irrigation methods, soil-water-plant relationships


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IRRIGATIONArtificial application of water to arid land for growing cropsSupplementary to rainfall when it is either deficient or comes irregularly or at unreasonable timesIrrigation engineering: Multi-disciplinary science encompassing hydrology, agriculture, geology, climatology, river engineering, agronomy, forestry, social science, hydraulics, river soil mechanics, snow hydrology and groundwater hydrology

NEED FOR IRRIGATIONDeficient rainfall:

Rainfall (cm)Irrigation requirement100Rainfall needs to be supplemented by irrigation100-50Rainfall is insufficient. Irrigation is essential.50-25Irrigation is essentially required.Less than 25No crop can be grown without irrigation.

ContdNon- uniformity of rainfallAugmentation of crop yieldsExacting water requirementCash crops cultivationAssured water supplyOrchards and gardens

ADVANTAGES OF IRRIGATIONDirect benefitsIncrease in food output through higher yieldCultivation of cash cropsLand value appreciates manifoldProtection from famine irrigation makes agriculture and economy drought proofPrevention of damage through floodsHydel power generation at dam sites and canal falls

ContdRise of subsoil water level in dry areasMeans of communication where navigation is possible in canalsRevenue from recreational facilitiesFish and wild life preservation and development of piscicultureLowers production risksMakes agriculture competitive and profitableReduced risks of crop failuresImprove the nutrition of people

ContdIndirect benefitsIncrease in GDPIncrease in revenue from sales tax on food grainsIncrease in employmentImprovement in groundwater storageIncrease in value of land propertyGeneral development of countryFarm laborers are benefited who get higher wagesRise to whole array of agro-based industries

DISADVANTAGES OF IRRIGATIONClimate becomes damp and cold, causing malarial diseasesOver-irrigation coupled with poor drainage in an area where water-table is high leads to water logging of the area, causes efflorescenceLow land revenue in certain cases Excessive seepage from unlined canals leads to water logging of lands adjacent to canals

SOIL-WATER PLANT RELATIONSHIPWater is the basic input influencing crop productionThe amount of water required for a given crop depends on:State of development of soilQuantity and type of fertilizer givenQuality of water usedClimatic conditions

ContdSoil- water- plant relationship Process that requires to be regulated for maximization of yields with a given unit of water Understanding of Soil- Water- Plant relationship is essential in order that water management principles are applied to various climatic, soil and cropping regions of both rain fed and irrigated lands

ContdSoil factors Infiltration:-Influences selection of irrigation methods, slope needed for the land, length of run, irrigation application time etc.Soil water parameters affecting infiltration rates:- Texture, Structure, Bulk Density, Sodium salts, Crop grown, Irrigation water, Temperature, Tillage and Water in soil

ContdPermeability:- Depends on soil texture and structure, presence of plant roots and changes in temperature of water K= QL/A(H1-H2) where, Q= Discharge/ unit time A= Cross sectional area through which water flows H1-H2 = Hydraulic head L = Percolation path length

ContdSoilGravel (clean)Coarse sand (clean)Sand (mixture)Fine sandSilty sandSiltClay K1.0 or more1.0-0.01.01-.005.05-.0010.002-0.00010.0005-0.000010.00001 or lesser

Measured using Constant head permeameter and Variable head permeameter

ContdDrainability and Leachability:-Principal factors in predicting the drainability of a soil is its permeability and hydraulic gradientLeachability is directly related to drainabilityErodibility

ContdPlant factorsRooting characteristics:-High water table limits the root growth due to lack of sufficient aerationEvapo-transpiration:-General rule is that 40,30,20 and 10 percent of the total ET is removed respectively from each successively deeper one-quarter of the rooting depth

ContdEffect of soil water level on crop growth and yield:-Crop growth and transpiration generally decreases as the wilting point approachesThe point at which growth or transpiration of a plant is retarded for want of soil water, crop characteristics, low or high evaporative demand etc.

