FLASHINESS OF HEADWATER STREAMS WITHIN THE CONTEXT OF SEASONAL CHANGES IN EVAPOTRANSPIRATION AND DIFFERING LAND USE

Streams in the Piedmont of North Carolina are often characterized by flashy discharge while land use is rapidly changing near population centers. Here, we examine discharge in 16 headwater streams and compare their drainage basins to determine the impacts of impervious surface cover and ET on stream flashiness seasonally. We hypothesized that forested streams would exhibit lower flashiness than suburban streams overall, but would exhibit a greater increase in winter flashiness, due to a greater seasonal decrease in ET. Discharge was collected at 15 minute intervals at each site using HOBO depth sensors and detailed ratings curves. To examine land use and seasonal ET variability, the Enhanced Vegetative Index (EVI) and Normalized Difference Vegetative Index (NDVI) were calculated in ArcGIS, using three Pleiades 50 cm 4-band satellite images, representing peak seasonality for two summers (2016, 2017) and one winter (2015-2016) in the study area. Percentage of impervious surface area in drainage basins was calculated in ArcGIS for each drainage basin. We categorized 3 drainage basins as forested (< 5% impervious surface) and the rest as suburban (13 to 39% impervious surface). There was a significant negative correlation between percent impervious surface and ET. Forested streams exhibited higher flashiness in the winter than they did in the summer, and discharged more water in the winter given similar precipitation. Additionally, we found that winter ET in forested drainage basins was similar to summer ET in suburban basins. We attribute these changes to variability in ET. Overall, these results suggest that even relatively natural streams in the Piedmont display seasonal flashiness. This finding suggests that streams in the area, especially headwater streams, may be predisposed to flashiness perhaps as a result of clay rich surface soils.