Irrigation

Irrigation is defined as the artificial application of water to the soil for the purpose of crop growth or crop production in supplement to rainfall and ground water contribution.

Purpose of Irrigation

• To add water to the soil for supplying the moisture essential for the plant growth
• To provide crop insurance against short duration drought
• To cool the soil atmosphere, thereby making more favourable environment for plant growth
• To washout or dilute salts in the soil
• To reduce the hazards of soil piping
• To soften the tillage pans

Importance of Irrigation

• Agriculture is the biggest user of water. The largest share of India’s freshwater consumption (78 per cent in 2010) comes from the agricultural sector (CWC, 2014).
• But the sector is facing competition for water due to increased urbanization and industrialization in the country.
• Unfortunately, even after 70 years of planned development and large public and private expenditure, only about 49 per cent of the cultivated area has access to irrigation.
• About 86 per cent of irrigated agriculture is through two main sources:
  • Groundwater (62 per cent)
  • Canals (24 per cent)
• India (1st) has highest area under irrigation in the world.
• Most irrigated state (Maximum area under irrigation of total) in India: Punjab
• Maximum total area is under Uttar Pradesh and which is by canal irrigation.
• The main source of irrigation in India is Tube-wells which provide 46% of water for irrigation. Other sources include Canals (24%), Other wells (16%), tanks (3%) and other sources (11%).

Advantages of Irrigation

• Irrigation plays a key role in increasing food production to feed the expanding population.
• Irrigation can ensure a stable production in traditional dryland farming systems, subjected to frequent vagaries of rainfall.
• Irrigation can prolong the effctive growing period in areas with dry seasons, plant permitting multiple cropping and employment generation.
• With the security of cropping under irrigation, additional inputs (tillage, fertilizers, plant protection etc) become economically feasible.
• Irrigation reduced the risk of expensive inputs being wasted by drought.

Adverse Effects of Excess Irrigation

• Irrigation without appropriate drainage leads to land degradation (waterlogging and soil salinization) leading to reduce crop productivity
• Ground water pollution, especially with nitrates, due to seepage of water carrying nitrate from applied fertilizer to the ground water
• Irrigation may lead to colder and damper climate conductive to outbreak of pests and diseases.

Life Saving Irrigation

• It is also known as contingency irrigation.
• Supplemental irrigation applied to dry land crops.
• The land is not prepared for irrigation.
• Irrigation water is applied to the fields by dividing them into plots of 20-25 m width with a small bund to guide the water.

Irrigation Management

• Irrigation water management is the act of timing and regulating irrigation water applications in a way that will satisfy the water requirement of the crop without the waste of water, soil, plant nutrients, or energy.
• It means applying water according to crop needs in amounts that can be held in the soil available to crops and at rates consistent with the intake characteristics of the soil and the erosion hazard of the site.
• In order to carry out the Irrigation Management effectively, we have to do a study regarding the following:
• The physical and chemical properties of soil are:
  • Biology of crop plants
  • Quantity of water available
  • Time of application of water
• The management of all the above said factors is known as Irrigation Agronomy.

👉🏻 Importance of Irrigation Management

• To store and regulate the water resources for further use or non-season use.
• To allocate the water with proper proportion based on area and crop under cultivation. (Balanced equity in distribution)
• To convey the water without much loss through percolation and seepage. (Efficiency in use)
• To apply sufficient quantity to field crops. (Optimization of use)
• To utilize the water considering cost-benefit (Economically viable management)
• To distribute the available water without any social problem (Judicial distribution)
• To meet the future requirement for other purposes like domestic use of individual and to protect against famine (Resource conservation).
• To protect the environment from over use or misuse of water (Environment safe use).

❓ Why we need irrigation?

• India gets most of its rain in the monsoon season. There is a lot of spatial and temporal variation when it comes to the pattern of rainfall in India.
• This unpredictability and variation have increased in the recent years due to the rampant climate change due to global warming.
• This has made dependence on rainfall for the growth of crops difficult.
• It is for this reason that we require irrigation.

The seasons of rainfall in India

Important Irrigation Concepts

Matric Potential

The total water potential that is attributed to the solid colloidal matrix of the soil system.

Capillary Potential

The energy with which water is held by soil is defined in terms of capillary potential.

Seepage

• It is the horizontal flow of water channel.
• Water loss from the irrigation channel or canal is mainly due to seepage.
• There are many factors that affect seepage from canals - texture of the soil in the canal bed and banks, water temperature changes, siltation conditions, bank storage changes, soil chemicals, water velocity, microbiological activity, irrigation of adjacent fields, and water table fluctuations.
• Proper design and construction of conveyance systems are necessary to minimize seepage, due to the limited available water supply and ever-increasing demand for water.
• Seepage is not only a waste of water, but also may lead to other problems such as waterlogging and salinization of agricultural land.

