Soil Water Dynamics

Change in soil moisture storage = inputs – outputs (eq. 1)

Water content increases (positive change in storage) when inputs including precipitation or irrigation exceed outputs. Water content decreases (negative change in storage) when outputs such as deep percolation, surface runoff, subsurface lateral flow, and evapotranspiration (ET) exceed inputs (eq. 1).

Water storage and redistribution are a function of soil pore space and pore-size distribution, which are governed by texture and structure (Childs 1940). Generally speaking, clay-rich soils have the largest pore space, hence the greatest total water holding capacity. However, total water holding capacity does not describe how much water is available to plants, or how freely water drains in soil. These processes are governed by potential energy. Water is stored and redistributed within soil in response to differences in potential energy. A potential energy gradient dictates soil moisture redistribution and losses, where water moves from areas of high- to low-potential energy (Hillel 1982). When at or near saturation, soils typically display water potentials near 0 MPa. Negative water potentials arise as soil dries resulting in suction or tension on water allowing the soil to retain water like a sponge.

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). 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. This mechanism of flow by the force of gravity occurs mainly in macropores. As the soil dries, field capacity is reached after free drainage of macropores has occurred. 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 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). 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. This free water is termed drainable porosity. In contrast, much of the water held at field capacity is available for plant uptake and use through evapotranspiration (Figure 2).

The point at which matric forces hold water too tightly for plant extraction (-1.5 MPa) is termed the permanent wilting point. The amount of water held between field capacity and permanent wilting point is considered plant available water (PAW) (Figure 2). Water held between these two states is retained against the force of gravity, but not so tightly that it cannot be extracted by plants. Mesopores and micropores supply most plant available water. Water held at potentials below permanent wilting point (< -1.5 MPa) is not available for use by most plants because it strongly adheres to mineral particles. Water held at permanent wilting point is associated with partially filled micropores and hydrated surfaces of soil particles (Figure 2).

Differences in soil properties (texture and structure) affect the water content at saturation, field capacity, and permanent wilting point. Texture and structure determine pore size distribution in soil, and therefore, the amount of PAW. Figure 3 illustrates how the magnitude of PAW changes with soil texture. Coarse textured soils (sands and loamy sands) have low PAW because the pore size distribution consists mainly of large pores with limited ability to retain water. Although fine textured soils have the highest total water storage capacity due to large porosity values, a significant fraction of water is held too strongly (strong matric forces/low, negative water potentials) for plant uptake. Fine textured soils (clays, sandy clays and silty clays) have moderate PAW because their pore size distribution consists mainly of micropores. Loamy textured soils (loams, sandy loams, silt loams, silts, clay loams, sandy clay loams and silty clay loams) have the highest PAW, because these textural classes give rise to a wide range in pore size distribution that results in an ideal combination of meso- and micro-porosity. Soil structure can increase PAW by increasing porosity. Soil depth and rock fragment content also affect water holding capacity because bedrock and rock fragments are assumed to be unable to hold plant available water and/or accommodate plant roots.

Water movement in soil is closely linked with storage because water potential is a function of water content (Richards 1931). Water flow is also influenced by texture and structure, and other factors such as the layering of soil profiles. The rate of water flow is a function of the potential energy gradient and the ease with which water is transmitted through soil, termed saturated hydraulic conductivity, which is governed by pore size distribution and tortuosity of flowpaths (Childs & Collis-George 1950, Vogel 2008). Clay-rich soils have low saturated hydraulic conductivity due to a highly tortuous flow path. Conversely, sandy soils have larger pores and lower tortuosity that facilitate rapid water flow. The National Cooperative Soil Survey identifies the following permeability classes based on soil texture (Table 1). These classes can be modified (qualitatively) by the degree of soil structure. Strong soil structure, consisting of very fine and fine aggregates (e.g., granular, fine and medium angular blocky and subangular blocky) facilitate rapid drainage of soil by increasing macroporosity. In contrast, weak structure or coarse-sized structural units (prismatic or blocky) and platy structure can inhibit flow, creating a more tortuous flow path constraining water to inter-structural voids (Figure 4). Soil structure is highly relevant to water management in soils because it is subject to change either through deterioration by improper management, or to improvement through additions of soil organic matter. In contrast, it is usually infeasible to change texture.