Search

From the Rocky Mountains in the US to the Sea of Cortez in Mexico, the Colorado River is one of the most managed and monitored rivers in the world. As decreasing river flows from drought and climate change continue to impact the Colorado River Basin, the US federal government, seven US states, and Mexico have signed agreements that would reduce deliveries, including a Drought Contingency Plan (DCP), an agreement among the US states and the Bureau of Reclamation, and Minute 323 (a treaty agreement between the US and Mexico). As part of the DCP, the US Bureau of Reclamation (Reclamation) committed to increasing conservation efforts to augment storage in Lake Mead. This paper presents an analysis of Reclamation's Decree Accounting Reports that record water deliveries and conservation savings from 1964 to 2019. Overall, this analysis illustrates that (1) the agency has implemented significant conservation actions over the last three decades, (2) the agency has responded to increased accounting needs as dictated by national and international management agreements, (3) some actions have resulted in environmental costs, and (4) future actions could be designed to avoid environmental harms while also augmenting Lake Mead levels. As climate change continues to constrain Colorado River water supply, detailed accounting may help reveal areas for potential efficiencies or demonstrate where the greatest levels of savings have been reached while ensuring that environmental and social benefits are preserved. Over the last 100 years, humans have changed the path and personality of the Colorado River by distributing water to cities, communities, and agricultural fields to support over 40 million people. Throughout the twentieth century, the US federal government, seven US, and Mexico signed agreements enabling development of Colorado River water. By the early 21st century, as it became evident that water demands and climate change were reducing available supplies, these same governments signed agreements to reduce pressure on the river. All these agreements have, in part, been tracked by the US federal agency that manages the river, the Bureau of Reclamation (Reclamation), through Decree Accounting Reports. Since 1964, these reports have recorded how water is or is not accounted for that might be important to the future of the river. This paper examines those reports and reveals four main findings: (1) Reclamation has implemented numerous water conservation projects, (2) they have responded to new management agreements, (3) some of their actions have impacted the environment, and (4) there are proactive ways they can reduce environmental impacts and bolster water supplies in the future. [ABSTRACT FROM AUTHOR]

Skillful mid-term temperature predictions (up to five years out) offer a potential opportunity for water managers, especially in the Colorado River Basin (CRB), where streamflows are sensitive to temperature. The purpose of this paper is to develop and demonstrate a framework for how mid-term temperature predictions can be incorporated into streamflow forecasting and operational projections. The framework consists of three steps. First, 5-year average temperature predictions are obtained from two large ensemble climate model datasets. Second, hindcasts from the Ensemble Streamflow Predictions (ESP), an operationally used forecast method in the CRB, are post-processed using the 5-year average temperature predictions; specifically, a tercile-based block bootstrap resampling approach generates weighted streamflow ensembles called WeighESP. Third, ESP and WeighESP are run through an operational model, the Colorado River Mid-term Modeling System (CRMMS). Compared to ESP, WeighESP marginally improves streamflow forecast accuracy in the multi-year hindcasts up to five years out (i.e., years 1-5, 2-5, 2-4, and 2-3). The multi-year hindcasts show median annual root mean square error (RMSE) improvements between 437,000 and 771,000 m3 (354 and 625 thousand acre-feet). Improvements in streamflow accuracy are more pronounced for the most recent hindcast run dates through 2016, partially due to ESP being run with climate time series data from 1981 to 2010. Next, CRMMS translates the streamflow forecasts into operational projections of end of calendar year (EOCY) pool elevations. WeighESP improves the accuracy of EOCY predictions, but mainly for longer leads of 3- and 4-years. For the 4-year lead, the median RMSE improves by 1.1 and 0.7 m (3.5 and 2.3 ft) for Lakes Powell and Mead, respectively. Although marginal improvements in pool elevation could be beneficial, not being realized until longer leads is a limitation. This study describes the need for better predictive tools at the mid-term timescale and underscores the importance of evaluating improvements in streamflow forecasts in decision-relevant terms. [ABSTRACT FROM AUTHOR]

Comprehensive assessment of hydrological alteration due to anthropogenic interventions is a prerequisite for sustaining the native biological profile and integrity of river ecosystems. This paper proposes a novel strategy to measurably evaluate how the dams have modified the natural flow regime at a downstream site. The Bhagirathi Basin, a part of the Himalayan ranges of the Ganga River Basin in India, is studied to comprehend the impact of multiple dams on natural river flow at Devprayag. The methodology for estimating the River Flow Health Index (RFHI) consists of four steps: (1) segregation of the flow data based on preimpact and postimpact periods, (2) identification of important hydrological parameters, (3) assessment of the alterations, and (4) development of an index indicating the health of the river flow during the altered period on a 0–1 scale. Results show that the different components of the flow regime have changed due to construction of dams across the river, and the hydrological alterations could be seen at the downstream site, Devprayag. The RFHI increased after subsequent addition of dams, i.e., 0.328 after commissioning of Maneri Bhali I hydro-electric projects (HEP), 0.374 after Maneri Bhali I HEP, Maneri Bhali II HEP, and Tehri HEP were commissioned, and 0.411 after Koteshwar HEP was commissioned in 2011. Our results may help upgrade the design and implementation of reservoir operation policies that consider downstream hydrological alterations. Further studies can be made to consider the coordination of hydrological alterations with different river ecosystem services important for the sustainable management of water assets. [ABSTRACT FROM AUTHOR]