ContdWater factorsWhen to irrigate:-Generally irrigation shall start when 50%, but not over 60% of the available moisture is used from the root zoneDesign frequency = (Field capacity of soil in effective crop root zone- moisture content of the same zone at the starting of irrigation)/ Moisture use of root of crop in peak period

ContdHow much water to apply:-The amount of water to be replaced is usually 40-50% of the available water in the root zone of the soils having a uniform available water capacity with depthWater application method:-Influenced by quantity of available water supply, type of soil, topography and crops to be grownMethods include controlled surface flooding method, sprinkler method and drip method.


A. Sub- surface irrigationWater applied beneath the ground by creating and maintaining an artificial water table30-75 cm below the ground surfaceConsists of main field ditches, laterals, laid 15-30m apartOpen ditches, mole drains or tile drains

Advantages of sub-surface irrigationMinimum water requirement for raising crops and high yieldMinimum evaporation and deep percolation lossesMost economical method of irrigation and suitable for most cropsInvolves no wastage of landNo interference in free movement of farm machineryCultivation operations can be carried out without concern for the irrigation periodLittle field preparation and labor

Disadvantages of sub-surface irrigationRequires a special combination of natural conditionsThere is danger of development of water loggingPossibility of choking of the pipe laid undergroundHigh cost

Classification of sub-surface irrigation

Natural sub-irrigation:Applicable to low lying lands where the water table is highWater table is charged by seepage from irrigation canalsArtificial sub-irrigation:Very expensive methodWater under pressure provided to crops by capillarity through a network of buried perforated pipes

B. Surface irrigationMost common type of irrigationWater is applied to the field in varied quantities at different timesFlow remains unsteadyDiverting a stream of water from the head of a field into furrows or borders and allows to flow downwardSupplemented with efficient water disposal system

Advantages of surface irrigationAllows use of machinery for land preparation, cultivation and harvestingHelps to store the required amount of water in the capillary zone of the soil for supply to the root zone of plants

Disadvantages of surface irrigationGreater loss of water by surface runoff and deep percolationLarger requirement of water per unit areaWater is lost in infiltration and deep percolationLow efficiency due to imperfect control over the water flowInferior quality crops with a low yieldWasteful use of waterCostly and time consuming land preparation

Classification of surface irrigationFlooding method:Water is allowed to cover the surface of land in a continuous sheetThe flooding may be:Wild flooding (uncontrolled flooding) :Primitive and most inefficient methodWater is spread over the smooth or flat field without much control over the flow or prior preparation

ContdWater distribution is quite uneven Advantage: Low cost and does not interfere with tillage Suitable for all medium to fine texture soilsDisadvantage: Wasteful use of water Non-uniform distribution of water Excessive soil erosion on steeper slopes Require drainage arrangement to reduce ponding

ContdControlled flooding :Free flooding (ordinary flooding) : Land is divided into plots or kiaries of suitable size depending on porosity of soilWater is spread over the field from water courseSpreading may vary from less than 15m to more than 60m

ContdBorder flooding: Field is divided into narrow strips by low parallel ridges on the sidesWidth if strip: 5-15m ; Length of strip: 60-100m for sandy loam, 100-120 for medium loam, 150-300 for clay loamLongitudinal gradient: 0.02-0.05% for clay to clay loam, 0.20-0.40% for medium loam, 0.25-0.60% for sandy loam to sandy soil

ContdCheck flooding: Applying water to relatively level check basins enclosed by small bundsSize of check basin: 3*2m to 3*3m or even largeContour lateral method: Best suited to steeper terrain Dense network of contour laterals are laid with spacing 15-50m Adopted mostly in close growing crops on sloping lands

ContdZig zag method: Suitable for relatively level fieldsUnsuitable for mechanical farming operationsLand is divided into square or rectangular plots; each plot further sub divided with low bundsBasin flooding: Check method of flooding adapted to orchardsBasins are made around one or more trees depending on the soil condition and topographyadapted essentially to flat lands

ContdContour farming:Adapted to hilly areas with steep slopes and quick falling contoursLand is divided into longitudinal curved plots, the bunds of the plots following the contoursReduces runoff and soil loss

ContdFurrow method:Used for row cropsA furrow consists of a narrow ditch between the rows of cropsWater is applied in small streams between rows of crops, grown on ridges or in furrows

C. OVER HEAD IRRIGATION (SPRINKLER IRRIGATION)Simulates natural rainfall to spread water in the form of rain uniformly over the land surface Water is spread i


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