HOW MUCH WATER DOES A FEN NEED? DEVELOPING ENVIRONMENTAL WATER REQUIREMENTS FOR GROUNDWATER-DEPENDENT ECOSYSTEMS

Groundwater-dependent ecosystems (GDEs) include lakes, wetlands, and rivers that are found at points of groundwater discharge, as well as subterranean and aquifer ecosystems. They are strongly adapted to relatively stable groundwater supply. Similar to groundwater sources for human consumption, GDEs are threatened by groundwater depletion and contamination. Yet unlike municipal and agricultural groundwater supply systems, little is known about thresholds for groundwater quality and quantity below which a GDE is irreversibly altered. The objective of this project is to develop quantitative thresholds for groundwater discharge, termed environmental water requirements, for groundwater-dependent wetlands called fens. This pilot project is being conducted in 3 fens in the Fremont-Winema National Forest in Oregon. The study sites are located in the northern part of the arid Upper Klamath Basin, where groundwater plays an important role in stream baseflow and wetland hydrology. The fens are in a grazing allotment where water is withdrawn for cattle, thus understanding thresholds for groundwater discharge and withdrawal is critical to manage these ecosystems. We are using two approaches to set upper limits on water withdrawals. In a “top-down” approach, we are developing hydrologic models to test the sensitivity of water budget parameters to change and to evaluate their effect on groundwater availability. In a “bottom-up” approach, we are developing quantitative relationships between indicator plant species, peat properties, and the timing and depth of water table fluctuations. For the latter, bottom-up approach, we are monitoring depth to water table, piezometric head at several depths within the peat profile, and total peat depth across the sites. Peat depth ranges from 0.5-2 m, and is closely related to consistent groundwater discharge. We identified a suite of potential indicator species, and we are monitoring their distribution with respect to seasonal depth to water table. We are supplementing our field data collection with data from the published literature on depth to water table requirements of these plants. A combination of botanical, hydrologic, and edaphic data will be used to define the ideal and minimum depths to water table, and this information will be used in the “top-down” hydrologic model.