The concept of integrated water resource management (IWRM) attempts to integrate all elements of water resources. Different tools are developed to assist in developing sound IWRM plans. One such tool is multiobjective analysis using an integrated hydro-economic model (IHEM). However, IHEM mainly deals with the optimization of river flow (blue water) in a river basin. This paper linked a distributed model of green water (landscape water uses) in the upper catchment with mainly blue water uses in the lower catchment of the Pangani Basin. The results show that agricultural water use has the highest water productivity and competes with all other objective functions in the catchment. The generation of firm energy competes with the downstream ecosystem requirements. The integrated study shows that improving rainfed cropping through supplementary irrigation has comparable marginal water values to full-scale irrigation but are much higher compared with hydropower. However, hydropower has more benefits if used in conjunction with the environment. The methodological approach has increased the understanding of trade-offs between green and blue water uses that are highly interdependent in African landscapes. [ABSTRACT FROM AUTHOR]

Ongoing changes in global climate are expected to alter the hydrologic regime of many river basins worldwide, expanding historically observed variability as well as increasing the frequency and intensity of extreme events. Understanding the vulnerabilities of water systems under such uncertain and variable hydrologic conditions is key to supporting strategic planning and design adaptation options. In this paper, we contribute a multiobjective assessment of the impacts of hydrologic uncertainty on the operations of multipurpose water reservoirs systems in arid climates. We focus our analysis on the Dez and Karoun river system in Iran, which is responsible for the production of more than 20% of the total hydropower generation of the country. A system of dams controls most of the water flowing to the lower part of the basin, where irrigation and domestic supply are strategic objectives, along with flood protection. We first design the optimal operations of the system using observed inflows and evaporation rates. Then, we simulate the resulting solutions over different ensembles of stochastic hydrology to partition the impacts of streamflow and evaporation uncertainty. Numerical results show that system operations are extremely sensitive to alterations of both uncertainty sources. In particular, we show that in this arid river basin, long-term objectives are mainly vulnerable to inflow uncertainty, whereas evaporation rate uncertainty mostly affects short-term objectives. Our results suggest that local water authorities should properly characterize hydrologic uncertainty in the design of future operations of the expanded network of reservoirs, possibly also investing in the improvement of the existing monitoring network to obtain more reliable data for modeling streamflow and evaporation processes. [ABSTRACT FROM AUTHOR]

Evidence from ecological studies suggests that the alteration of river flows downstream of reservoirs can threaten native aquatic ecosystems and the services they offer. Innovative revisions to water management practices are required to improve the health of aquatic species while maintaining the benefits from current infrastructure projects. The impacts of individual reservoir operations on ecosystem vitality are often masked by the uncoordinated and compounding influences of several impoundments upstream, undermining the examination of environmental impacts from particular reservoirs in a large watershed system. This paper presents a large-scale optimization model that investigates the value of coordinated reservoir management practices for ecological benefits in a large watershed with several major reservoir systems operating for a range of management objectives. An application of the model is presented for the Connecticut River watershed, the largest river basin in New England and one of the most impounded rivers in the United States. The model can examine trade-offs between the maintenance of ecologically acceptable, daily streamflows at key locations throughout the watershed and traditional reservoir objectives, including flood risk reduction, municipal and riparian water supply, hydropower production, and recreation. The ecological streamflow targets are designed specifically for the basin's ecology in a collaborative process engaging regional experts and constitute a unique and innovative component of the modeling approach. This study focuses on the re-operation of a network of federal flood control dams for the restoration of environmental flows. Results suggest that coordinated changes to current flood control reservoir operations can restore aspects of the natural hydrologic flow regime necessary for ecosystem persistence without significantly reducing current flood risk reduction capabilities. [ABSTRACT FROM AUTHOR]

The preponderance of evidence in the scientific community supports the premise that global climate is changing. The precise impacts of climate change on natural and man-made systems remain less certain. Municipal water supplies, particularly those that rely on summer snowmelt to augment storage capacity, may experience significant changes in their flow regimes in the future. This paper presents an evaluation of climate change impacts on a water resource system using a three-stage modeling approach: General circulation models (GCMs) to simulate global climate, basin scale hydrology models, and water resource system simulation models. This approach is applied to two river basins in the Puget Sound Region of the Pacific Northwest: The Cedar and South Fork Tolt Rivers, which are the principal sources of water for the Seattle metropolitan region. The greatest source of uncertainty associated with climate change impacts arises from the range of future scenarios produced by GCMs. This uncertainty is addressed by incorporating multiple climate models at every stage of the process and using the values produced to generate an ensemble average that quantifies the most likely impact. The ensemble average is characterized by an envelope of uncertainty based on the range and spread of the individual GCM ensemble members. Hydrologic impacts of climate change such as alteration of historic streamflow patterns and snow accumulation are explored, as well as impacts to the water supply system’s yield. [ABSTRACT FROM AUTHOR]