Amber Kuss, San Francisco State University;
William T. Brandt, California State University, Monterey Bay;
Joshua Randall, Arizona State University;
Bridget Floyd, University of California, Berkeley;
Abdelwahab Bourai, Cupertino High School;
Michelle Newcomer, San Francisco State University;
Cindy Schmidt, NASA Ames Research Center;
J.W. Skiles Ph.D., NASA Ames Research Center
Abstract — The role of the NASA Applied Sciences DEVELOP student internship program is to use NASA satellite missions to explore Earth-based research questions in collaboration with state and federal agencies. One recent project focused on the use of the Gravity Recovery and Climate Experiment (GRACE), to estimate changes in groundwater storage in the Central Valley aquifer in collaboration with the California Department of Water Resources (DWR). GRACE can be used to measure gravitational anomalies on Earth which are further processed to remove other gravitational effects, and are obtained as measurements of equivalent water thickness, which relate to changes in total water storage (TWS) throughout the world. For this study, GRACE TWS anomalies were obtained from October 2002 to September 2009, for two hydrologic regions, the Sacramento River Basin and the San Joaquin River Basin, including the Tulare Lake Basin, encompassing the Central Valley aquifer. To calculate monthly groundwater storage estimates, additional variables such as soil moisture, snowpack, and surface water storage were combined with GRACE TWS values to estimate groundwater storage anomalies. The GRACE-derived changes in groundwater storage at the basin and regional level or the two basins combined, were then compared to modeled values calculated using the California Department of Water Resources’ Central Valley Groundwater-Surface Water Simulation Model (C2VSIM) and a Geographical Information Systems Change in Storage Tool (GIS CST). Groundwater storage estimates from GRACE and C2VSIM were comparable for the entire Central Valley, showing similar seasonal, annual, and long-term trends. This work has the potential to improve California’s groundwater resource management and the use of existing hydrologic models for the Central Valley.
I. INTRODUCTION TO NASA DEVELOP
The NASA Applied Sciences’ DEVELOP Program sponsors paid internships located at Ames Research Center for students to extend science research to local communities. DEVELOP is a NASA Science Mission Directorate Applied Sciences Program training and development internship. Students work on Earth science research projects, mentored by science advisors from NASA and partner agencies to extend research results to local communities. The NASA Ames DEVELOP program hosts graduate, undergraduate and high school students in a ten week summer internship. Students are involved in a wide range of projects such as vegetation mapping, air quality assessments, wetland restoration, and water availability. The DEVELOP program is student run and student lead, and is supported by science advisors and mentors.
II. CALIFORNIA’S GROUNDWATER
One pressing issue within California is the amount of available groundwater to support a growing population, agriculture and industry . Groundwater, one of the most important resources on Earth, is difficult to understand and manage. The majority of groundwater in California is found in the Central Valley aquifer system (Figure 1). This aquifer supports agriculture that supplies nearly 7 percent of the United States’ food supply, with an estimated annual value of $21 billion . California does not regulate groundwater pumping at the state level. Groundwater management is implemented at the local level and includes groundwater monitoring, basin management and water-use restrictions. Understanding groundwater availability is complicated and may benefit from using different methods. The California Department of Water Resources (DWR) has developed tools to assess groundwater storage changes within the Central Valley with the use of the Central Valley Groundwater-Surface Water Simulation Model (C2VSIM) and a Water Data Library (WDL) Geographic Information System (GIS) change in groundwater storage tool (CGST) .
The Central Valley aquifer is a basin, which is about 80 kilometers wide and 650 kilometers long, and is bounded by the coast ranges on the east and the Sierra Nevada on the west (Figure 1). The Central Valley aquifer is contained within three hydrologic regions ÛÒThe Sacramento River Basin, the San Joaquin River Basin, and the Tulare Lake Basin (Figure 1). For this study, the San Joaquin River Basin and the Tulare Lake Basin were combined and will be referred to as the San Joaquin River Basin. The C2VSIM model also incorporates inflow from watersheds within the Sierra Nevada, and although the study areas from C2VSIM and GRACE data are significantly different, the Sierra Nevada is not conducive to groundwater storage, and it is assumed that the majority of changes within the GRACE defined study area are occurring within the Central Valley aquifer .
University of Colorado’s GRACE Data Analysis website . The Sacramento and San Joaquin River Basins were defined using a model  (Figure 3).The GRACE satellite can detect anomalies and deviations from normal values in the Earth’s gravity field . Changes in gravity over short time periods on Earth are often attributed to changes in water, ice and snow. Data were obtained from GRACE and processed to represent changes in total water storage (TWS). This included the removal of other factors such as atmospheric effects, postglacial rebound and large-scale topographic processes, with units expressed as equivalent water thickness   . The GRACE sensor operates with two identical satellites that fly in the same orbit (Figure 2). These satellites move further or closer together based on changes in the Earth’s gravitational field. These measurements can then be used with other satellites and ground measurements to calculate how much of that change may be attributed to groundwater. Previous studies from all over the world demonstrate the use of this satellite and how to calculate changes in groundwater storage      . For this study, GRACE data were collected from the
Other sensors and field measurements can be used to detect surface water storage changes in California, and can aid in calculating changes in groundwater storage that may be due to pumping, drought or natural climate variability. The most important variables to use when converting from changes in TWS to changes in groundwater storage are changes in lakes or reservoirs, soil moisture and snowpack (Figure 4). For this study, changes in reservoir storage were obtained from the California Data Exchange Center (CDEC) , soil moisture can be estimated from AMSR-E) , and snowpack was obtained from data collected from various sources including satellite data by the National Oceanic and Atmospheric Administration (NOAA) . The monthly anomalies for soil moisture (SMë±), surface water (SWë±) and snow pack (SPë±) were then subtracted from monthly TWSë±,GRACE values to calculate monthly GWë± for the Central Valley, the Sacramento River Basin and the San Joaquin River Basin.
