The effect of macropores on bi-directional hydrologic exchange between a stream channel and riparian groundwater

Abstract

Macropores and soil pipes in stream banks are common geomorphic features. Macropores and soil pipes that are open to the channel (i.e. "bank face-connected" macropores) are inundated when channel stage is elevated (e.g., from precipitation, snowmelt, dam release). However, previous studies have not investigated macropore impact on bi-directional water exchange between the channel and bank/riparian groundwater under variable hydrologic conditions. We monitored two transects of riparian groundwater wells perpendicular to the bank of a 2 nd order stream for a year: one with bank face-connected macropores (M transect) and one without bank face-connected macropores (NM transect). Fluctuations in water level and temperature during storms in those wells closest to the channel were on average 139% and 29% higher, respectively, in the presence of macropores. Rising head tests in the same wells indicated that hydraulic conductivity was 61 to 140 times higher in the presence of macropores. Bank storage, indicated by gradient reversals between channel and riparian zone, occurred on two temporal scales. Bank storage during storms was more frequent in the M transect (occurred all year) than in the NM transect (occurred just in winter and spring). Smaller magnitude gradient reversals at the M transect are consistent with faster head equilibration and greater exchange volume. Bank storage also occurred on an annual basis, with channel water entering storage during summer and fall and returning to the channel during winter and spring. Taken together, these results suggest that macropores act as preferential flow paths that enhance the connectivity between channels and riparian groundwater that enhance bank storage. Where bank macropores are present, conceptual models of hyporheic and groundwater flow should account for their effects.

Vertical surface water–groundwater exchange processes within a headwater floodplain induced by experimental floods

“…Where soils are heterogeneous, a third type of vertical connectivity (preferential flow) occurs where water moves faster than bulk Darcy flow. Preferential flow can be either Darcy flow through regions of higher K or possibly non‐Darcy flow at higher Reynolds numbers in void spaces or macropores such as those from animal burrows or root channels (Aubertin, ; Beasley, ; Beven and Germann, ; Menichino et al ., ; Menichino and Hester, ). Preferential flow is likely occurring from the surface downward toward XS1‐Centre‐30 cm because the pressure signal at depth mirrors that at the surface (e.g.…”

“…During overbank floods, surface water can enter the floodplain vertically across the floodplain surface or laterally across the channel banks, with the later process referred to as bank storage or 'lung model' hyporheic exchange (Pinder and Sauer, 1971;Sawyer et al, 2009). All such exchange flow between channel and floodplain groundwater can occur by Darcy flow and potentially non-Darcy flow (Menichino et al, 2014;Menichino and Hester, 2015). Floodplain groundwater levels can also increase in response to elevated stream stage with a response time too quick to be explained by Darcy or non-Darcy flow (Käser et al, 2009;Vidon, 2012).…”

Filling the Void: The Effect of Stream Bank Soil Pipes on Transient Hyporheic Exchange During a Peak Flow Event

“…Comparison of field data from Slate Branch (Menichino & Hester, ) versus our model simulations using their boundary conditions. Panels (a) and (b) are field data from transects with and without soil pipes, respectively, and panels (c) and (d) are model results for domains with and without soil pipes, respectively.…”

Physical and Chemical Connectivity of Streams and Riparian Wetlands to Downstream Waters: A Synthesis

“…Downstream transport of peak flows following storms can be delayed via bank storage or the transfer of stream water to an alluvial aquifer for temporary storage. Bank storage is gradually released after flooding subsides, augmenting baseflow (Whiting and Pomeranets 1997;Menichino and Hester 2015). Kondolf et al (1987) investigated the role of bank storage on streamflow in the Carmel River in central California.…”