Physiographic features

This site occurs within the basin floor of the San Luis Valley. Elevation ranges from 7500 to 7800 feet. Landforms include flood plains on valley floors and alluvial flats.

Table 2. Representative physiographic features

Climatic features

The climate that typifies the High Intermountain Valley, ranges from arid to semi-arid, and is characterized by cold winters, moderate summers, and much sunshine. Average annual precipitation ranges from 6 to 10 inches along the valley floor and throughout most of the resource area. Approximately 55-60 percent of the annual precipitation falls between May 1 and September 1. May and June are normally dry. Precipitation comes mostly from short duration high intensity thundershowers in July and August. Wide seasonal and yearly variations are common. The San Juan mountain range to the west and the Sangre de Cristo Mountains to the east intercept much of the precipitation causing a two-way rain shadow effect.

Cold air from the encompassing mountain ranges drain into the valley and settle. This phenomena results in long cold winters and moderate summer temperatures. Mean average annual temperature ranges between 42 to 44 degrees F. July is the hottest month and January is the coldest. Summer temperatures range from highs in the upper 70’s and low 80’s and occasionally reach to the mid 90 degrees F. Summer nights are cool. Average frost-free period is 90-115 days, from late May or early June to September. There is a 50% probability that the first frost in the fall will occur near September 16, and the last frost in the spring on about June 9. Most major plant species initiate growth between mid May and late July, but growth may extend into September. Some cool season plants begin growth earlier and complete growth by mid June. There may be late re-growth on some of the plants.

Wind speeds average 7 miles per hour annually. Wind that often reaches high velocities are common, especially in the spring. Relative humidity is usually low. Even so, evaporation rates average lower than those of many dry regions because of the cooler climate. Snow cover is often light and is sometimes lacking through much of the winter. There is usually some snow, though, during the coldest weather.

Table 3. Representative climatic features

Climate stations used

• (1) ALAMOSA SAN LUIS AP [USW00023061], Alamosa, CO

• (2) WAVERLY 1W [USC00058860], Alamosa, CO

• (3) MANASSA [USC00055322], La Jara, CO

• (4) MONTE VISTA 2W [USC00055706], Monte Vista, CO

• (5) CENTER 4 SSW [USC00051458], Center, CO

Influencing water features

Seasonal high water table exists from 4 to 6 feet below the surface. The water table can be found usually June thru September. Lower areas may receive low amounts of run-on from adjacent higher areas.

Soil features

Soil surface textures are fine-loamy. Clay content in the surface ranges from 25 to 35%. They are strongly alkaline. They are usually underlain by sand and gravel and this shows are a restrictive feature in the soil at 20 to 40 inches (50 to 100 cm) . Large areas of "slick spots" are mottled throughout the site where particles are highly dispersed causing impermeable soils with virtually no penetration of water. Runoff from rain or snow melt sit in these lower slick spot areas until evaporation occurs.

Typical soil correlated to this site is Hooper.

Table 4. Representative soil features

Animal community

This site offers only limited value for livestock grazing because of the low production, high percentage of unpalatable species, and because of the vast areas of "slick spots" which have little or no vegetation. These grasses and shrubs can provide fair nutrition for grazing animals, however, if properly managed. Care should be exercised when grazing pastures which are dominated by this site, to avoid nutritional stress.

A planned grazing system should be used which benefits both early and later season grasses. Grazing periods should be kept relatively short, and adequate rest provided prior to re-grazing. This type of rotational grazing, if carefully monitored, will provide the highest level of production and nutrition for grazing animals. Spring rest for May 1 to June 20 benefits cool-season plants such as western wheatgrass and creeping wildrye. While rest from June through July would be most beneficial to alkali sacaton and alkali cordgrass. Dormant season grazing greatly favors the plants of this site and allows for recovery during the growing season. Chemical brush control would effectively reduce black greasewood, however, adequate grass cover of alkali sacaton should exist prior to spraying. If rubber rabbitbrush is part of the community, the brush control must be conducted just as the rabbitbrush is beginning to flower, otherwise it will respond to the disturbance with an increase in population. A least greasewood provides good browse. Most of this site, consequently, is not suited for brush management. Any attempts at reseeding would result in total failure.

b. Guide to Initial Stocking Rates:

(1) Stocking rates given below are based on continuous use for the entire growing season, and are intended only as an intitial guide. Forage needs are calculated on the basis of 900 lbs of air-dry forage per animal unit month (AUM). To maintain proper use and allow for forage that disappears through trampling, small herbivore use, weathering, etc., 35% of the palatable forage produced is considered avaialable for grazing by large herbivores.

