ABSTRACT
Water in soil occupies pore spaces that arise from the physical arrangement of the particulate solid phase, competitively and oen concurrently with the soil gas phase (Chapters 1, 2, and 8). While hidden from casual view, substantial volumes of water are commonly stored in soils. For example, one hectare of mediumtextured soil 1 m deep and having eld capacity water content of 20% by volume may store su¬cient water to ll 4000 200 L barrels. ¤is reservoir serves as a resource pool for plant growth and a substantial bu¦er in areas having scattered or sporadic precipitation. Other soil organisms, many of which are bene- cial, especially to biogeochemical cycling, also rely heavily on the water-holding characteristics of soils for their existence. On the other hand, soil water is a highly dynamic entity, exhibiting substantial variation in both time and space (Western et al., 2002). ¤is is particularly true near the soil surface and in the presence of active plant roots. Changes in soil water content and its energy status a¦ect many soil mechanical properties including strength, compactibility, and penetrability, and may cause changes in the bulk density of swelling soils. ¤e liquid-phase characteristics a¦ect the soil gaseous phase and the rates of exchange between these phases, as well as other important soil properties such as the hydraulic conductivity that governs the rate of water and soluble chemical ¥ow.
