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Morphology, Porosity, and Hydraulic Conductivity of Weathered Granitic Bedrock and Overlying Soils
Author(s) -
Graham R. C.,
Anderson M. A.,
Sternberg P. D.,
Tice K. R.,
Schoeneberger P. J.
Publication year - 1997
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1997.03615995006100020021x
Subject(s) - bedrock , geology , entisol , alfisol , saprolite , horizon , soil water , loam , pedogenesis , hydraulic conductivity , silt , soil horizon , regolith , geochemistry , loess , mineralogy , geomorphology , soil science , physics , astrobiology , astronomy
Granitic 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 ( K sat ) of weathered granitic bedrock and overlying soils and to relate the measured K sat 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 K sat , 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 K sat , 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 K sat 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 K sat of the Bt horizon.