Demonstration of simulating agricultural water demands using the MODFLOW-OWHM2 Farm Process with an example from Paso Robles Basin

The Paso Robles groundwater basin (PR) encompasses approximately 500,000 acres and is located in the upper portion of the Salinas River watershed in the San Luis Obispo (SLO) and Monterey counties, Central Coast region of California (.\ancillary\Figures\Figure 1. Site Map.tif from this data release; California Department of Water Resources, 2003). Agriculture makes up a large part of the regional economy and agricultural water demand is the single largest component of total water demands in San Luis Obispo County (SLO; Fugro West, Inc and others, 2005). As such, future water supply sustainability is an important water management objective. To quantitatively evaluate long-term trends in groundwater levels, inflow, and outflow to and from the basin, and to implement as a groundwater resource management tool, a hydrologic model, called the Paso Robles Basin Model (PRBM), was developed and updated by several consulting firms (GEOSCIENCE Support Services, Inc and others, 2014; GEOSCIENCE Support Services, Inc, 2016; Montgomery & Associates, 2018). The PRBM was developed with MODFLOW-2005 (Harbaugh, 2005) software and incorporates external spreadsheet-based methods for estimation of agricultural water demands, associated groundwater pumpage, groundwater recharge, and riparian evapotranspiration. The PRBM can be downloaded from the County of San Luis Obispo Groundwater Sustainability website (GEOSCIENCE Support Services, Inc and others, 2014; GEOSCIENCE Support Services, Inc, 2016; Montgomery & Associates, 2018). This data release documents the conversion of a single year of the Paso Robles Basin Model (PRBM) to the MODFLOW-One Water Hydrologic Flow Model (MF-OWHM2) software (Boyce and others, 2020) and illustrates how to set up the MODFLOW Farm Process (FMP; Boyce and others, 2020) using the input file structures from MF-OWHM2. This update to the PRBM, referred to as the Paso Robles Demonstration (PRDS), improves the ability to maintain, modify, and extend the model for future work. The updated PRDS model input file structures will provide future studies with the flexibility to easily incorporate new model input data. This data release shows how climate data, watershed inflows, groundwater extraction and injection wells, and agricultural wells can all be easily updated. This ability to update the model will keep the model relevant to decision-making and contribute to improved understanding of the source, movement, and use of surface water and groundwater.