Montane meadow evapotranspiration: implications for restoration and impacts on downstream flow.

Abstract:

Meadows comprise less than 1 percent of the area in the Sierra Nevada Mountains, CA, but play an important role in hydrologic processes of Sierra catchments. This study constrains meadow evapotranspiration (ET) and places meadow ET in the context of the greater watershed ET. Meadows in the Sierra Nevada range from pristine to restored to degraded—decades of stock grazing, road building, and logging, have left incised stream channels and lowered water tables in many meadow s. The State of California has prioritized meadow restoration; these efforts are motivated, at least in part, as a means to increase groundwater storage (California Water Action Plan; 2014). Information on how restoration affects meadow and catchment hydrology and ET is lacking. We combined eddy covariance measurements (2013 and 2014) and meteorological records (2008 to 2014) with an Artificial Neural Network to quantify meadow ET over a 7 year period. We focused on Long Meadow, which is a pristine meadow located at 2210 m in Sequoia National Park, CA. ET was highest in drought years 2012-2014 (532 mm in 2013) and lowest in a particularly wet year (470 mm), 2011. This trend is opposite that observed for nearby upland Sierra Nevada forest by the SSCZO eddy flux stations, where annual ET declined from wet to dry years. Meadow ET was generally lower and less variable than forest ET. Long Meadow ET is 25-50% greater than ET rates reported in systems similar to degraded meadows such as bitterbrush/sagebrush dominated areas (Mauer et al.; 2006); this ET discrepancy is consistent with previous studies that suggest healthy meadow ET can be 25-50% more than ET in a degraded meadow s (Hammersmark et al.; 2008). Because meadows are a small fraction of the land area, however, restoring meadows to healthy conditions results in only a small increase in ET in the greater watershed. Restoring a meadow similar in size (~4.6 hectares) and associated catchment size (~140 hectares) to Long Meadow from a degraded to a healthy system, for example, would increase the associated catchment ET by only 0.03-0.05%. We conclude that when evaluating the ET effect of a meadow restoration, it needs to be done in context of the whole catchment.