Scott Valley
Groundwater: Dr. Thomas Harter, a professor and water expert from U.C. Davis, recently
gave a presentation on the status of phase one of a groundwater study that is being
implemented in the Scott Valley. The study was requested by the North Coast Regional Water
Quality Control Board as part of the Scott River Total Maximum Daily Load action plan for
temperature. The TMDL had determined that conditions in the Scott River do not meet the
requirements of cold water salmon. The study’s intent is to explore the
inter-relationship of groundwater to Scott River temperature and flows, and to riparian
shade vegetation along the river. Phase one of the study has incorporated basic data into
a preliminary computer model. It will be used as a tool to understand how groundwater
works in Scott Valley.
In 2006, before the TMDL went into effect, 35 groundwater users
throughout the valley volunteered to become part of a long term RCD (Resource Conservation
District) study of static well water levels. This well study tracks how water levels
respond seasonally and annually to changes. The wells used in the study are domestic,
abandoned and ag wells that are only periodically used so that the pumping does not
immediately impact the water level. Most are measured on a monthly basis. This study has
provided additional data to that previously gathered by Seymour Mack in the 1940s and 50s.
The CA Department of Water Resources has also
measured five of the wells semi-annually for many decades. In addition, graduate student
Ryan Hines measured temperature differences in the water column in a 2,000 ft. section of
the river where groundwater was accreting (coming into the river.) All of this data was
inventoried.
Harter stated that the data indicates that there is not an emptying
out of the aquifer occurring in Scott Valley. It is not about the groundwater going dry,
but about timing. The computer modeling can be used as a tool to help people understand
what we know and to support problem solving.
In explaining the aquifer, Harter described a bathtub partially
filled with sand, gravel and soil. If you slope the sides into the center and then fill
the tub with water until it comes to the surface, it would emerge in the center part. This
would be representative of the river. He described how water moves through different kinds
of soils, stating: “Envision a small flower pot with your thumb covering the hole in
the bottom. Fill the pot with sand and then water. When you remove your thumb the water
will come out quickly. Imagine the pot filled with potting soil. The water would drip out
the bottom.” This is called the “hydro-connectivity” of the sediment.
Harter also talked about how gradient or slope will also affect how fast the water will
travel.
During the winter, the Scott River is a “gaining stream.”
That means that groundwater feeds into or “accretes” into the river as it makes
it way down to join the Klamath. However, in the summer in several places along the river,
it loses water back to the groundwater because the aquifer water level is lower than that
of the stream. The model will help identify those dynamics. Possible solutions such as
flooding an area in winter to elevate groundwater levels in summer could be tried.
The conceptual model is able to reflect the great variance in soil
deposits throughout the valley. Hines researched well records and was able to identify
about 550 well drilling logs. Prior studies had indicated that in one area of the valley,
sediments went down to about 450 feet before hitting bedrock. The model is comprised of
blocks representing 150 feet square overlying the geography of the valley and
characterized by their soil type. It illustrates in what direction water will flow and how
fast it will flow as it moves from block to block. The scientists can approximate what
will happen when precipitation is added in an area, when irrigation moves water from
surface sources back to groundwater and when vegetation removes water through
evapotranspiration (ET.)
The model also includes data on the adjudicated water use rights in
the system and the crop cover type in cultivated areas. It does not currently illustrate
the uplands or Quartz Valley as there is no well data for these areas. Domestic wells are
not plotted because they constitute less than 2% of the water use in the valley.
Graduate student Tyler Hatch will be refining the model. Harter said
he would welcome working with a Board of Supervisor’s Groundwater Committee on
improving underlying assumptions in the model and using it as a tool to understand more
about groundwater in the Scott Valley. |
