17 The climate of the Texas High Plains AVA is semi-arid, with hot summers and mild winters. Temperatures generally decrease with increasing elevation from southeast to northwest (Fig. 2a). Most of the region receives an average annual precipitation of 43.7 to 52.6 cm; the range throughout the AVA is 41.4 to 63.7 cm, increasing from west to east (Table 1; Fig. 2b). Cumulative GDD (Fig. 2c) follows the pattern of elevation: the lowest GDD is in the northwestern corner of the AVA, averaging 2028, and the highest in the southeastern region, at 2653. For comparison, the low range of degree-days is comparable to the Sierra Foothills region of California and the high range to Fresno, California (Table 2). It is noteworthy that the broad range of degree-days within the Texas High Plains AVA overlaps with the degree-days range of many well-established wine regions in California, including Napa Valley, Lodi, Paso Robles, and Temecula Valley (Jones et al. 2010). Grape RPMT ranges from 23.8 to 26.7°C in August and 19.9 to 22.6°C in September (Table 1; Fig. 2d). Night time minimum temperatures range from 16.7 to 19.4°C in August and from 12.2 to 15.6°C in September. Average daily temperature in January, the coldest month, ranges from 2.1 to 5.4°C, and minimum temperatures are -5.6 to -2.2°C. Freeze injury to grapevines occurs sporadically, usually in association with a rapid drop in temperature while vines are in early stages of cold acclimation in late autumn or de-acclimation in late winter. The region experiences 1 to 6 days of frost in April, presenting a risk of frost damage, particularly to grape cultivars with early budburst.

18 The Texas High Plains AVA is underlain by the Blackwater Draw Formation (Fig. 3a), a sheet of Quaternary eolian sediment that is up to 27 m thick (Holliday 1989). These sediments vary in texture from sandy in the southwest to silty and clayey in the northeast; the textural fining is apparently the result of downwind sorting. The formation comprises up to six well-developed buried soils, indicating episodic sedimentation, that are similar to the regional surface soils. There are two narrow west–east trending fields of eolian sand dunes overlying the formation, and much of the southern region of the AVA is generally overlaid with upper Quaternary sand sheet deposits (Holliday 2001). The Ogallala Formation, of Miocene to Pliocene age, underlies the Blackwater Draw Formation and consists of sediments (sand, silt, clay, gravel, carbonate) derived from eolian and fluvial deposits. This formation is host to the important Ogallala Aquifer, the primary source for agricultural irrigation and drinking water in the region.

19 The region is characterized by very deep (>152 cm), well-developed soils with accumulations of calcium carbonate in the subsoil and clay increasing with depth. Distribution of general soil types in the region corresponds closely to geology, with texture becoming increasing finer from southwest to northeast (Fig. 3b). The Texas High Plains AVA contains 31 soil associations, five of which constitute 75% of the area: Pullman–Randall–Lofton 22.8%, Amarillo–Acuff–Olton 19.5%, Patricia–Amarillo–Gomez 15.5%, Olton–Acuff–Amarillo 9.5%, and Pullman–Olton–Randall 7.7% (Table 3). Pullman–Randall–Lofton soils have clayey subsoil horizons with shrink-swell potential and are not commonly planted to grapes (Fig. 3b).

20 The Amarillo–Acuff–Olton soil association covers almost 700 000 ha (Table 3), primarily in the west-central part of the AVA (Fig. 3b). It comprises deep soils with sandy loam texture from 0 to 30 cm depth, transitioning into sandy clay loam from 30 to 200 cm. Permeability of the upper 30 cm averages about 8.5 cm/hr; within the underlying sandy clay loam it is approximately 3.2 cm/hr. Soil pH increases with depth, from approximately 7.3 in sandy loam to 7.9 to 8.1 in sandy clay loam. Available water capacity ranges from 15 to 28 cm, increasing with depth (Table 3).

21 The Patricia–Amarillo–Gomez soil association predominates in the southern part of the AVA (Fig. 3b). These soils have a sand texture from 0 to 40 cm depth and sandy clay loam from 40 to 200 cm (Table 3). Permeability within the sand is high (21 to 24 cm/hr), decreasing to 3.5 to 6.5 cm/hr in the sandy clay loam. The pH ranges from 7.2 to 7.8, and available water capacity ranges from 12 to 26 cm, both increasing with depth.

