Dynamic streambed fluxes during rainfall‐runoff events

Abstract

Observed 3-D temperature distributions within a streambed were used to analyze the effects on exchange fluxes between groundwater and the stream during rainfall-runoff events. By combining a dense vertical and lateral monitoring network of streambed temperatures with coupled surface/subsurface 3-D flow and heat transport modeling, we demonstrate how temperature can be used directly as a calibration target. Three model setups with different hydraulic conductivity distributions were evaluated in an optimization approach using temperature and hydraulic head data. The hydraulic conductivity distributions were based on slug test surveys within the streambed and aquifer. A detailed characterization of the hydraulic conductivity of the streambed and aquifer is needed to accurately simulate observed temperatures. Hence, the most sensitive parameters, the vertical hydraulic conductivity and the thermal conductivity, were calibrated within different conductivity zones of the heterogeneous model. Simulated exchange fluxes across the streambed showed variations up to a factor of four within just a meter. Such differences may not have been correctly predicted using 1-D heat transport models due to lateral conduction amongst the different flow paths. During the rainfall-runoff event, fluxes decreased substantially (250%) due to a decrease in the hydraulic gradient with increased stream stage. Although no flow reversals were observed during the studied conditions, it is possible that these can occur during larger rainfall-runoff events. We show that with the current sampling and modeling techniques, 3-D temperature data can be used to estimate dynamic exchange in heterogeneous flow fields encountered in the field.

Detecting groundwater discharge dynamics from point-to-catchment scale in a lowland stream: combining hydraulic and tracer methods

“…Weaker groundwater gradients could potentially allow for a temporarily weakening of the groundwater discharge to the stream during large rain events, entailing a temporary dominance of surface and event water. Similar mechanisms were observed by Karan et al (2014) where large rain events temporarily decreased groundwater discharge to Holtum stream. Also Gerecht et al (2011) observed highly dynamic responses to rapid stage changes in terms of shifting between gaining and losing conditions in a groundwater influenced river.…”

Advances in understanding river‐groundwater interactions

“…D. H. Käser et al [] provided a detailed description of many aspects of streambed structure, including hydraulic conductivity, gradient, topography, and exchange fluxes. Other noteworthy examples are Karan et al [], who measured hydraulic conductivity, temperature profiles, and hydraulic gradients at a large number of locations along the Holtum stream in Denmark. Rosenberry and Pitlick [] measured vertical and horizontal hydraulic conductivities, seepage rates, hydraulic gradients, and shear stress and related their results to the bedforms present.…”

Combined use of thermal methods and seepage meters to efficiently locate, quantify, and monitor focused groundwater discharge to a sand‐bed stream

“…Temporally evolving physical features, such as meander bends, bar forms, pool‐riffle sequences [ Käser , ; Tonina and Buffington , ], and highly transient ripples and moving bedforms [ Käser et al ., ] grow and shrink at multiple temporal and spatial scales. This continually evolving porous medium imparts nested hyporheic‐scale exchange superimposed on longer flowpaths that confounds determination of larger‐scale distribution of groundwater discharge to a stream or river [ González‐Pinzón et al ., ; Hatch et al ., ; Karan et al ., ; Menció et al ., ; Rosenberry and Pitlick , ]. In addition to diffuse groundwater discharge controlled by reach‐scale hydraulic gradients and geologic properties, groundwater discharge also is focused in discrete areas where flow is orders of magnitude faster than diffuse flow [ Hare et al ., ; Kidmose et al ., ].…”