary water source that sustains human habitation, agriculture and riparian systems. To most efficiently utilize groundwater it is important to know both its source and how the groundwater is used both by pumping and by naturally occurring vegetation. The largest use of groundwater by non-irrigated vegetation occurs in riparian areas. However, it is very difficult to estimate the amount of groundwater that riparian vegetation uses over a large area. Measurements were made in this study to estimate the amount and source of water used by two dominant types of vegetation (sacaton grassland and mesquite) in the riparian floodplain of the San Pedro River in southeastern Arizona. The results indicate that the grassland relied primarily on recent rainfall, while the mesquite obtained water from deeper in the soil profile. Neither appears to be drawing significant water from the regional groundwater. This suggests that the dominant, natural groundwater withdrawals in this basin are mainly confined to the narrow cottonwood/willow gallery.
Technical Abstract: Consumptive water use from riparian evapotranspiration is a large component of many semiarid basins' groundwater budgets. 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). 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 this basin are mainly confined to the narrow cottonwood/willow gallery.