The most notable geographical characteristic of the Coombsville viticultural
area is a horseshoe-shaped, elevated landform, part of the Vaca Range.[9] The west-facing horseshoe comprises a ring of volcanic mountains, according to the petition. The elevated "cup-and-saucer" landform lies partially within the curvature of the horseshoe on the western side of the viticultural area. A small flood plain lies along the western portion of the boundary line near the Napa River and Milliken Creek. The petition states that gentle slopes and rolling terrain
extend westward from the Vaca Range and the opening of the horseshoe to the
Napa River and Milliken Creek, and that most viticultural activity occurs within
this area. The petition states that the Milliken-Sarco-Tulocay watershed, named after the three main creeks in the region, lies within the Coombsville viticultural area. The cup-and-saucer landform presents a drainage
obstacle, making Sarco Creek detour to the north and Tulocay Creek flow to the
south. Eventually, all drainage flows to the southwest and joins with the south-
flowing Napa River, the petition explains. According to USGS maps, elevations
within the Coombsville viticultural area vary from about 10 feet (3 m)
along Milliken Creek and the Napa River shoreline to 1,877 feet (572 m) at the peak of Mt. George, at the northeast corner of the Coombsville viticultural area along the western ridge of the Vaca Range. The landforms along the
remaining caldera wall that forms the edge of the "saucer" vary from
approximately 500 to 1,200 feet (152–366 m) in elevation, some having steep terrain. The raised "cup" portion of the cup-and-saucer formation exceeds 400 feet (122 m) in elevation in some areas. The surrounding gentle slopes and rolling
terrain which form the bottom of the "saucer" vary from approximately 100 to 200 feet (30–61 m) in elevation. The flood plain along the western boundary line varies
in elevation from 10 to 20 feet (3–6 m) along Milliken Creek and the Napa River. The combination of unique landforms and large elevation differences gives the
Coombsville viticultural area a fog-protected partial basin with high surrounding ridges. The aerial photograph submitted with the petition shows Coombsville as an isolated niche within the larger, more open terrain of the Napa Valley viticultural area. Also, the USGS maps indicate that the Vaca Range to the east provides a natural
geographical boundary for the viticultural area. According to the USGS maps and the
petition, the regions surrounding the Coombsville viticultural area have different geographies. To the northwest of the viticultural area lies the Oak Knoll District of Napa Valley viticultural area, which can be distinguished from the Coombsville viticultural area by its low valley floor elevations and the dry creek alluvial fan. To the west lies the City of Napa. To the southwest lies the Los
Carneros viticultural area, which can be distinguished from the viticultural area by its low rolling hills, flatlands, and mountainous terrain. To the southeast lies the Solano County Green Valley viticultural area, with a more rugged terrain than the Coombsville viticulture area. To the east lies the Wild Horse Valley viticultural area, which can be distinguished from the viticultural area by its isolated valley and the surrounding steep, rugged terrain and high
elevations. To the northeast are the Vaca Mountains, which can be distinguished
from the viticultural area by their rugged terrain.[1]
Coombsville viticultural area has climatically unique features, including precipitation and heat summation. The petition provides statistical information on the microclimates of the adjacent Los Carneros and Oak Knoll District of Napa
Valley viticultural areas, which are both within the larger Napa Valley viticultural area.[10] According to
the petitioner, the isolated Wild Horse Valley and Solano County Green Valley
viticultural areas, to the immediate east of the Coombsville viticultural area, lack available weather station data. In considering this petition, TTB obtained historic weather station data for surrounding north, east, south, and west regions within 15 miles (24 km) or less of the Coombsville viticultural area (Lake Berryessa, Fairfield, Napa State Hospital, and the City of Napa, respectively) from the Western Regional Climate Center (WRCC) Web site,
created in partnership with the National Climatic Data Center, Regional Climate
Centers, and State Climate Offices. The petition's table data presents average annual precipitation amounts and heat summation range totals for the Coombsville region, the Los Carneros and Oak Knoll District of Napa Valley viticultural areas, and the surrounding north, east, south, and west weather station areas. The table data is based primarily on petition documentation and also TTB’s WRCC Web site data
research. The table shows that precipitation in the Coombsville viticultural
area averages 19.14 inches (486 mm) annually, and varies from the surrounding viticultural microclimates. The Coombsville region is warmer and wetter than the Los Carneros viticultural area to the southwest and cooler and drier than the Oak Knoll District of Napa Valley viticultural area to the northwest, according to Michael Wolf, owner of Michael Wolf Vineyard Services. To the northwest, the Oak Knoll District of Napa Valley viticultural area averages 2.5 inches (64 mm) more annual rainfall. To the southwest, the Los Carneros viticultural area has about 2 inches (51 mm) less rainfall annually. The data in the table indicates that the Coombsville viticultural area averages 3.63 to 5.47 inches (92–139 mm) less precipitation annually than the four surrounding areas for which weather station data was obtained by TTB. The growing season in the Coombsville viticultural area is measured in the Winkler climate classification system.[11] In the Winkler system, heat accumulation per year defines
climatic regions. As a measurement of heat accumulation during the growing season, 1 degree day accumulates for each degree Fahrenheit that a day’s mean temperature is above 50 degrees, which is the minimum temperature required for grapevine growth. Climatic region I has less than 2,500 growing degree days (GDD) per year; region II,
2,501 to 3,000; region III, 3,001 to 3,500; region IV, 3,501 to 4,000; and region V,
4,001 or more. According to the table, the Coombsville region is a low Winkler
