Riparian hydroecology: A coupled model of the observed interactions between groundwater flow and meadow vegetation patterning

Abstract

[1] Stream incision is altering the hydroecology of riparian areas worldwide. In the Last Chance watershed in the northern Sierra Nevada, California, logging, overgrazing, and road/railroad construction have caused stream incision, which resulted in drainage of riparian meadow sediments and a succession from native wet meadow vegetation to sagebrush and dryland grasses. Restoration efforts have been initiated to reestablish the ecosystem function of these systems. Original field data including stream stage records, water table hydrographs, sediment hydraulic properties, topographic transects, and aerial imagery of vegetation patterning were used to develop a model of an archetype meadow. Hydrologic behavior was simulated with a finite element model of variably saturated groundwater flow. This model was coupled to an empirical, time-dependent, vegetation threshold relationship between vegetation type and depth to the water table. This was a two-way coupling requiring an iterative approach because water table depth is a determinant of vegetation type, yet the vegetation regime influences water table depth through evapotranspiration. The hydrology and vegetation patterns were analyzed under pristine, degraded (incised), and restored conditions. For the case of deep streambed incision, our hydroecological model predicts the observed shift from mesic (wetter) to xeric (drier) vegetation communities and reproduces their imaged longitudinal zonation. This patterning is explained as a response to groundwater drainage to the stream, which creates dry zones with xeric vegetation adjacent to the stream, while preserving sufficient moisture at the margins of the meadow to support holdout populations of mesic vegetation. The model further predicts the reestablishment of meadow vegetation when the incised channel is filled and a new shallow channel is restored. The coupling of a near-surface hydrologic model to a vegetation response model may be used to design stream restoration projects by predicting vegetation patterning.

Changes in carbon storage with land management promoted by payment for ecosystem services

“…There are over 30 communities in the entire project area, with an estimated 49.3 km 2 of total land area affected by restoration activities after accounting for regeneration rates in NonProject communities. Other wet meadow restoration studies found that grade control structures that accumulate sediment in incised gullies raised water tables and contributed to vegetation recovery in a buffer zone up to 100-m on either side of the channel (Shields et al 1995;Schilling et al 2003;Loheide & Gorelick 2007;Loheide & Booth 2011). We speculate that check dams affect bofedal and grassland vegetation in a broader area by raising water tables relative to surface vegetation, a conclusion that deserves further research.…”

“…Land degradation impacts bofedales in the study area through the following mechanisms: (1) reduced vegetative cover, increased runoff, and decreased infiltration rates on slopes reduce groundwater recharge, causing dry season water stress for wetland plants (Trimble & Mendell 1995;Salvador et al 2014); (2) gullies that incise through bofedales lower water tables, changing species composition, also causing dry season water stress for wetland plants (Wright & Chambers 2002;Schilling et al 2003;Loheide & Gorelick 2007;Loheide & Booth 2011); (3) increased flow velocities in channels can increase vegetation scour, leading to rapid development and destruction of bofedal vegetation in channels and floodplains (Earle et al 2003); and (4) increased sediment transported from slopes can cover bofedales that are not incised by gullies, causing plant mortality, changes in species composition, and increased soil elevation relative to the water table (Werner & Zedler 2002;Miller et al 2012).…”

“…Check dams in gullies reduce water flow velocity and increase sediment deposition, soil moisture, and riparian vegetation (Boix-Fayos et al 2008;Bombino et al 2008;Zeng et al 2009). Check dams and other grade control structures can also raise water tables to restore wet meadows incised by gullies (Shields et al 1995;Schilling et al 2003;Loheide & Gorelick 2007).…”

Hydroecological model predictions indicate wetter and more diverse soil water regimes and vegetation types following floodplain restoration

“…Feedbacks related to water use by a particular plant functional type influencing hydrology and subsequently impacting the composition of plant functional types are important at other floodplain sites [e.g., Loheide and Gorelick, 2007]. This feedback is central to the discipline of ecohydrology, which studies the two-way interaction between water and ecosystems [Hannah et al, 2004].…”

“…Previous researchers have simulated these processes using physically based saturated [Lowry et al, 2011;Rains et al, 2004;Springer et al, 1999] and variably saturated [Hammersmark et al, 2010;Loheide and Booth, 2011;Loheide and Gorelick, 2007] groundwater flow models to generate predictions of water table depth that are then linked with vegetation models to provide quantitative predictions of vegetation composition. Although modeling techniques varied among these previous studies, all used groundwater metrics as predictor variables.…”