Evaluating the Groundwater Data Record for California's Central Valley

The U.S. Bureau of Reclamation (USBR) has been involved in the collection of groundwater data in California's Central Valley for over 40 years. These data have been maintained in a relational database with an interface to ArcInfo and ArcView for analysis and display. USBR is using ArcInfo and ArcView to assess changes in groundwater elevations for individual basins in the Central Valley utilizing historic groundwater level data. This paper discusses the procedures used to develop time sequences of snapshots for a sample basin from 1960 to 2000. Requirements for maintenance of these data are also presented.

Early in the development of water conveyance systems in California, the Bureau of Reclamation (USBR) and the California Department of Water Resources (DWR) recognized the need to assess impacts on groundwater as a result of providing surface water via the Central Valley Project (CVP), the State Water Project (SWP) and other related projects. Through the cooperative efforts of USBR, DWR, irrigation districts, farmers, and other local entities, groundwater level data have been collected continuously since project conception. In some localities, groundwater records go back for more than sixty years. With increasing demands for water supply, the ability to quantify the groundwater resource accurately is imperative. State and Federal programs (CVPIA and CALFED) are relying on groundwater components to meet water demands. These components include water transfers, water conservation, and conjunctive use. Groundwater accounts for approximately 20 percent, or 15,000,000 acre-feet of applied water in California.

Similar to surface water flow measurements, groundwater level measurements provide a snapshot in time showing an aquifer's response to a changing environment, including precipitation, irrigation practices, and pumping. Groundwater measurements can indicate trouble ahead, such as groundwater overdraft, ground-surface subsidence, drainage problems, or poor water quality. Groundwater level measurements can be used to construct maps -- groundwater surface elevation, depth to groundwater, and groundwater level change between two time periods.

The Central Valley groundwater monitoring program was developed to measure the impacts of the CVP and SWP surface water deliveries on groundwater levels. This voluntary program utilizes existing irrigation wells and a few domestic wells scattered throughout the valley. The wells in this program are generally constructed in the shallow aquifers above 300 feet in the Sacramento Valley and in the upper unconfined aquifer in the San Joaquin Valley. The data are collected semi-annually, with a few wells measured monthly. Measurements generally occur in the fall (September, October, November) after irrigation season pumping has ended with the groundwater elevations at the lowest level and again in the spring (February, March, April) when the groundwater elevations should be at the highest level. Complete groundwater data coverage for the San Joaquin Valley, collected by USBR, DWR, and cooperators, provide basin-wide groundwater contour maps compiled and distributed by DWR.

The Data Management Team within the Environmental Affairs Division manages the Mid-Pacific Region groundwater information system. Activities include coordinating the data collection effort, entering data into a relational database, reviewing data for accuracy, preparing district groundwater level status reports, and providing basic data to cooperators. As field sheets and logbooks are received from the field, data are entered into a relational database and then immediately retrieved from the system. The data are then checked against original forms. Any inconsistencies are corrected. Standard reports with current and previous measurements are generated and sent out following each data collection period. Reports go to the participating irrigation districts, DWR, and to USBR Staff. Requests for historic well measurements are retrieved and provided to requestor(s). Data are used as input into various analysis programs to create well hydrographs, area contour maps, and other analysis. All of USBR's groundwater level data in the USBR's computer are considered public, as well as all of the data collected and compiled by DWR. Shared USBR and DWR data are currently available on the Internet.

The primary objective of this paper is to demonstrate the ability to quantify the groundwater resource from the public record utilizing ArcInfo tools. Additional purposes include a description of the process to get the well measurements into the public system, and finally to describe some of the weaknesses of the existing data.

The Madera Basin (figure 1 and figure 2), within the San Joaquin hydrologic basin, was selected for this demonstration. Selection was based on the area possessing a relatively homogeneous aquifer (unconfined), a good spatial distribution of groundwater wells, and a continuous 40 year data record for these wells. The geology of the Madera basin consists of lands overlying the younger alluvium (definition: alluvium - geologic term describing beds of sand, gravel, silt and clay deposited by flowing water; younger alluvium - sand, gravel, silt, and clay deposits of recent geologic age). The basin is bounded on the south by the San Joaquin River, on the west by the eastern boundary of the Delta-Mendota basin, on the north by the southern boundary of the Chowchilla basin, and on the east by the Sierra Nevada foothills. A 1975 survey of basin overdraft indicated that the annual overdraft was 100,000 acre feet, based upon the DWR Mid-Valley Canal Areal Study.

