The Ecohydrology of Degraded and Restored Meadow Systems

Meadow systems play a critical buffering role in the hydrology of the northern Sierra Nevada, Plumas County, CA. They serve as surface and subsurface storage zones that mitigate flood flows resulting from the spring snowmelt. They also provide reservoirs of water that contribute to baseflow and transpiration demands during the dry summer in this semi-arid environment. Unfortunately, land use practices such as grazing, logging, and railroad construction have caused stream incision and subsequent draining of meadow sediments. This watertable drop causes a succession from wet meadow species such as sedges and rushes to dryland grasses and sagebrush. Based on water level records at 22 piezometers, we present the critical depth to water hydrograph that determines whether the xeric or mesic/hydrophytic vegetation communities will dominate a site. We call this the "vegetation-threshold hydrograph" (VTH); sites with water levels above those of the VTH support meadow species whereas sites with water levels below those of the VTH are dominated by sagebrush. The most important feature of the VTH is that water levels are very near, or above, the land surface during April and May. If the water table is within 0.5m of the surface during this period of growth initiation, the very high moisture contents in the root zone will cause water-logging and mortality of xeric vegetation. Additionally, the water table must remain within 1m of the surface through June and July, to maintain sufficient soil moisture for the mesic/hydric species to reach full bloom during the annual drought period. Through August and later, the mesic/hydric species begin to senesce and no longer require a shallow watertable. Knowledge of the VTH has potential to improve the success rate of restoration in this environment by providing a minimum target hydrograph. Examples will be used to discuss the hydrologic function of degraded and restored meadow systems.