We were talking last month about the California Sustainable Groundwater Management Act (SGMA), which was passed in 2014. As you may recall, this has required Napa County to take a close look at the way we use groundwater. Last summer the County began an intensive process to develop a Groundwater Sustainability Plan (GSP) for the Napa Valley subbasin of the Napa River, scheduled for completion in November 2021, exactly one year from now. The process includes a public advisory committee; the Friends of the Napa River are represented on that committee.
I mentioned the distinction between gaining and losing streams. These terms describe the relationship between flow in a surface channel like the Napa River and the underlying water table. If the two are in hydraulic contact - if there is no intervening geological barrier - then either the channel is fed by groundwater or the reverse happens, depending on whether the water table slopes toward the stream (for a gaining stream) or away from it (for a losing stream).
The water table near a flowing stream is typically adjacent to it. But sometimes the two are disconnected, as in the first diagram below. In this picture, the ground all around the streambed is not saturated with groundwater, so you can see that water is going to soak into it at some rate.
More commonly, the relationship is more direct than that, and changes in the water table can clearly affect flow in the stream. The next diagram illustrates the depletion of surface flow by groundwater pumping. This stream might be a losing stream anyway, but the effect is intensified by pumping. The wells shown in the diagram each create what is called a cone of depression, as the surface of the water table slopes more steeply downhill in the vicinity of the wells. Depending on various factors, including how close the hydraulic connection between stream and well is, this situation could lead to undesirable depletions of surface water, one of the possible undesirable results the GSP aims to prevent.
Napa County's GSP will include an expanded network of monitoring wells, and there will be new modeling and analytic tools as well. It will be important to monitor surface water flow as well, to fine-tune our understanding of the interface between surface and ground water. And there is also the issue of groundwater-dependent ecosystems, which must also be considered in the sustainability plan.
There's a lot of work to do here. The Friends appreciate the efforts of County staff and their consultants, and of the many citizens contributing to the work of the public advisory committee. Thanks also to the Nature Conservancy, for the use of these excellent visuals.
Bob Zlomke