Evaluating Flow Diversion Impacts to Groundwater‐Dependent Riparian Vegetation with Flow Alteration and Groundwater Model Analysis

Abstract

An approach for assessing the potential ecologic response of groundwater‐dependent riparian vegetation to flow alteration is developed, focusing on change to groundwater. Groundwater requirements for riparian vegetation are reviewed in conjunction with flow alteration statistics. Where flow alteration coincides with groundwater‐related vegetation sensitivities, scenarios are developed for groundwater simulation. Groundwater depths and recession rates in the riparian zone are simulated for baseline and altered stream hydrographs, with changes to river stage and width represented with a transient, flow‐dependent boundary condition. Potential flow diversion from the Upper Gila River in New Mexico is examined. Statistical flow alteration analysis, applying prospective diversions to a 76‐year record of daily flow, shows that flows in the winter‐spring months and within the high‐pulse to small flood range are subject to greatest potential change. Groundwater simulation scenarios are developed for these flow conditions in representative dry, near‐average, and wet years. Differences in groundwater elevations, generally less than 0.25 m during the flow alteration period, dissipate rapidly following cessation of diversion. Relating groundwater depth, recession rates and range of fluctuations to riparian vegetation needs, we find adverse ecological response is not expected from groundwater impacts for the flow alteration examined.

Delineating and mapping riparian areas for ecosystem service assessment

“…In this work, we show that different delineation approaches greatly influence the total predicted riparian area within a subcatchment, their spatial land patterning, and the subsequent distribution of habitats present within these areas. In reality, however, riparian boundaries are rarely discrete, and no single approach can be expected to adequately capture all the features of riparian areas, particularly as our mechanistic and quantitative understanding of some riparian functions is still lacking (e.g., hyporheic filtering of nutrients, groundwater flow and recharge rate, and riparian biodiversity; Hanula, Ulyshen, & Horn, ; Hathaway, Barth, & Kirsch, ; Doble & Crosbie, ; Swanson, Kozlowski, Hall, Heggem, & Lin, ). Further, riparian zones are typically both spatially heterogeneous (vertically and horizontally) and temporally dynamic with strong interactions between the aquatic and terrestrial component (Broder, Knorr, & Biester, ).…”

Discussion

“…In similar situations, a good solution is to use a “telescope” approach to inset a site‐specific model into a comprehensive, well‐conditioned regional model (Mehl and Hill ), an approach which was recommended in a previous study of the same area (SSPA ) but not adopted in the “riparian model” of Hathaway et al. ().…”

“…The above‐cited article (Hathaway et al. ) centers on evaluating flow diversion impacts to riparian vegetation in the Cliff‐Gila Valley, New Mexico, by developing a “riparian model” approach. The authors concluded that proposed stream diversions would not negatively affect riparian habitat.…”

Reply to Discussion

“…In our article, “Evaluating Flow Diversion Impacts to Groundwater‐Dependent Riparian Vegetation with Flow Alteration and Groundwater Model Analysis” (Hathaway et al. ), we describe a study approach for relating the potential response of riparian vegetation to changes in groundwater conditions associated with flow alteration. The approach involves several elements: (1) review of scientific literature and field studies to identify groundwater conditions that may signal adverse groundwater‐induced impacts to vegetation; (2) statistical flow alteration analysis to identify more highly altered flow characteristics (i.e., magnitude, frequency, duration, timing, and fall/rise rates); (3) framing of scenarios where more highly altered flows coincide with groundwater‐related vegetation sensitivities; and (4) simulation of changes to groundwater under identified scenarios with a groundwater flow model.…”