Gilbert Barth, Ph.D., Hydrologist, S. S. Papadopulos and Associates, Inc.
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| Figure Reproduced from Butler et al, 2001. |
Introduction
The riparian zone is to most of us a visible continuum clearly demonstrating an abundance and variety of both flora and fauna made possible by a surface-water feature. The visible continuum of a classic riparian zone starts with a river and moves through a series of ecosystems, from aquatic to terrestrial, that are progressively removed from the river. As a result, the riparian zone encompasses a wide range of species, from aquatic to almost xeric. However, hidden below the surface is a critical component of the riparian continuum, often overlooked when assessing impacts to the riparian zone: the subsurface hydrology.
While a river-and-flora continuum provides vivid imagery of a river’s influence, only a small fraction of a riparian zone utilizes water within, or directly from, the river. The majority of the riparian zone relies on the transmission of water through the subsurface. Subtle changes in this hidden component of the riparian zone can affect water transmission thereby altering the entire riparian zone.
With continually increasing water demands, historic river conditions are commonly modified. Such changes require consideration of connections between the aquatic, terrestrial and subsurface components of a riparian system. Simply targeting river-flow rates or water temperatures overlooks the contiguous nature of the aquatic, terrestrial and subsurface components of a riparian zone. This article explores one way in which river changes might modify the river-subsurface interaction, thereby impacting the riparian zone.
River-Subsurface Interaction
The extent of a riparian zone varies with space and time. Spatial variability is typically linked with factors such as soil types and topography. Temporal changes are most commonly linked to subsurface water-level changes. In most cases, maintaining an historic set of flows and/or water levels will tend to preserve the subsurface water-levels and riparian-zone extent. Recent trends of increasing diversions and decreasing flows are often accompanied by efforts to modify the river channel, creating geometry capable of sustaining fish and invertebrate habitat at reduced flow levels. While channel modifications address some well-quantified objectives, such as maximum water temperature, it may be considerably more complicated to characterize and quantify the subsurface component of the riparian zone. However, it is possible to demonstrate potential river-channel-modification impacts using well-established hydrologic principles..In riparian areas, a river will always have some interaction with the subsurface below and adjacent to the channel. This will consist of water gains(losses) from(to) the subsurface. Plants within the riparian zone “pump” water from the subsurface and use the water for transpiration. A wide range of solutions have been derived to quantify the interaction of rivers with the pumping stresses in the nearby subsurface (e.g.,Theis, 1941, Glover and Balmer, 1954; Hantush, 1965; Jenkins, 1968; Hunt, 1999; Butler et al., 2001). Of course there are limitations to their application since the geometry of a well-bore drilled into a river-alluvium aquifer is different than the roots of riparian vegetation, and the solutions involve a number of other simplifying assumptions, but these types of analytical solutions are effective and efficient tools for gaining preliminary insight.
Butler et al.’s (2001) solution for stream-aquifer interaction with pumping can be used to examine potential impacts when river flows have been reduced and the channel has been modified. In general terms, the solution can be used to determine the ratio of water contributed by the river to riparian transpiration, versus the amount pulled from the pore space of the alluvial aquifer. A river-water/pore-space ratio of 1.0 indicates that all the water consumed by transpiration originates from the river, a value of 0.5 would indicate an equal split with half of the water from the river and the other half from the alluvial aquifer pore space, and a value of 0.0 indicates that no water comes from the river. It is also important to realize that the river-water/pore-space ratio varies with time, always starting at zero when pumping or transpiration starts, and then rising to some long-term limit, less than or equal to 1.0. Starting from a ratio of zero represents the very first bit of water drawn up by the riparian vegetation. This water will originate immediately adjacent to the roots. As more water is transpired, more water is pulled in and the river begins to contribute. Over time it would be reasonable to expect that the river contributes most of the water to the vegetation: it is riparian vegetation and would not be there without the river.
Consider an example where, by reducing channel width, channel modifications have managed to preserve the historic pattern of river stage despite decreased river flows. Using figure 7 from Butler et al. (2001), it is possible to evaluate the impact of the reduced channel width on the river-water/pore-space ratio for water moving to the riparian-vegetation roots. The reduced channel width results in a river-water/pore-space ratio that takes significantly longer to rise to its long-term limit. This means that, as a result of reduced channel width, riparian vegetation must rely more on the capacity of the alluvial aquifer to supply water until the reduced river channel can provide enough. This simplified example provides a clear demonstration that the riparian vegetation and the alluvial aquifer will need to compensate for the river-channel modifications and associated slower leakage of water from the river. To make any reasonable predictions of riparian impacts, it will be important to quantify the extent to which the aquifer and vegetation must compensate for the river-channel modifications.
Unfortunately, analytical solutions accounting for river-channel width do not account for the limited capacity of the alluvial aquifer. The alluvial aquifer may not have the capacity to compensate for delayed transmission of water from the river. Riparian vegetation, especially at the limits of the riparian zone, may not tolerate the resulting drop in groundwater levels. To address these kinds of issues, it is important to go beyond a simplified analytical solution and develop both a site-specific conceptual model of the riparian zone and water-budget estimates in terms of evapotranspiration, river losses/gains and subsurface losses/gains. The conceptual model and water budgets may demonstrate that the limitations of the analytical solution are acceptable, or the need to develop a numerical model capable of accounting for specific water-budget components and spatial and temporal variability.
Conclusions
When modifying historic conditions it is important to consider the connections between aquatic, terrestrial and subsurface components of a riparian system. Limiting impact-assessment and mitigation-design criteria to a few quantified targets associated with the aquatic environment may not provide a solution conducive to sustaining the riparian zone. Sensitivity of the riparian zone to relativity small changes in water levels requires consideration of the complete riparian-zone water budget: aquatic, terrestrial and subsurface.
References
Butler, J. J, V. A. Zlotnik, and M.-S. Tsou, Drawdown and stream depletion produced by pumping in the vicinity of a partially penetrating stream, Ground Water, 39:5, p. 651 – 659, 2001.
Glover, R.E., and G. G. Balmer, River depletion resulting from pumping a well near a river. Trans. Am. Geophys. Union 35, 468-470, 1954.
Hantush, M.S., Wells near streams with semi-pervious beds, J. Geophys. Res. 70, no. 12:2829-2838, 1965.
Hunt, B., Unsteady stream depletion from ground water pumping, Ground Water 37, no. 1:98-102, 1999.
Jenkins, C. T., Techniques for computing rate and volume of stream depletion by wells, Ground Water 6, no. 2:37-46, 1968.
Theis, C.V., The effect of a well on the flow of a nearby stream, Trans. Am. Geophys. Union 22. no. 3:734-738, 1941.