Morphology, Porosity, and Hydraulic Conductivity of Weathered Granitic Bedrock and Overlying Soils

Abstract

AbstractGranitic bedrock, generally weathered several meters deep, is widespread in upland areas of California. Sound management of these areas requires knowledge of the hydraulic properties of both soils and weathered bedrock. The objectives of this research were to determine the saturated hydraulic conductivity (Ksat) of weathered granitic bedrock and overlying soils and to relate the measured Ksat to the regolith morphology 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‐loamy Alfisol ≈ 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 excavated by chopping with a spade and crushed into grains using bare hands. In situ Ksat, measured using constant‐head permeameters, was highest (29 cm h−1) in the Entisol AC horizon, which also had the greatest macrovoid (>0.1 mm diam.) space (24%), a low clay content (6%), and abundant roots to provide pore continuity. A pedogenic horizon also had the lowest Ksat, with the Alfisol Bt horizon averaging 0.9 cm h−1. This horizon had 9% macrovoid space and 21% clay. The Cr horizons had intermediate Ksat values (1.4–3.7 cm h−1). Low clay contents, joint traces, and macrovoids (7–9%) largely unimpeded by illuvial clay help make weathered granitic bedrock an effective conductor of water under saturated conditions. On a landscape scale, water should percolate readily into regolith on Entisol sites, whereas runoff is more likely on Alfisol sites due to the low Ksat of the Bt horizon.