Irrigation Methods & Drainage

Irrigation systems

Tank irrigation — small storages (common in southern India); well irrigation — open wells and tube wells, the largest source of irrigation in India today.

Surface irrigation methods

In surface methods water is applied over the soil surface and flows by gravity.

Uncontrolled (wild) flooding

Water is spread over the land without control of depth or flow — cheap but wasteful and non-uniform; used where water is plentiful and land relatively flat.

Border strip method

The field is divided into long, narrow strips (commonly 3–20 m wide and 60–300 m long) separated by low ridges (borders). Water released at the upper end flows down the gentle slope as a sheet.

• Suited to close-growing crops (wheat, fodder, pastures) and medium-to-low infiltration soils on uniform slopes.
• Needs precise land levelling along the length.

Check basin method

The field is divided into nearly level plots (checks) surrounded by low bunds; water is ponded to the required depth. Suited to rice, permeable and heavy soils, and flat land.

Basin (ring) method

Circular or square basins around individual plants — orchards and trees; water is fed through a supply channel into each basin.

Furrow method

Water runs in small channels (furrows) between crop rows; only part of the surface is wetted (about one-fifth to one-half).

• Suited to row crops — maize, cotton, sugarcane, potatoes, vegetables.
• Less evaporation loss and less puddling/crusting than flooding.
• Corrugation method — small closely spaced furrows for close-growing crops on uneven land.
• Contour furrows on sloping land.

Subsurface irrigation

Water is applied below the surface to the root zone by raising the water table or through buried perforated pipes.

• Natural — seepage from canals or a naturally high water table maintained by controlled drainage.
• Artificial — open-jointed or perforated pipes laid below the root zone.
• Needs a permeable top layer over an impervious layer; little evaporation; high cost; risk of salt accumulation.

Sprinkler irrigation

Water is sprayed into the air under pressure and falls on the crop like rain.

Types

• Rotating head (rotary) sprinklers — nozzles on rotating heads cover circles; most common.
• Perforated pipe system — pipes with small holes; low pressure, small areas (nurseries, gardens).
• By portability: portable, semi-portable, semi-permanent and permanent (solid-set) systems; centre-pivot and travelling gun systems for large areas.

Components

Pump unit, main line and sub-mains, laterals, risers and sprinkler heads, pressure gauge, valves, filters, fertiliser applicator.

Merits: suited to undulating land and shallow or sandy soils where levelling is difficult; saves land used by channels and bunds; uniform application and higher efficiency than surface methods; light, frequent irrigation possible; fertilisers and pesticides can be applied; frost protection.

Limitations: high initial and energy cost; wind distorts distribution; evaporation loss in hot dry climates; not suited to very heavy soils with low infiltration or to crops sensitive to wet foliage; saline water can scorch leaves.

Drip (trickle) irrigation

Water is applied drop by drop, at low rates and low pressure, directly to the root zone of each plant through emitters.

Components

1. Head unit — pump, filters (sand/media filter for organic matter, screen or disc filter for fine particles, hydrocyclone for sand), fertiliser tank or venturi injector (fertigation), pressure regulator, valves, non-return valve.
2. Main line and sub-main (PVC/HDPE).
3. Laterals (LDPE), laid along plant rows.
4. Emitters (drippers) — online or inline; pressure-compensating types give uniform discharge on slopes; typical discharge a few litres per hour.
5. Flush valves, air-release valves.

Variants: micro-sprinklers, bubblers (for trees), subsurface drip.

Water requirement per plant

($E{T}_c$ = crop evapotranspiration in mm/day, $A_p$ = area per plant in m², $W_f$ = wetted-area/canopy factor.) Operating time = $V$ ÷ (number of emitters × emitter discharge).

Merits: highest water-use efficiency (very little evaporation, runoff or deep percolation); water saving and higher yields; fertigation — nutrients applied precisely; less weed growth (dry inter-row space); works on undulating land and poor soils; saline water can be used with care (continuous moisture keeps salts at the wetting front edge); lower labour and energy (low pressure).

