Integrated Hydrologic Model of Pajaro Valley, Santa Cruz and Monterey Counties, California

Increasing population, agricultural development (including shifts to more water-intensive crops), and climate variability are placing increasingly larger demands on available groundwater resources in the Pajaro Valley, one of the most productive agricultural regions in the world. This study provided a refined conceptual model, geohydrologic framework, and integrated hydrologic model of the Pajaro Valley. The goal of this study was to produce a model capable of being accurate at scales relevant to water management decisions that are being considered in the revision and updates to the Basin Management Plan (BMP).

The Pajaro Valley Hydrologic Model (PVHM) was designed to reproduce the most important natural and human components of the hydrologic system and related climatic factors, permitting an accurate assessment of groundwater conditions and processes that can inform the new BMP and help to improve planning for long-term sustainability of water resources. Model development included a revision of the conceptual model of the flow system, reevaluation of the previous model transformed into MODFLOW, implementation of the new geohydrologic model and conceptual model, and calibration of the transient hydrologic model.

The conceptual model identified inflows and outflows that include the movement and use of water from natural and human components. The groundwater flow system is characterized as a layered geologic sedimentary system through which downward flow is driven by the combined effects of the application of irrigation water at the land surface and the pumping of groundwater from deeper in the system.

Overall, groundwater meets most of the agricultural demand in the initial part of the growing season, augmented by precipitation during wet winter and spring seasons. In addition, the amount of groundwater used for irrigation varies from year to year in response to climate variation and can increase dramatically in dry years.

Data on agricultural pumpage is a major component of simulated outflow that was previously unavailable; therefore, a coupled farm-process model was used to estimate historical pumpage for Pajaro Valley by subregion (water-balance subregions) as well as the delivery of surface water to and from the Harkins Slough Aquifer-Storage-and-Recovery System (HS-ASR) and related Coastal Distribution System (CDS) since 2002.

The new, integrated hydrologic model includes new water-balance subregions; delineation of natural, municipal, and agricultural land use; streamflow networks; regions of tile drains; and, the groundwater flow system. The redefinition of the geohydrologic framework and incorporation into the simulation revealed the importance of the confining units at the base of the alluvial deposits and between the upper and lower Aromas Sand for regional groundwater flow.