Hydrologic indicator statistics used to examine changes in streamflows associated with changing land use practices in Minnesota, 1945-2015

Hydrologic indicator statistics were computed for 82 selected surface water sites located throughout Minnesota using daily streamflow data from the U.S. Geological Survey (USGS) National Water Information System (NWIS). The 187 hydrologic indicator statistics were computed in RStudio version 3.5.0 using the EflowStats version 5.0.0 (Mills and Blodgett, 2017) and NWCCompare version 5.0 (Blodgett, 2017). The computed hydrologic indicator statistics encompass the five components of hydrologic conditions: magnitude, frequency, duration, timing, and rate of change. Magnitude is the amount of water moving past a fixed location in a given unit of time. Frequency refers to how often streamflows above a given magnitude recur over a specified time interval. Duration is the period of time associated with a specific streamflow condition. Timing refers to the regularity with which streamflows of a given magnitude occur, and rate of change refers to how quickly the magnitude of streamflow changes (Poff and others, 1997). Site selection was based on sites previously selected in three other studies evaluating long-term streamflow records for trends (Novatny and Stefan, 2007; Peterson, Nieber, and Kanivetsky, 2011; Ziegeweid et.al, 2015). Nontrend sites were shown to not have trends in streamflow that were not related to precipitation. Hydrologic indicator statistics were computed for two periods: 1) the pre-period from 10-1-1944 through 9-30-1979 and 2) the post-period from 10-1-1980 through 9-30-2015. Exact dates of the start of trends varied among sites, but 1980 was the selected cutoff period based on an approximation of the largest cluster and on other anecdotal evidence of changes in farming practices. Both categories also had at least 10 water years with complete streamflow data. Blodgett, D., 2017, NWCCompare: Returns NWC comparison stats for two daily data sets version 5.0, https://github.com/USGS-R/NWCCompare. Mills, J., and Blodgett, D., 2017, EflowStats: Hydrologic Indicator and Alterations Stats version 5.0.0, https://github.com/USGS-R/EflowStats. Novotny, E.V., and Stefan, H.G., 2007, Stream flow in Minnesota: Indicator of climate change, Journal of Hydrology 334: 319-333. Peterson, H.M., Nieber, J.L., and Kanivetsky, R., 2011, Hydrologic regionalization to assess anthropogenic changes, Journal of Hydrology 408: 212-225. Ziegeweid, J.R., Lorenz, D.L., Sanocki, C.A., and Czuba, C.R., 2015, Methods for estimating flow-duration curve and low-flow frequency statistics for ungaged locations on small streams in Minnesota: U.S. Geological Survey Scientific Investigations Report 2015–5170, 23 p., http://dx.doi.org/10.3133/sir20155170.