It is the artificial application of water to the agricultural field for the purpose of cultivation i.e. in accordance with crop requirement throughout the crop period.
Note- Crop period is the period from instant of sowing to the instant of harvesting.
Advantages of irrigation-
- Prevention from draught and famine conditions.
- Elimination of mixed cropping.
Mixed cropping means growing two or more crops simultaneously in the field.
Advantage of mixed cropping-
It is found economical and necessary when irrigation facilities are lacking i.e. if weather condition are not suitable for one of the crop they may be suitable for other crop and hence farmer will get some yield.
Disadvantage of mixed cropping-
Different crops require different types of field preparation, watering and manuring. It will be difficult to satisfy need of both the crops simultaneously in the field. It results in low yield.
Also at the time of harvesting crop would get intermixed which reduces purity and value of crops in the market.
- Power generation
- Flood control
- Transportation ( Roads and inland navigation )
- Ground water recharge
- Industrial and domistic water supply
- Employment generation
Disadvantage of irrigation-
- Water logging
- Intense irrigation results in cold and damp climate which causes spreading of diseases like dengu and malaria
- Ground water pollution due to percolation of fertilizer.
Note- Bad effect of irrigation can be eliminated by economical scientific use of water.
Types of irrigation-
1- Surface irrigation
2- Subsurface irrigation
1- Surface irrigation-
It is a method in which water is directly applied to the soil surface either by gravity or by pumping.
It is best suitable for soil having moderate infiltration capacity and areas with gentle slope.
Surface irrigation further can be classified as-
A- Flow irrigation-
If water is available at higher elevation and it is supplied to lower elevation under the action of gravity.
B- Lift irrigation-
If water is lifted up by some mechanical or manual means, supplied to the agricultural field it is called as lift irrigation.
Examples- Pumps, wells and tubewell.
It is costlier than flow irrigation.
Flow irrigation can be further classified as-
1- Perennial irrigation-
If a constant and continuous water is supplied to the field as per requirement of crop throughout the crop period it is called as perennial irrigation.
(A)- Direct irrigation-
By diverting river water into canal with the help of weir or barrage.
(B)- Storage irrigation-
By system of dam and channel.
(C)- Combined irrigation-
Combination of direct and storage irrigation.
2- Flood/ inundation irrigation-
In this system of irrigation a large quantity of water flowing in a river during flood is allowed to flood or inundate the area to be cultivated, causing saturation of the area. When excess water is drained off area is used for cultivation.
2- Subsurface irrigation-
In this type of irrigation system water does not wet the soil surface rather it is directly apply to the root zone.
It is classified into two types-
A- Natural subsurface irrigation-
In this system water seeping through channel and water bodies makes irrigate crops grown on the lower area by capillarity.
B- Artificial subsurface irrigation-
In this system water is directly applied to the root zone of the crops by a network of percolated pipe laid below the ground surface.
Techniques of water distribution-
1- Free flooding / Ordinary flooding-
In this method ditches are excavated in the field and water from these ditches flows across the field.
After water leaves the ditches no attempt is made to control the flow hence it is also called as wild flooding. It is suitable for rolling terrain. Field preparation is less.
Water application efficiency is less, most suitable for closed growing crop. Example- Pasture.
Lateral ditches are space 20 to 50 m apart depending on slop, type of soil and crop to be grown.
2- Border flooding-
Area is divided into number of strips separated by low levees called border along the slope of ground.
Relation between discharge through supply ditch Q, Rate of infiltration f, Average depth of flow over the strip y, Area to be irrigated and time required to irrigate the area (t)
t= 2.303 (y/f ) log (Q/(Q-Af))
Maximum area to be irrigated
Amax = Q/f
Note- Size of strip will depend on discharge Q, slope of area and soil characteristics.
3- Check flooding / Method of irrigation by plots-
In this method area to be irrigated is divided into small plots or Check area ( 0.2 to 0.8 hectare ) with low flat levees having vertical height of 5 cm.
