The overall purpose of the project is to develop a methodology for watershed managers to successfully target and release recommendations to growers that could benefit from improved on-farm irrigation management. Initially it was thought that the methodology could rely on remote sensing of actual crop evapotranspiration, effective rainfall estimates, and water application information (pumped volumes and surface water deliveries) to simply evaluate if fields and farms that applied significantly more water than plants needed could be identified. However, it became clear that, in this region specifically, additional information on water quality, crop sensitivity to salinity, and an assumed good irrigation distribution uniformity must also be incorporated to obtain a more reliable targeting method.

The 2010 calendar year was selected to examine if a methodology could be developed that would identify/target farms for improved irrigation management. This year was selected because complete datasets of applied water could be obtained from the local groundwater management agencies (Fox Canyon Groundwater Management Agency and United Water Conservation District). Remote sensing was used to compute actual evapotranspiration using the Irrigation Training and Research Center (ITRC) modified METRICTM (Mapping EvapoTranspiration at high Resolution with Internal Calibration program developed by Richard Allen at the University of Idaho). Twenty-one LandSAT 5 and seven images were processed using the ITRC-METRIC surface energy balance methodology (procedures are described in Attachment A and in the body of this report). These images were used with local weather data from three weather stations to develop monthly and annual actual ET images.

Effective rainfall estimates were necessary even though nearly all rainfall occurs from October through April, since crops are grown year-round in this area. Vegetable, sod, nursery, and fruit row crops are continuously rotated through the winter and summer, generally with 2-3 planting harvest cycles in a single calendar year. Fruit trees primarily consist of avocados, lemons, and other citrus that utilize water year-round. Previous work by Styles and Howes (2010a) examined effective precipitation in the area and was used as the basis for the estimates in this study. Some modification was necessary since nearly half of the precipitation during 2010 occurred from late November through December, much of which would be non-effective (at least in 2010).

Groundwater is the primary urban and agricultural water supply within the two watersheds. However, there is some surface water available in certain areas. Many growers are required to provide pumping records and planted acreages along with crops grown to local water agencies. In certain areas, such as Fox Canyon Groundwater Management Agency, growers are limited on how much can be pumped based on the crops grown, area planted, salinity management needs, and assuming a good irrigation system distribution uniformity. Approximately 560 individual parcels with surface and groundwater deliveries were examined within the watersheds. The pumping and delivery information was integrated into the assessor’s parcel map for Ventura County so that these could be overlaid on the actual ET images in GIS. Using GIS the actual ET was averaged by parcel.


Bioresource and Agricultural Engineering

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URL: https://digitalcommons.calpoly.edu/bae_fac/127