title: Groundwater response to precipitation events, Kalaloch, Olympic Peninsula, Washington
Creators: name: Hanell, Casey R.
Publisher: name: Western Washington University
Year: 2011

description: <p>Tens of thousands of square kilometers of forestland in Washington are managed as working forests, primarily for timber production. The effects of timber harvesting on physical watershed processes continue to be the subject of intense research throughout the Pacific Northwest. Watershed analyses completed in Washington during the mid-1990s resulted in significant modifications to Washington's Forest Practices Act and Rules. These measures mandate rigorous evaluation of potential effects of timber harvesting on slope stability. Although timber harvesting has been linked to an increase in surface erosion and mass wasting in the Pacific Northwest, most studies have focused on shallow landslide processes. The loss of canopy interception and evapotranspiration associated with timber harvesting and the resulting effects on groundwater levels and stability of deep-seated landslides are not well understood. In this study, I use field measurements to analyze subsurface water level rise and attenuation in response to precipitation events, and the Distributed Hydrology Soils Vegetation Model (DHSVM) to model potential changes in hydrology resulting from clearcut timber harvesting. The research site is a portion of a moderately steep watershed (2 sq-km) located 6 km southeast of Kalaloch, WA on the coast of the Olympic Peninsula. Slope gradients generally measure between 30 and 50 percent, with localized steeper and gentler slopes. Ten wells at the site are instrumented with pressure transducers that record hourly subsurface water levels. I use onsite and nearby precipitation measurements and pressure transducer data to characterize groundwater level response characteristics at the site between February 2005 and February 2007. This analysis shows subsurface water levels rise and attenuate rapidly in response to precipitation and usually reach peak levels within hours after the onset of precipitation and attenuate within days, regardless of the magnitude of the event. I also use Kendall's Ï„ correlation analysis (Kendall, 1938) to evaluate the relationship between cumulative precipitation for a given event and peak well water level for the same event. Kendall's τ values were most significant for between 3 hours and 13 hours of cumulative onsite precipitation in 6 of the 10 wells, with all of the most significant correlations falling between 3 hours and 48 hours with the exception of one well which had no significant correlations. Kendall's τ correlation between subsurface water levels and open air precipitation measurements made at the nearby Black Knob Weather Station and linear regression analysis were also most significant with hours of cumulative precipitation as opposed to days or weeks. The DHSVM results show a 27.4 percent reduction in evapotranspiration when the research basin vegetation is converted from an entirely forested condition to an entirely shrub-covered condition with all other variables constant. This reduction in evapotranspiration is modeled to result in a slight increase in streamflow and a slight increase in soil moisture and groundwater level for the two year period from February 2005 through February 2007. The majority of this decrease in evapotranspiration and increase in streamflow and subsurface water occurs during the spring and early summer when evapotranspiration rates are high and water levels are below modeled maximum peak levels.</p>