During an event with low antecedent storage, activation of a shallow, perched, transient water table dominated runoff production. Groundwater dynamics during precipitation events with different input characteristics and contrasting storage states showed distinct shallow and deep groundwater flow path behavior could produce similar runoff magnitudes. Within these states, distinct flow paths were activated, resulting in divergent hydrograph recessions. Our results indicated two dominant catchment storage states driven by seasonal evapotranspiration. This Piedmont region is gently sloped with highly weathered soils characterized by shallow impeding layers due to decreases in saturated hydraulic conductivity with depth. We addressed this by characterizing dominant runoff generating flow paths by monitoring the timing and magnitude of precipitation, runoff, shallow soil moisture, and shallow and deep groundwater dynamics in a 3.3 ha ephemeral-to-intermittent drainage network in the Piedmont region of North Carolina, USA. Runoff generation processes in low relief landscapes with deep soils remain less understood. Channels become dry when runoff ceases and/or seepage rates exceed streamflow (Woessner, 2020).Most field-based approaches that address runoff generation questions have been conducted in steep landscapes with shallow soils. Streamflow seeps into the channel (influent). c) Ephemeral streams contain no streamflow until precipitation causes runoff. During a portion of the year groundwater inflows decrease and stream leakage (orange channel arrow) increases (right diagram of b) such that all or portions of the channel will become dry (red dot and dashed blue line). Sections of the stream may be gaining or losing during periods of full channel flow (left diagram). b) Intermittent streamflow is driven by precipitation and groundwater discharge. a) Perennial effluent stream where groundwater discharges to the stream and streamflow is maintained year-round. Small arrows indicate general exchange directions of groundwater (black arrows) and surface water (orange arrows). Figure 22 – Map view of stream channels (long blue solid and dashed arrows surface flow is from the upper left to the lower right) illustrating groundwater exchange in three settings. These streams remain dry when no runoff occurs. Most of the time influent conditions occur during channel flow. Ephemeral streams only flow in response to runoff as the water table generally remains below the channel bottom (Figure 22c). If channel leakage is high, all or portions of the channel become dry for a period of time. However, there are periods when the water table drops below the effluent portions of the channel and streamflow recharges the underlying groundwater. Intermittent streams flow only when sufficient groundwater discharge (effluent) and/or precipitation support flows. Streams referred to as perennial have flows year-round and are most often supported by base flow (effluent conditions) when runoff is insufficient to maintain discharge (Figure 22a). In general, streamflow conditions can be described in terms of the presence/duration of flow, as continuous and discontinuous, and in most cases, are directly related to the nature of the groundwater exchange process. 2.5 Perennial, Intermittent and Ephemeral Streams
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