ESTIMATING GROUNDWATER-DISCHARGE IN OREGON’S HARNEY BASIN

Increasing groundwater demand in Oregon’s Harney Basin, along the northwest edge of the Great Basin, has prompted a reevaluation of the groundwater resource and its ability to sustain existing uses and future development. A major objective of this reevaluation is to develop a basin-scale water budget that incorporates measured and remotely sensed data. Water budgets account for recharge to and discharge from the aquifer system and provide a basic mechanism to evaluate the occurrence and movement of groundwater. In the Harney Basin, a closed surface water basin, evapotranspiration (ET) is the principle mechanism by which water exits the groundwater system, either directly via phreatophytic vegetation or indirectly via direct ET losses from wetlands and open water sustained by spring discharge or base flow. Reconnaissance-level estimates of groundwater ET in Harney Basin were determined in the early 1930s from ground-based vegetation mapping, crop consumptive-use estimates made in the Harney Basin, and native vegetation consumptive-use estimates made in Southern California. In the current study, natural groundwater ET is being estimated using (1) remotely-sensed satellite imagery and field observations to map and characterize vegetation type and vigor, (2) recent groundwater ET measurements made across multiple vegetation types throughout the northern Great Basin, and (3) remote-sensing based ET prediction models developed for agriculture and native vegetation areas. Improved groundwater-discharge estimates will constrain recharge estimates and planned groundwater-flow models, and provide stakeholders and water-resource managers with information to make informed groundwater-resource decisions.