Theme for 2013:
Improving Precipitation and Runoff Forecasts and Implications for Reservoir Operations
University of California, Davis
Why this theme?
Reservoir operations will need to adapt to a changing climate. The Sierra Nevada snow pack is a vital resource for reducing floods by storing water. California's Mediterranean climate, with a distinct hot summer without rainfall and a winter with snow in the mountains and rain at lower elevations, presents a problem. A warmer climate means less snow and more rain. Multi-purpose reservoirs built for water supply and flood water storage will be challenged by changing elevation distributions of precipitation types, precipitation amounts, and runoff timing. Cutting edge work is being done at Folsom Dam and Reservoir to use forecasts to more actively inform reservoir operations for adapting to these changes. This work has the potential to assist other reservoir operators in thinking about how to better use realtime forecasting to inform decision-making regarding reservoir operations.
Special Recognition Award
The 2013 Special Recognition Award was presented to California-Nevada River Forecast Center of NOAA's National Weather Service for their history of of providing excellent public service and exemplifying the best in government agencies. See the award language and hear the presentation on the Symposium's CNRFC award page.
Hydrology for Folsom Dam Water Control Manual Update
The purpose of the Folsom Dam Water Control Manual is to lay out a detailed plan for operating Folsom Dam for flood control and management. The update to the Manual is motivated by the additional capabilities offered by the Joint Folsom Project's auxiliary spillway. Additionally, as part of the directive authorizing the update, Congress charged the Corps to examine how measurements of basin wetness and inflow forecasts might inform operations. The Hydrology Section of the Sacramento District of the U.S. Army Corps of Engineers has been tasked with providing a framework for evaluating the degree of protection afforded by the new structure and operations. This framework will be used to develop operating rules and to evaluate the added performance benefits from basin wetness indices and forecasts.
One way of depicting the performance is a regulated frequency curve, in which the magnitudes of peak regulated flows are plotted against their probabilities. Two lines of analysis come together to produce the final regulated frequency curve. We first assess the probability of unregulated flows in the American River by applying the procedures of Bulletin 17B to them. In a parallel effort, unregulated hydrographs based on the floods of 1955, 1964, 1986 or 1997 and scaled by multiple factors (ranging from 0.1 to 3.0) are run through an HEC-ResSim model of Folsom Dam. From the model output, regulated peak outflows are taken. The critical duration of each model run is also assessed. For the Manual Update, the volume-window method for selecting critical durations was developed. The volume-window method determines the critical duration based on of the percentage of the maximum n-day flow that has entered the reservoir by the time of maximum storage. The frequency of the critical duration's volume is then assigned to the regulated peak outflow. From conditional, pattern-specific regulated frequency curves, a composite frequency curve may be derived by applying the Total Probability Theorem. The final curve serves as our best estimate of regulated peak probabilities before the inclusion of risk and uncertainty.
Using Real-Time Watershed Information in Reservoir Operations—A Case Study of Folsom Reservoir
This case study evaluates the potential benefits of variable index rule curves that incorporate current precipitation and snowpack into the operation of Folsom Reservoir in the American River watershed. Over 100 synthetic flood hydrographs generated from seven historical flood events are used to assess each rule curve's flood management performance. Water supply performance is also evaluated over 53 water years in the period of record. Trade-offs between flood control and water supply are analyzed using the probability of exceeding downstream channel capacity, and the probability of refill.
Two types of variable rule curves are presented. The first type of alternative rule curve used a precipitation-based index (Type P curves), and the second type used a precipitation index and a snowpack index (Type S curves). In total, 91 Type P curves and 55 Type S curves were evaluated to determine the effects of varying the index range, flood pool size range, and reservoir refill start and end dates.
In general, Type P curves were found to improve water supply benefits while maintaining or reducing flood risk. Type P curves with lower precipitation index ranges performed better for flood management while those with higher ranges performed better for water supply. Larger flood pool sizes functioned best in balancing water supply and flood management performance. Adjusting the precipitation index during the refill period using normalized snowpack data to produce Type S curves generated small but noticeable improvements in refill.
