Grantee Research Project Results

 

Two case studies were undertaken to demonstrate the effectiveness of the tool. The first was in support of a state rule-making effort in Connecticut, where the Reservoir Yield model was extensively used by stakeholders on a technical advisory committee to inform recommendations in draft regulations that are currently out for public comment. See: http://www.ct.gov/dep/streamflow. The second was to support efforts by the Town of Scituate, Massachusetts, and the North and South River Watershed Association, to evaluate the feasibility of re-operating their water supply system to restore herring migration to the First Herring Brook.

 

Optimization of Human and Ecological Flows: Our initial efforts were directed at developing an optimization-based decision support system (DSS), which could be used by water supply managers to balance water supply needs. The need for such an optimization-based DSS results from the ever increasing need and attention being given to balancing human water needs and ecological flow needs. Our literature review revealed that the problem of optimal water allocation has received significant attention in the literature; however, nearly all previous literature has focused primarily on allocating water among competing human needs. There is an extensive literature relating to each of the following individual subjects: (1) instream flow needs, (2) regulatory policy for water withdrawals, and (3) optimal allocation of water withdrawals; however, only a few studies cited by Homa et al. (2005) and Vogel et al. (2007) introduced a methodology that integrates these three areas. Our work on the development of an optimization approach differs significantly from previous studies because previous work employed the more complex, time-consuming and data intensive habitat-based approaches to determining instream flow needs that were primarily focused on low flow protection for game fish. Instead, our work focused on the concept of an ecodeficit (introduced by Homa et al. (2005) and evaluated by Vogel et al. (2007) and Gao et al. (2009) to enable a broader evaluation of instream flow needs, while simultaneously exploiting one of the most common methods of water allocation, the flow duration curve (FDC). Our early work by Homa et al. (2005) shows that use of optimization, along with sensible regulatory strategies, can result in improvements in both water supply reliability and in instream flow.

 

Generalized SRY Relations The study by Vogel et al. (2007) represents one of the first studies that attempts to generalize our understanding of the storage-yield-instream flow relationship and to improve our understanding of the impact of reservoir operating policies on the tradeoff between properties of instream flow and water supply yield. A simple case study documents that the choice of a reservoir operating policy can have an enormous impact on the properties of both yield and instream flow and that different types of policies may be suited to different size reservoir systems (based on storage ratio). Overall, our simulations reveal that for larger reservoir systems (storage ratios above 0.4, approximately), both the fraction of inflow (FOI) and flow components (FC) policies in combination with drought management policy offer significant promise for aiding future negotiations between yield and instream flow with the FOI parameter used as a policy parameter. For smaller reservoir systems with storage ratios less than about 0.4, a minimum release policy with or without augmentation, combined with drought management, may offer promise, though we caution that the modification to the low flow variability of the instream flows needs attention in these cases. Here drought management involves reductions in releases for both human and instream flows when reservoir storage levels are low.

 

In general, drought management was shown by Vogel et al. (2007) to be an effective management strategy for reducing both human and instream flow water uses, and interestingly, for reservoir systems with storage ratios below unity we found additional gains in overall reservoir yield that exceed the magnitude of the reductions in demand. This extra benefit associated with drought management results from the fact that the demand curtailments in both human and instream flow releases occur at the time when the reservoir is drawn down, leading to overall increases in net or effective safe yield. Our findings also indicate that drought management is likely to have its greatest impact for reservoir systems with relatively small storage ratios, but that it is always a useful tool. Viewed only from the perspective of human water use requirements, the FOI and FC policies combined with drought management can lead to human water yields that are commensurate with yields for systems with no instream flow requirements for small storage ratios. Formalizing this type of drought management into actual reservoir operational guidelines is a challenge that appears to be worth pursuing.

 

Vogel et al. (2007) also documented that it is much more difficult to maintain instream flows for systems with large storage ratios than for systems with small storage ratios. On the other hand, for a fixed storage ratio, we document the somewhat counterintuitive result that reservoir systems in temperate regions (low inflow variability) will generally have a lower fraction of water available for instream flow (relative to the mean annual flow) than similar systems in arid regions, which are subject to greater inflow variability.

 

The Ecodeficit-Ecosurplus: There is an increasing need to account for natural differences in flow variability among rivers and to understand the importance of such differences for the protection of freshwater biodiversity and maintenance of goods and services that rivers provide. On the other hand, there is a need to develop a reduced suite of indices to replace the commonly used 33 IHA parameters or the 171 parameters in more recent USGS National Hydrologic Assessment Tool (NHAT) and to provide an accurate overall determination of the impact of hydrologic alteration. The use of a single or just a few indices of hydrologic alteration can minimize statistical redundancy and lead to significant reductions in the complexity associated with the formulation and development of optimal reservoir operation policies and other river regulation schemes.

