Estimating Average Annual Runoff from Montana Streams

TitleEstimating Average Annual Runoff from Montana Streams
Publication TypeConference Proceedings
Year of Conference2007
AuthorsFarnes, P. E.
Conference Name75th Annual Western Snow Conference
Series TitleProceedings of the 75th Annual Western Snow Conference
Date PublishedApril 2007
PublisherWestern Snow Conference
Conference LocationKailua-Kona, HI
KeywordsRunoff prediction, Montana, SNOTEL, precipitation, cover type, fire history

Procedures for estimating runoff from un-gaged streams will be presented. First and most important is to have an accurate average annual precipitation map for the current base period. The precipitation map needs to be based on a maximum elevation grid of 500 meters and preferably a 100 to 200 meter grid. It needs to be based on a procedure that looks at the nearest two or three neighbors and one that assigns measured values to measured points. The Montana SILC3 Land Cover Classification provides the cover types for most areas. Some of the 36 different cover types that have similar hydrologic responses can be combined for hydrologic analysis. Data from SNOTEL sites and climatological stations in the area was used to determine the relationship between average annual precipitation, average April 1 snow water equivalent, and average April through July precipitation for the current base period. Average annual precipitation and runoff from gaged streams in the area and percent forested area for each gaged watershed are used to determine a runoff vs. precipitation curve for drainages if all of the forest was removed. By applying factors from the cover type vs. forest species age curves, the runoff was determined for the area of each cover type in each precipitation zone. These were accumulated to determine runoff for each watershed or HUC of interest. Using fire history or logging by decades, usually available from land managing agencies, it was possible to determine changes in runoff by decade from any given watershed.