Each plot has nearly fair level surface. Irrigation water is applied by filling the plots with water upto desire depth without over toppling the levees and water is retained their to allow it infiltrate into the soil.
4- Basin flooding / Ring basin method-
This method is special form of check basin which is used for irrigation of Orchards ( enclosure of fruit trees ). In this method a separate circular basin is made for each tree.
5- Furrow irrigation-
In this method water is applied to the area to be irrigated by long and narrow field channels called furrows which are excavated at regular interval.
Water flowing in the furrow infiltrate into the soil and spreads horizontally to irrigate the area between furrows.
It is a controlled type of subsurface irrigation. In this method 20 to 50% of the area wetted which result in less evaporation. It is suitable for row crops such as maize, cotton, groundnut etc.
6- Sprinkler irrigation-
In sprinkler irrigation water is applied in the form of spray through the network of pipe and pumps.
Advantages of sprinkler irrigation-
- It is similar to rain hence uniform application of water is possible.
- It can be used for wide range of topography soil and crops i.e irregular topography steep slopes and areas in which soil is easily errodable.
- No field preparation is required hence labour cost is less
- Surface runoff and percolation losses are eliminated.
- Field application efficiency is high close to 80%.
- Fertilizer, insecticide can be mixed with water and supplied.
- It can be used when soil is excessively permeable or less permeable.
- About 15 to 20% crop area is increased because no cultivation area is loss for levees and furrow.
- It can be used even when water table is high.
Disadvantage of sprinkler irrigation-
- Evaporation loss is high.
- It causes interference in farming operations due to network of pipe.
- Wind may disturb sprinkler pattern which result in non uniform application of water.
- High initial cost.
- It requires large electrical power and constant water supply.
- It can not be used for crops required large depth of water. Example- Paddy.
- Water shall be clean from silt and sand to prevent chocking of the system.
7- Drip irrigation / Trickle irrigation-
In this method water is directly and slowly applied to the root zone of the plants using small diameter plastic pipes with drip nozzles commonly called as emmiters or drippers.
Water is applied at very low rate 2 to 10 l/ hour to keep soil moisture within the desired range of plant growth.
Irrigation efficiency is equal to 90%. Evaporation, seepage and surface runoff are completely eliminated.
Fertilizer can be mixed with water and supplied. Cost of whole system is very high hence less preferred but its is very useful in the areas where availability of water is less. It is useful for cultivation of fruits and vegetables.
Quality of irrigation water-
Quality of irrigation water is decided on following factors.
Effect of sediment on the quality of irrigation water depends on nature of sediment and characteristics of soil receiving that water.
If sediment contain large amount of plant nutrient and/ or it comes from fertile area then it is quite beneficial particularly for the agricultural area which has low amount of plant nutrient and low water holding capacity.
If sediment is not rich in plant nutrient and it is deposited on the surface of the fertile area then it will reduce permeability of soil which causes difficulty in irrigation.
2- Concentration of soluble salts-
When salts present in irrigation water are in excess quantity, they Increase the osmotic pressure of soil solution which causes high soil moisture stress in the root zone. This affects growth of crops and yield.
Bad effect of salts on the plants depends on concentration of salts left in the soil.
Concentration of salts in water may not appear to be harmful in the beginning but with the passage of time, salt concentration in the soil may increase to harmful level.
Soil solution gets concentrated by evaporation.
Salinity concentration of soil solution (Cs ) after consumptive use ( Cu) has been given by
Cs= C.Q/ Q- (Cu – Reff ) mg/l or ppm.
Reff= Effective rainfall
Cs= Salinity concentration of soil solution
Q= Quantity of water applied
Cu= Consumptive use of water i.e. total quantity of water used by the crop for its growth.
C= Concentration of salt in applied water.
If Cs > 700 ppm – Harmful to some crop
Cs < 2000 ppm – Harmful to all crops
Salt concentration is easily measured by determination of electrical conductivity ( Ec ) which is represented in milli Mho/ cm or micro Mho/ cm.