Effects of changing channel morphology on vegetation, groundwater, and soil moisture regimes in groundwater-dependent ecosystems

Abstract

Channel incision, channel widening, and excessive floodplain sedimentation are major causes of riparian ecosystem degradation across the country. Although the causes and consequences of these processes vary significantly, the resulting morphology in all cases results in a lower stream stage relative to the floodplain surface for any given discharge. This change in channel morphology alters surface water–groundwater interactions between the stream and the riparian aquifer and affects the soil moisture and groundwater regimes differentially across the floodplain. The distribution of vegetation is altered as the hydrologic regime shifts between the hydrologic niches of potential species. We simulate the hydroecologic response of a groundwater-dependent ecosystem on the floodplain of an archetypical watershed under three scenarios representing changes in channel morphology: a base case, a widened channel case, and an incised channel case. Stochastically-driven synthetic rainfall records are used to drive a rainfall–runoff model of the archetype watershed. The resulting hydrograph is transformed to stage via Manning's equation for the three channel morphologies considered. This stage record is used as a boundary condition for a finite-element model simulating 2-dimensional, variably-saturated groundwater flow in the riparian aquifer. The model predicts soil moisture and groundwater regimes lateral to the channel. The 7-day moving average high water level is determined for ten growing seasons and used to predict the distribution of three species across the floodplain. Plant frequency is predicted for Carex emoryi (an obligate wetland species), Carex crawei (facultative wetland species), and Carex duriuscula (a facultative upland species) for each of the channel morphologies considered. The frequency is predicted using existing nonlinear parametric species response curves determined empirically for these species using direct gradient analysis. Results show the sensitivity of the distribution of vegetation on floodplains to channel morphologic changes. Our linked hydrologic and ecological model provides a solid framework for considering potential hydroecologic impacts of channel disturbance and/or restoration on vegetation communities which are often affected by channel degradation and targeted in restoration efforts.