Forecasting Advances at California-Nevada River Forecast Center
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The California-Nevada River Forecast Center (CNRFC) in Sacramento, CA is one of thirteen RFCs in the U.S. operated by NOAA's National Weather Service. The CNRFC was established in June of 1963 and has always functioned as a collaborative center with the State of California's Department of Water Resources (DWR).
RFCs benefit from both national and regional NWS support, but much of the progress and development is a function of local initiative for the purpose of meeting specific customer needs. The CNRFC pioneered advances related to watershed modeling (Sacramento Soil Moisture Accounting Model) and real-time data collection (ALERT). More recently, the CNRFC has focused on strengthening and improving forecasts through better QPF investments, better model calibrations, an improved modeling infrastructure, collaborative activities, and the development of probabilistic forecast. These improvements facilitate improved emergency response, flood management, reservoir management decisions, and longer-term water management objectives.
Over the past 10 years, the CNRFC has been aggressively pursuing ensemble techniques in order to provide risk-based information to decision makers. Those investments are realized for longer-term forecasts and are anticipated for short-term flood products within the next 3-5 years. Also on the horizon, the CNRFC envisions the development of a collaborative Delta forecasting function that dramatically expands the information available to effectively manage Delta resources in the future.
Supporting Reservoir Management Through Hydrologic Forecasting Services in American River Basin
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The California-Nevada River Forecast Center (CNRFC) has recently expanded its hydrologic services in the American River Basin to assist reservoir management decision support. A total of six new sub-basins have been created to support the needs of the Sacramento Municipal Utility District (SMUD) and Placer County Water Agency (PCWA). Both real-time deterministic and ensemble forecast products are being utilized by SMUD and PCWA to assist in short and long-range reservoir management decision making. This newly developed model is also being used to support the Folsom Joint Federal Project (JFP) water control manual update. The CNRFC is working closely with the JFP multi-agency team to develop procedures that utilize hydrologic model antecedent conditions and meteorological forecasts to establish dynamic flood control requirements.
Atmospheric River Impacts California June 24-25, 2013
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This is an extra presentation on the potential rainfall from an atmospheric river in the normally dry June in California. Because it is unusual to have a rain-producing atmospheric river this time of year, this presentation was held during part of the lunch break for those attendees who might be interested. This was the second time in three years (2011-2013) that an atmospheric river produced rain while the California Extreme Precipitation Symposium was being held.
Satellite images of the atmospheric river heading for California, Washington, and Oregon are shown. Forecast model results and estimates of precipitation amounts are provided.
Potential to Improve Forecasts and Reservoir Operations with a 21st Century Observing Network
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In 2008, the California Department of Water Resources (CA-DWR) signed a five-year agreement with NOAA's Earth System Research Laboratory (ESRL). The joint project between CA-DWR, ESRL, and the Scripps Institute for Oceanography is part of CA-DWR's Enhanced Flood Response and Emergency Preparedness (EFREP) Program. The underlying goal of the joint project is to improve precipitation monitoring and prediction, especially for extreme events. The statewide deployment of observing systems and suite of highly detailed weather forecast models builds on NOAA's Hydrometeorology Testbed (HMT) project carried out in the North Fork of the American River.
During northern hemisphere winters, the western coast of North America is battered by land-falling storms. The impact of these storms is a paramount concern to California, where water supply and flood protection infrastructure is being challenged by the effects of age, increased standards for urban flood protection, and projected climate change impacts. In addition, there is a built-in conflict between providing flood protection and the other functions of major water storage facilities in California: water supply, water quality, hydropower generation, water temperature and flow for at-risk species, and recreation. In order to improve reservoir management and meet the increasing demands on water, improved forecasts of precipitation, especially during extreme weather events, will be required. The following observing networks are being installed throughout California to improve short term forecasts of extreme events.
Water vapor fuels precipitation, and GPS technology provides a viable method of measuring the vertically integrated water vapor (IWV). HMT is partnering with UNAVCO, the operators of the Plate Boundary Observatory, where many GPS receivers already exist for geodetic purposes, to provide IWV measurements from 45 locations in or near California. The snow level is important with respect to flooding in mountainous watersheds because it determines the surface area throughout the watershed that is exposed to snow versus rain. ESRL engineers have invented a new compact radar designed to measure the snow level at a much reduced cost compared to other radars used for this purpose. These "snow-level radars" are being installed in ten key watersheds across California.
