Document Type

Thesis

Date of Degree Completion

Summer 2023

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Carey Gazis

Second Committee Member

Lisa Ely

Third Committee Member

Clay Arango

Abstract

In the past two decades, stream restoration work, primarily in the form of wood emplacement, has been undertaken in the Taneum Creek watershed, resulting in increased channel-floodplain connectivity. One of the goals of stream restoration was to boost dry season groundwater storage in the shallow floodplain aquifer. However, any gains in groundwater due to increased connectivity may be nullified by increased evapotranspiration (ET) losses because of denser floodplain vegetation. Within the floodplain aquifer budget, ET is a major flow of water out of the system and is not well quantified.

In order to quantify ET, a monitoring site was established to measure relevant hydrological and environmental parameters along lower Taneum Creek in a meadow which contains a new side channel and a series of beaver dams. The monitoring site includes an evaporation station, and two soil monitoring locations, “riparian” and “meadow”, representing relatively wet and relatively dry areas within the Taneum floodplain respectively. This monitoring equipment provided ground-truthing estimates of ET that were compared to satellite-based estimates generated by OpenET.

Estimates of ET and evaporation using different methods were generated for a six week period in May to July, 2023. Two ET estimates were calculated using the Penman-Monteith FAO-56 method, using soil heat flux data collected in the field and local weather data. These two estimates, (1-7 mm/day at the riparian location and 0-6 mm/day in the meadow location), overlap significantly. These soil heat flux estimates increased as the season progressed, but were consistently lower than the satellite-based estimates, which range from 4-7 mm/day. This difference between estimates from different methods may be because the remote-sensing based estimates include transpiration from large trees that were not captured in the localized soil heat flux measurements. Evaporation calculated from pan evaporation, which excludes transpiration, displayed the lowest rate of water loss, at an average of 2 mm/day across the study period. Magnitude of ET fluxes suggests that any increase in inputs to groundwater storage [as a result of increased channel-floodplain connectivity] may well be negated by increased vegetation growth and associated evapotranspiration.

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