Willcox American Viticultural Area

Much of the Willcox AVA has a Growing Season Temperature¹ between 66°F and 70°F (19°C and 21°C), based on data² from 1981 through 2010. Isolated areas and the southwestern part are warmer and have long-term normal values between 70°F and 75°F (21°C and 24°C).

Growing Season Temperature is the average temperature between April 1 and October 31. It is a common climate-viticulture classification used to compare winegrape-growing regions and to gauge which varieties might do well in an area. Different varieties require different amounts of heat accumulation during the growing season to ripen fruit.

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¹ Jones GV and colleagues (2005)
² TopoWx ("Topography Weather")

Much of the Willcox AVA has Winkler Index³ values between 2222 and 2700, based on data² from 1981 through 2010. Some northern, central, and southeastern areas have long-term normal values that are lower and between 1944 and 2222.

Winkler Index values are cumulative growing degree days (10°C-based) between April 1 and October 31. It is a common climate-viticulture classification used to compare winegrape-growing regions and to gauge which varieties might do well in an area. Different varieties require different amounts of heat accumulation during the growing season to ripen fruit.

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² TopoWx ("Topography Weather")
³ Winkler AJ and colleagues (1974)

Many southwestern and central areas of the Willcox AVA have Huglin Index⁴ values greater than 3000, based on data² from 1981 through 2010. The northern and southeastern parts have long-term normal values that are lower and between 2700 and 3000.

Huglin Index values are cumulative growing degree days (10°C-based) between April 1 and September 30 that additionally account for maximum temperature and daylength. It is a common climate-viticulture classification used to compare winegrape-growing regions and to gauge which varieties might do well in an area. Different varieties require different amounts of heat accumulation during the growing season to ripen fruit.

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² TopoWx ("Topography Weather")
⁴ Huglin P (1978) Nouveau mode d’évaluation des possibilités héliothermiques d’un milieu viticole. Comptes Rendus de l’Académie d’Agriculture de France, 64, 1117-1126

Almost all of the Willcox AVA has values of Biologically Effective Degree Days⁵ between 1600 and 1800, based on data² from 1981 through 2010.

Values of Biologically Effective Degree Days are cumulative growing degree days (10°C-based) between April 1 and October 31 that additionally account for how vine growth rate differs at different temperatures, daylength, and diurnal temperature range. It is a common climate-viticulture classification used to compare winegrape-growing regions and to gauge which varieties might do well in an area. Different varieties require different amounts of heat accumulation during the growing season to ripen fruit.

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² TopoWx ("Topography Weather")
⁵ Gladstones J (1992)

Much of the central part of the Willcox AVA from northwest to southeast has an average date of the last spring freeze during the second half of April, based on data² from 1981 through 2010. Many areas along the Willcox AVA boundary have dates during the first half of April.

The last spring freeze is the latest occurrence of daily minimum temperature below 32°F (0°C) from January 1 through July 31. Freezing temperatures close to the start of the growing season can damage vines and reduce yield.

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² TopoWx ("Topography Weather")

Much of the central part of the Willcox AVA from northwest to southeast has an average date of the first fall freeze during the second half of October, based on data² from 1981 through 2010. Many areas along the Willcox AVA boundary have dates during the first half of November.

The first fall freeze is the earliest occurrence of daily minimum temperature below 32°F (0°C) from August 1 through December 31. Freezing temperatures close to the end of the growing season can damage vines and unharvested fruit, as well as curtail post-harvest photosynthesis and carbohydrate accumulation.

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² TopoWx ("Topography Weather")

Igneous rock is a known parent material occurring in areas throughout the Willcox AVA.¹ Limestone appears along the eastern, southern, and southwestern AVA boundaries. Mixed parent material also appears along the southern and southwestern boundaries.

Parent material is the type of bedrock from which soil forms. As a component of vineyard soils, it influences water infiltration, storage, and drainage, erodibility, pH, nutrient availability, and heat absorption, which affect root, canopy, and fruit growth.

¹ Gridded Soil Survey Geographic (gSSURGO) Database for Arizona, United States Department of Agriculture, Natural Resources Conservation Service, July 2020 Release. Not all areas have complete or available data.

Sand and gravel are the most abundant rock types for almost all of the Willcox AVA.² Dacite, rhyolite, granodiorite, granite, conglomerate, and sandstone occur in isolated areas close to the southern boundary.

Major rock types are the most common solid, loose, and unstratified rocks, defined by composition, texture, and origin. As a component of vineyard soils, they influence water infiltration, storage, and drainage, erodibility, pH, nutrient availability, and heat absorption, which affect root, canopy, and fruit growth.

² US Geological Survey Mineral Resources, Arizona geologic map data

Known depths to bedrock are less than 12 inches for areas in the central part and along the northern and southern boundaries of the Willcox AVA.¹ Depths between 24 and 36 inches exist in the north-central part.

Depth to bedrock represents the distance from the soil surface to the top of a restrictive layer, such as bedrock and other dense layers. As a component of vineyard soils, such restrictive layers can impede water drainage and restrict root depth.