A major finding from HMT is the role that atmospheric rivers, narrow regions of enhanced water-vapor transport, have in creating heavy precipitation that can lead to flooding. A picket fence of atmospheric river observatories (AROs) is being deployed along the California coast. The AROs provide critical information on water vapor transport aloft and the snow level. Antecedent soil moisture can determine whether a storm produces a flood, so soil moisture sensors with other associated meteorological equipment are being placed at 43 new sites across California.
Taking full advantage of the new measurements requires a complementary effort in data assimilation and weather forecast modeling. Decision support tools also are being developed to integrate the new information provided by the observations and models into flood forecasts and water management decisions. This talk will review the current status of the observation installation and discuss how these observations will improve rainfall and runoff forecasts for improved reservoir operations.
Wireless-Sensor Technology for Basic-Scale Hydrology
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A basin-scale observatory in being developed in the main seasonally snow-covered portion of the American R. basin, which is over 2000 square kilometers in area, to make comprehensive water-balance measurements in support of research by multiple investigators. This basin-scale "instrument cluster" will consist of over 20 local sensor groups, following the design for a local sensor group, or "headwatercatchment- scale observatory" developed at the Southern Sierra Critical Zone Observatory (CZO).
Each local instrument group will have 10-20 "sensor nodes" consisting of a snow-depth and temperature sensor, with solar radiation, soil moisture, soil temperature and sap flow measured at a subset of the nodes and local groups. All local groups will have a meteorological station, including precipitation measurements. Individual sensors within local sensor group will be deployed to measure the quantities of interest across variable aspect, slope and vegetation cover. Local sensor groups will be arrayed within the basin to measure attributes across the same variables, plus account for elevation and soil differences.
The inter- and intra-group communications will be provided by a sophisticated wireless sensor network. Locations for the local sensor groups for measurement of snow depth and water equivalent was evaluated using rank-based clustering, which proved superior to random placement or geographic clustering. Rank-based clusters remained stable inter-annually, suggesting that rankings of pixel-by-pixel snow water equivalent exhibit stationary features that can be exploited by a sensor-placement algorithm. Locations of wireless-sensor nodes within each local sensor group follows a three-phase design procedure to overlay a wireless network onto strategically placed sensor nodes. An iterative procedure is necessary, with specific metrics for network performance, as network performance in vegetated, complex terrain can be much different from that in flat, open areas. Data and analysis from these catchment-scale and basinscale observatories provides unprecedented, detailed accounting of water fluxes and storage, and insight into catchment-scale and basin-scale processes.
Airborne Snow Observatory and Distributed Hydrologic Modeling: Next Generation of Snowmelt Runoff and Operations Forecasting
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Snow cover and its melt dominate regional climate and water resources in many of the world's mountainous regions. However, we face significant water resource challenges due to the intersection of increasing demand from population growth and changes in runoff total and timing due to climate change. Moreover, increasing temperatures in desert systems will increase dust loading to mountain snow cover, thus reducing the snow cover albedo and accelerating snowmelt runoff.
The two most critical properties for understanding snowmelt runoff and timing are the spatial and temporal distributions of snow water equivalent (SWE) and snow albedo. Despite their importance in controlling volume and timing of runoff, snowpack albedo and SWE are still poorly quantified in the US and not at all in most of the globe, leaving runoff models poorly constrained.
Recognizing this need, JPL developed the Airborne Snow Observatory (ASO), an imaging spectrometer and imaging LiDAR system, to quantify snow water equivalent and snow albedo, provide unprecedented knowledge of snow properties, and provide complete, robust inputs to snowmelt runoff models, water management models, and systems of the future. The ASO is being evaluated during a multi-year Demonstration Mission of weekly acquisitions in each of the Uncompahgre River Basin (Upper Colorado) and the Tuolumne River Basin (Sierra Nevada) beginning in spring 2013.
The ASO data will be used to constrain spatially distributed models of varying complexities and integrated into the operations of the O'Shaughnessy Dam on the Hetch Hetchy reservoir on the Tuolumne River. Here we present the first results from the ASO Demonstration Mission.