Investigating the Link Between Surface Water and Groundwater in the Tule Lake Subbasin, Oregon and California

摘要

Water allocation in the upper Klamath Basin of Oregon and California has been challenging. Irrigators have increasingly turned to groundwater to make up for surface water shortages because of shifts in allocation toward in-stream flows for Endangered Species Act listed fishes. The largest increase in groundwater pumping has been in and around the Bureau of Reclamationu27s Klamath Irrigation Project, which includes the Tule Lake subbasin in the southern part of the upper Klamath Basin. Previous groundwater flow model simulations indicate that water level declines from pumping may result in decreased flow to agricultural drains in the Tule Lake subbasin. Agricultural drains on the Klamath Project are an important source of water for downstream irrigators and for the Tule Lake and Lower Klamath Lake National Wildlife Refuges. To better assess the impact of increased pumping on drain flow and on the water balance of the groundwater system, flow data from agricultural drains were evaluated to investigate the changes that have taken place in groundwater discharge to drains since pumping volumes increased. Additionally, a fine-grid groundwater model of the Tule Lake subbasin was developed based on the existing regional flow model. The fine-grid model has sufficient vertical and horizontal resolution to simulate vertical head gradients, takes advantage of time-series data from 38 observation wells for model calibration, and allows agricultural drains to be more explicitly represented. Results of the drain flow analysis show that the groundwater discharge to agricultural drains has decreased by approximately 4000 hectare-meters from the 1997-2000 average discharge. Most of this decrease takes place in the northern and southeastern portions of the subbasin. Results of the groundwater model show that the initial source of water to wells is groundwater storage. By 2006, approximately 56% of the water from wells is sourced from agricultural drains.