region II (2,550 GDD units), which is cooler by 61 to 683 degree units than the
four surrounding areas from which weather station data was obtained by TTB. The coolest of the four areas is Lake Berryessa to the north at 2,611 GDD units (region II), and the warmest is the City of Napa to the west at 3,233 GDD units (region III). Also, the adjacent Oak Knoll District of Napa Valley viticultural area is significantly warmer at 2,888 GDD units, a high Winkler region II. The adjacent Los Carneros viticultural area is cooler than the Coombsville region (region l) at 2,435 GDD units. The petition states that significant viticultural factors for the Coombsville region growing season include the amount of solar radiation and daytime
heating. The solar radiation and heating are affected by the dissipation rate of
morning fog, followed by the number of hours of sunshine, and then the onset of
afternoon cooling bay breezes from San Pablo Bay. The petition states that the effects of the presence and disappearance of fog from the Napa Valley region in the day alters the temperature rise in the grape-growing season. Temperature and
sunlight have subtle effects on grape development that, over the growing season, affect grape ripening times and flavors. The pace of sugar accumulation and the pace of the lessening of acidity during grape ripening are two examples of how the fog affects grape development. The petition notes that grape growers in the cooler Los Carneros viticultural area, to the south and closer to the foggy bay, harvest grapes with similar sugar and acidity levels for the same varietal as in the Coombsville
region, but do so later in the growing season. To the north of the Coombsville
region, in the warmer and less foggy Oak Knoll District of Napa Valley viticultural area, the same varietals with similar sugar and acid levels are harvested earlier than in the Coombsville and Los Carneros areas. The petition explains that the
Coombsville region has more sunlight and daytime heat during the growing season than the Los Carneros viticultural area to the southwest and less than the Oak Knoll District of Napa Valley viticultural area to the northwest. The morning fog generally dissipates about 1 to 2 hours earlier in the Coombsville region than in the Los Carneros viticultural area to the southwest, and an hour later than in the Oak Knoll District of Napa Valley viticultural area to the northwest. Also, in the afternoon, the bay breezes first cool the Los Carneros viticultural area, then spread slowly northward through the Coombsville region into the Oak Knoll District of Napa Valley
viticultural area, and eventually continue northward up the Napa Valley.
According to the petition, as the San Pablo Bay afternoon breezes reach northward to each micro-climate in the Napa Valley region, the air temperature incrementally stops rising, or slightly decreases. These cool breezes contribute to the differences in maximum daytime temperatures during the growing season for the south-to-north locations in the Los Carneros viticultural area, the Coombsville region, Oak Knoll District of Napa Valley viticultural area, and other Napa Valley viticultural areas.[1]
The petition describes the ancient volcanic and crustal uplift events in the geologic history of the Coombsville region. The initial geological event was the eruption and collapse of a volcano that was part of the Napa Valley-Sonoma volcanic series. The collapse of the volcano created a bowl-shaped structure known as a caldera, which formed the basis for the "cup and saucer" topography within the Coombsville region.[12] The next important geologic process began when crustal forces started to uplift and wrinkle the earth crust in the Vaca Range. The uplift progressed from east to west through the Vaca Range. When the uplift passed through the Coombsville region, the western front of the caldera collapsed and slid westward
as a large landslide into the valley below. The ancient Napa River removed most of the landslide debris from the Napa Valley.[12] The remaining debris formed a raised structure in the valley, and the remaining portion of the caldera formed a horseshoe-shaped ridge to the east. This area is referred to on USGS maps of the Coombsville area as the "cup and saucer," since the raised area resembles a teacup sitting within the curved "saucer" formed by the remaining ridge
of the caldera. The petition states that the earth surface materials that cover the Coombsville viticultural area originated in a variety of ways. A thin coat of residual debris on volcanic bedrock covers the hills. Within the remains of the caldera, alluvial gravels of the Huichica Formation occur in the northern part and diatomaceous lake deposits occur along the northeast edge. The remainder of the surface material is a variety of alluvial deposits laid down since the ancient volcanic collapse.[12] The petition did not include data on
the geology of the surrounding areas.[1]
The soils of the Coombsville viticultural area are generally well drained and of volcanic origin. Upland soils are weathered from their primary volcanic source, while lowland soils are alluvial in nature.[13] The Hambright-Rock outcrop complex makes up 28.5 percent of the Coombsville area and is found in lesser concentrations to the north, east, and south. The complex is found in the Vaca Range and makes up most of the cup-and-saucer landform soils. Coombs gravelly and stony loams represent 24.1 percent of the soils in the Coombsville area, and are found in lesser concentrations to the north, east, and west. In addition, those soils are the main types appropriate for grape growing in the Coombsville region. They are alluvial, well drained soils at elevations of 50 to 500 feet (15–152 m). The Coombs soils are "relatively unique to the area," and they were likely first identified in the Coombsville area, according to the petition. Coombs soils make up only 1.7 percent of the soils in Napa County, but they account for almost a quarter of the Coombsville region soils. As shown in the petition, Sobrante soils make up 15.5 percent of the Coombsville region, 16 percent to the east in Wild Horse Valley, and a much lesser concentration to the northwest. These soils are well drained and are at elevations of 120 feet (37 m) and higher. As shown on the petition tables, soils found in lesser concentrations in the Coombsville viticultural area include Haire and Cole, which have higher concentrations in three of the surrounding areas.[1]
