Abstract
Introduction
Early in the development of water conveyance systems in California,
USBR and DWR recognized the need to assess impacts on groundwater from
surface water deliveries. 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.
Importance of Groundwater Monitoring
Similar to information gleaned from continuous surface water flow measurements,
groundwater level measurements can provide equally important information
describing an aquifer's response to changes precipitation, irrigation practices,
and pumping. Groundwater monitoring can indicate trouble ahead, such
as groundwater overdraft, ground-surface subsidence, drainage problems,
or poor water quality. Groundwater level measurements are used to
construct individual well hydrographs, to develop area wide contour maps,
and to prepare groundwater level change reports.
Central Valley Monitoring Program
The Central Valley groundwater monitoring program was developed to
measure the impacts of the CVP and SWP surface water deliveries on groundwater
levels. This monitoring program utilizes existing irrigation wells (figure
1) 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.
Groundwater Information System
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 requesters). 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.
Objectives
The primary objective of the demonstration project is to quantify the
groundwater resource from the public record and to determine the adequacy
of the result. Description of the processing steps are included.
Additional purposes include a description of the process to get the well
measurements into the public system, and finally to describe some of the
challenges lying ahead to improve our groundwater information system.
Demonstration Area
Criteria for selecting the demonstration area was based on several
factors: relative homogeneous aquifer in the unconfined layer, good spatial
distribution of groundwater wells, and continuous 40 year data record.
The particular selected area was in the north central part of the San Joaquin
Valley consisting of the Madera, Chowchilla, and El Nido Irrigation Districts,
as well as the Chowchilla and Madera groundwater basins as depicted by
DWR (figure
2, figure
3). The geology of the area 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 area is bounded on the south and southwest by the San Joaquin River,
on the west by the eastern boundary of the Delta-Mendota basin, on the
north by the southern portion of the Merced Basin and the county line between
Merced and Madera, and on the east by the Sierra Nevada foothills.
Based on the 1975 DWR Mid-Valley Canal Areal Study, this area had an annual
overdraft of 100,000 acre feet.
Selection of Wells
The selected area contains approximately 528,132 acres as determined
from the boundary polygon (figure
4). Once the demonstration area was selected, all wells within
the boundaries were retrieved using both the USBR and DWR systems.
Over 500 wells were within the selected polygon. Following review
of the selected wells based on the previous mentioned criteria, only 332
wells made the final cut (figure
5). 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 would be necessary for additional
"micro" analysis.
Data Record
Groundwater level measurements currently reside in two relational databases.
The USBR system has data from 1970 through 2000 while DWR system contains
records from "the beginning" through 2000. Four jurisdictions were
involved within the selected area: El Nido Irrigation District, Chowchilla
Irrigation District, Madera Irrigation District, and USBR. 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 date
(figure
6). For this demonstration, only the spring measurements were
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 332 wells were examined for positional accuracy. All of the 332 wells fell within the proper township, range and section. Review was not made to see if the well was within the 40-acre tract. 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 40-acre tracts designated by am alpha character. Within each 40-acre 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.
Analysis
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.
Locations of sample well hydrographs within the study area are shown in
figure
7. Sample well hydrographs are shown in figure
8, figure
9, figure
10, figure
11, figure
12, and figure
13. These hydrographs provide an example of the basin.
Another hydrograph, representing the basin average from the 332 wells,
is shown in figure
14. 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 332 wells within the
demonstration area, indicate a decrease of 1.8 million acre feet of groundwater
storage (table
1). A specific yield value of 12 percent was used to represent the
entire demonstration area.
Groundwater surface elevation contour coverages
Groundwater surface elevation contours were made for 1960, 1970, 1980,
1990 and 2000. 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
15), 1970 (figure
16), 1980 (figure
17), 1990 (figure
18), and 2000 (figure
19) using 10-foot intervals are provided.
Grid analysis
Additional analysis using ArcInfo GRID procedures produced basin level
storage volume changes (table
2). Grid cell size was about 40 acres or 1/4 mile. The
purpose of using GRID to prepare the computations is that a variable distribution
of specific yield values can be used over the demonstration area instead
of a constant. Specific yield values could range from from 10 to
20 percent within a study area.
Conclusions
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:
1. Well locations need to be verified.
2. Historic well measurements need to
be reviewed
3. Some level of quality control needs
to be established and made part of the record.
4. Well depths need to be incorporated
into the record.
5. Well screening intervals need to
be determined and made part of the record.
6. Aquifer characteristics underlying
each well need to be determined and made part of the record.
Analyzing the data record "as is" can provide for broad brush groundwater basin trends. For more in-depth scientific studies, the above steps need to be done. Individual water agencies/consultancies are doing that now for localized requirements. That is, they are taking the basic public record, performing the quality control and validations, and performing additional rigorous analyses.
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 also hampered greater understanding of the hydrology within 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. Many previous impediments (lack of political will, limited public awareness, inadequate planning, lack of incentives, lack of adequate data, lack of understanding of groundwater and surface water interactions, and the lack of coordinated efforts between land use planners and water suppliers) toward understanding and managing our groundwater resource are slowly eroding away. Water managers need to know how to quantify the resource. Local water agencies, state, federal and other cooperating agencies should form partnerships to gather, synthesize, model, and share groundwater data, analysis, and results.
Acknowledgments
The author wishes to thank Barbara Simpson and Mike Sebhat for participating
in this joint endeavor. The continued technical support provided
by Mid-Pacific GIS Service Center is invaluable. Technology transfer
of GIS methodologies continue to be a high priority within the Region.
Disclaimer
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.
References
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.
U.S. Bureau of Reclamation (USBR). 1993. Forty Second Annual Water Supply Report, 1992. Mid-Pacific Region. Fresno, CA.
U.S. Geological Service (USGS). 1997. A Quality-Assurance plan for district
groundwater activities of the U.S. Geological Survey. U.S. Geological Survey,
Open-file Report 97-11. Reston, Va.
U.S. Geological Service (USGS). 1998. Environmental Setting of the
San Joaquin-Tulare Basins, California. Water Resources Investigations
Report
97-42025. Sacramento, CA.
Author Information
Robert W. Young, MP-150
Bureau of Reclamation
2800 Cottage Way
Sacramento, California 95825
916.978.5042
ryoung@mp.usbr.gov
This paper, as well as all of the graphics, can be viewed from the following
source:
http://www.mp.usbr.gov/mp400/geopage/Esri2001/byoung/p0447.htm
An updated version is also available and can be viewed from the following
source:
http://www.mp.usbr.gov/mp400/geopage/Esri2001/byoung/p0447u.htm