The selected basin was further reduced in area based on the well records. The Madera basin contains approximately 370,000 acres as described by DWR. Figure 3 depicts the spatial distribution of the 226 wells used for the demonstration and contains around 300,000 acres. Most of the wells are private farm irrigation wells with depths ranging from 50 to 500 feet. The data record contains well depths for about half of the wells. Additional information, not available in the record at this time, is screening intervals and well logs. Well depths, logs, and screening intervals are necessary components for a more detailed "micro" analysis.

Groundwater level measurements currently reside in two relational databases. The USBR system has data from 1970 through 2000 while DWR has records from "the beginning" through 1998. Four jurisdictions were involved within the selected basin. These are the Madera Irrigation District, Chowchilla Irrigation District, USBR, and DWR. Data records were retrieved from the two sources and reviewed for consistency. For this demonstration project, very little screening, filtering, or editing of data was done. An effort was made to ensure that only wells with a relatively complete data record from 1960 through 2000 would be included. Individual data files for years 1960 through 2000 were created (41 files). The data record consisted of the state well identification number, water surface elevation (feet), depth to water from ground surface, and sampling year. For this demonstration, only the spring measurements have been used.

A separate file was created to produce the groundwater well location coverage. That file contained the state well number and UTM coordinates for both easterly and northerly directions (meters, zone 10). The coordinate file was used to create a well location map. Using the state well number as a check, all 226 wells were examined for positional accuracy. All of the 226 wells fell within the proper township, range and section. Review was not made to see if the well was within the 40-acre tract. Definition of the state well numbering system follows:

Wells monitored by the USBR and DWR along with cooperating agencies are identified according to the state well numbering system. The numbering system is based on the public land grid, and includes the township, range, and section in which the well is located. Each section is further subdivided into sixteen 40acre tracts, which are assigned a letter designation as shown in the figure below. Within each 40acre tract, wells are numbered sequentially. The final letter of the State Well Number refers to the base line and meridian of the public land grid in which the well is located. "M" refers to the Mount Diablo baseline and meridian; "S" refers to the San Bernardino baseline and meridian; "H" refers to the Humboldt baseline and meridian. An example follows: State Well Number: 10S/17E30B02M designates the following:

This number identifies and locates the well. In the example, the well is located in Township 10 South, Range 17 East, Section 30 Track K, in the Mount Diablo baseline and Meridian. A section is divided into 40-acre tracts as follows:

Sequence numbers in a tract are generally assigned in chronological order. In the example, the '02' designates the second well to be assigned a number in Tract B.

Traditional non-spatial analysis was performed on the existing data set. Well hydrographs were prepared for each well from 1960 through 2000. The hydrographs were used to further examine the quality of the data as well as begin to see existing patterns in the basin. Three sample well hydrographs are shown in figure 4, figure 5, and figure 6 providing an example of the basin. Another hydrograph, representing the basin average from the 226 wells, is shown in figure 7. An estimate of the change in groundwater elevations and storage volumes within the demonstration area was made for the time period 1960 through 2000. The computations, based on the 226 wells within the demonstration area, indicate a decrease of 1.2 million acre feet of groundwater storage (table 1). A specific yield value of 13 percent was used to represent the entire demonstration area.

Spatial analysis using ArcInfo modules produced the following products/results:

As discussed previously, a file containing well identification and UTM coordinates was used to produce the well location coverage.

A groundwater surface elevation contour coverage was made for each (41) year. Separate data files including well identification, groundwater surface elevation, depth to water, and year provided the information. Contouring was created using ArcInfo's TIN procedure. The TIN's were then converted to grids for presentation. Groundwater contour maps for 1960 (figure 8), 1970 (figure 9), 1980 (figure10), 1990 (figure 11), and 2000 (figure 12) using 10-foot intervals are provided.

