Remote Sensing Consumptive Use Analysis

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Owner: California Department of Water Resources
Location: 
Central Valley, California
Funding Source: California Department of Water Resources

Actual ET for Delta study area, March to September, 2009. Streaks represent gaps in Landsat 7 satellite imagery.

The Sacramento-San Joaquin River Delta is located along the western edge of the Central Valley of California, extending from near Tracy in the south to near Davis in the north and encompassing an area of more than 1,300 square miles. The Delta provides conveyance of water to more 25 million Californians as well as three million acres of agricultural lands and supports the region’s overall environmental health and an economy of $400 billion. Several models are being used to estimate water demand and consumptive use in the Delta. Improving estimates of water demand and consumptive use for crops and native vegetation in the Delta is important in planning for future water conveyance options through the Delta.  The California Department of Water Resources (DWR) completed a detailed land use survey for the area in 2007 to support the computation of daily evapotranspiration of applied water (ETaw) using three, existing models.

To support the on-going efforts of DWR to obtain reliable estimates of consumptive use within the Delta region, DWR engaged Davids Engineering to estimate ETa for crops and native vegetation for the 2007 crop growing season.  Nine Landsat multispectral satellite images acquired between March and September 2007 were analyzed using the Surface energy Balance Algortihm for Land (SEBAL®).  SEBAL quantifies surface energy fluxes at the Earth’s surface including net solar radiation (Rn), soil heat flux (G), and sensible heating (H).  Then, based on the principle of conservation of energy, the latent heat flux (L) is calculated, which is directly proportional and readily converted to ETa. SEBAL has been extensively validated in the U.S. and worldwide over more than 15 years and has been found to consistently provide estimates of ETa that agree within 5% of reliable ground-based estimates on a seasonal or annual basis. Additional information describing the SEBAL model and its validation is available at www.sebal.us.

Spatially distributed ET data from SEBAL were used as a standard against which the other three models were compared.  Results of the comparison to SEBAL suggest that the models DWR has been using have underestimated the water consumed by native vegetation and over estimated the water consumed by crops.

Cumulative distributions of actual ET for selected Delta land uses, March to September, 2009.

To determine if different water year types resulted in different actual ET of the various crops grown in the Delta, DWR commissioned Davids Engineering to complete an additional ET analysis for 2009. Using National Agricultural Statistics Survey (NASS) crop layer data for 2009, DWR is currently analyzing the SEBAL ET results to identify potential changes in the timing and amount of ET for individual crops in the 2009 irrigation season as compared to 2007.