Infiltration

• Entry of water from the upper layer of the soil is called infiltration.
• It occurs in unsaturated soil.
• It is a process which is also used to measure the speed with which water enters the soil in case of rain or when water is supplied to the ground through human made means.
• The speed of infiltration is measured in the amount of water absorbed per hour. This amount is in inches or in millimeters. The infiltration capacity of a soil is high at the beginning of a storm and has an exponential decay as the time elapses.
• The infiltration characteristics of the soil are one of the dominant variables influencing irrigation.
• Infiltration rate is the soil characteristic determining the maximum rate at which water can enter the soil under specific conditions, including the presence of excess water.
• The actual rate at which water is entering the soil at any given time is termed the Infiltration velocity.
• The infiltration rate decreases during irrigation. The rate of decrease is rapid initially and the infiltration rate tends to approach a constant value. The nearly constant rate that develops after some time has elapsed from the start of irrigation is called the basic infiltration rate.
• The major factors affecting the infiltration of water into the soil are the initial moisture content, condition of the soil surface, hydraulic conductivity of the soil profile, texture, porosity, and degree of swelling of soil colloids and organic matter, vegetative cover, duration of irrigation or rainfall and viscosity of water.
• Infiltration rates are generally lower in soils of heavy texture than on soils of light texture.
• Infiltration rates on grassland is substantially higher than bare uncultivated land. Cultivation influences the infiltration rate by increasing the porosity of the surface soil and breaking up the surface seals.
• Additions of organic matter increase infiltration rate substantially.
• The high rates of infiltration in the tropics under otherwise comparable soil conditions is due to the low viscosity of warm water.

Percolation

• Downward movement of water through saturated or nearly saturated soil in response to the gravity or we can put it as the descending motion of infiltered water through soil and rock layers. ^{AFO 2017}
• Thus, the Percolation process represents the flow of water from unsaturated zone to the saturated zone.

emoji="👀" ❓ Difference between Infiltration and Percolation Infiltration occurs closer to the surface of the soil. Infiltration delivers water from the surface into the soil and plant rooting zone while Percolation moves it through the soil profile to replenish ground water supplies or become part of sub-surface run-off process.

Leaching

Downward movement of nutrients and salts from the root zone with the water is called leaching.

Runoff

The flow of excess water from the field after saturation of soil.

Saturation Capacity

This is the maximum water holding capacity of the soil where all the soil pores (Macropores and Micropores) are completely filled with water.

Field capacity (FC)

• The soil moisture content after 2-3 days of irrigation and after drainage of gravitational water has become very slow and soil moisture content has become relatively stable.
• At the field capacity, the large pores are filled with air and the micro pores are filled with water.
• It is considered as the upper limit of water availability to plants.

Permanent Wilting Point (PWP)

• The concept of PWP was proposed by Briggs and Shantz in 1912.
• They utilized dwarf sunflower as an indicator plant.
• It is the soil moisture content at which plants can no longer obtain enough moisture to meet their requirement and remain wilted unless water is added to the soil.
• It is the lower limit of available water to the plant.
• In this case, though the plants are not dead, they are in a permanently wilted condition and would die if water is not added.

Wilting Coefficient

The percentage of moisture in root zone at the permanent wilting of plants is called wilting coefficient or critical moisture point.

Available water

• This concept was given by Veihmayer and Hendrickson in 1981.
• It is the moisture available for maximum plant use.
• It is arrived at by subtracting the water at Field Capacity and water at the Permanent Wilting Point (PWP).

Ultimate Wilting Point (UWP)

• The moisture content at which the wilting is complete, and the plants die is called UWP.
• At UWP, the soil moisture tension is as high as -60 bars.

Soil Moisture Tension

Soil moisture tension is a measure of the tenacity with which water is retained in the soil and shows the force per unit area that must be exerted to remove water from a soil.

