Upload Now
ADVERTISEMENTS:
After reading this article you will learn about:- 1. Source of Soil Water 2. Forms of Soil Water 3. Permeability and Percolation 4. Free Energy 5. Potential 6. Suction and Tension.
Source of Soil Water:
The original source of soil water is rainfall, which occurs during the months of June to September and December-January. All parts of India do not receive equal rainfall.
The moisture bearing south-west monsoon wind blows over to the Arabian Sea and the Bay of Bengal and strikes the Western Ghats Mountains on the west coast of India and the Himalayan mountain ranges at Assam. Thus it rains heavily on the western side of the Western Ghats mountain ranges and Assam beginning from the last week of May.
ADVERTISEMENTS:
A broad and rather irregular belt of low rainfall exists on the eastern side of the Western Ghats Mountains, which stretches from the interior of Tamil Nadu to the Deccan Plateau and Madhya Pradesh. The south-west monsoon wind is deflected by the Himalayan mountain ranges towards upper India, during June to September when it rains heavily in West Bengal, Bihar, Uttar Pradesh and northern Punjab.
Since the Aravalli mountain ranges lies parallel to the direction of the south-west monsoon wind, so it passes over Gujarat, Rajasthan and Haryana where it rains little during June to September. The pressure increases in northern India during the latter part of the monsoon season.
This obstructs and pushes back the monsoon wind towards southern India. So the heavy rainfall occurs on the east coast of Tamil Nadu during October-November. The original source of irrigation water is the river system.
The six major river systems of India have been assessed to supply the following quantities of water:
Irrigation systems are being developed in order to utilize the above water resources of the river system. Well water and pond water are also used for irrigation at many places of India.
Forms of Soil Water:
Soil water is commonly of the following three forms:
(i) Gravitational or free water than moves under the influence of gravity, down the macrospores. It is held by the soil with a force of less than 0.1 bars. It is harmful to crops because it removes air and nutrients from the soil. Hence crops suffer from lack of enough oxygen and nutrient. Moreover, they cannot use this kind of water.
(ii) Capillary water is held on the surface of the soil particles and within the microspores by surface tension forces (0.1 to 31 bars). Plants can use it.
ADVERTISEMENTS:
(iii) Hygroscopic water is held with a force greater than 31 bars, mostly in non-liquid or vapour form. Higher plants cannot absorb it.
Infiltration:
Infiltration means the downward entry of water into the soil. A loose porous well aggregated surface soil allows more water to infiltrate it than a compact soil. If the land surface is covered with vegetation, it remains loose and porous.
Hence, more water will infiltrate it than a bare unprotected soil. More water infiltrates to clayey soils when they dry. So more water initially infiltrates clayey soil. But later, infiltration decreases, when the clayey soils swell and cracks are closed. Infiltration rates are classified in Table 5.1.
Permeability and Percolation of Soil Water:
Soil characteristics that determine the rate of movement of water and air in the soil is called permeability of the soil. The soil characteristics that determine the rate of movement of water down into the soil are called the hydraulic conductivity of the soil. Permeability and hydraulic conductivity of soils depends mainly on the amount of macrospores (non-capillary pores) in the soil.
Permeability and hydraulic conductivity of the surface soil is more than that of the sub soil because the surface soil contains more macrospores than the sub soil. If the soil possesses an impermeable layer, then the permeability of the whole soil is reduced.
These impermeable layers must be broken by sub-soiling to improve the permeability of the whole soil. Permeability of soils is red used by continuous tillage and improved by growing deep rooted crops. Permeability rates of soils are shown in Table 5.2.
Percolation means the downward movement of water down the column of the soil. An excess of rain or irrigation water that percolates down the column of soil also carries away considerable amounts of plant nutrients and recharges the ground water in the humid region.
ADVERTISEMENTS:
Percolation is negligible in the arid regions. More water percolates down the sandy soil than the clayey soils. If crops are grown on the field, they will utilize the water and less water will percolate down the soil. Permeability rates are classified as has been shown in Table 5.2.
