The intensively irrigated Arkansas River Valley in Colorado is underlain by a valley‐fill aquifer resting in a U‐shaped trough cut in relatively impermeable Cretaceous rocks. Ground water is pumped to supplement surface water; in the last 10 years pumping has more than doubled. Ground water is closely related to the Arkansas River; percolation from irrigation recharges the aquifer, which discharges into the river. Pumping has resulted in a reduction in streamflow because it intercepts water that ordinarily would have reached the river. The 1,500 irrigation wells in the Arkansas Valley withdrew 230,000 acre‐feet of water in 1964. An analog model is being used to evaluate the relation of ground water to surface water and to predict effects of changes in water management. The model, simulating a 150‐mile reach of the Arkansas Valley (Pueblo to the State line), has a resistor spacing of 8 per modeled mile. The framework for the model was a transmissibility map; transmissibility ranges from less than 50,000 to 700,000 gallons per day per foot. Specific yield averages about 0.2. Hydrologic boundaries, such as the Arkansas River, and the bedrock valley‐fill contact were simulated. Applied water, precipitation, evapotranspiration, and ground‐water pumping were the independent variables programed. The model is being verified by comparing predicted changes in water level and river discharge with observed changes.

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Harris County, in the West Gulf Coastal Plain in southeastern Texas, has one of the heaviest concentrations of groundwater withdrawal in the United States. Large quantities of water are pumped to meet the requirements of the rapidly growing population, for industry, and for rice irrigation. The water is pumped from artesian wells which tap a thick series of sands ranging in age from Miocene (?) to Pleistocene. The water-bearing sands, many of which contained slightly saline water, are interbedded with clays. Subsequent artesian circulation has flushed the sands, probably to the limit determined by the Ghyben-Herzberg principle. The base of the freshwater sands ranges in depth from about 100 feet over the salt dome near Hockley to more than 3,000 feet in the northeastern part of the county. Before large-scale groundwater withdrawals were begun, the hydraulic gradient sloped gently toward the coast. Then, as large quantities of water were withdrawn a large cone of depression was established, the hydraulic gradient was reversed, and salt water began to move slowly toward the centers of pumping. The rate of movement of the salt water is very slow and the closest salt water is probably 5 miles from centers of pumping in the deeper sands. However, the threat of saltwater intrusion is present and the rate of advance of the salt water should be watched by means of strategically placed observation wells. Other less probable potential sources of saltwater contamination which are discussed include upward movement of salt water from below, vertical movement around salt domes or along faults, downward seepage from surface sources, and contamination through leaking wells.