Soil Water Potential

• Most of the issues about soil-water relate to its energy state and its movement (e.g. Evapotranspiration and Deep Drainage).
• Classical physics recognizes kinetic (i.e. movement) and potential (i.e. position) energy. In soil, water does not move rapidly so kinetic energy is negligible.
• Therefore, water moves constantly in direction of potential energy (i.e. wet to dry soil), where the gradient of potential energy with distance is the moving force causing flow.
• An indication of the tendency of soil water to move is expressed by the Soil Water Potential (Ψ).
• Ψ is defined as the work water can do as it moves from its present state to the reference state. The reference state is the energy of a pool of pure water at an elevation defined to be zero.
• In soil, the reference state is the energy level of water in the soil at saturation. That is, when all pores are filled with water. At this point soil water potential (Ψ) is nominally zero (0).
• In most cases, however, soil water potential (Ψ) is less than zero. This is indicated by giving soil water potential (Ψ) a negative sign (-ve).
• In practical terms, and as the soil dries out soil water potential (Ψ) decreases and becomes increasingly more -ve.
• So that when soil water potential (Ψ) is high it means Ψ is less -ve and is therefore very close to 0. When soil water potential (Ψ) is high it means soil water is held loosely, highly available and ready to move somewhere else.
• There are three important factors affecting total soil water potential:
  • Ψg Gravitational
  • Ψo Osmotic
  • Ψm Matric
• The general relationship between Total Soil Water Potential (Ψt) and the various factors is expressed as Ψt = Ψg + Ψo + Ψm
• The force of gravity acts on soil water as it does on all other bodies. In a soil profile the gravitational potential (Ψg) of water near the soil surface is always higher than Ψg in the subsoil. As a result of heavy precipitation or irrigation, therefore, the difference in Ψg causes downward flow of water deeper into the soil profile.
• The Osmotic Potential (Ψo) is attributable to the attraction between a water molecule and various ions (e.g. cations) and solutes (e.g. soluble salts) in the soil solution. The presence of large amounts of soluble salts results in osmotic potentials (Ψo) that reduce soil water potential. This makes it difficult for plants to remove soil water even though water may be present. This is known as physiological drought and is why plants wilt and appear stunted in saline soil profiles.
• Finally, adhesion (attraction) of water to the soil matrix, provides a matric force (i.e. adsorption and capillarity) which reduces energy of water particles near surfaces. Effects of surface adsorption on ability of water to do work.
• For example, water adsorbed to soil or held in capillary pores by H-bonding. In saturated soil, water free to flow, Ψm is not a factor and value is 0.

Note: Matric and Osmotic potentials are negative and reduce the free energy level of the soil water. These negative potentials are referred as suction or tension. The force of gravity is always positive.

Methods of expressing suctions

There are two units to express differences in energy levels of water:

Atmospheric pressure or Bars

• It is another common mean of expressing suction.
• Atmosphere is the average air pressure at sea level.
• If the suction is very low as occurs in the case of a wet soil containing the maximum amount of water that it can hold, the pressure difference is of the order of about 0.01 atmospheres or 1 PF equivalent to a column of water 10 cm in height.

pF Scale

• The concept of the pF curve for expressing the relation between the amount of water in a soil and the force with which it is held there was introduced by Schofield.
• The free energy is measured in terms of the height of a column of water required to produce necessary suction or pressure difference at a particular soil moisture level.
• The pF, therefore, represents the logarithm of the height of water column (cm) to give the necessary suction.

Soil condition and the corresponding pF value and Pressure

Moisture Regime (MR)

The percentage of moisture in the soil at atmospheric pressure is known as moisture regime.

Moisture Equivalent (ME)

Moisture equivalent is defined as the amount of water retained by a sample of initially saturated soil material after being subjected to a centrifugal force of 1000 times that of gravity for a definite period of time, usually half an hour.

Puddling

Irrigation before sowing of crops to reduce percolation of water.

Base period

The period (days) during which irrigation water is supplied to the crop.

Delta

Delta is the total depth of water (cm) required by a crop during its duration in the field.

Duty of water

Volume or quantity of water required for irrigation to bring a crop to maturity.

Gross Duty of Water

Area commanded by the flow of water as measured at the source of supply. It includes wastage in channel in addition to what is used for measuring crops.

Net Duty of Water

• Area commanded by water delivered at field. It includes the losses of water in the field.
• The difference between gross and net duty of water gives efficiency of distributaries.

Palco

Palco is the first irrigation before sowing the crop for seed germination and seedling establishment.

Kor watering

Crop water requirement is not uniform all through base period. The first watering is known as Kor watering.

Rostering/ Water regulation

The process of distribution of irrigation water.

Water Stress

Water stress is both shortage of water and water logging.

Classification of Irrigation Projects / Works Major

Irrigation

Irrigation is defined as the artificial application of water to the soil for the purpose of crop growth or crop production in supplement to rainfall and ground water contribution.

Purpose of Irrigation

• To add water to the soil for supplying the moisture essential for the plant growth
• To provide crop insurance against short duration drought
• To cool the soil atmosphere, thereby making more favourable environment for plant growth
• To washout or dilute salts in the soil
• To reduce the hazards of soil piping
• To soften the tillage pans

Importance of Irrigation

• Agriculture is the biggest user of water. The largest share of India’s freshwater consumption (78 per cent in 2010) comes from the agricultural sector (CWC, 2014).
• But the sector is facing competition for water due to …

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