¹ Gridded Soil Survey Geographic (gSSURGO) Database for Arizona, United States Department of Agriculture, Natural Resources Conservation Service, July 2020 Release. Not all areas have complete or available data.

For much of the Willcox AVA, soil texture is fine sandy loam, loam, sandy clay loam, sandy loam, silt loam, and silty clay loam.¹ Areas of sand, coarse sand, loamy sand, and loamy fine sand occur throughout the AVA, as do areas of clay and silty clay along the western and southern boundaries.

Texture represents the relative proportions of mineral grains in soil based on their size. As a component of vineyard soils, it influences water infiltration, storage, and drainage, erodibility, pH, nutrient availability, and heat absorption, which affect root, canopy, and fruit growth.

¹ Gridded Soil Survey Geographic (gSSURGO) Database for Arizona, United States Department of Agriculture, Natural Resources Conservation Service, July 2020 Release. Not all areas have complete or available data.

Much of the Willcox AVA has well-drained soils.¹ Excessively, somewhat excessively, moderately well, and somewhat poorly drained soils occur near the Willcox Playa. An area of somewhat poorly and poorly drained soils exists in the southeastern part of the AVA, and an area of somewhat excessively drained soil exists in the northern part.

Drainage class refers to the rate at which water drains from soil, how frequently water occurs in soil, and at which soil layers water is found. As characteristics of vineyard soils, these conditions influence soil moisture and nutrient and oxygen availability, which affect root, canopy, and fruit growth.

¹ Gridded Soil Survey Geographic (gSSURGO) Database for Arizona, United States Department of Agriculture, Natural Resources Conservation Service, July 2020 Release. Not all areas have complete or available data.

Many areas in the Willcox AVA have available water storage in the top approximately 40 inches of soil greater than four inches.¹ Areas with values between three and four inches occur throughout the AVA, and those with values less than three inches exist in the central and extreme south-central parts of the AVA.

Available water storage represents the amount of water soil can store, in this case in the top approximately 40 inches (1 meter), for plants. As a characteristic of vineyard soils, it influences how much rain, melted snow, and irrigation can be absorbed as well as irrigation frequency.

¹ Gridded Soil Survey Geographic (gSSURGO) Database for Arizona, United States Department of Agriculture, Natural Resources Conservation Service, July 2020 Release. Not all areas have complete or available data.

Soil pH is between 8 and 9 across much of the central part of the Willcox AVA, particularly to the north and south of the Willcox Playa.¹ Much of the rest of the AVA has soil pH between 7 and 8. Soil pH for some areas along the AVA boundaries are between 6 and 7.

Soil pH reflects the relative acidity (values less than 7) or alkalinity (values greater than 7) of soil. As a component of vineyard soils, it influences nutrient availability, which affects root, canopy, and fruit growth.

¹ Gridded Soil Survey Geographic (gSSURGO) Database for Arizona, United States Department of Agriculture, Natural Resources Conservation Service, July 2020 Release. Not all areas have complete or available data.

Elevations are between 4,000 and 4,750 feet (1,219 and 1,448 meters) for almost all of the Willcox AVA.^1,2 Higher elevations, some of which exceed 7,000 feet (2,134 meters), occur just outside of the western and eastern boundaries.

Elevation creates pronounced differences in temperature and precipitation. Average temperatures at higher elevations are cooler than those at lower ones. Average precipitation amounts at higher elevations are greater than those at lower ones.

¹ US Geological Survey, 1 arc-second (approximately 30 meters) digital elevation model
² relative to the North American Vertical Datum of 1988 (NAVD 88)

The central part of the Willcox AVA from northwest to southeast is dominated by slopes that are less than 1%.¹ Slopes between 1% and 5% occur along the eastern and western boundaries.

Slope represents the change in elevation across an area. It can influence the amount of sunlight directly reaching vines and thus temperatures within a vineyard, as well as cold-air drainage, water infiltration, soil erosion, and use of vineyard equipment.

¹ US Geological Survey, 1 arc-second (approximately 30 meters) digital elevation model

Many areas along the western boundary of the Willcox AVA face north or east.¹ Much of the rest of the area faces south or west.

Aspect is the cardinal direction that a slope faces. Like slope, it can modulate sunlight that directly reaches vines and thus temperatures within a vineyard. Locations with south- and west-facing slopes experience higher values of these environmental conditions.

¹ US Geological Survey, 1 arc-second (approximately 30 meters) digital elevation model

Numerous creeks, washes, and ephemeral streams drain into the Willcox AVA along its northern, eastern, and western boundaries.³ The rest of the AVA is largely devoid of such features, but does include the centrally located Willcox Playa.

Natural features that are perennial, intermittent, or ephemeral and engineered features like canals, ditches, and reservoirs make up surface water. In addition to directing water across a landscape, these features can channel cooler air from higher elevations to lower ones on nights with little to no wind, influencing spring and fall freeze risk as well as diurnal temperature range during summer.

³ US Geological Survey, National Hydrography Dataset