The 41 contour grids were used to create an animation file (figure 13). The visualization procedure provided a clear picture of where the average 30-foot groundwater decrease took place over the 41 years. The animation file also provides a quick way of presenting all 41 years of contouring data.

Additional work will be done using ArcInfo GRID procedures. This includes the algebraic computations describing storage volume changes (table 2). Groundwater volume changes using GRID will be compared with the results described in table 1. The purpose of using GRID to prepare the computations is that better spatial values of specific yield can be used over the demonstration area instead of the constant of 13 percent that was used to create results in table 1. Specific yield values might range from 10 to 20 percent.

ArcInfo can be used effectively to quantify the groundwater resource in California's Central Valley. However, resource managers need to be assured that the data does indeed represent the basin and or aquifer correctly. In order for that to happen, the following steps need to be done:

Analyzing the data record "as is" can provide groundwater basin trends. For more in-depth scientific studies, the above steps need to be done. Individual water agencies/consultancies are doing that now localized requirements. That is, they are taking the basic public record, performing the quality control and validations, and performing additional rigorous analyses.

A three-day workshop was held on April 5-7, 1995 in Sacramento, CA with over 100 stakeholders representing all Regions of California. Purpose of this "California Assembly" process was to describe and document the issues relative to groundwater management in California. The following statement summarizes significant findings.

Groundwater has received relatively little attention from water managers in the past for several reasons. The law of the western states generally places a high value on landowners' rights to use underground supplies. The quantity of groundwater in California is so enormous, that in the past, it has been thought of largely as a free resource, like air. Incomplete and questionable data has hampered understanding of the hydrology of many basins. Common threads across the state include the following: 1. Data necessary for effective groundwater management are incomplete and of questionable validity, 2. Conjunctive use is central to more efficient management of water, and 3. Basin management programs are increasing, especially in regions where access to imported water has been decreasing. A comprehensive local management approach is preferred. Elements would include the following: 1. Alignment of groundwater management plans with basin boundaries, 2. Integration of groundwater management plans with water transfer planning, 3. Collection of reliable data, 4. Leadership training and public education, and 5. Watershed of origin and area of origin protection concepts need to be reviewed and strengthened.

Many obstacles impede progress on California's groundwater management programs. The obstacles include fear of change, loss of local agency control, property rights, water supply, income, and autonomy, lack of political will, limited public awareness of the problem, inadequate planning, information, lack of incentives, lack of adequate data and understanding of basin-specific groundwater and surface water interactions, lack of coordinated efforts between land use planners and water suppliers. Local water agencies, state, federal and other cooperating agencies should form partnerships to gather, synthesize, model, and share groundwater data, analysis, and results.

The author wishes to thank Barbara Simpson, Mike Sebhat, Michelle Prowse, and Dave Hansen for participating in this joint endeavor. The continued technical support of the Mid-Pacific GIS Service Center is invaluable in the sometimes long and tedious process of transferring GIS technology to the user community.

The use of trade, firm, or corporation names is for the information and convenience of the reader. Such use does not constitute an official evaluation, conclusion, recommendation, endorsement, or approval of any product or service to the exclusion of others which may be suitable.

California Department of Water Resources (DWR). 1995. Lines of Equal Elevation of Water in Wells, San Joaquin Valley, 1989 and 1993. State of California. Fresno CA.

California Department of Water Resources (DWR). 1975. California's Ground Water Bulletin 118. State of California. Sacramento, CA.

California Department of Water Resources (DWR). 1980. Ground Water Basins in California, A Report to the Legislature in Response to Water Code Section 12924, Bulletin 118-80. State of California. Sacramento, CA.

Public Officials for Water and Environmental Reform (POWER). 1995. Groundwater Assembly 95. Assembly Statement. April 5-7, 1995. Sacramento, CA.

U.S. Bureau of Reclamation (USBR). 1993. Forty Second Annual Water Supply Report, 1992. Mid-Pacific Region. Fresno, CA.

U.S. Geological Service (USGS). 1998. Environmental Setting of the San Joaquin-Tulare Basins, California. Water Resources Investigations Report 97-42025. Sacramento, CA.