GWë± = TWSë±,GRACE – ( SWë± + SMë± + SPë± )
GWë± = groundwater storage anomaly
TWSë± = total water storage anomaly
SWë± = surface water storage anomaly
SMë± = soil moisture storage anomaly
SPë± = snowpack storage anomaly
Changes in storage were then calculated for each of the anomalies TWS, SW, SM, SP, and GW over the length of the study. Anomalies were plotted and a trend line was fitted. The slopes of the graphs were then converted into a total volume of water lost or gained over the course of the study.
Changes in groundwater storage in the Central Valley were also calculated using two DWR tools, the C2VSIM model and groundwater level measurements that are incorporated into a GIS CGST. The C2VSIM also calculates changes in groundwater storage using a variety of inputs including precipitation, river discharge and aquifer characteristics. The C2VSIM then calculates the change in groundwater storage for each groundwater basin. Also, the GIS CGST uses groundwater elevation measurements taken each spring to estimate changes in groundwater storage. This tool was used in the Sacramento River Basin only. The groundwater storage estimates from each method were compared.
The processing of GRACE data can also have significant impacts on the groundwater storage results  . For this study, a Gaussian smoother of 300 kilometers was used to reduce noise to an acceptable level without significantly altering the signal. GRACE also has two types of error, a measurement error and a leakage error, and for this study, the combined measurement and leakage error was found to be 1.01 kilometers3, 1.00 kilometer3, and 1.00 kilometer3 for the Sacramento River Basin, and the San Joaquin River Basin, and the Central Valley, respectively. It should be noted that this was a best-estimate approximation of the errors, and recent studies suggest the errors for smaller regions are correlated and may be larger than currently reported errors . We also used an error of 15 percent to estimate the error involved in the hydrologic variables of SM, SP, and SW, and for the C2VSIM and the GIS CST.
GRACE and C2VSIM groundwater storage anomalies exhibited similar trends for the Central Valley region from October 2002 through September 2009 (Figure 5) . This finding is important as it validates the usefulness of GRACE at scales that are less than or equal to 150,000 kilometers2. It should be noted that GRACE and C2VSIM groundwater anomalies display marked differences during the seasonal peaks and troughs. The GRACE data appears to be more variable than C2VSIM, although the trends for the entire time period are similar. Also, climate variability is observed, and the drought period beginning in 2007 has a distinct negative trend in available groundwater.
Water storage anomalies produced from GRACE were also examined to understand climatic trends and long term differences in each basin. The San Joaquin River Basin exhibited the largest loss in TWS (-21.92 å± 1.92 kilometers3), snowpack (-3.90 å± 0.59 kilometers3) and groundwater storage anomalies (-16.43 å± 2.04 kilometers3) using the GRACE method. In contrast, the Sacramento River Basin displayed the largest loss in surface water storage (-4.22 å± 0.63 kilometers3) compared to the San Joaquin (-2.40 å± 0.36 kilometers3) . Soil moisture storage values, however, remained relatively unchanged throughout the study period in both regions. The results presented here are consistent with the results presented by  and .
The change in groundwater storage estimates from GRACE for the Sacramento River Basin and the San Joaquin River Basin were not comparable with those from C2VSIM, thus illustrating the usefulness of GRACE on very large scales rather than smaller basins . The GIS CGST estimated a loss of groundwater storage for the Sacramento River Basin of -0.67 å± 0.1 kilometers3 while C2VSIM and GRACE showed losses of -7.70 å± 1.49 kilometers3 and -2.55 å± 0.38 kilometers3, respectively . The apparent differences in the results by the two methods will need to be investigated in the future.
Groundwater storage loss was calculated for the entire Central Valley, the Sacramento River Basin and the San Joaquin River Basin which includes the Tulare Lake Basin using GRACE and C2VSIM. The calculations of changes in groundwater storage for each river basin using both the GRACE-derived and the C2VSIM estimates did not produce comparable results. Although there are large differences on the hydrologic region level, both tools were similar for the entire Central Valley. For the study period, GRACE calculated a total loss of groundwater storage of -14.47 å± 1.49 kilometers3 or -11.7 å± 1.2 million acre-ft, and C2VSIM calculated a total loss of -15.01 å± 2.25 kilometers3 or -12.2 å± 1.8 million acre-ft, for the Central Valley (Figure 6) . For a detailed analysis of results refer to Kuss et al., 2011 . The large losses in groundwater storage in the Central Valley during the study period most likely resulted from a period of extended drought from 2007 to 2009.
V. CONCLUSIONS AND IMPLICATIONS FOR GROUNDWATER MANAGEMENT
The usefulness of the GRACE satellite to monitor changes in groundwater storage for the Central Valley aquifer was examined through a comparison approach. Satellites have been shown to be useful to detect changes in groundwater storage over large areas, and could be an important tool for groundwater management on large scales. The GRACE satellite and other sensors can supplement current groundwater management techniques for the Central Valley aquifer. This study also supports DWR’s modeling efforts for the Central Valley, and illustrates the usefulness of multiple techniques. Current spatial downscaling limitations of GRACE may result in decreased usefulness for smaller scale basin management. Estimated changes in groundwater storage by the C2VSIM and GRACE are comparable, while the estimated changes in groundwater storage for the Sacramento River Basin by the GIS CGST are significantly less than the estimates by the other two methods. Understanding how groundwater storage is changing in the Central Valley is an important aspect of sustainable groundwater resource management in California. This understanding will enhance the ability to make informed decisions on proper management techniques such as curbing water use, implementing artificial recharge and maximizing conservation and water-use efficiency efforts that will aid in sustainable groundwater practices.
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