Stream flows recorded by the USGS from 1946 to 2005 at 42 gauging stations in the five
major river basins of Minnesota and tributaries from neighboring states were analyzed and
related to associated climate data. Goals of the study were (1) to determine the strength of the
relationships between annual and seasonal runoff and climatic variables in these river basins, (2)
to make comparisons between the river basins of Minnesota, and (3) to determine trends in
stream flows over time. Climatic variables were air temperature, precipitation, the Palmer
Drought Severity Index (PDSI), and the Palmer Hydrological Drought Index (PHDI); the latter
are common indices of soil moisture. Water year averages showed stronger correlations than
calendar year averages. Precipitation was a good predictor of stream flow, but the PDSI was the
best predictor and slightly better than PHDI when linear regressions at the annual timescale were
used. With an exponential regression PDSI gave a significantly better fit to runoff data than
PHDI. Five-year running averages made precipitation almost as good a predictor of stream flow
(runoff) as PDSI.
A seasonal time scale analysis revealed a logical stronger dependence of stream flow on
precipitation during summer and fall than during the winter and spring, but all relationships for
seasonal averages were weaker than for annual (water year) averages. Dependence of stream
runoff on PDSI did not vary significantly by season.
On a monthly timescale the strength of correlation between precipitation and runoff
dropped off significantly, while PDSI was still a decent predictor in all months but the spring.
Annual stream flow in the Upper Mississippi River Basin, including the Minnesota River
Basin, had the strongest dependence on precipitation and PDSI. The Red River of the North
Basin showed lower than average dependence on precipitation and average dependence on PDSI.
The Rainy River Basin and the Lake Superior Basin showed the weakest dependence of annual
stream flow on precipitation and PDSI.
The relationship between stream flow and precipitation can be expressed most easily by
an annual average runoff coefficient, i.e. the ratio of runoff to precipitation in a year. Runoff
coefficients vary significantly across the state of Minnesota, from more than 0.4 in the northeast
to less than 0.1 in the northwest. Trends in runoff coefficients were estimated from averages for
20-year periods from 1926-1945 to 1986-2005, although data for 1926-1945 were sparse.
According to our analysis, runoff coefficients in some of the major river basins of Minnesota
have increased significantly during the last 40 years.
The Lake Superior and Rainy River Basins have high and invariant characteristic runoff
coefficients around 0.35. The Red River Basin has the lowest characteristic runoff coefficient at
~0.14 but its value has consistently increased from the beginning of the record. The Mississippi
Headwaters Basin characteristic runoff coefficient has increased to ~0.24. The Minnesota River
Basin runoff coefficient (from the Minnesota River at Jordan, MN station) has also increased
significantly and consistently to 0.19. The largest increases in runoff coefficients were found in
the Red River and the Minnesota River Basins, the two basins with the lowest runoff
coefficients; runoff coefficients in some tributary or sub-watersheds have doubled. In the Lake
Superior and Rainy River Basins, and in the St. Croix River watershed, little change in runoff
coefficients was found.
Overall runoff coefficients drop significantly from east to west in Minnesota. This
distribution does not seem to have changed over time. Increases in runoff coefficients over time
have been highest in the west, and lowest in the east of Minnesota. One can hypothesize that
changes in stream flow in Minnesota’s west are mainly due to land use changes that have lead to
faster and easier surface runoff from the land since the beginning of European settlement. An
explanation based on climatological factors can, however, also be offered. Precipitation has
increased in all of the river basins of Minnesota over the time period of 1926 to 2005, but the
largest changes have occurred in the south and west and little change in the northeast of
Changes in total annual runoff (in/yr) between 1946 - 1965 and 1986 – 2005 increased at
38 of 42 stream gaging stations analyzed. Only 4 gaging stations, 3 in the Lake Superior and
Rainy River Basins showed decreases, with all being less than 3%. The largest increases in
average annual runoff were at 19 gaging stations in the Red River and Minnesota River Basins;
at 17 of these, increases were from 60% to 132%, and at the remaining two stations the increases
were 19% and 20%. The southern Minnesota watersheds with the largest increases in runoff also
had the largest increases in precipitation.
Overall, stream flow, expresses as annual runoff (in/yr), has increased since the
beginning of stream gaging in Minnesota and the Upper Midwest, although periods of
substantially lowered stream flows have occurred, e.g. in the drought period of the 1930s. Not
only has the runoff (cm/yr) increased, but runoff coefficients, i.e. the ratio of runoff to
precipitation, have also increased. When viewed as a percent change of annual runoff, the largest
stream flow changes have occurred in the western part and the lowest in the eastern part of
Minnesota. Increases in absolute values of annual runoff, percent of runoff, and runoff
coefficients have been quantified in this study.
Vandegrift, Todd R.; Stefan, Heinz G..
Annual Stream Runoff and Climate in Minnesota’s River Basins.
St. Anthony Falls Laboratory.
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