Soil Moisture

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Soil Moisture

Soil Moisture

Soil moisture is difficult to define because it means different things in different disciplines. For example, a farmer’s concept of soil moisture is different from that of a water resource manager or a weather forecaster. Generally, however, soil moisture is the water that is held in the spaces between soil particles. Surface soil moisture is the water that is in the upper 10 cm of soil, whereas root zone soil moisture is the water that is available to plants, which is generally considered to be in the upper 200 cm of soil.

Methods of determination of soil moisture

Measuring soil moisture content in laboratory

1. Gravimetric method: This consists of obtaining a moist sample, drying it in an
oven at 105°C until it losses no more weight and then determining the percentage
of moisture. The gravimetric method is time consuming and involves laborious
processes of sampling, weighing and drying in laboratory.

2. Electrical conductivity method: This method is based upon the changes in
electrical conductivity with changes in soil moisture. Gypsum blocks inside of with
two electrodes at a definite distance are apart used in this method. These blocks
require previous calibration for uniformity. The blocks are buried in the soil at
desired depths and the conductivity across the electrodes measured with a modified
Wheatstone bridge. These electrical measurements are affected by salt
concentration in the soil solution and are not very helpful in soils with high salt
contents.

Measuring soil moisture potential insitu (field)

Suction method or equilibrium tension method: Field tensiometers measure the
tension with which water is held in the soils. They are used in determining the need
for irrigation. The tensiometer is a porous cup attached to a glass tube, which is connected to a mercury monometer. The tube and cup are filled with water and cup
inserted in the soil. The water flows through the porous cup into the soil until
equilibrium is established. These tension readings in monometer, expressed in
terms of cm or atmosphere, measures the tension or suction of the soil.

Characterstics Of Soil Moisture

1. Describes relationship between soil-water potential and volumetric water content

2. Can be determined by simultaneous measurement of water content and pressure potential

3. As soil drains, the largest soil pores empty first since the capillary forces are smallest in these pores. As the soil drains further, the maximum diameter of the water-filled pores further decreases, corresponding with pores that have decreasing values for the pressure potential (water is held by larger capillary forces)

4. Soil-water retention is unique and is a function of pore size distribution

5. At a pressure potential of zero, the soil’s volumetric water content is defined as the saturated water content

6. Maximum pressure potential at which soil begins to de-saturate (starting at saturation) is defined as the air entry value of the soil and is determined by the largest pores in the soil.

Soil moisture characteristic curve is more strongly affected by soil texture. Greater the clay content, greater the water content at any particular suction and more gradual the slope of the curve.

Infiltration

This refers to the amount of water that passes through the soil surface in terms of depth (inches) in a given time period (usually 1 hour). Lighter textured soils (such as sands or sandy loams) have desirable infiltration rates. This is important in the fact that the turf can have water penetrate into the soil relatively quickly so no runoff or puddling occurs. ‘Heavy’ textured soils which have a lot of clay and/or silt often have poor water penetration (infiltration), because the space that the soil occupies is relatively dense. Organic matter helps infiltration because of the soil aggregation that occurs with organic matter (makes for larger soil particles)! The addition of turfgrass to the turf-system generally increases infiltration rates. For essentially all soil-types, the initial intake of water is greatest at the beginning of an irrigation or rainfall (when the surface is driest). After the first 1/2″ becomes wet, the infiltrate rate will slow down.

Infiltration rate
Soil texture inches/hr.

Sand-coarse 1.00-8.00
Sand-very fine 0.50-3.10
Sandy loam 0.40-2.60
Loam 0.08-1.00
Clay loam 0.04-0.60
Clay 0.01-0.10

Percolation:

The movement of water through a column of soil is called
percolation. It is important for two reasons.
i) This is the only source of recharge of ground water which can be used through
wells for irrigation
ii) Percolating waters carry plant nutrients down and often out of reach of plant
roots (leaching)

Water Potential

Water potential is the potential energy of water in a system compared to pure water, when both temperature and pressure are kept the same. It can also be described as a measure of how freely water molecules can move in a particular environment or system. It is measured in kilopascals (kPa) and is represented by the Greek letter Psi (Ψ). Water potential is never positive but has a maximum value of zero, which is that of pure water at atmospheric pressure. When it comes to impure water, or water that has solutes in it, the more solute there is, the more negative Ψ becomes, since the solute molecules will attract the water molecules and restrict their freedom to move.

Soil Retention

The soils hold water (moisture) due to their colloidal properties and aggregation
qualities. The water is held on the surface of the colloids and other particles and in
the pores. The forces responsible for retention of water in the soil after the drainage
has stopped are due to surface tension and surface attraction and are called surface
moisture tension. This refers to the energy concept in moisture retention
relationships. The force with which water is held is also termed as suction.

Soil Water Movement

i) Saturated Flow
ii) Unsaturated Flow
iii) Water Vapour Movement

i) Saturated flow: This occurs when the soil pores are completely filled with water. This water moves at water potentials larger than – 33 kPa. 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.

Factors affecting movement of water
1. Texture, 2.Structure, 3.Amount of organic matter, 4.Depth of soil to hard pan,
5.Amount of water in the soil, 6.temperature and 7. Pressure
Vertical water flow .The vertical water flow rate through soil is given by Darcy’s law. The law states that the rate of flow of liquid or flux through a porous medium is proportional to the hydraulic gradient in the direction of floe of the liquid.

(ii) Unsaturated Flow
It is flow of water held with water potentials lower than- 1/3 bar. Water will
move toward the region of lower potential (towards the greater “pulling” force). In
a uniform soil this means that water moves from wetter to drier areas. The water
movement may be in any direction .The rate of flow is greater as the water
potential gradient (the difference in potential between wet and dry) increases and
as the size of water filled pores also increases. The two forces responsible for this
movement are the attraction of soil solids for water (adhesion) and capillarity.
Under field conditions this movement occurs when the soil macropores (noncapillary) pores with filled with air and the micropores (capillary) pores with water and partly with air.
Factors Affecting the Unsaturated Flow
Unsaturated flow is also affected in a similar way to that of saturated flow. Amount of moisture in the soil affects the unsaturated flow. The higher the percentage of water in the moist soil, the greater is the suction gradient and the more rapid is the delivery.
(iii) Water Vapour Movement

The movement of water vapour from soils takes place in two ways: (a)
Internal movement—the change from the liquid to the vapour state takes place
within the soil, that is, in the soil pores and (b) External movement—the
phenomenon occurs at the land surface and the resulting vapour is lost to the
atmosphere by diffusion and convection.
The movement of water vapour through the diffusion mechanism taken
place from one area to other soil area depending on the vapour pressure gradient
(moving force).This gradient is simply the difference in vapour pressure of two
points a unit distance apart. The greater this difference, the more rapid the
diffusion and the greater is the transfer of water vapour during a unit period.


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