Using Net Groundwater Extractions for Farm Level Groundwater Sustainability Monitoring

The Cal Poly Irrigation Training and Research Center (ITRC) has developed a method for computing net groundwater extraction and recharge at the farm level for district management and regulation of sustainable/safe yields. This method is called Net To/From Groundwater (NTFGW). Net groundwater extraction is preferred for assessing sustainable yield in unconfined aquifer systems over direct metering of gross groundwater pumping. A recent pilot project with the Lower Tule River and Pixley Irrigation Districts’ Groundwater Sustainability Agencies (GSAs) compared actual metered groundwater pumping, surface deliveries, and evapotranspiration to the NTFGW outputs on 19 farms within the GSAs over a 3-year period (2014-2016). In nearly all cases gross metered pumping was greater than net groundwater use, as it should be. In the few instances where this was not the case, intensive investigations identified the issues, which will be presented. The average difference between gross and net groundwater extractions was approximately 14”. The variation of this difference was substantial between farms, indicating the difficulty in using gross pumping from flow/volume metering of actual pumping for sustainability. The NTFGW can incorporate seepage and recharge basin operation on a GSA level. It is also capable of tracking banked groundwater supplies on a farm level