Seasonal estimates of riparian evapotranspiration using remote and in situmeasurements

In many semi-arid basins during extended periods when surface snowmelt or s torm runoff is absent, groundwater constitutes the primary water source for human habitation, agriculture and riparian ecosystems. Utilizing regional groundwater models in the management of these water resources requires accu rate estimates of basin boundary conditions. A critical groundwater boundar y condition that is closely coupled to atmospheric processes and is typical ly known with little certainty is seasonal riparian evapotranspiration (ET) , This quantity can often be a significant factor in the basin water balanc e in semi-arid regions yet is very difficult to estimate over a large area. Better understanding and quantification of seasonal, large-area riparian E T is a primary objective of the Semi-Arid Land-Surface-Atmosphere (SALSA) P rogram. To address this objective, a series of interdisciplinary experiment al campaigns were conducted in 1997 in the San Pedro Basin in southeastern Arizona. The riparian system in this basin is primarily made up of three ve getation communities: mesquite (Prosopis velutina), sacaton grasses (Sporob olus wrightii), and a cottonwood (Populus fremontii)/willow (Salix goodingi i) forest gallery. Micrometeorological measurement techniques were used to estimate ET from the mesquite and grasses. These techniques could not be ut ilized to estimate fluxes from the cottonwood/willow (C/W) forest gallery d ue to the height (20-30 m) and non-uniform linear nature of the forest gall ery. Short-term (2-4 days) sap flux measurements were made to estimate cano py transpiration over several periods of the riparian growing season. Simul taneous remote sensing measurements were used to spatially extrapolate tree and stand measurements. Scaled C/W stand level sap flux estimates were uti lized to calibrate a Penman-Monteith model to enable temporal extrapolation between synoptic measurement periods. With this model and set of measureme nts, seasonal riparian vegetation water use estimates for the riparian corr idor were obtained. To validate these models, a 90-day pre-monsoon water ba lance over a 10 km section of the river was carried out. All components of the water balance, including riparian ET, were independently estimated. The closure of the water balance was roughly 5% of total inflows. The ET model s were then used to provide riparian ET estimates over the entire corridor for the growing season. These estimates were approximately 14% less than th ose obtained from the most recent,groundwater model of the basin for a comp arable river reach. Published by Elsevier Science B.V.