What is Wet Meadow Re-watering?
What is wet meadow re-watering?
Wet meadows are a type of wetland, and can include both seasonally inundated floodplains or spring-fed upland areas. Wet meadows occur where groundwater is at or near the surface most of the growing season, following spring runoff. Wet meadows at all elevations generally have a simple structure consisting of a layer of herbaceous plants. Shrub or tree layers are usually absent or very sparse but may be an important feature of the meadow edge. Hydrologically, they occupy lotic, sunken concave, and hanging sites (Ratliff 1985).
- Lotic sites are those with main input flow (other than precipitation) from upstream sources (Gosselink and Turner 1978). Downstream runoff is the principal output flow. Lotic sites are topographic basins but have a slight slope, which permits drainage of surface water. Percolation is nil due to the saturated or slowly permeable nature of underlying materials (Ratliff 2003).
- Sunken concave sites also receive water input from upstream sources, but evapotranspiration is the main output flow. Percolation is slowed by heavy-textured soils and/or shallow bedrock; however, in contrast to lotic and hanging sites, soil of sunken concave sites may dry to considerable depth by fall (Ratliff 2003).
- Hanging sites are watered by hydrostatic flows as springs or seeps. They frequently occur on rather steep slopes, and downstream runoff is the main output flow (Ratliff 2003).
The single most important characteristic of a wet meadow is its hydrology. Seasonality and reliability of yearly water inflows and outflows largely determine the vegetational stability of wet meadows. Hydrology is the primary driving force in wetland and wet meadow systems and is the critical element in wetland and wet meadow system restoration and management efforts.
Wet meadows provide important ecological benefits including breeding and foraging habitat for birds and invertebrates and habitat for wetland plants. Meadows near small thermal hot springs in Napa County support the remaining two populations each of Calistoga popcornflower (Plagiobothrys strictus) and Napa bluegrass (Poa napensis). These two seasonal wetlands are underlain by a gravelly loam intermixed with clay, and water tables are close to the surface. Concentrations of boron, arsenic, and sulphates are high in these areas and a unique flora has evolved in them. Urbanization and viticulture have extirpated one historic occurrence and eliminated much of the species' habitat.
Streambank instability caused by degradation of riparian vegetation can be mitigated in part by meadow vegetation. Measurements by Micheli and Kirchner (2002) indicate that streambanks colonized by ‘wet’ graminoid meadow vegetation were on average five times stronger than those colonized by ‘dry’ xeric meadow and scrub vegetation.
The introduction of livestock grazing can denude vegetation and increase channel erosion, causing subsequent channel incision and lowering of the water table. Overgrazed wet meadows have more forbs and fewer grasses and grasslike species than properly grazed or ungrazed (by livestock) meadows, and taller species are replaced by lower growing types. Channel erosion lowers the water table, causing succession to species characteristic of dryer habitats. Meadows can become dewatered due to a drop in the groundwater table, often driven by changes in land use. Water diversion can also damage wet meadows by lowering the water table. Thus, meadow re-watering often necessitates large-scale stream restoration as a mechanism to restore the local water table to pre-disturbance conditions.
Meadow re-watering involves the restoration of complex hydrologic, vegetative, and geomorphic processes. Meadow re-watering projects may include a variety of actions to restore the pre-disturbance water table, including:
- changes in livestock grazing regime
- channel and floodplain reconstruction
- changes in water diversion regime
- revegetation of meadow surface