Sonoma County Water Agency, California Active

Authors: Larisa Serbina and Holly Miller

The lands along the Russian River of California are home to over 61,000 acres of vineyards. The wineries, offering tours and tastings as well as producing wine, contribute greatly to the economic prosperity of the region. The valley’s grapes produce some of the highest quality Chardonnay, Cabernet Sauvignon, and Pinot Noir wines. A thriving economy in the region depends on flourishing agricultural production while grape production relies on timely water availability for irrigation and frost control. The Sonoma County Water Agency (SCWA) manages flows in the Russian River during most of the growing season and controls releases from two reservoirs. The SCWA is responsible for managing the reservoir storage water collected in the winter to support habitat, recreation, and water-supply needs throughout the year.

To ensure the most efficient use of the limited water supplies in the reservoirs, the SCWA has an ongoing program to enhance water-management strategies. Recent studies have focused on developing a better understanding of total water demands on the Russian River. Given the large number of vineyards along the river and a lack of coordination of associated river diversions, the SCWA faces operational challenges in predicting demands on the river while maintaining a required minimum flow in the river. Better information on agricultural water use in the watershed is not only improving current operations, but is also important for long-term planning for the water resource.

To assess agricultural water demands, the SCWA contracted with Davids Engineering in Davis, California, to develop a methodology that uses Landsat imagery to determine crop coefficients based on data for a full growing season. Twelve Landsat scenes obtained for the 2008 growing season were used in the project. Davids Engineering, working with SEBAL North America, compiled the Landsat scenes and ancillary meteorological data sets to calculate consumptive water use using the physically-based SEBAL method for calculating actual evapotranspiration (fig. 1). Using crop field mapping, crop types were correlated with consumptive use to derive local crop coefficients that could be applied to estimate crop water use in different years under different meteorological conditions.

With crop water use established, Davids Engineering constructed a soil-moisture balance model to develop estimates of applied water use. Since the early growing season can coincide with the region’s rainy season, a model was needed to predict when the growing crops would consume the stored water in the soil and require the supplement of irrigation water. Assessments at the field level were made possible due to Landsat’s relatively high spatial resolution. The project developed an agricultural water-demand model that can be used to estimate river diversions for irrigation under various growing conditions. Historical Landsat imagery provided the base data to derive requirements for crop water use (Todd Schram, Sonoma County Water Agency, oral commun. and written commun., 2013).

The project provided the SCWA with a better understanding of how water is currently being used in the Russian River watershed. It developed the methodology, tools, and data to conduct long-term evaluations of the reliability of the available water resources in the face of projected increases in water demands and predicted changes due to climate change. The SCWA faced a few challenges when working with Landsat data. The primary challenge involved the scan-line gaps in Landsat 7 scenes, which were used to get better coverage for periods when Landsat 5 data were unavailable. The data gaps due to the satellite’s scan-line corrector (SLC) failure resulted in limiting coverage of the consumptive use calculations at some fields for a few periods during the growing season. However, not only did the thermal band and relatively high resolution make Landsat the obvious choice, but the costs and benefits of using Landsat highly outweighed those of any other alternatives. In the case that Landsat data are not available, the California Department of Water Resources conducts land-use surveys approximately every 10 years and maps irrigated agricultural lands. From this, the Department develops irrigated land estimates of applied water use by crop over large watershed-planning areas, which does not capture the variability of the vineyard irrigation practices and micro-climates (Todd Schram, Sonoma County Water Agency, oral commun. and written commun., 2013).

Reference: 

Todd Schram, Agency Engineer, Water Resources Planning, Sonoma County Water Agency, California.