 

In an effort to develop an overall measure of habitat alteration based on streamflow data, Homa et al. (2005) and Vogel et al. (2007) first introduced the concept of an "ecodeficit" and "ecosurplus." These metrics provide an overall numerical and graphical representation of the tradeoff between human and ecological needs for available water. A subsequent study by Gao et al. (2009) evaluated the ability of the "ecodeficit" and "ecosurplus" statistics to provide an overall measure of streamflow alteration.

 

Gao et al. (2009) sought to evaluate the ability of a set of generalized indices of hydrologic alteration to describe the variations in stream discharge resulting from reservoir operating release rules. Their goal was to develop a set of independent and representative hydrologic indicators that can best characterize hydrologic alteration caused by reservoirs and other forms of river regulation. Two sets of pre- and post-dam streamflow records were used: (1) based on artificial simulations of a wide range of reservoir release rules and (2) streamflow records for 189 gaging stations throughout the United States. Principal component analysis was used to address the intercorrelation among the IHA parameters. Results revealed that the recently introduced metrics termed ecodeficit and ecosurplus can provide a good overall representation of the degree of alteration of a streamflow time series. Across both datasets, 32 individual IHA statistics and several potential generalized indices, three indices based on the ecodeficit and ecosurplus explained the most variability associated with the ensemble of 32 IHA statistics.

 

Importantly, Gao et al. (2009) found that the eco-flow statistics termed the ecodeficit and the ecosurplus can provide good overall measures of hydrologic alteration. The annual ecodeficit appears to be the best generalized index among all the indices in the simulated data set. On the other hand, winter ecosurplus and summer ecosurplus appear to perform best in the empirical data set. In addition, total seasonal ecochange appears to be a good generalized index in both data sets because it accounts for all the seasonal deficits and surpluses and because it accounts for seasonal changes, thus taking timing of the flow into consideration.

 

Generally, small values of the ecodeficit/ecosurplus correspond to low values of hydrologic alteration. However, unlike other measures commonly used such as DHRAM scores, which enable water resources managers to determine the level of risks that a particular reservoir regulation scheme has on a river, the ecodeficit/ecosurplus does not yet include the level of risks. Future research should (1) investigate the hydrologic and ecological significance of the values of ecodeficit/ecosurplus needed to fully address the ecologically based environmental flow requirement; and (2) establish a system to classify what level of ecodeficit/ecosurplus is acceptable and unacceptable for a particular reservoir operation in a river.

Initial research associated with this project performed carefully controlled computer experiments that evaluated a wide range of reservoir system operating rules, over a wide range of operating conditions, for their ability to balance the needs of both human and ecological flow requirements. That research was published in the form of a research paper in the journal Water Resources Research (Vogel et al. 2007) and served the basis for the development of a decision support system that focused on reservoir management to meet both human and environmental needs. The model, built on the WEAP modeling platform, integrates ecological flow requirements, conservation and demand management requirements, and reservoir operations into an easily accessible and flexible modeling tool that allows testing of numerous scenarios. The model includes a reservoir yield tool that determines reservoir yields for human uses for multiple scenarios involving release rules and demand restrictions. The tool determines the maximum dependable yield based on a long period of record of inflows to the reservoir. The tool allows for operating rules to be constructed in both simple and complex ways, offering broad computing flexibility to analyze systems of varying complexity. The model can be used on single and multiple reservoir systems and can include groundwater withdrawals as part of basin or regional analyses.

 

Our work included two case studies. The first was in conjunction with the Connecticut Department of Environmental Protection (CT DEP) to support the development of a proposed new streamflow protection regulation in the State of Connecticut. The WEAP model and Reservoir Yield tool was used by the technical advisory committee to develop and test numerous reservoir releases and drought management policies. The result of this work is a set of presumptive reservoir release standards that are included in the draft regulation (see: http://www.ct.gov/dep/streamflow).

 

A second case study was completed in conjunction with the Town of Scituate, Massachusetts, and the North & South River Watershed Association. In this application, we provided modeling in support of the town's effort to assess the feasibility to restore river herring to the First Herring Brook, the location of the town's two water supply reservoirs. The model included several groundwater sources within the First Herring Brook watershed. The work estimated the amount of additional water, if any, necessary to meet various environmental goals and continue to meet town water needs. Various alternatives were analyzed, including new groundwater sources, enlarging one or both reservoirs through dredging, installation of new, more water efficient fish ladders and the addition of out-migration, and the addition of seasonal releases to support resident aquatic organisms.