MORPHOLOGY, POROSITY, AND HYDRAULIC CONDUCTIVITY OF WEATHERED GRANITIC BEDROCK AND OVERLYING SOILS

Granitic bedrock, generally weathered several meters deep, is widespre ad in upland areas of California. Sound management of these areas requ ires knowledge of the hydraulic properties of both soils and weathered bedrock. The objectives of this research were to determine the satura ted hydraulic conductivity (K-sat) of weathered granitic bedrock and o verlying soils and to relate the measured K-sat to the regolith morpho logy and porosity. The study sites in the foothills of the San Jacinto Mountains represent common regolith conditions in granitic terrain of southern California: a coarse-loamy Entisol <50 cm thick and a fine-l oamy Alfisol approximate to 100 cm thick, both over weathered granitic rock. The underlying bedrock (Cr horizon) at each site has a low clay content (<6%) and is weathered such that chunks of it can be excavate d by chopping with a spade and crushed into grains using bare hands. I n situ K-sat, measured using constant-head permeameters, was highest ( 29 cm h(-1)) in the Entisol AC horizon, which also had the greatest ma crovoid (>0.1 mm diam.) space (24%), a low clay content (6%), and abun dant roots to provide pore continuity. A pedogenic horizon also had th e lowest K-sat, with the Alfisol Bt horizon averaging 0.9 cm h(-1). Th is horizon had 9% macrovoid space and 21% clay. The Cr horizons had in termediate K-sat values (1.4-3.7 cm (h-1)). Low clay contents, joint t races, and macrovoids (7-9%) largely unimpeded by illuvial clay help m ake weathered granitic bedrock an effective conductor of water under s aturated conditions. On a landscape scale, water should percolate read ily into regolith on Entisol sites, whereas runoff is more likely on A lfisol sites due to the low K-sat of the Bt horizon.