Soil moisture

Soil moisture is the key state variable in climate of the second kind and the terrestrial branch of the hydrologic cycle. It controls the partitioning of available energy at the surface into sensible heat and latent heat exchanges with the atmosphere, and it links the water and energy balances of the surface through the moisture and temperature states of the soil. Soil water is the immediate source of moisture that evaporates and transpires from the soil and vegetation into the atmosphere (Robock, 2003), and it is the switch that controls the proportion of precipitation that percolates vertically into the soil, evaporates from the land, or eventually becomes runoff. Soil moisture integrates precipitation and evaporation over extended periods, and it, along with snowcover, provides a significant memory component for the atmosphere-land system.

Soils form in response to physical, chemical, and biological processes modifying parent material on the land surface. The soil resulting from these natural processes is a mixture of solid, liquid, and gaseous materials. The solid materials include particles of different sizes, shapes, and mineral composition along with organic matter. Solid particles commonly comprise the majority of the mass volume of the soil and constitute the soil matrix. Soil water and soil air occupy the void spaces or pores between the solid particles. The volume of air and water in pores is complementary in that as one increases the other decreases. Porosity is an index of the relative pore space in a soil, and the size of pores varies depending on soil texture and soil structure. Soil texture is determined by the relative amounts of the mineral particles of sand, silt, and clay making up the soil matrix. Soil structure is the arrangement of the soil particles into granules or soil aggregates of different shapes, sizes, and volumes of pore spaces. General values of porosity range from 30% to 60%. The wettest possible soil condition in which all the pores are filled with water is saturation (Hillel, 2004).

Soil moisture is commonly regarded as the amount of water in the upper layer of the soil typically down to a depth of one meter that interacts with the atmosphere. The amount of moisture in this soil layer is a function of precipitation, soil texture, soil structure, soil porosity, organic matter content, and the rate of moisture withdrawal from the soil, and soil moisture varies considerably on scales of only a few meters (Miller et al., 2005). Fine sands hold much less water per unit depth than silt or clay soil, but moisture storage for most soils is around 15% of the bulk soil volume of the root zone, which is the layer from which plant roots can extract water during transpiration (Dingman, 1994). Precise in situ soil moisture measurements are sparse, each value represents a small area, and the available data are relatively recent. The Global Soil Moisture Data Bank contains soil moisture observations for over 600 stations, but all of the stations are in the Northern Hemisphere (Robock et al., 2000). Details of soil-water relationships and direct and indirect field methods for determining soil moisture appear in the soil science and hydrology literature (e.g. Maidment, 1993; Ward and Elliot, 1995; Hillel, 2004; Rose, 2004). An overview of soil properties relevant to soil moisture and selected soil moisture measurement techniques are presented here.

4.7.1 Soil water properties

Soil water in the soil profile is important for moisture storage and for influencing energy and moisture fluxes in land surface hydroclimatic processes. The soil profile is a vertical cross-section through the soil commonly comprising a number of soil layers having different physical characteristics. Soil texture and soil structure are the physical characteristics most relevant to the soil's ability to store and release water. In the following discussion, unsaturated soil conditions are assumed.

Two basic approaches aid in characterizing and measuring soil water. A physically based approach emphasizes the moisture status of the soil resulting from the interaction of forces related to the solid, liquid, and gaseous soil components. A second approach employs the energy state of soil water. Although the energy state is most often associated with soil water movement, it is relevant to soil wetness (the contemporary term for soil water content) because it is used to

Adsorbed water

Soil particle

Capillary water

Fig. 4.8. Simplified and magnified view of water in soil pores.

Adsorbed water

Soil particle

Capillary water

Fig. 4.8. Simplified and magnified view of water in soil pores.

estimate soil water storage and to define the work required to remove water from the soil by evaporation and plant transpiration (Hillel, 2004).

Soil wetness

Under natural conditions, soil moisture is dependent on time since the last precipitation event. Water entry into the soil through the soil surface occurs by infiltration as discussed in Section 4.7.4.

