Developing Drought Curtailment Criteria for a Groundwater Basin Based on a Model of Deep Recharge

Water-supply agencies declare curtailments during droughts that consist of voluntary conservation goals, water-use restrictions, and/or mandatory cutbacks.  For agencies that rely on surface water, the criteria for declaring a curtailment are typically based on the projected ability of runoff and storage to meet that year's demand.  For agencies that primarily rely on groundwater, developing drought curtailment criteria is not straightforward because a period of lower precipitation does not immediately lead to a reduction in water supply.  Although a period of lower precipitation would lead to decreased deep recharge of aquifers, groundwater-supply agencies often maintain water production by pumping groundwater storage.

Groundwater-supply agencies may wish to avoid overpumping groundwater storage for a variety of reasons.  For example, the Soquel Creek Water District (SqCWD) in Santa Cruz County has a basin management objective of maintaining coastal groundwater levels high enough to prevent seawater intrusion.  Reducing pumping during droughts would help SqCWD meet this objective during periods of decreased deep recharge.  However, using coastal groundwater level data to declare drought curtailments is infeasible because the groundwater levels are far more sensitive to pumping variations than recharge variations.

In order to develop defensible drought curtailment criteria for SqCWD, the time series of historical deep recharge results from a calibrated Precipitation-Runoff Modeling System (PRMS) model of the Soquel-Aptos Basin was evaluated.  The PRMS model showed that only 36% of the years have greater than average deep recharge.  The frequency of single year deep recharge shortfalls led to the recommendation that cumulative shortfall amounts be used as drought curtailment criteria.  The shortfall criteria are based on the probabilities estimated from the PRMS model that the shortfall can be overcome the following year.

In addition, the PRMS model results provide a rainfall-recharge relationship that can be used by SqCWD to establish rainfall criteria for drought curtailments.