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Two approaches for estimating discharge on ungauged basins in Oregon, USA
Citation:
WIGINGTON JR, P. J., S. G. LEIBOWITZ, R. COMELEO, J. L. EBERSOLE, AND E. S. Copeland. Two approaches for estimating discharge on ungauged basins in Oregon, USA. Presented at 2009 American Geophysical Union Fall Meeting, San Francisco, CA, December 14 - 18, 2009.
Impact/Purpose:
Detailed information on the hydrologic behavior of streams is available for only a small proportion of all streams
Description:
Detailed information on the hydrologic behavior of streams is available for only a small proportion of all streams. Even in cases where discharge has been monitored, these measurements may not be available for a sufficiently long period to characterize the full behavior of a stream. In this presentation, we discuss two separate approaches for predicting discharge at ungauged locations. The first approach models discharge in the Calapooia Watershed, Oregon based on long-term US Geological Survey gauge stations located in two adjacent watersheds. Since late 2008, we have measured discharge and water level over a range of flow conditions at more than a dozen sites within the Calapooia. Initial results indicate that many of these sites, including the mainstem Calapooia and some of its tributaries, can be predicted by these outside gauge stations and simple landscape factors. This is not a true “ungauged” approach, since measurements are required to characterize the range of flow. However, the approach demonstrates how such measurements and more complete data from similar areas can be used to estimate a detailed record for a longer period. The second approach estimates 30 year average monthly discharge at ungauged locations based on a Hydrologic Landscape Region (HLR) model. We mapped HLR class over the entire state of Oregon using an assessment unit with an average size of 44 km2. We then calculated average statewide moisture surplus values for each HLR class, modified to account for snowpack accumulation and snowmelt. We calculated potential discharge by summing these values for each HLR within a watershed. The resulting monthly hydrograph is then transformed to estimate monthly discharge, based on aquifer and soil permeability and terrain. We hypothesize that these monthly values should provide good estimates of discharge in areas where imports from or exports to the deep groundwater system are not significant. We test the approach by comparing results with discharge measurements from 30 long-term US Geological Survey gauge stations. The advantage of this HLR approach is that it can be applied to watersheds or basins in Oregon without additional information. However, results represent monthly averages. In contrast, the approach we demonstrate in the Calapooia provides detailed, daily hydrographs, but requires field measurements over a range of hydrologic conditions for calibration.