University of Wisconsin–Madison

Physically-Driven Flood Frequency Analysis

UW-Madison Project Leads: Guo Yu, Daniel Wright

Outside Collaborators: Kathleen Holman (US Bureau of Reclamation), Ricardo Mantilla (University of Iowa/Iowa Flood Center)

Floods are the product of complex interactions among processes including precipitation, soil moisture, and watershed morphology. Conventional flood frequency analysis (FFA) methods such as design storms and discharge-based statistical methods offer few insights into these process interactions and how they “shape” the probability distributions of floods. We present an alternative “process-based” FFA approach that couples remote sensing precipitation, stochastic storm transposition (SST; e.g., RainyDay), and hydrologic model. The methodology is applied to the 4002 km2 Turkey River watershed (Fig. 1) in the Midwestern United States, which is undergoing significant climatic and hydrologic change.

Figure 1. Study region. (a) Contiguous United States with the state of Iowa highlighted in grey. (b) Zoomed-in map showing Iowa (black outline) and the Turkey River watershed (red) and the extent of the stochastic storm transposition region (blue dash line). (c, d) Turkey River watershed showing land surface elevation (based on the USGS National Elevation Dataset) and land use (based on the USGS 2012 NLCD), respectively.

The hydroclimate of Turkey River is changing. Since 1948, annual precipitation and discharge show increases and their variability has also increased, while annual maximum daily discharge has decreased. It is important to note, however, that there are two counteracting seasonal trends (Fig. 2): annual daily discharge maxima have decreased significantly in March-April, but have increased somewhat in May-September. Thus, the lack of statistically significant change in annual maximum daily discharge in Turkey River masks changes in the seasonality of flooding.

Figure 2. Linear trends for two groups of annual maxima daily discharge: March-April floods (blue) and May-September floods (red).  The October-February maxima daily discharge are in black dots and its trend line is not calculated because only nine annual maxima occur during this period.

Process-based flood frequency distributions for Turkey River using Stage IV precipitation are compared with the distribution based on statistical analyses of discharge observations using 1933-2016 USGS annual maxima daily streamflows (Fig. 3). When streamflow observations are divided into two groups (1933-1989 and 1990-2016), it becomes clear that the recent peak flood observations align well with the Stage IV-based SST results. This suggests that, despite the relatively short (15-year) rainfall record used, Stage IV- driven process-based FFA adequately reflects flood frequency in the wetter recent climate Bulletin 17B methods based on much longer data records fail to do so.

Figure 3. Peak discharge analyses for Turkey River. Process-based FFA (red) using Stage IV (2002-2016) rainfall are compared with USGS frequency analyses (grey) using Bulletin 17B methods. 

You can find more about this study in our HESS paper (https://www.hydrol-earth-syst-sci.net/23/2225/2019/).