Condition Class/ % Climax Vegetation/ Ac/AUM 1/ AUM/Ac 1
excellent/ 76-100/ 15/ .07
good/ 51-75/ 25/ .04
fair/ 26-50/ 40/ .03
poor/ 0-25/ 55/ .02

Due to the high amount of bare ground in the mapping units, the stocking rates have been adjusted accordingly.

Adjustments to the initial stocking rates should be made as needed to obtain proper use. With specialized grazing systems, large livestock breeds, uncontrolled big game herbivores, inaccessability, dormant season use, presence of introduced species, etc., stocking rate adjustments will be required.

Major Poisonous Plants to Livestock:

Plant Common Name/ Livestock Affected/ Type of Poisoning/ Season Serious
Black greasewood/ sheep, cattle/ acute/ spring


The relative value of this site for wildlife is low. As ecological degradation occurs, however, food availability for jackrabbits and other small herbivores would decrease. Value for habitat would not be decreased.

Hydrological functions

Soils in this site are grouped into "D" hydrologic group, as outlined in the "Soils of Colorado Loss Factors and Erodiblity Hydrologic Groupings" handbook. Field investigations are needed to determine hydrologic cover conditions and hydrologic curve numbers. Refer to "Peak Flows in Colorado" handbook, and SCS "National Engineering Handbook", Section 4, for hydrologic curve numbers and determining runoff quantities.

Recreational uses

Recreation values are limited to hunting of rabbit species and the incidental use of this site in hunting or observing waterfowl near artesian wells, and other wetland areas associated with this site.

Inventory data references

Counties where this ecological site occurs include:
Alamosa, Conejos; Costilla. Rio Grande and Saguache

Field Offices in Colorado where the site occurs:
Alamosa, San Luis, and Center

References

• . 2021 (Date accessed). USDA PLANTS Database. http://plants.usda.gov.

Other references

Chapman, S.S., G.E. Griffith, J.M. Omernik, A.B. Price, J. Freeouf, and D.L. Schrupp. 2006. Ecoregions of Colorado. (2-sided color posterwith map, descriptive text, summary tables, and photographs). U.S. Geological Survey, Reston, VA. Scale 1:1,200,000.

Cleland, D.T.; Freeouf, J.A.; Keys, J.E.; Nowacki, G.J.; Carpenter, C.A.; and McNab, W.H. 2007. Ecological Subregions: Sections andSubsections for the conterminous United States. Gen. Tech. Report WO-76D [Map on CD-ROM] (A.M. Sloan, cartographer). Washington,DC: U.S. Department of Agriculture, Forest Service, presentation scale 1:3,500,000; colored.

Natural Resources Conservation Service (NRCS). 2017. Ecological Site Description for Chico Land #264(ESIS) : USDA, Denver Colorado.

United States Department of Agriculture, Natural Resources Conservation Service. 2006. Land Resource Regions and Major Land Resource Areas of the United States, the Caribbean, and the Pacific Basin. U.S. Department of Agriculture Handbook 296.

Contributors

C. Villa, H. Garcia, S. Woodall
Suzanne Mayne-Kinney

Approval

Kirt Walstad, 9/07/2023

Acknowledgments

Project Staff:
Suzanne Mayne-Kinney, Ecological Site Specialist, NRCS MLRA, Grand Junction SSO
Alan Stuebe, MLRA Soil Survey Leader, NRCS MLRA Alamosa SSO

Program Support:
Rachel Murph, NRCS CO State Rangeland Management Specialist, Denver
Eva Muller, Regional Director, Rocky Mountain Regional Soil Survey Office, Bozeman, MT
B.J. Shoup, CO State Soil Scientist, Denver
Eugene Backhaus, CO State Resource Conservationist, Denver

--Site Development and Testing Plan--:
Future work to validate and further refine the information in this Provisional Ecological Site Description is necessary. This will include field activities to collect low-, medium-, and high-intensity sampling, soil correlations, and analysis of that data.

Additional information and data are required to refine the Plant Production and Annual Production tables for this ecological site. The extent of MLRA 51 must be further investigated.

Field testing of the information contained in this Provisional ESD is required. As this ESD is moved to the Approved ESD level, reviews from the technical team, quality control, quality assurance, and peers will be conducted.