THE ROLE OF EVAPOTRANSPIRATION IN DECADAL-SCALE CHANGES IN GROUNDWATER-SURFACE WATER DYNAMICS IN THE WILLIAMSON RIVER BASIN, OREGON

Decoupling of the groundwater and surface-water systems in the Williamson River basin have caused a precipitous decline in the area of open water in Klamath Marsh (KM) since the early 2000s. The 18,000 ha KM in the northern Klamath River basin of Oregon is a culturally significant wetland for The Klamath Tribes, a National Wildlife Refuge, and an important stopover for migratory birds on the Pacific Flyway. Since 2001, the area of open water in KM has declined at a mean rate of 83 ha/yr, principally due to increased infiltration and loss of support from the underlying groundwater system. Concurrent with the declines in open water, the groundwater level beneath the marsh has declined about 0.3 m/yr and gaged streamflow into the marsh has declined about 480,000 m³/yr.

Prior to about 2005, the groundwater level beneath KM was at land surface. Relatively steady declines since then have resulted in an increasingly thick unsaturated zone under the marsh which has led to increased infiltration of incoming streamflow and incident precipitation. Compounding the effect of the declines, widespread ditching of the marsh for drainage during the 20^th century breached the surficial peat soils which enhances infiltration.

The declines in groundwater level and streamflow cannot be attributed to changes in anthropogenic water use or management over the previous 25 years, and no trends in precipitation were observed. The declines are well-correlated with an increase in air temperature, however, which may indicate that they are related to increasing evaporative demand (ED) in the basin. Additional increases in ED also have resulted from the regrowth of about 23,000 ha of forest land in the Williamson River basin during 2001–21, following a substantial reduction in timber harvests around 1990. Ongoing work in the basin is attempting to quantify changes in ED over time and to understand the processes through which increasing ED affects recharge and runoff processes.