title: Hydrologic and geomorphic assessment of Ebey's Prairie, Central Whidbey Island, Washington
Creators: name: Larrabee, Michael A. (Michael Allen)
Publisher: name: Western Washington University
Year: 2011

description: <p>Ebey's Prairie, Washington, was once bisected by a broad riparian corridor consisting of waterlogged soils, swampy areas, seasonal ponds, and intermittent flows, which helped recharge the local aquifer. By the mid-1900s, agriculture drainage tiles, drainage ditches, and fill were being installed by landowners to increase tillable acreage. The extent and location of these drainage tiles or the effects these tiles have had on surface water and subsequently on aquifer recharge in the area remains uncertain. In this study, I characterized the modern and historic surface hydrologic conditions of Ebey's Prairie and their relationship to the local geomorphology. I used the Distributed Hydrology-Soil-Vegetation Model (DHSVM) to reconstruct the pre-agricultural surface hydrology and evaluate the effects agricultural drainage tiles have had on surface hydrologic conditions and aquifer recharge. A model representing Ebey's Prairie watershed with was created, calibrated, and validated to stream discharge measured during my study. A second model was created to represent Ebey's Prairie watershed without drainage tiles. Simulations for water years 2001-2010 for each basin condition were executed and compared to quantify the influence of drainage tiles on hydrologic regimes. Additionally, I mapped the local geomorphology, relating landforms to hydrologic regimes, and used lake sediment coring to improve the understanding of the sequence of events that created the unique landscape and its paleo-environment. Average annual surface discharge for Ebey's Prairie watershed increased by 41,540 m3 (10.97 million gal.) when artificial drainage was present in the model, an increase of 163 percent over the pre-disturbance basin. The general shape of hydrographs was similar for both watersheds; however the basin with drainage tiles typically had peak flows 2-3 times larger than the basin without tiles, in addition to greater hourly baseflows and a longer recessional curve. Average recharge for the entire Ebey's Prairie watershed with drainage tiles was 19.9 cm/yr. and without tiles was 20.3 cm/yr., an increase in recharge of 41,420 m3 or 1.65 percent, which is within the margin of error for the model. It was determined that the effective drainage area of the Ebey's Prairie watershed was smaller than the watershed boundaries as delineated by DHSVM. The effective drainage area largely contained both the silty loam and loam soils or in the silty loam soil only. The silty loam is coincident with the majority of the drainage tiles network and two closed depressions identified as relict marshes. The distribution of an additional 41,418 m3 of recharge across a smaller effective drainage area would result in an increase of between 1.0 to 9.8 cm/yr., which is significant. A geomorphic map of Ebey's Landing National Historical Reserve was created, identifying 20 distinct landforms covering an area of 72.7 km2. Eighty-six percent of the map area is composed of four map units: glaciated uplands, ice-marginal deltas, marine terrace and kame-kettle topography. Two sediment cores, 6.64 m and 9.24 m long, were collected from the Lake Pondilla kettle pond. I attempted to numerically date sediments deposited after kettle collapse to constrain the timing of events associated with the formation of the local geomorphology. Lack of extension rods during coring prevented recovery of deeper sediments. The recovered cores indicated a rapid sedimentation of 1.26 - 1.37 mm/yr through the mid and late-Holocene. A tephra at 7.81 m could not be identified based on chemical analysis, however it is likely Mazama ash based on thickness, character and position within the sediment sequence.</p>