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t:'- I."- ("lj C' '--, The Grand Valley is underlain by Mancos shale formation and covers about 126,000 acres, of which 66,000 acres are irrigated cropland. About 6,000 acres of cropland are not cultivated in anyone year. Irrigation water is obtained mostly from the Colorado River and is delivered to the fields through Federal and private canal systems involving more than 200 miles of canals and over 500 miles of laterals. 3. salinity In The Grand Valley The Grand Valley contributes about 600,000 to 700,000 tons of salt annually to the Colorado River. It is assumed that most of these salts (gypsum, calcite and sodium sulfate) are leached from the soil and underlying Mancos shale and carried to the river by deep percolation through over irrigation and by seepage from water delivery and tailwater collection ditches. Seepage losses from unlined canals and farm delivery ditches, tailwater collection ditches and deep percolation from field irrigation are considered to be major sources of water that carries salt to the river. Salt loading is the major cause of salinity increase in the Colorado River and has caused water quality problems for downstream water users. 4. Climatic Conditions Climatic conditions vary in different parts of the Grand Valley and there is difference in temperature and precipitation between the east and west end of the valley. In general, the area has hot summers, mild winters, and a high percentage of sunny days. The hottest temperatures occur in July and August and can exceed 100 degrees for several days. The coldest temperatures occur in January and February and readings below zero are not unusual. The climatic summary for the Grand Junction area is presented in Appendix A. Average monthly temperatures and precipitation since the M&E program started are provided in this Appendix table. The average growing season varies from 154 days to 190 days in different parts of the Grand Valley. The Grand Valley has an arid climate and receives between 8 to 10 inches of moisture annually depending on the area. In the last three to four years, extreme record low temperatures in February have been the norm. As a result, in the last two years the peach crop in the valley has been destroyed. Other fruit trees and grapes have also succumbed to the cold. In 1991, the temperatures were colder than normal (April/May) in the spring and delayed crop planting (e.g. corn) and growth by about two weeks. Harvesting of corn was delayed by several weeks because of cold and wet fall (refer to climatic summary in Appendix A). 3