![]() Figure 3 illustrates how the magnitude of PAW changes with soil texture. Texture and structure determine pore size distribution in soil, and therefore, the amount of PAW. In contrast, much of the water held at field capacity is available for plant uptake and use through evapotranspiration (Figure 2).ĭifferences in soil properties (texture and structure) affect the water content at saturation, field capacity, and permanent wilting point. This free water is termed drainable porosity. Water held between saturation and field capacity is transitory, subject to free drainage over short time periods, hence is it is generally considered unavailable to plants. As water content decreases, soil matric potential decreases, becoming more negative, and as a result, water is held more strongly to mineral surfaces due to cohesive forces between water molecules and adhesive forces associated with water and mineral particles ( capillary forces). Field capacity represents the soil water content retained against the force of gravity by matric forces (in micropores and mesopores) at tension of -0.033 MPa (Figure 2). As the soil dries, field capacity is reached after free drainage of macropores has occurred. This mechanism of flow by the force of gravity occurs mainly in macropores. When the soil is at or near saturation the direction of the potential energy gradient is downward through the soil profile or laterally down slope. Three soil moisture states, saturation, field capacity and permanent wilting point are used to describe water content across different water potentials in soil and are related to the energy required to move water (or extract water from soil) (Figure 2). Negative water potentials arise as soil dries resulting in suction or tension on water allowing the soil to retain water like a sponge. When at or near saturation, soils typically display water potentials near 0 MPa. A potential energy gradient dictates soil moisture redistribution and losses, where water moves from areas of high- to low-potential energy (Hillel 1982). Water is stored and redistributed within soil in response to differences in potential energy. These processes are governed by potential energy. However, total water holding capacity does not describe how much water is available to plants, or how freely water drains in soil. Generally speaking, clay-rich soils have the largest pore space, hence the greatest total water holding capacity. Water storage and redistribution are a function of soil pore space and pore-size distribution, which are governed by texture and structure (Childs 1940). These concepts are integrated with a case study that describes the interplay between hydrologic processes, water dynamics in soil, and soil genesis. The learning objectives discussed here include: 1) an introduction to the soil water balance equation 2) the factors and soil properties that govern water potential and plant available water holding capacity and 3) soil morphologic features and classification methods to describe the fate of water in soil. This article focuses on soil water dynamics and introduces concepts of soil moisture storage, water flow and the soil properties that influence these processes. Water dynamics in soil are governed by many factors that change vertically with depth, laterally across landforms and temporally in response to climate (Swarowsky, et al., 2011). Thus, most aspects of terrestrial- and freshwater aquatic-life depend on hydrologic processes in soil (O'Geen et al. As such, the sustainability of water resources (considering both quantity and quality) is directly influenced by soil. Water percolating through soil is filtered, stored for plant utilization, and redistributed across flow paths to groundwater and surface water bodies. The capacity of soil to regulate the terrestrial freshwater supply is a fundamental ecosystem service.
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