Estimated Depth to Groundwater in the Portland, Oregon Area

This application provides access to estimates of depth-to-water and water-table elevation for the Portland, Oregon, metropolitan area, along with estimates of their relative uncertainty that were developed in Snyder (2009). The method of analysis used to determine the water-table configuration relies on water-level data from shallow wells and surface-water features that are representative of the water table (Dates of data used and when data were pulled). The water-table configuration data represent average conditions in the study area during 1929-2004. The precise position of the water table may differ from the estimated position at specific locations, and short-term, seasonal, and long-term variations in the differences can be expected (see Disclaimer). The values reported in this application represent the average water-table position for a 250 feet by 250 feet square area (as represented by the interpolation grid). This grid was generated from datasets of depth-to-water and water-table elevation for the current study were interpolated independently based on kriging as the method of interpolation with parameters determined through the use of semivariograms developed individually for each dataset. Resulting interpolations were then combined to create a single, averaged representation of the water-table configuration. Kriging analysis also was used to develop a map of relative uncertainty associated with the values of the water-table position. Site-specific estimates of the water-table position in areas of steep land-surface relief, steep water-table gradients, abrupt local change in hydrogeologic properties, or strong stresses to the groundwater system such as focused recharge or pumping, may result in water-table positions that differ from the average.

Each estimate of the water-table position has an associated uncertainty. The relative uncertainty data set provides a measure of the relative uncertainty associated with the estimates of the water-table position at any location. The values of the relative uncertainty range from 0 to 1, where 0 represents a low relative uncertainty and 1 represents a high relative uncertainty. These values provide a method to compare and evaluate the reliability associated with the values of the water-table position. The relative uncertainty data set addresses some but not all possible errors associated with the analysis of the water-table configuration and does not depict all sources of uncertainty. In particular, perched groundwater zones may be present in some areas resulting in the actual depth to water being less than estimated using the results of this study.

Additional information on the sources and description of data, study methods, estimates and information on seasonal water-table fluctuations, and contour maps of the depth-to-water, water-table elevation, and relative uncertainty, for the Portland, Oregon metropolitan area can be found in the associated report, "Estimated Depth to Ground Water and Configuration of the Water Table in the Portland, Oregon Area."

Elevation is referenced to the North American Vertical Datum of 1988 (NAVD88) in feet/meters above NAVD88. Depth to groundwater displayed in feet.