To characterize soil moisture and infiltration, the physical controls on these processes must be considered. The primary soil controls will be considered in this chapter; however, other factors such as soil chemistry, thickness, soil layering or horizons, and preferential flow paths, as well as vegetation cover, tillage, roughness, topography, temperature, and rainfall intensity also exert important controls.4
A soil's particle size distribution has a large impact on its hydraulic properties. Soil particles less than 2 mm in diameter are divided into three texture groups (sand, silt, and clay) that help to classify broad soil types and soil water responses (Fig. 2).5 The type of clay and the coarse material over 2 mm in diameter can also have a
Handbook of Weather, Climate, and Water: Atmospheric Chemistry, Hydrology, and Societal Impacts, Edited by Thomas D. Potter and Bradley R. Colman. ISBN 0-471-21489-2 © 2003 John Wiley & Sons. Inc.
significant impact on soil water properties. An overview of methods for determining particle size properties is given by Gee and Batider.f>
Bulk density, ph (M/L3) is the ratio of the weight of dry solids to the bulk volume of the soil, and porosity, (p (M3/M3), is the total volume occupied by pores per unit volume of soil:
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