Water Flow in Soil

  Soil Moisture


Water contained in soil is called soil moisture. The water is held within the soil pores. Soil water dissolves salts and makes up the soil solution, which is important as medium for supply of nutrients to growing plants.

Importance of Soil Water

  • Soil water serves as a solvent and carrier of food nutrients for plant growth
  • Yield of crop is more often determined by the amount of water available rather than the deficiency of other food nutrients
  • Soil water acts as a nutrient itself
  • Soil water regulates soil temperature
  • Microorganisms require water for their metabolic activities
  • Soil water helps in chemical and biological activities of soil
  • Water is essential for photosynthesis

 

  How water is retained in soil?


Water is retained in soil basically due to following forces:

1.Cohesion and adhesion forces : These two basic forces are responsible for water retention in the soil. One is the attraction of molecules for each other i.e., cohesion. The other is the attraction of water molecules for the solid surface of soil i.e. adhesion. By adhesion, solids (soil) hold water molecules rigidly at their soil - water interfaces. These water molecules in turn hold by cohesion. Together, these forces make it possible for the soil solids to retain water

2.Surface tension : elastic tendency of a fluid surface which makes it acquire the least surface area possible.  

3.Polarity or dipole character: The water molecules are held by electrostatic force that exists on the surface of colloidal particles

  Factors Affecting Soil Water


  • Texture: Finer the texture, more is the pore space and also surface area, greater is the retention of water
  • Structure: Well-aggregated porous structure favors better porosity, which in turn enhance water retention
  • Organic matter: Higher the organic matter more is the water retention in the soil
  • Density of soil: Higher the density of soil, lower is the moisture content
  • Temperature: Cooler the temperature, higher is the moisture retention
  • Salt content: More the salt content in the soil less is the water available to the plant

 

  Classification of soil water


Gravitational water: 

Gravitational water occupies the larger soil pores (macro pores) and moves down readily under the force of gravity. Water in excess of the field capacity is termed gravitational water. Gravitational water is of no use to plants because it occupies the larger pores. It reduces aeration in the soil. Thus, its removal from soil is a requisite for optimum plant growth

 

Capillary water: 

Capillary water is held in the capillary pores (micro pores). Capillary water is retained on the soil particles by surface forces. It is held so strongly that gravity cannot remove it from the soil particles. The molecules of capillary water are free and mobile and are present in a liquid state. Due to this reason, it evaporates easily at ordinary temperature though it is held firmly by the soil particle; plant roots are able to absorb it. Capillary water is, therefore, known as available water

Hygroscopic water: 

The water that held tightly on the surface of soil colloidal particle is known as hygroscopic water. It is essentially non-liquid and moves primarily in the vapour form. Hygroscopic water held so tenaciously by soil particles that plants can not absorb it. Some microorganism may utilize hygroscopic water.

Biological Classification of Soil Water

There is a definite relationship between moisture retention and its utilization by plants.

Available water: The water which lies between wilting coefficient and field capacity. It is obtained by subtracting wilting coefficient from moisture equivalent

Unavailable water: This includes the whole of the hygroscopic water plus a part of the capillary water below the wilting point

Super available or superfluous water: The water beyond the field capacity stage is said to be super available. This water is unavailable for the use of plants. The presence of super-available water in a soil for any extended period is harmful to plant growth because of the lack of air.

Terminology

Field capacity:

Assume that water is applied to the surface of a soil. With the downward movement of water all macro and micro pores are filled up. It is the amount of water held in the soil when all pores are filled.

Sometimes, after application of water in the soil all the gravitational water is drained away, and then the wet soil is almost uniformly moist. The amount of water held by the soil at this stage is known as the field capacity or normal moisture capacity of that soil. It is the capacity of the soil to retain water against the downward pull of the force of gravity

Wilting coefficient:

As the moisture content falls, a point is reached when the water is so firmly held by the soil particles that plant roots are unable to draw it. The plant begins to wilt. The stage at which this occurs is termed the Wilting point and the percentage amount of water held by the soil at this stage is known as the Wilting Coefficient. It represents the point at which the soil is unable to supply water to the plant

Hygroscopic coefficient: 

The hygroscopic coefficient is the maximum amount of hygroscopic water absorbed by 100 g of dry soil under standard conditions of humidity (50% relative humidity) and temperature (15°C).

 

  Soil Moisture Constants


Infiltration: 

Infiltration refers to the downward entry or movement of water into the soil surface. Soil surface with vegetative cover has more infiltration rate than bare soil. Warm soils absorb more water than colder ones. Coarse surface texture, granular structure and high organic matter content in surface soil, all help to increase infiltration.

The infiltration rate is the velocity or speed at which water enters into the soil. It is usually measured by the depth (in mm) of the water layer that can enter the soil in one hour. An infiltration rate of 15 mm/hour means that a water layer of 15 mm on the soil surface, will take one hour to infiltrate.

