Evaluating Variation in Hydrologic Transport in Steep-Forested and Low Gradient-Agricultural Streams

Abstract:

Understanding hydrologic exchange between streams and the subsurface is critical to assessing transport influence on the fate of solutes. In many systems, land cover and management have impacted channel morphology, hydrology, and nutrient loading to streams. Deciphering interactions between land cover change and stream processes is important to managing impacts of land use on lotic ecosystem function. This study characterizes hydrologic exchange through a series of conservative tracer injections at various flows in two contrasting stream ecosystems: Como Creek, CO, a high-gradient, forested, snowmelt-driven environment and Clear Creek, IA, an incised, agricultural, storm-driven environment. We analyzed tracer breakthrough curves (BTCs) and compared metrics quantifying shorter hydrologic flowpaths including the transient storage index, and temporal moments measuring advective transport, spreading and tailing behaviors to those quantifying longer flowpaths measured through tracer mass loss. Clear Creek, IA is a straightened, incised channel with deep bank cuts (~ 3.5 meters), likely due to agricultural tile draining. Tile draining has enhanced lateral hydrologic connectivity, while channel incision into clay substrate has decreased vertical hydrologic exchange. Consequently, transient storage is more influenced by surface storage as opposed to hyporheic exchange, resulting in more symmetric and mesokurtic tracer BTCs. In contrast, Como Creek, CO is a steep (average slope of 9.8 %), multi-thread, step-pool channel with substrate ranging from small gravel to bedrock. Decreasing discharge along Como Creek indicates net loss through hydrologic exchange with the subsurface. Tracer BTCs at Como Creek are heavily skewed with long tails, which indicate substantial transient storage and long hyporheic flowpaths. The hydrologic transport characteristics at Como Creek increase hydraulic residence times relative to transport dynamics in the agricultural system, Clear Creek. These variations are likely to impact stream water quality and the potential for in-stream reductions of terrestrial nutrient load. Moving forward, this hydrological assessment will be coupled with nutrient spiraling metrics to determine what role hydrology plays in stream-hyporheic nutrient retention in these systems.