Water Movement in Soils

Direction of Water Movement: The total potential energy of water is the sum of the potentials from all sources. Potential energy per unit mass or per unit volume or per unit weight is known as the potential of the water. So water free to move will move from a region where it has higher total potential to one of lower total potential. The potential due to gravity is known as the gravitational potential and that due to the soil particles is the matric potential. Soils whose pores are not filled have matric potentials less than zero. Saturated soils under the influence of external hydrostatic pressures have matric potentials which are greater than zero. The total potential at any point is just the sum of the gravitational and matric (or pressure) potentials at that point. The distribution of total potential within a soil allows us to determine if water will move and the direction of movement for any soil system. If the total potentials are equal, no movement will occur. Details about water potential and how it can be determined in soils are available here.

Rate of Water Movement: The section above introduces water potential and the manner in which it can be used to determine the direction of water movement. In this section, we will examine factors that determine the rate of movement. It is useful to think of water  movement as the product of a driving force causing water to move and a factor representing the ease with which water moves in the soil.  This was formalized by Henry Darcy in 1856 as 

where q is the volume of water flowing through a unit cross-sectional area of soil per unit time, K is the saturated hydraulic conductivity of the soil, TH is the total hydraulic head and x is the position coordinate in the direction of flow. This equation is known as Darcy's Law. For uniform saturated soils, it is useful to write this equation as

where TH_A is the total head at the inlet end of the soil, TH_B is the total head at the outlet end of the soil column, and L_AB is the distance between the inlet and outlet. The hydraulic conductivity, K, represents the ease with which water flows through a soil. Its value depends upon the soil properties and the properties of the soil water. The driving force, df,  is represented by  

Hydraulic Conductivity: Since some of the volume of a soil is occupied by soil particles, water does not flow through a soil as easily as it does through an open pipe. The ease of water movement is known as the hydraulic conductivity of the soil. Soils with large pores such as sandy soils tend to conduct water more easily than soils with smaller pores such as clay soils. Thus sands have higher saturated conductivity values than clays (Examples). As soils dry out, water in the largest pores is lost first. Water movement must then occur through smaller pores and in films near soil particles. These pores and films present more resistance so the conductivity of a soil decreases as it dries out (Examples). In nature the wetness of the soil often changes with position in the soil. Thus, the conductivity changes with position as well. 

Driving Force: The rate of water movement in soils increases with increasing driving force. In saturated soils the driving force is a result in differences in elevation and positive external pressures in the soil. In unsaturated soils, the attraction of the soil surfaces for water is often a major component to the driving force. Driving forces are commonly much larger in unsaturated soils than in saturated soils. The driving force commonly changes in magnitude with changing soil properties and wetness. Examples of this are available here.

For more insight into driving forces in soils under one-dimensional steady-state flow, utilize the Steady-state Water Movement software and associated exercises. 

Transient Water Movement

The following sections introduce dynamic flow processes that occur in nature.  The CHEMFLO-2000 software can be used to simulate most of these transient processes. The mathematical model, computational methods, and suggested numerical experiments are included in the software manual.

Infiltration Rate: Infiltration is a very dynamic process. Water applied to the surface of a relatively dry soil infiltrates quickly due to the affinity of the soil particles for water. As time passes and the soil becomes wet, the force of gravity becomes the dominant force causing water to move. The infiltration rate gradually decreases with time and approaches the value of the saturated conductivity of the soil as shown at the right. This example is for the Cobb sandy loam in Caddo County Oklahoma.

Many other factors influence the shape of these infiltration functions. They include the manner in which the unsaturated conductivity changes with water content or matric potential, the initial water content distribution in the soil, and changes in soil hydraulic properties with depth or time.

Clearly the answer is no. Forces which were present to cause water to move are still present. Those forces will continue to move water. The difference is that in order to wet the soil deeper in the profile, water must now be removed from upper wet regions. The figure at the right shows the predicted distributions of water at three times during redistribution (no water enters or leaves the soil surface during this time). Infiltration stopped at 12 hours. At 24, 36, and 48 hours, the water contents near the surface decrease and the wet front advances down the profile. The advance of the wet front is much slower than while water was being applied. The rate of advance and the  rate of drying of the surface soil both decrease with time. This redistribution will continue, but at slower and slower rates. Because the rates decrease rapidly with time, soils have the ability to store water for plant use.

Evaporation: We know that water which is free to move moves from regions of higher potential to lower potential. The sources of potential for unsaturated soils are primarily the attraction of soil surfaces for water and the force of gravity. Water movement is not limited to downward movement nor to movement in the liquid phase. Evaporation from the soil surface can be large as well. Evaporation of water requires that water be present in the soil at the evaporation site. It also requires a substantial source of energy to convert liquid water to a vapor. Without that energy, evaporation cannot occur. Finally, sustained evaporation requires that the water vapor above the evaporation site must be moved away from that site. If that does not occur, the air becomes saturated with water so evaporation stops.

Soon after infiltration, the soil surface is wet so plenty of water is available at the surface. In this early stage, the atmospheric demand or energy available to convert water to vapor are most likely to limit the evaporation rate. As the soil dries out, the ability of the soil to move water upward to the soil surface limits the evaporation rate.

Plant Uptake: Water moves from soil to plant roots and on through plants as a result of potential differences, just as it does elsewhere. Water potentials in plants and the conductivity of the plant parts change rapidly and vary over large magnitudes depending upon the atmospheric conditions and the physiology of the plant.

Plants are capable of removing large quantities of water from soils. Details at the microscopic scale of individual roots is difficult to study, but this basic work is being carried out. For most water management purposes, it is sufficient to quantify the average rate of water loss per unit area. This is often called the evapotranspiration rate or ET since it combines direct evaporation from the soil surface and evaporation of water taken up by plants and transpired through the plant system. Numerous methods have been developed to measure and estimate these evapotranspiration rates. Potential evapotranspiration rates or PET values are generally calculated for a reference crop growing under well watered conditions. ET values for a specific crop are obtained by multiplying this PET value by a crop coefficient to account for differences in plant species and stage of growth and by a soil factor reflecting differences in soil wetness and hence the soil potential and conductivity in the vicinity of the roots.

Drainage: Water moving below the root zone of the plant is available to move the chemical deeper into the profile and to recharge the underlying aquifer. This amount depends upon the soils ability to store water, the evapotranspiration of the area, and the amount of water infiltrating the soil from rainfall or irrigation. An interactive program is available to examine this in more detail.