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(J' ~ - !: q /. Llli~ 22. commonly are necessary to determine the very few have been made in Colorado. extent of such areas and ., " Most of the water in the pe rmeable consolidated formations in the State is confined under artesian pressure by overlying dense beds of clay or shale. Some of the permeable beds crop out and are recharged at a considerably higher eleveation than a part of the area where they are confined. The artesian pressure in the areas of low elevations may in places be great enough to cause wells to flow. . Artesian aquifers have substantially different storage character- istics than most of the unconsolidated aquifers. Large amounts of water can be pumped from them without dewatering the materials near the well. Local dewatering occurs only when the pumping level is below the top of the water-bearing zone. Even in cases where the pumping level is low enough to cause dewatering, the zone will fill almost immed- iately when pumping ceases. The water is yielded largely from "ar- tesian storage, " a term that is used to describe the water yielded when the aquifer compresses and the water expands. After a period of time, which depends largely on the distance of the outcrop from the point of withdrawal, water stored in the out- crop will move toward the well. Even though the total amount moving toward the well may be large, the rate of movement in the outcrop generally is slow because the area contributing water generally is large. Thus, the water levels in the outcrop tend to decline over a large area, but at a slow rate. .., If perennial streams cross the outcrop in the area subject to deline and if they are hydraulically connected with the aquifer, the flow in them will be reduced. The loss in flow will tend to equal ultimately the ground-water withdrawal rate although it may take many year in some instances before the two are nearly equal. In many, per- haps in most, places in Colorado the effect on stream flow will be negligible owing to the scarcity of perennial streams, and the lack of extensive areas where they are hydraulically connected. As a res.llt, water withdrawn from artesian aquifers generally would ultimately result in a decreasing amount of water in storage in the area of out- crop, but the affected area probably would be small. I " From a practical standpoint, the amount of ground water that can be withdrawn from an artesian aquifer is not closely related to the amount of ground water in storage nor the amount of replenishment - 6 -