AN ANALYSIS OF RAINFALL, FLOODING, AND IMPACTS ON FEDERALLY-LISTED SPECIES IN A SOUTHERN APPALACHIAN WETLAND

This study focused on a channelized and ditched groundwater-fed wetland in the Southern Appalachians that holds a rare, federally-endangered species, the Bunched Arrowhead (Sagittaria fasciculata). The Bunched Arrowhead lies within and along the banks of a ditch along the wetland boundary, and when the wetland floods the plant is submerged and becomes coated with layers of sediment. The overall goal of the study was to evaluate the frequency of these flooding events and whether the number of flooding events was increasing due to upstream development and/or more frequent heavy storms. Hourly precipitation and water levels have been recorded since December 2017, and over the past six years, there were 23 flooding events that raised water levels in the wetland at least 0.4m above baseflow conditions. These floods occurred irregularly, however, as there were nine floods in 2020 alone, almost double the amount of any other year.

Most flooding events followed rainfall of at least 70mm over the previous 48 hours, though some rain events of this magnitude did not cause significant flooding. For example, there was just one recorded flood in 2018, but seven instances where rainfall exceeded 70mm without a flood. Unfortunately, there were some gaps in the onsite precipitation data; to fill in these gaps–and estimate historic rainfall-flooding frequencies–we evaluated two open sources of high-resolution precipitation data. The first, Daymet, from NASA Earthdata, Oak Ridge National Laboratory, provides daily precipitation data at a square kilometer resolution. Our analysis showed that Daymet was consistent with the on-site rain gauge. However, daily precipitation aggregated in 24-hour periods does not capture the rainfall intensity of short storms. Hourly precipitation data from another free open-source data set, Multi-Radar Multi-Sensor (MRMS), was downloaded through NOAA to assess precipitation intensity. MRMS data can be downloaded for real-time data for a square kilometer area but requires a week to download each year of data. Effectively using open-source precipitation data for modeling rainfall-runoff relationships allows for reduced travel to field sites, analysis of rainfall events before onsite data collection began, and assessment of the rainfall amount and intensity that causes flooding in this wetland.