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Show PART II PRESENT STATUS OF IRRIGATION Drainage Most of the drainage problems in the irrigated lands of the Upper Colorado Region have been caused by one or more of the following factors: (l) improper water management, including continuous irrigation and seep- age from unlined canals and laterals; (2) restricted permeability in soil layers or underlying materials, and (3) topographic position. Poor drainage results in inadequate aeration in the plant root zone and some soluble salt accumulations in the upper soil horizons. As mentioned previously, 68 percent of the region's irrigated crop land is in grass hay and pasture and these are produced primarily in mountain meadows of Colorado and Wyoming at elevations ranging from 6,000 to 9*000 feet. It is common practice for farmers to flood-irrigate the meadow lands during the early season when streamflow is high and discon- tinue irrigation about the first of August to allow harvesting. High water tables occur throughout the bottom lands of the high mountain val- leys as a result of almost continuous application of irrigation water dur- ing this period. Most of the meadow lands are naturally well-drained. Their favorable topography and adequate depths of permeable soils and subsurface materials effectively control water table levels for normal cropping. The exten- sively grown, shallow-rooted meadow hay species tolerate high water tables for prolonged periods during the irrigation season and the excellent qual- ity of the water supply in the headwaters of the streams permits aeration and little or no accumulation of salts in the soils. When irrigation is discontinued, the ground water usually recedes rapidly to depths well be- low levels that are injurious to crops. Meadow crop production has been materially improved in a few local areas where drainage, land preparation, reseeding, fertilization, and controlled irrigation are practiced. Where land preparation is inadequate, the high water table permits capillary rise of water in the root zone in high spots in the meadows and results in the development of seeped areas and phreatophyte growth in the low areas. A few shallow drains have been installed in these low spots to provide better surface drainage and some degree of control of the ground water table. As a result the forage is improved and the farmer is able to perform haying operations. Alfalfa, small grains, and other annuals or speciality crops are pro- duced on about 32 percent of the irrigated acreage in the Upper Colorado Region. These crops are grown on lands along the lower reaches of stream valleys of the region, primarily between elevations of *l-,000 and 7*000 feet. The growing season is relatively longer than that in the upper reaches, permitting a wider selection of crops, many of which have a higher irrigation demand. The cropping systems require stringent ground water controls for sustained productivity. Most irrigated lands in lower river valleys occur on large, gently sloping fans, on prominent mesas and terraces, and to a lesser extent on low, flat-lying river flood plains. 37 |