The Colorado ELOHA project demonstrates (1) using flexible approaches to develop flow-ecology curves based on studies reported in the literature and (2) using flow-ecology curves to inform basin-scale water-resource planning.
In 2005, the Colorado Legislature passed the Colorado Water for the 21st Century Act, mandating that representatives of cities, farms, and other water users join conservation and recreation interests at “basin roundtables” to assess future water supply needs for their watersheds. These assessments are framing discussions about future water allocations and must address both consumptive and non-consumptive (recreation and environmental flows) water needs.
The Colorado Water Conservation Board funded this 1-year, approximately $200,000 project to help two pilot basin roundtables—the Roaring Fork watershed in western Colorado and the Fountain Creek watershed in eastern Colorado—understand tradeoffs between consumptive and non-consumptive water uses. In 2008, the consulting firm Camp Dresser & McKee (CDM) worked with scientists from Colorado State University and The Nature Conservancy, staff from the Colorado Water Conservation Board, and representatives of the Colorado Basin Roundtable to apply ELOHA to estimate flow-related ecological risk at the basin scale. The Watershed Flow Evaluation Tool (WFET) displays the results under various water management scenarios.
A hydrologic foundation existed before this pilot project began. The State of Colorado’s water supply model, StateMod (CDWR and CWCB 2009), is a monthly water accounting program that begins with gaged streamflow data under current conditions. Reservoir storage changes, water diversions, and return flows are incorporated to obtain baseline flows. Simple water accounting, weighted by drainage area and precipitation, is then used to calculate baseline flows at ungaged sites. Groundwater withdrawals and return flows are simulated with aquifer-dependent time delay.
Indicators of Hydrologic Alteration (IHA) software (The Nature Conservancy 2009) used output from StateMod to calculate changes in five ecologically relevant flow statistics—mean annual flow, mean August flow, mean September flow, mean January flow, and mean annual peak daily flow—for water use scenarios.
Relationships were developed between streamflow and fish, riparian vegetation, invertebrates, and recreation, based on a literature review. The technical team then identified 3-5 risk classes for each ecological attribute. Using the flow-ecology relationships, they determined the range of flow values associated with each ecological risk class. These are incorporated into a Watershed Flow Evaluation Tool (WFET) (Sanderson et al 2011), which Basin Roundtable groups use to map [Figure 2] the levels of flow-related ecological risk for all river segments in their watersheds under different future water use scenarios.