Energy is involved in the retention of water by the soil, its movement, its absorption by crops and loss to the atmosphere. Potential energy is mainly involved. Potential energy = mgh where m is the mass of water, h is the height or distance the water has been moved and g is the acceleration due to gravity.
When work is done on soil water, for example it is raised to some higher elevation, and then its potential energy is increased. But when the soil water itself does work, for example it is absorbed by the clay particle or exchangeable cations, and then its potential energy is decreased. Water has a tendency to move from a state of higher potential energy to a state of lower potential energy.
Free Energy of Soil Water:
Free energy of soil water is a measure of its tendency to change, react or escape. Water has a tendency to move from state of higher free energy to a state of lower free energy.
Free energy is the same as potential energy because like potential energy, free energy of soil water is increased if work is done on it. If, for example, it is raised to a higher elevation and free energy of soil water is decreased, if it itself does work, for example, if it is absorbed by the clay particles or exchangeable cations.
Factors Affecting Free Potential Energy of Soil Water:
(i) Gravity tends to pull water down. Work is done to raise water to higher elevation when the applied kinetic energy is converted to potential energy. Hence potential or free energy of soil water is increased. Therefore, the potential or free energy of soil water located at a higher elevation is always higher than the same of the soil water located at some reference point at the lower elevation.
(ii) Water molecules are attracted by the clay particles and exchangeable cations. Water molecules, while moving towards them, do work. So the potential or free energy is decreased. Hence potential or free energy of soil water that has been held by clay particles and exchangeable cations is always less than the pure free water located at the same elevation as the soil water.
Potential of Soil Water:
Potential energy possessed by a unit quantity of water is called potential of soil water. It is very difficult to measure the absolute level of potential of soil water. So the difference of potential or free energy of soil water and that of pure free water at some reference point, usually at the lower edge of the soil profile is usually measured. Potential or free energy of water at this reference point is always considered zero.
If the potential or free energy of water at a particular point is more than the pure free water at the reference point, it is considered a positive, if it is less, it is considered negative. The total potential of soil water is tire sum of the potential of soil water due to gravity and due to the attraction between it and clay and humic particles, and exchangeable cations.
The total potential of soil water = Gravitational potential + Matric potential + Osmotic potential + Gravitational potential. It is the amount of work that must be done per unit quantity of pure water in order to transport isothermally (at the same temperature) and reversibly (which can also be put in a backward direction) an infinestimal quantity (extremely small quantity) of water from the reference point to the concerned point in soil.
Gravitational potential is usually positive because work has been done on the water to raise it from the lower edge of the soil profile, to the surface from where it tends to flow down, under the influence of gravity. Matric potential may be defined as the amount of work that must be done per unit quantity of pure water in order to transport isothermally and reversibly an infinestimal quantity of water from the reference point, to the concerned point located at the same elevation as the reference point in the soil.
It is due to the attraction of water by clay and humic particles which decreases the potential of water at the concerned point. Hence matric potential is always negative.
Osmotic potential is the amount of work done per unit quantity of pure water in order to transport isothermally and reversibly and infinestimal quantity of water from the reference point to the concerned point located at the same elevation as the reference point in the soil. It is due to the attraction of water by exchangeable cations, and a soluble salt which decreases the potential of soil water. Hence the Osmotic potential is always negative.
Suction and Tension of Soil Water:
Both the matric and osmotic attractive forces reduce the potential or free energy of water with respect to potential of pure water at the reference point, considered zero. Thus these are negative and are also called suction or tension of soil water.
Method of Expressing the Negative Potential or Suction or Tension of Soil Water:
Most commonly, the negative potential is expressed in terms of the height in centimeters of a column of water of one square centimetre cross-sectional area, whose weight is just equal to the negative potential or tension of soil water.
PF is the logarithm to the base 10 of the height in cms of a water column of 1 sq. cm. cross-sectional area whose weight is just equal to the negative potential or tension of soil water. Another method is to express it in terms of Bars.
One Bar is the pressure exerted by a column of water of length 1023 cm and of cross-sectional area of one sq. cm. One bar is approximately equal to one atmosphere, which is equal to a pressure of 14.7 pounds/sq. inch or 760 mm. of mercury at the mean sea level.
Comments are closed.