This case study demonstrates (1) the use of flow-ecology relationships to determine groundwater withdrawal limits, (2) ecological goal setting through river basin classification, and (3) the adoption of an ecologically-based presumptive flow standard.
The Rhode Island Stream Depletion Method (SDM) was developed in 2010 from a series of initiatives that began in 1999, driven by an increasing need for water supply in the state. In 1999, the RI General Assembly granted the Water Resources Board (WRB) sole authority to devise a fair and equitable allocation of water resources among users and uses to ensure that long-range considerations of water supply prevail over short-term considerations (Rhode Island Gen. Laws §46-15.7).
In 2002, the Water Resources Board (WRB) formed the Water Allocation Program Advisory Committee (WAPAC) and launched an inclusive water allocation planning effort, which brought together 150 people from 66 participating organizations. The Committee recommended, and in March 2004 the Board approved, the establishment of the Streamflow Working Group, a partnership between WRB and the Rhode Island Department of Environmental Management (DEM) to address streamflow issues such as aquatic base flow and the further development of a statewide streamflow gaging network.
The Rhode Island Stream Depletion Method (SDM) provides resource agencies with a withdrawal or streamflow depletion allowance that establishes the volume of water that can be extracted as direct stream withdrawals or as indirect groundwater withdrawals, while still leaving sufficient streamflow to maintain habitat conditions essential to a healthy aquatic ecosystem. This methodology is currently being applied to all new or increased groundwater withdrawals, and is being applied in the Rhode Island Water Resources Board’s Statewide planning process to assess build-out scenarios and the potential for future water resource availability.
In order to arrive at a set of watershed condition goals for water management, watersheds were classified based on landscape and site characteristics. Each sub-watershed was scored according to degree of human alteration and then grouped into larger watersheds. Each watershed was assigned to one of the goal classifications based on its total score. By applying the biological condition gradient (Davies and Jackson, 2006), appropriate management practices could be applied to each watershed.
The methodology allows for a fairly simple calculation of allowable streamflow depletion by considering existing withdrawals and returns, their locations within the watershed, time of year, watershed characteristics, and natural low-flow conditions. Six hydroperiods were defined to represent the biological need for seasonal flow variability, and typical high, medium, medium-low, and low flow conditions were defined for each hydroperiod. Finally, literature-based allowable streamflow depletion
[Table 1] levels were defined for each river condition class by hydroperiod under each flow condition.
When an applicant applies for a new wetlands permit, regulators use SDM as a presumptive approach. If the proposed increased withdrawal is at or below the allowable depletion level (taking into account existing uses), then the withdrawal request is reviewed for other freshwater wetlands impacts and National Pollutant Discharge Elimination System (NPDES) flow needs. Wetlands biologists evaluate the analysis and, if acceptable, the permit is issued. If the applicant’s increased withdrawal exceeds the allowable depletion, or wetland impacts are unacceptable, then the applicant must conduct a site-specific study, reduce the request, or be subject to shutoffs.
