Abstract
AbstractTransmission losses from the beds of ephemeral streams are thought to be a widespread mechanism of groundwater recharge in arid and semi‐arid regions and support a range of dryland hydro‐ecology. Dryland areas cover ~40% of the Earth's land surface and groundwater resources are often the main source of freshwater. It is commonly assumed that where an unsaturated zone exists beneath a stream, the interaction between surface water and groundwater is unidirectional and that groundwater does not exert a significant feedback on transmission losses. To test this assumption, we conducted a series of numerical model experiments using idealised two‐dimensional channel‐transects to assess the sensitivity and degree of interaction between surface and groundwater for typical dryland ephemeral stream geometries, hydraulic properties and flow regimes. We broaden the use of the term ‘stream–aquifer interactions’ to refer not just to fluxes and water exchange but also to include the ways in which the stream and aquifer have a hydraulic effect on one another. Our results indicate that deep water tables, less frequent streamflow events and/or highly permeable sediments tend to result in limited bi‐directional hydraulic interaction between the stream and the underlying groundwater which, in turn, results in high amounts of infiltration. With shallower initial depth to the water table, higher streamflow frequency and/or lower bed permeability, greater ‘negative’ hydraulic feedback from the groundwater occurs which in turn results in lower amounts of infiltration. Streambed losses eventually reach a constant rate as initial water table depths increase, but only at depths of 10s of metres in some of the cases studied. Our results highlight that bi‐directional stream–aquifer hydraulic interactions in ephemeral streams may be more widespread than is commonly assumed. We conclude that groundwater and surface water should be considered as connected systems for water resource management unless there is clear evidence to the contrary.
Highlights
Key to developing improved understanding of such dryland processes is a better appreciation of the degree and extent of interactions between surface water and groundwater (SW–GW) within ephemeral stream systems
Ephemeral streams are under-represented in existing hydrological research into SW–GW interactions, with much greater emphasis being placed on interactions under perennial streamflow conditions (Jarihani, Larsen, Callow, McVicar, & Johansen, 2015)
We set out to understand the process controls on transmission losses from idealised ephemeral stream beds in dryland environments
Summary
Loss of water through the streambeds of ephemeral streams is thought to be a key pathway of aquifer recharge in arid and semi-arid dryland regions (Costa, Bronstert, & de Araújo, 2012; Cuthbert et al, 2019; Keppel & Renard, 1962; Lerner, Issar, & Simmers, 1990; McCallum, Andersen, Giambastiani, Kelly, & Ian Acworth, 2013; Qin et al, 2012; Renard & Keppel, 1966; Wang, Pozdniakov, & Vasilevskiy, 2017; Wang, Yu, Pozdniakov, Grinevsky, & Liu, 2014; Wheater, Sorooshian, & Sharma, 2008). In ephemeral streams we anticipate that the development of the IWT should be controlled by factors such as the degree of saturation, initial water table depth, the magnitude, timing and sequencing of streamflow events and hydraulic properties, including anisotropy, of the streambed sediments. We propose that the following factors will be most important in controlling the degree of bi-directional hydraulic interactions: water table depth, stream stage, hydrograph shape, time between events, channel shape, channel boundary permeability and water retention characteristics of the subsurface materials All these factors may vary individually or in combination in real systems. Variations from the base case were simulated in order to assess the sensitivity of the stream–aquifer interactions to the aspects hypothesised to be important (see conceptual model description, Section 2) as follows: streamflow duration, dry period length between flow events and hydraulic properties.
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