22 Vineyards in the Texas High Plains AVA are predominantly planted on deep, reddish-brown Patricia and Amarillo loamy fine sands (205.9 ha) and Amarillo fine sandy loam (115.3 ha) soils (Fig. 4). The Amarillo and Patricia series are classified taxonomically as fine-loamy, mixed, superactive, thermic Aridic Paleustolls, and are favoured for their depth and combination of good internal drainage and subsoil with enough clay for adequate water-holding capacity (Table 4). Both soils are characterized by a friable calcareous zone in the subsoil; depth to this zone for Patricia soils is typically 152 to 203 cm (Natural Resources Conservation Service 1999), whereas in Amarillo soils it is 76 to 150 cm (Natural Resources Conservation Service 2010). Figure 5 illustrates a typical soil profile of an Amarillo fine sandy loam having an apparent calcareous zone at 100 to 140 cm depth. Favourable soil-water characteristics, low annual rainfall (see above), and low to moderate fertility of these soils enables viticulturists to manage grapevine vigour through irrigation practices, which is an important tool in optimizing canopy micro-climate for fruit quality.

23 The Texas High Plains AVA is home to approximately 60 vineyards totalling 440 ha, and six wineries, including the second-largest producer in the state, the Llano Estacado Winery. Vineyards tend to be relatively large (>9 ha), mechanized (Fig. 6), and not associated with a winery; grapes are sold to wineries both within and external to the Texas High Plains AVA. The region first established its winegrowing reputation with Cabernet Sauvignon and Merlot, frequently producing medal-winning wines that were commonly vineyard-designated on the label in recognition of local terroir. Recent planting trends have increased production of grape cultivars common to warm and hot climates, including Sangiovese, Vermentino and Tempranillo, and the Rhone cultivars Viognier, Grenache, Mourvèdre, and Syrah. The high quality of varietal wines or Rhone-type blends of these cultivars suggests that the region may be better suited to such warm-climate cultivars.

24 Climatic and edaphic conditions of the Texas High Plains AVA are conducive to high-quality grape production; although the region is considered to have a hot climate based on GDD, temperatures become favourably moderate at night during the fruit ripening period because of the high elevation and low relative humidity. Good tannin and colour development is characteristic of fruit from the region and vine fruitfulness is high, which are attributable to abundant sunlight and possibly an enhanced ratio of red to far red light wavelengths reflected from red soils (Gladstones 1992). Relatively low annual precipitation enables grapevine vigour management through irrigation practices. Low rainfall and low relative humidity also provide an environment conducive to few fungal diseases of grapes.

26 The Escondido Valley AVA is the warmest and second-driest growing region among west Texas AVAs. Air temperature, GDD, and precipitation variability are associated with elevation (Fig. 8a to d). Annual precipitation (Table 1) ranges from 34.1 to 38.5 cm, with greater precipitation occurring at higher elevations along the southern border (Fig. 8b). Cumulative GDD is very high, ranging from 2768 at the higher elevations to 3035 at the lowest elevations in the north (Fig. 8c), and average about 2970 at Mesa Vineyards, somewhat warmer than Bakersfield, California (2822 GDD; Winkler et al. 1974). However, grapes ripen in July and August in the Escondido Valley AVA, suggesting that comparisons with standard full-season (April to October) degree-day accumulations may not be appropriate. Grape RPMT averages 27.6°C in July and 27.1°C in August (Table 1). Winter temperatures are the mildest among west Texas AVAs; January minimum temperature averages about -0.6°C (Table 1). Spring frost frequency averages about 3.8 days in March and 0.6 days in April at the intermediate elevations where vineyards are planted.

27 The Escondido Valley AVA lies within a transition zone between the southern extent of Quaternary, alluvial eolian sand, silt, clay, gravel and carbonate deposits common to the High Plains, and the western extent of the Cretaceous Edwards Limestone (Fig. 9a). Quaternary deposits account for almost 40% of the total area and predominate in the central part of the region, whereas Edwards Limestone underlies a slightly larger proportion (43.6%), associated with higher elevations mainly along the southern and eastern boundaries and with a mesa in the northeast corner. A prominent east–west band of Quaternary–Tertiary alluvial deposits of gravel and conglomerate fill the lowest elevations (772 to 790 m) along the northern boundary of the AVA, corresponding to the bed of present day Tunas Creek. North of this band, there are three occurrences of Cretaceous deposits (limestone, mudstone, dolomite, and chert) of the Fredericksburg Group (Fig. 9a).