In dry soil, water infiltrates rapidly. This is called the initial infiltration rate. As more water replaces the air in the pores, the water from the soil surface infiltrates more slowly and eventually reaches a steady rate. This is called the basic infiltration rate 

The infiltration rate depends on soil texture (the size of the soil particles) and soil structure and is a useful way of categorizing soils from an irrigation point of view

The most common method to measure the infiltration rate is by a field test using a cylinder or ring infiltrometer.


Percolation:

The movement of water through a column of soil is called percolation. It is important for two reasons;

  • This is the only source of recharge of ground water which can be used through wells for irrigation
  • Percolating waters carry plant nutrients down and often out of reach of plant roots (leaching). Percolation is dependent of rainfall. In dry region it is negligible and under high rainfall it is high. Sandy soils have greater percolation than clayey soil. Vegetation and high water table reduce the percolation loss

Permeability:

It indicates the relative ease of movement of water with in the soil. The characteristics that determine how fast air and water move through the soil are known as permeability. The term hydraulic conductivity is also used which refers to the readiness with which a soil transmits fluids through it.

  Soil Water Movement


Water movement in soil is mainly of three types:

  • Saturated Flow
  • Unsaturated Flow
  • Water Vapour Movement

Saturated flow: This occurs when the soil pores are completely filled with water. Saturated flow is water flow caused by gravity’s pull. It begins with infiltration, which is water movement into soil when rain or irrigation water is on the soil surface. When the soil profile is wetted, the movement of more water flowing through the wetted soil is termed percolation.

Hydraulic conductivity can be expressed mathematically as

  V = k f 
Where, 
          V = Total volume of water moved per unit time     
          f = Water moving force 
          k = Hydraulic conductivity of soil

Factors affecting movement of water

  • Texture
  • Structure
  • Amount of organic matter
  • Depth of soil
  • Amount of water in the soil
  • Temperature
  • Pressure

  Soil Moisture Retention Curve


KE negligible as flow is very slow in soil, PE is governing energy ; flows from zone of higher potential energy to zone of lower PE

WP and FC are Wilting Point and Field Capacity

Soil water potential :

Expression of specific PE of soil water relative to that of water in standard reference state (hypothetical reservoir of pure and free water at atmospheric pressure, same temperature and constant elevation)

This is called Hydrostatic pressure ; in saturated conditions which is generally greater then that of reference state thus positive.

 

  Darcy Law


Darcy Law for flow of liquids in soil. The law has following assumptions:

  • Consider a salt-free soil system
  • The soil matrix is subjected to a hydraulic potential head (H) difference with head at both ends of the soil column maintained constant.
  • For a condition when a steady flow occurs through the soil matrix from left to right, the hydraulic gradient (ΔH) across the soil matrix is Hin – Hout
  • and L is the length of flow or soil matrix
  • If volume of water flowing through the soil matrix is V in time t, then the volumetric flow rate  Q =V/t
  • If the cross-sectional area of flow is A; and the soil system is homogeneous and isotropic (soil properties uniform in all direction) then the volumetric flow rate through soil matrix is given by the following relationships

Flow types – Darcy Law

Vertical Flow

When flow is vertical both the pressure and gravitational head may vary and therefore, flow or gradient of flow may change

Horizontal Flow

the gravitational head is constant everywhere in the soil system and the pressure head is the only driving force

Validity of Darcy’s Law

There are two distinct regimes of fluid flow: laminar and turbulent. Laminar flow is a state of flow when water flows like a sheet with uniform velocity throughout. Each parcel of flow is nearly parallel to adjacent ones. The forces, which can cause acceleration, are nonexistent or insignificant. In turbulent flow portions of fluid move radially and axially.

Darcy law is dependent on Reynolds number (NRe)

Validity of Darcy’s Law

Limitations of Darcy’s Law

Darcy's Law is valid for laminar flow through the soil. Coarse-grained soils also behave similarly but in very coarse-grained soil, the flow is of turbulent nature. Hence Darcy's Law is not valid in such soils.

For flow through commercial pipes, the flow is laminar when Reynolds number is less than 2000, but in some soils it has been found that flow is laminar when the value of Reynolds number is less than unity

Darcy law is only applicable in laminar flow conditions. NRe values of less than 1 correspond to laminar flow.

 

  Hydraulic Conductivity


The hydraulic conductivity of a soil is a measure of the soil's ability to transmit water when submitted to a hydraulic gradient.

The hydraulic conductivity depends on

  • the soil grain size,
  • the structure of the soil matrix,
  • the type of soil fluid, and the relative amount of soil fluid (saturation) present in the soil matrix.

The important properties relevant to the solid matrix of the soil include pore size distribution, pore shape, specific surface, and porosity. In relation to the soil fluid, the important properties include fluid density and fluid viscosity 

When the soil is saturated with water (all pores filled) the hydraulic conductivity has a value called the saturated hydraulic conductivity

The saturated hydraulic conductivity (Ks) of a porous medium, such as soil, refers to its ability to conduct water when all pores are full of water. As the pores drain, the conductivity falls rapidly. With half the pore space drained (roughly field capacity) the conductivity has decreased, typically, by a factor of a thousand.

When three- fourths of the pore space has drained (roughly permanent wilting point) the conductivity is only one-millionth of its value at saturation.

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