Using Geospatial Technologies to Differentiate Between Land Use and Climate Impacts on Washington State Semi-arid Hydrologic Systems

Agricultural land use, combined with weather/climate, plays a major role in surface runoff hence lake/pond and stream dynamics (e.g., Detenbeck et al, 2002). Land use and weather/climate patterns may result in similar hydrologic responses in settings that are further complicated by different geomorphic surfaces. The overall objective of this research is to test the utility of geospatial techniques in assessing these relationships over time on Washington’s semiarid Northern Columbia Plateau (NCP). The NCP includes traditional summer fallow and minimum tillage grain agriculture, conservation reserve program (CRP), and range lands and a semiarid, winter precipitation-maximum climate that varies inter-annually (e.g., El Nino) and inter-decadally (e.g., Pacific Decadal Oscillation). Late Pleistocene glaciation and flooding resulted in a variety of geomorphic surfaces (i.e., stripped bedrock, bars, till, outwash, lake sediments, and loess) and hydrographic patterns (i.e., open drainages characterized by streams and lake/pond-dominated closed drainages). Lakes and ponds have long-term significance to migratory and resident waterfowl while streams of the area are potential salmonid habitat.

This project will select a sub-set of the NCP based on the variety of land uses, geomorphic surface types, and drainage patterns using current airphotos. The boundary and area of each lake/pond and stream system watershed will be determined using digital elevation models (DEM’s). Landsat Thematic Mapper (TM) imagery and digital orthophotos will be used to calculate the proportion of each watershed comprised of particular land use and geomorphic surface types. In closed drainage settings, we will identify lake/pond changes over time by quantifying the extent of water bodies using TM imagery taken on or near bi-annual anniversary dates (e.g., Beeri and Phillips, 2007). Discussions with local residents will also help us determine when lakes/ponds typically reach their maxima in spring and minima in late summer/early fall thus allowing us to better determine imagery dates. In open drainage systems, we will identify changes in stream channels over time using satellite imagery taken on or near annual anniversary dates. While Landsat imagery is preferable for this analysis because it predates the advent of the CRP, we will use 10 m resolution SPOT imagery that became available after 1986 if the 30 m resolution TM imagery is insufficient (Lillesand and Kiefer, 2000).

Bi-annual satellite images will allow us to construct general hydrographs for the study area’s lakes/ponds and assess stream channel changes over the past ~25 years, as well as determine the relationship of lake/pond level and stream channel changes to land uses, geomorphic surfaces, and weather and climate patterns over time. The blend of satellite-derived data with digital orthophotos, DEM’s, and climate data will allow us to determine the causes of small-watershed hydrographic changes over time. With the successful completion of this pilot project, we will be able to apply this methodology to the entire NCP, furthering understanding of hydrographic dynamics over the past 25 years as well as hindcasting historic hydrographic patterns and forecasting future hydrographic patterns under different land use and climate change scenarios.