28 Three soil associations, all of which are calcareous and alkaline (pH 8.1 to 8.2), characterize the Escondido Valley AVA (Table 5): Ector–Rock outcrop–Dev, Reagan–Hodgins–Iraan, and Lozier–Rock outcrop–Upton. The most suitable vineyard soils are found within the Reagan–Hodgins–Iraan association, which occupies 38.5% of the AVA, generally overlying the Quaternary and Tertiary deposits in the north and central areas (Fig. 9a, b). Mesa Vineyards is located within this soil association, somewhat northeast of the AVA centroid (Fig. 10). The Reagan–Hodgins–Iraan association comprises very deep (>152 cm) soils having silty clay loam texture from 0 to 20 cm, and loam texture beneath. Permeability is moderately high throughout, ranging from 2.56 to 2.68 cm/hr. Available water capacity is good and increases with depth as the proportion of clay increases, from 14 cm within the top 100 cm to 26 cm at depth (Table 5). Soil pH is high (8.1), requiring vineyard management practices to avoid iron and zinc deficiency in grapevines.

29 The Ector–Rock outcrop–Dev soil association predominates, underlying 58% of the total area, and is generally associated with Edwards Limestone geology and the higher elevations of the southern and eastern portions of the AVA (Fig. 9b). Because this association is mostly shallow gravely loam and rock outcrop, it has inadequate depth for grape production and very low available water capacity, and is therefore not suitable for grape growing. A small area (3.5%) along the northern border of the AVA contains the Lozier–Rock outcrop–Upton soil association (Fig. 9b); these soils are developed on Fredericksburg Group substrate, and have characteristics very similar to the Ector–Rock outcrop–Dev soil association. The soils are gravely loams too shallow for grape production, and have low available water capacity.

30 As noted above, Ste. Genevieve is Texas’ largest wine producer (58 000 hL annually) and is located, along with the 288 ha Mesa Vineyards, within the Escondido Valley AVA (Figs. 7 and 10). Unlike other Texas wineries, Ste. Genevieve has no tasting room and wine is sold only through distribution to retailers, almost all within Texas. The winery began operation in 1987, producing Cabernet Sauvignon, Merlot, Zinfandel, and other red wines, but has been more recognized for Sauvignon blanc, Chenin blanc, and Chardonnay. The hot, dry climate seems particularly favourable for these white wine cultivars. With the abundant sunlight and high temperatures in this AVA, fruit readily attains high sugar content. The long growing season begins in March, earlier in Escon-dido than the other west Texas AVAs, and presents a risk of frost injury to grapevines after budburst. High GDD leads to early ripening, and harvest occurs in July and August.

32 The varied elevation (1125 to 2546 m; Fig. 11a) and topography of the Texas Davis Mountains creates the broadest range of climatic variables of all AVAs in west Texas (Table 1). Precipitation is greatest at the highest elevations, which receive almost 50 cm annually (Fig. 11b); vineyards are at elevations of 1500 to 1600 m, and average 42 to 43 cm annual precipitation. Higher elevations experience only 1430 GDD compared to 2672 at low elevation (Fig. 11c). Favourable vineyard locations in the southeast have approximately 2190 GDD, comparable to the west central area of the Texas High Plains AVA and Lodi, California (Table 2). Temperatures during fruit ripening are the lowest among Texas AVAs, and exhibit a similarly broad range associated with elevation (Fig. 11d); RPMT ranges from 18.8 to 25.6°C in August and 16.7 to 22.6°C in September. Risk of mid-winter freeze injury to grapevines is low, as January minimum temperatures average about -2°C at vineyard locations. The number of frost days is also associated with elevation, ranging from 1.2 to 7.2 days in April. Vineyards are at elevations that experience frost on average about 2.4 days in April, but strategic location of vineyard sites on slopes can mitigate the risk of frost injury.