Water molecules adhere to soil particles as a film of water in response to forces related to capillarity, adsorption, and osmosis. Capillary forces are the result of surface tension between soil air and soil water and the attraction of water molecules to compatible surfaces provided by soil minerals with oxygen atoms to share with water's hydrogen atoms. Liquid molecules are attracted more to each other than to water vapor molecules in the air, and this causes contraction of the liquid surface. A curved liquid surface results from the pressure differences experienced by the liquid and vapor molecules. The pressure on the liquid molecules is likely to be lower than the atmospheric pressure acting on the vapor molecules (Hillel, 2004). Withdrawing water from the soil increases the pressure difference as the liquid surface becomes more curved and is maintained only in the smaller pores (Fig. 4.8). Soils with larger pores retain less water than soils with smaller pores because the smaller pores act as narrower capillaries and exert greater force on soil water than the force exerted by larger pores.

Capillary water is augmented by water molecules adsorbed upon the surface of soil particles largely due to electrostatic forces. The forces involved are only effective very close to the soil particle surface and only a thin film of water is held in this way. Adsorptive forces are greatest for the first layer of water molecules, and the second layer is attached to the first by hydrogen bonding so that the attractive force diminishes rapidly with distance as each additional layer is added (Hillel, 2004). Differences in soil particle size are important in determining differences in adsorbed water. Large soil particles are able to adsorb more water than small particles due to their greater surface area. Adsorbed water has a degree of mobility that becomes important at low soil wetness (Catriona et al., 1991).

Capillary and adsorptive forces are regarded as being in equilibrium and are not easily measured separately (Ward and Robinson, 2000). The water pressure in pores is less than atmospheric pressure in unsaturated soils, and both capillary and adsorption forces are regarded as exerting tension or suction on the soil water. Therefore, it is usual to consider their combined effect on water in the soil matrix as matric suction or matric potential. The terms tension, suction, and pressure are used interchangeably in soil water studies. Tension or suction is negative pressure in that the is less than atmospheric pressure (Hillel, 2004).

Osmotic pressure is a third force acting to retain soil water. This force is due to the presence of solutes in soil water and the dipolar nature of water molecules. Ions in solution are attracted by the electric field around individual water molecules. The influence of osmotic pressure is often ignored unless there is a difference in solute concentration across a permeable membrane. In addition, osmotic pressure applies to water alone and not the soil solution (Hillel, 2004).

The total force holding water in the soil is the sum of the matric and osmotic forces. These forces vary with soil wetness, and soil wetness varies with differences in the shapes and sizes of soil particles. Soils with large pores empty at low suction, but soils with small pores empty at higher suctions. The relationship between soil wetness and soil moisture suction is fundamental to understanding soil moisture behavior and measurement (Ward and Robinson, 2000).

In theory, the capillary/adsorptive attraction is greater than the force of gravity and this water does not drain away. Additional water entering the plant root zone percolates vertically through the soil once the film of capillary water has reached a maximum diameter. The soil water remaining after internal drainage ceases is traditionally known as field capacity, but this is a subjective concept that lacks a universal physical basis since drainage typically continues for long periods (Hillel, 2004). The water moving vertically in response to gravity passes through the zone of air and capillary water storage and continues downward as long as void spaces in the strata are available to provide transmission routes. Eventually, this gravity water may fill all of the void spaces in the strata. The subsurface water driven by gravity is groundwater that commonly discharges into rivers and streams and sustains streamflow between rainfall events.

Capillary water adhering to soil particles is vital to plants that are able to extract the water by the suction action created by their vascular structure and transmitted to their roots. The root depth of plants varies from a few millimeters to tens of meters, and the water accessible to plants varies with the root depth and the physical characteristics of the soil. A capillary water flow toward the surface can occur under extreme drying conditions at the surface, but the quantity of water responding in this manner is very small. Consequently, the common convention is to assume that capillary water is available to plants only, and the quantity of this water is estimated by the plant's rooting depth. Within the root zone, plants extract 40% of their needed water from the top quarter of the root zone, and only 10% comes from the bottom quarter of the plant root zone.

Soil wetness can be expressed either on a mass or volume basis. In either case, the derived value is dimensionless and can be regarded as a fraction or expressed as a percentage. Mass wetness (Og) is expressed as

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