The Varying Effects of Climate And Landscape Changes on Increased River Flows In Minnesota Watersheds
2023-05
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The Varying Effects of Climate And Landscape Changes on Increased River Flows In Minnesota Watersheds
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2023-05
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Minnesota has experienced increases in precipitation and river flows, particularly in its southern, heavily agricultural region, since 1940. However, these areas have also experienced significant increases in the extent of corn/soybean cropping and artificial drainage. Currently, the relative contributions of these factors to increased flows are not well understood. There is also some indication that intense rain events have increased across this region and may coincide with increased flood events. However, most climate studies have not considered the roles of watershed antecedent moisture conditions and runoff storage in their proxies of potential flood impact, nor explored any direct observational evidence of intense rain event links with flooding. Therefore, to better understand the relationships between these complicating factors, this study pursued the following three objectives: (1) Apportion the relative contributions of increases in climate, corn/soybean agriculture, and artificial drainage to increased river flows in 21 agricultural watersheds in southern Minnesota, (2) Analyze trends in intense rainfall, antecedent moisture conditions and predicted runoff at 132 climate stations across Minnesota to estimate potential flooding hotspots and, (3) Apportion the relative contributions of intense rain events of different magnitudes, snowmelt, and antecedent moisture conditions to flooding in the Cottonwood River watershed -- a heavily agricultural watershed that has seen exceptional increases in intense rain events and flooding since 1950.To apportion the relative contributions of increases in climate, corn/soybean agriculture, and artificial drainage to increased river flows, trends in precipitation, crop conversions, and extent of drained depressional area in 21 Minnesota watersheds were compared from 1940-2009. Watersheds with large land-use changes had increases in seasonal and annual water yields of >50% since 1940. On average, changes in precipitation and crop evapotranspiration explained less than one-half of the increase, with the remainder highly correlated with artificial drainage and loss of depressional areas.
Trends for intense rain events, antecedent moisture conditions and watershed storage were analyzed in 132 precipitation stations within 65 major watersheds in Minnesota. Resulting runoff was predicted using a simplified rainfall-runoff model. Results show widespread increases in intense rain events across southern Minnesota since 1950, but markedly fewer trends in antecedent moisture conditions and predicted runoff. However, exceptional intense rain events, antecedent moisture conditions, and predicted runoff increases are concentrated in and around specific hotspot watersheds in Minnesota. The study shows the importance of considering watershed hydrology when diagnosing potential impacts of intense rain event increases.
Relative contributions of intense rain and antecedent moisture conditions to flooding in the Cottonwood watershed were determined using an empirical, data-driven approach which apportioned the roles of snowmelt, intense rain events and antecedent moisture conditions in each every flow (and flood) event 1950-2021. Results suggest that recent major flooding in the Cottonwood has been driven by intense rain events (≥ 75/mm/day), and to a lesser degree by event antecedent baseflow, which reduces channel storage and increases flood stage. Less severe flooding is driven roughly equally by (less) intense rain events (≥ 25mm/day & <75/mm/day) and both antecedent base- and quickflows.
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University of Minnesota Ph.D. dissertation. May 2023. Major: Water Resources Science. Advisor: Bruce Wilson. 1 computer file (PDF); vii, 85 pages.
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Ulrich, Jason. (2023). The Varying Effects of Climate And Landscape Changes on Increased River Flows In Minnesota Watersheds. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/257044.
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