Annual Stream Runoff and Climate in Minnesota’s River Basins

Abstract

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 Minnesota.

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.