33 The Davis Mountains were formed by volcanic activity approximately 37 million years ago (Swanson 1995). Mount Livermore is the highest peak at 2555 m. The geology of the Texas Davis Mountains AVA is dominated by a large area of older volcanic rocks (rhyolite, trachyte, lava flows) at lower elevations in the north and east, and younger volcanic rocks (trachyte, rhyolite, tuff, latite) at the higher elevations in the central and southwest areas, including Mount Livermore, Black Mountain and surrounding peaks (Fig. 12a). Quaternary alluvium (sand, silt, clay, gravel) and alluvial fan deposits occur along the southern border, Quaternary–Tertiary gravel and conglomerate in the northeast corner, and scattered areas of Oligocene intrusive rocks (basalt, trachyte, felsic volcanic rock, rhyolite, phonolite) are located in the west (Fig. 12a).

34 The relatively recent volcanic activity in this AVA results in a predominance of rock outcrop, and only a limited area of soils suitable for vineyards. The two most prevalent soil associations, Rock outcrop–Mainstay–Liv and Brewster–Rock outcrop–Liv mostly comprise non-soil surface bedrock, and together constitute 82.5% of the total area (Table 6). Both of these associations, as well as the Puerta–Madrone– Rock outcrop association, are considered too shallow (44 to 58 cm to bedrock) for grape production.

35 The Musquiz–Boracho–Santo Tomas soil association is the most favourable in the AVA for grape production. It underlies 9% of the area, primarily along the southern boundary where vineyards have been established, and to a lesser extent along the northwestern boundary (Fig. 12b). Soils in this association are deep (up to 144 cm), with loam texture in the upper 20 cm, and clay and clay loam at lower depths. Permeability is high (4.12 cm/hr) in the upper 20 cm, but only moderately high (3.07 cm/hr) at depth as clay content increases. Soil pH is favourable at 7.5 and available water capacity is good: 12 cm within the top 100 cm and 18 cm from 100 to 150 cm depth.

36 The Phantom–Rockhouse–Hodgins soil association accounts for only 1% of the area and is located in the northeast corner at the lowest elevation (1124 to 1300 m) in the AVA. It has a clay loam to loam texture with only moderately high permeability (2.84 cm/hr) in the upper 20 cm, and clay loam below having high permeability (6.25 cm/hr) to a total depth of 142 cm. The pH is favourable and slightly higher than Musquiz–Boracho–Santo Tomas, at 7.6 to 7.8. Available water capacity is similarly favourable at 13 cm within the top 100 cm and 20 cm from 100 to 150 cm depth.

37 The limited area suitable for vineyards in the Texas Davis Mountains AVA has restricted winery development in the region. Grapes were first planted in 1977 at Blue Mountain Vineyard near Fort Davis; the winery operated from 1995 to 2006, producing award-winning wines from estate-grown Cabernet Sauvignon and Merlot that were notable for good colour and tannins. Although there are no operational wineries in the region today, a vineyard that cultivates Cabernet Sauvignon and Sauvignon blanc provides grapes to Pleasant Hill Winery in Brenham, Texas for appellation-labelled wines.

38 The high-altitude combination of GDD and relatively cool RPMT of the Texas Davis Mountains AVA has been demonstrated to be favourable for traditional Bordeaux grape cultivars. Experimentation with warm-climate grape cultivars, as has been done in the Texas High Plains AVA, has not been conducted in the region, but results at the latter AVA suggest similar opportunities for the Texas Davis Mountains AVA. Low precipitation reduces the risk of fungal diseases, but restricts grape production to areas with irrigation wells. Suitable soils in the Musquiz–Boracho–Santo Tomas association generally correlate with availability of irrigation water, practically limiting grape production to the southern border of the AVA. Although suitable vineyard locations are limited, there is potential for additional vineyard and winery development to take advantage of the unique environmental conditions of this region.

