How Much Groundwater does a Wetland Need? Setting Ecological Water Requirements for Groundwater-Dependent Ecosystems

Presented at The Oregon Water Conference, May 24-25, 2011, Corvallis, OR.Many freshwater ecosystems are sustained by a continuous supply of clean groundwater. For example, groundwater may provide late season baseflow to rivers or a sustained high water table in wetlands. These ecosystems, termed GDEs, often are affected by management activities that reduce, interrupt, or contaminate their groundwater supply, including groundwater pumping and waste disposal. Despite these issues, no methods exist to set limits to groundwater extraction or contamination to protect these ecosystems. To address this need, The Nature Conservancy and USDA Forest Service are developing methods for setting thresholds to groundwater change, termed Ecological Water Requirements. Here we discuss this method applied to three wetlands from which water is extracted to water livestock in the Fremont and Winema National Forests.\ud We evaluated the Ecological Water Requirements for GDEs in two steps. First, we developed empirical relationships between the groundwater-driven hydroperiod, and two key ecological parameters: indicator plant distributions and peat accretion. We did this by collecting data on hydroperiod fluctuations, species distributions, and peat depths. We then used these quantitative relationships to determine water table thresholds beyond which these ecological parameters could be impaired. Second, we used MODFLOW to evaluate the potential dewatering effects of pumping on these wetlands. A 4-layer model was constructed (peat, muck, pumice, bedrock). The muck is simulated as a thin, leaky confining layer between the peat and pumice. Pumping was simulated in the pumice layer to evaluate effects to water table depth in the peat. This was compared to the threshold data obtained in the first step.\ud We identified 14 indicator species and peat accretion rates which only occur where the depth to water table was less than 15cm, thus we established this as our threshold for the model. Hydraulic parameters were varied, but for each scenario, pumping at predicted rates of 0.009 L/sec for 75 days produced a maximum drawdown in the peat that did not exceed the 15cm drawdown threshold. These data are being used to inform grazing management on this national forest. We plan to refine these methods to help evaluate ecological water requirements more broadly in a variety of settings