San Joaquin Valley Water Supply Curtailment Impact Analysis

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Owner: State and Federal Contractors Water Agency
Location:
 San Joaquin Valley, California
Funding Source: State and Federal Contractors Water Agency
Completion Date: 2011

The State and Federal Water Contractors Agency (SFCWA) was formed in 2009 as a joint powers authority by water agencies that rely upon surface water transported through the Sacramento-San Joaquin River Delta delivered under contracts with the State Water Project and the Central Valley Project. As part of its mission, the Agency supports the development of sound science to understand issues affecting its members and the Delta as a whole. Reductions in surface water supplies during 2009 and 2010 resulting from drought and regulatory constraints on Delta conveyance prompted SFCWA to seek objective quantification of drought impacts with respect to fallowing in the San Joaquin Valley.

Fallowed field near Corcoran, one of approximately 170 fields inspected to ground-truth interpretation of NDVI calculated from satellite data.

The Agency engaged Davids Engineering to quantify the effects of reduced surface water supplies on irrigated acreage in the San Joaquin Valley. Davids Engineering designed and executed a remote sensing analysis to identify lands fallowed in 2009 and 2010, respectively, years in which water supplies were relatively limited compared to a base year of 2005 when supplies were more abundant.  The analysis encompassed approximately 82,000 individual fields representing over five million acres. Remote sensing provides a means of objectively quantifying changes in the intensity of cropping over time due to changes in water supply or for other reasons. Remote sensing also provides the benefit of being more cost-effective than a land-based, visual data gathering process and the ability to evaluate impacts retrospectively using archived satellite imagery.

In irrigated areas, analysis of the intensity of cropping over the course of a growing season can be used to assess whether a field is irrigated or fallowed for the season as a whole.  For this investigation, Davids Engineering defined cropped areas as those agricultural areas with green vegetation present for at least three consecutive months during the primary growing season, from late April through early October.  Areas without green vegetation for at least three consecutive months were considered fallow. The determination of the presence of green vegetation for a given field was made based on a threshold value of the Normalized Difference Vegetation Index (NDVI), determined through field visits to approximately 170 strategically selected fields between Stockton and Bakersfield during the 2010 growing season.

Change in summer irrigation status from base year 2005 to 2009.

The investigation revealed that the areas experiencing the greatest proportional increases in fallowing were areas with historical reliance on imported surface water supplies and limited groundwater availability. Other areas with relatively reliable surface water supplies or available groundwater appeared to experience little or no reduction in irrigated area and in some cases a modest increase in irrigated area.

Grower response to changes in water supplies may include reductions in cropping intensity. For example, under a limited water supply, a grower may shift to crops that require less water but continue to irrigate the land. This study focused on identifying irrigated lands, regardless of crop intensity; however, the vegetation indices quantified could also provide insight into the magnitude of reduced cropping intensity, for example through crop shifting.