41 The Mesilla Valley AVA is by far the driest in west Texas, with annual precipitation ranging from 20.4 to 29.3 cm (Table 1). Rainfall and other climatic parameters are closely associated with elevation (Fig. 13a); precipitation is least at the lowest elevations in the west and increases with elevation toward the Franklin Mountains in the east (Fig. 13b). The GDD ranges from 2480 to 2849 (Fig. 13c) and averages about 2830 in vineyard areas, somewhat lower than Escondido Valley and similar to Bakersfield, California (2822 GDD; Winkler et al. 1974). As in the Escondido Valley, mild winters and a warm growing season lead to early budburst and fruit ripening in July and August. RPMT is highest in the lower elevation western part of the region (Fig. 13d), ranging from 26.7 to 28.1°C in July and 25.5 to 26.8°C in August. January minimum temperatures present little risk of freeze injury to grapevines, averaging about -1.9°C for the AVA. Spring frost days increase with elevation, ranging from 4.1 to 6.0 days in March, and 0.7 to 1.1 days in April.

42 The major geological feature of the region is the Rio Grande rift, which results from a period of crustal extension that began between 32 and 27 million years ago, and is today distinguished by faults and volcanoes and associated mountains and basins (Chapin 1979). The ancestral Rio Grande developed about 4 million years ago, following the course of the pre-established north-trending rift valley that extends from Colorado to Mexico. The rift valley today consists of a series of basins deeply filled with sediments from the nearby eroding mountains. Beginning about 780 000 years ago, the ancestral Rio Grande began to erode through its own alluvial plain, and episodes of down-cutting alternated with sediment deposition (Mack 1997). The river now occupies an entrenched valley about 100 m lower than the surrounding, older sedimentary rocks.

43 The Texas portion of the Mesilla Valley AVA is mostly underlain by Quaternary alluvium (sand, silt, clay and gravel) at lower elevations close to the river in the west, and by older Quaternary alluvial deposits (gravel, sand, silt) to the east as elevation climbs toward the Franklin Mountains (Fig. 14a). Deposits of Quaternary–Tertiary clay, silt, sandstone, and conglomerate occur locally at higher elevations, where they underlie the older Quaternary alluvium. Some Quaternary sand sheet deposits are seen along the transition zone between younger and older alluvium, where elevation begins to climb (Fig. 14a).

44 The spatial distribution of the three predominant soil associations in the Mesilla Valley parallels the local geology (Fig. 14b). The Glendale–Armijo–Harkey and Pintura–Blue-point– Wink associations correspond to areas underlain with younger alluvium at lower elevation close to the Rio Grande; Delnorte– Canutio–Nickel is found at higher elevations, generally corresponding to the area of underlying olderalluvium.

45 Vineyards have been primarily located within the area of the Glendale–Armijo–Harkey association, which comprise deep alluvial soils of clay loam texture in the top 30 cm, and sandy loam beneath (Table 7). These soils have a combination of drainage and water holding capacity that is well-suited to grape production. Permeability is high: 5.68 cm/hr in the upper 30 cm, and 9.09 cm/hr at lower depths. Available water capacity is 13 cm in the upper 100 cm and 37 cm in the lower 150 cm. The Pintura–Bluepoint–Wink association has higher permeability and somewhat lower available water capacity, but is similarly favourable for grape production. These alluvial soils are also deep, but have a shallow (10 cm) upper layer of sand, underlain by sandy loam and more sand below. The pH of both these soil series is somewhat high for grape production, ranging from 7.9 to 8.1, which may necessitate management practices to avoid iron and zinc deficiency problems.

46 The Delnorte–Canutio–Nickel soil series comprises deep sandy loam soils with high permeability (15.6 cm/hr) and low available water capacity (Table 7). The generally dry nature of these soils can be difficult to manage for grapes, requiring frequent irrigation and careful monitoring of soil and vine water status.

47 The Texas portion of the Mesilla Valley AVA has seen less vineyard and winery development than within this AVA in New Mexico. There are currently two small wineries in this AVA in Texas, one with an estate vineyard (Zin Valle Vineyards; Fig. 15) producing Zinfandel and Malvasia Bianca. The potential for development of new vineyards is highest in the western part of the AVA, at lower elevations closer to the Rio Grande; these areas have the most favourable combination of good soils and availability of irrigation water. This area also features the lowest precipitation, highest GDD, and highest RPMT. These factors result in good fruit ripening conditions leading to high sugar accumulation, and low disease pressure from fungal pathogens.