A significant limitation of the "Water Evaluation and Planning" model

The collection and integration of a rich data stream during the CA 2009 Water Plan Update process relates directly to the degree of confidence the public can have in the results of models being used to quantify "uncertainty" in future water planning.

Groundwater users became very concerned upon learning that the Water Evaluation and Planning (WEAP) model "assumes an unlimited groundwater supply." [See notes]

In a surface water model, groundwater is "an externality" and therefore, unconstrained. But in the real world groundwater is not "an externality"; not to the natural system that produces 86% of California’s water supply, not to the commercial and recreational fishing industry dependant on the successful reproduction of native fish on year-round streams connected to healthy groundwater aquifers, not to the farmers with thousands of acres of high value crops in the north state, and certainly not to people who are directly dependent on private wells for their drinking water. All this is occurring in the context of CVP and SWP plans to extract of hundreds of thousands of acre feet from the Lower Tuscan Aquifer to facilitate surface water sales.

Historical evidence shows the groundwater in the Sacramento Valley has been dropping for decades. Streams that once flowed year round have become seasonal and long time residents have repeatedly deepened wells. Only the construction of Oroville and Shasta dams (allowing year-round irrigation) has kept our groundwater from being as badly over-exploited as has happened with devastating consequences in the San Joaquin.

Unlike the shallow alluvial aquifers of Kern County that recharge when water is spread out over sandy soils, our deep confined aquifer has physical constraints that limit recharge. We know that pumping in the Sacramento Valley exceeds recharge, but are we pumping only ten years of recharge annually, or are we like the unfortunate folks on the Washington-Oregon border who recently learned that every year they had been extracting 10,000 years of recharge?

Current theory is that the Lower Tuscan is recharged by the streams coming out of the foothills, possibly where the formations cross the Chico Monocline. Chico is located where Big Chico Creek enters the valley. Durham is just below where Butte Creek leaves the foothills. If "recharge follows the pumping," it would surely occur in those two locations. Yet, these communities have permanent and growing cones of depression which have recently merged together.

DWR estimates the Lower Tuscan Formation holds up to 30 maf., but there is no published estimate of the percentage of the water actually accessible before subsidence cuts connections and permanently removes storage capacity. There is no peer reviewed estimate of the recharge rate to be compared to pumping. How can any meaningful predictions be made involving future groundwater extractions?

This major uncertainty about supplies in the WEAP surface water model must be made explicit, certainly before using WEAP for planning, operations or legislative activities involving the state’s water projects. And the model should be expanded to include realistic estimates of aquifer performance, including recharge. Following the scenario system already used in the Water Plan, you might begin by developing at least three alternatives for the annual net loss to the aquifer and apply these both to current conditions and to the future scenarios being examined.


Slide 6 shows that as less surface water is available more groundwater will be pumped.

The need for explicit acknowledgement about the uncertainties of water supplies in models used by the state is found in this comment letter from 2004.