Development of a Digital Aquifer Permeability Map for the Pacific Southwest in Support of Hydrologic Landscape Classification: Methods
Stratton, L., R. Comeleo, S. Leibowitz, AND P. Wigington. Development of a Digital Aquifer Permeability Map for the Pacific Southwest in Support of Hydrologic Landscape Classification: Methods. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-16/063, 2016.
Hydrologic landscape (HL) maps categorize areas based on five key indices of macro-scale hydrologic behavior: climate, seasonality, aquifer permeability, terrain, and soil permeability. Under the Air, Climate, and Energy National Program, work has been done to expand the HL concept to other geographic areas and to develop an approach for using it to address streamflow vulnerability from climate change. This work has included: development of the HL classification in Oregon, revision of the HL classification to be more broadly applicable to the Pacific Northwest (Oregon, Washington, and Idaho), use of the HL classes to predict where a simple lumped hydrologic model accurately predicts daily streamflow, use of HL information to model the presence of cold-water patches at tributary confluences, and combining Oregon HL results with temperature and precipitation predictions to examine how HLs would vary as a result of climate change. This report expands the aquifer permeability map to the Pacific Southwest (California, Arizona, and Nevada) and revises the methodology to better account for additional geologic variability and uncertainty within the source data. These improvements create a product that can be used effectively both in the HL classification and as a valuable dataset for groundwater and climate models. This contributes to the ACE CIVA-2.3 product “Hydrologic landscape national watershed vulnerability assessment.”
Researchers at the U.S. Environmental Protection Agency’s Western Ecology Division have been developing hydrologic landscape maps for selected U.S. states in an effort to create a method to identify the intrinsic watershed attributes of landscapes in regions with little data. Each hydrologic landscape unit is assigned a categorical value from five key indices of macro-scale hydrologic behavior, including annual climate, climate seasonality, aquifer permeability, terrain, and soil permeability. The aquifer permeability index requires creation of a from-scratch dataset for each state. The permeability index for the Pacific Southwest (California, Nevada, and Arizona) expands and modifies the permeability index for the Pacific Northwest (Oregon, Washington, and Idaho), which preceded it. The permeability index was created by assigning geologic map units to one of 18 categories with presumed similar values of permeability to create a hydrolithologic map. The hydrolithologies were then further categorized into permeability index classifications of high, low, unknown and surface water. Unconsolidated, carbonate, volcanic, and undifferentiated units are classified more conservatively to better address uncertainty in source data. High vs. low permeability classifications are assigned qualitatively but follow a threshold guideline of 8.5x10-2 m/day hydraulic conductivity. Estimates of permeability from surface lithology is the current best practice for broad-scale assessment of groundwater flow characteristics in regions with little data, but assumptions are broad and may not be met due to lithologic variability with depth and intra-category variation in primary and secondary porosity. The permeability maps for each state were completed at the resolution of the best-available geologic map and should not be used to perform analysis on specific units or at scales finer than the primary dataset.