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In two documented cases, pumping of large‐capacity, 13 to 16 l/s (200 to 250 USgpm) irrigation wells completed in fractured granodiorite of Jurassic age resulted in the occurrence of trough‐like cones of depression extending up to 1,500 m (4,900 ft) away from the pumped wells. The longitudinal axes of the drawdown troughs developed along trends coinciding with areal intersections of the two prominent bedrock joint sets prevalent in the region and bisecting the angles between the sets. The drawdown troughs appear to extend preferentially away from the pumped wells in the general direction of increasing regional potentiometric head towards local recharge areas. Occurrence of this phenomenon indicates that fracture anisotropy in this terrain has a pronounced effect on the magnitude and direction of drawdown interference about a pumped well. To be effective, several monitor wells strategically placed may be required to adequately assess drawdown interference in crystalline terrain. Although large‐capacity wells may be effective in dewatering extended narrow zones in the granitic bedrock, several closely‐spaced pumping wells sited in series normal to the regional potentiometric gradient in conjunction with monitoring of peripheral observation wells are recommended as a first step in engineering situations where dewatering a large area of this type of fractured granitic body may be desired.

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