Energy considerations in groundwater‐ridging mechanism of streamflow generation

Abstract

Abstract:Groundwater ridging is the rapid rise of a shallow water table during a rainfall event, in an environment where, in the pre-event period, the capillary fringe extends to the ground surface. Groundwater ridging is widely cited to account for the observed significant appearance of pre-event water in a stream stormflow hydrograph. Various hypotheses have been advanced to explain the groundwater-ridging mechanism; and most recently, from a field study site in South Africa, an energy hypothesis was proposed, which explains that groundwater-ridging water-table rise is a result of rapid introduction and transmission of additional pressure head into the capillary fringe from an intense rainfall at the ground surface. However, there is a need for further analysis and evidence from other field study sites to confirm and support this newly proposed energy hypothesis. The objectives of this paper are, therefore, as follows: to review previous observations on groundwater ridging, from other study sites, in order to deduce evidence of the newly proposed energy hypothesis; to present and evaluate a one-dimensional diffusion mathematical model that can simulate groundwater-ridging water-table rise, based on the newly proposed energy hypothesis; and to evaluate the importance of a capillary fringe in streamflow generation. Analysis of previous observations from other study sites generally indicated that the rate of groundwater-ridging water-table rise is directly related to the rainfall intensity, hence confirming and agreeing with the newly proposed energy hypothesis. Additionally, theoretical results by the mathematical model agreed fairly well with the field results observed under natural rainfall, confirming that the rapidly rainfall-induced energy is diffusively transmitted downwards through pore water, elevating the pressure head at every depth. The results in this study also support the concept of a three-end-member stream stormflow hydrograph and contribute to the explanation of how catchments can store water for long periods but then release it rapidly during storm events.

Hydroclimatic controls on runoff activation in an artificially drained, near‐level vertisolic clay landscape in a Prairie climate

“…Rapid GWT activation was also observed in summer but appeared to be related to a combination of precipitation intensity/soil infiltration capacity and AMC (either as indicated by AP or antecedent GWT position) as the water table is most frequently below the tile drains during the dry Prairie summers. Early GWT activation in this landscape could have been attributed to GWT ridging (reverse Wieringermeer effect) or the Lisse effect in the summer, where the water table rises sharply after medium‐intensity to high‐intensity rainfall events due to the rapid addition and transfer of extra pressure head into the capillary fringe (Khaled, Tsuyoshi, Kohei, Taku, & Hiromi, ; Waswa & Lorentz, ). This is often followed by a sharp decline in water table position.…”

Dynamics of groundwater flow and upwelling pressure heads at a wetland zone in a headwater catchment

“…It follows that the conversion of the capillary fringe into phreatic water requires only an addition of pressure head (energy), a process that will automatically elevate the water table, not necessarily to the ground surface but, to a position that will depend on the amount of pressure energy added. Certainly, results of field observations in the Weatherley Research Catchment in South Africa [10] and other study sites [11] found a direct relationship between the intensity of rainfall and the water table rise in groundwater ridging.…”

“…It should be noted that in groundwater ridging the source of pressure head is at the ground surface and the pressure head diffuses downward through the pore water [10,11]. Inversely, at U2, it appears that the source of pressure head is from below and the pressure head diffuses upwards through pore water.…”

Modelling of shallow water table dynamics using conceptual and physically based integrated surface water-groundwater hydrologic models

“…In shallow water table regions, as the water table rises the specific yield decreases nonlinearly and its value approaches zero when capillary fringe is extended up to the soil surface. This means that, in such conditions a small amount of rain may result in a significant rise in water table (up to the soil surface if the water table is close enough to the surface such that the capillary fringe reaches the surface) as investigated by Gillham, 1984, 1989;Buttle and Sami, 1992;Sklash and Farvolden, 1979;Waswa and Lorentz, 2015). The schematic illustration of the method is given in Fig.…”