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.

 

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