On the arid Mountain Home Plateau of Idaho more than 400,000 acres of arable land is largely unused except for grazing. The Boise Valley, adjoining on the north, was reclaimed from similar land and irrigated with Boise River water. The productivity of about 100,000 acres of the valley land is lowered or threatened, however, by an ever-rising water table, caused to rise by infiltration of excess irrigation water. Surface drains alleviate the drainage problem but areas of water-logging, alkaline land, and inferior vegetation continue to spread.

Idaho↗

USGS1955

Ground-water and drainage problems in the Whitney terrace area, Boise, Idaho

Ground-water levels in the Whitney terrace area, after many years of rise owing to downward percolation of water from irrigated land, became nearly stabilized by 1935. Strong seasonal water-level fluctuations of 8 to 10 feet, however, are common in the Boise Valley. In recent years, the Whitney terrace has been changing rapidly from a farming to an urban community. The 3,200-acre area now has about 15,000 inhabitants and the land area is about 35-percent occupied by urban developments. Population trends indicate that urban expansion will continue. Coincident with the change from rural to urban conditions on the Whitney terrace, the patterns of water use were drastically altered. The amount of irrigated land has decreased but the amount of ground-water recharge from irrigation and yard water may have increased. Liquid sewage effluent disposed in the ground also has increased the yearly volume of recharge. The estimated average yearly amount of recharge from all sources now is about 2.4 acre-feet per acre. Full urban development of the area may increase recharge from all sources to about 3.3 acre-feet per acre. Ground-water and drainage problems can be relieved by reducing excessive recharge to the ground-water reservoir. Reduction can be accomplished by economical use of water by individuals, establishment of a water-tight public sewage system to transport all sewage to a central plant outside of the area, and by drainage works. These measures would cause a net decline of ground-water levels in the area. They can be undertaken separately or collectively.

Idaho↗