Limitations: high initial cost; clogging of emitters (needs filtration and periodic acid/chlorine treatment); salt accumulation at the periphery of the wetted zone; damage by rodents; needs technical skill and maintenance; limited to high-value row crops, orchards, vegetables, sugarcane, plantations.

Comparison of methods

In India, micro-irrigation (drip and sprinkler) is promoted under the "Per Drop More Crop" component of the Pradhan Mantri Krishi Sinchayee Yojana (PMKSY).

Worked examples

Frequently tested points

• Flow vs lift; perennial vs inundation; storage vs diversion.
• Indian scale: major > 10 000 ha CCA; medium 2000–10 000 ha; minor < 2000 ha.
• Border strip: close-growing crops; $t=(y/f)\ln [Q/(Q-fA)]$; $A_{max}=Q/f$.
• Check basin for rice; ring basin for orchards; furrow for row crops (cotton, sugarcane, maize).
• Sprinkler: undulating/sandy land; application rate $q/(S_l{S}_m)$ ≤ infiltration rate; wind affects uniformity.
• Drip: highest efficiency, fertigation, filters prevent clogging; salts accumulate at wetting-front periphery.
• Micro-irrigation promoted under PMKSY "Per Drop More Crop".

Waterlogging

Land is waterlogged when the water table rises so close to the surface that the root zone becomes saturated, air circulation in the soil is cut off and crop yields fall.

A commonly adopted Indian criterion classifies areas with a water table within about 2 m of the ground surface as waterlogged, and 2–3 m as potentially waterlogged (critical depth also depends on soil type, crop and groundwater salinity).

Causes

1. Seepage from unlined canals and distributaries — the main cause in canal commands.
2. Over-irrigation and intensive irrigation (excessive water application, no volumetric pricing).
3. Inadequate natural drainage — flat topography, obstructed natural drains.
4. Obstruction of drainage by roads, railway embankments, canal banks without adequate cross-drainage.
5. Impervious sub-soil (hard pan) close to the surface preventing percolation.
6. Heavy rainfall and flooding; submergence of low-lying land.
7. Seepage from reservoirs and irregular topography.

Harmful effects

• Lack of aeration in the root zone — plants cannot respire; bacterial activity (nitrogen fixation) is reduced.
• Soil remains cold — delayed germination; difficult tillage and delayed sowing.
• Growth of weeds and aquatic plants.
• Salinisation (efflorescence) — capillary rise of saline groundwater and evaporation leaves salts at the surface (white crust) → sterile land.
• Restricted choice of crops; reduced yields.
• Unhealthy climate — breeding of mosquitoes (malaria).
• Weakening of foundations of buildings and roads.

Prevention and remedies

1. Lining of canals and watercourses to reduce seepage.
2. Intercepting (seepage) drains alongside canals.
3. Efficient water use — reducing irrigation intensity in affected areas, volumetric assessment, proper land levelling, improved methods (sprinkler, drip).
4. Optimum use of groundwater — tube well pumping (vertical drainage) and conjunctive use of surface and groundwater.
5. Improving natural drainage and providing adequate cross-drainage under embankments.
6. Surface and subsurface drainage systems.
7. Bio-drainage — plantations of high water-using trees (e.g. eucalyptus) along canals and in affected areas.
8. Changing cropping pattern (less water-intensive crops).

Soil salinity and alkalinity

Irrigation water always contains some salts; where drainage is poor, salts accumulate.

• Saline soils ("white alkali") — excess soluble salts (chlorides, sulphates of Na, Ca, Mg); white surface crust; soil structure is usually good. Plants suffer because high osmotic pressure reduces water uptake.
• Sodic (alkali) soils ("black alkali") — high exchangeable sodium; clay disperses, soil becomes impermeable, hard when dry and sticky when wet; dark surface due to dispersed organic matter; high pH (often above 8.5).
• Locally known in India as reh, kallar or usar lands.

Reclamation

• Saline soils — leaching with good-quality water (ponding or intermittent) combined with effective drainage to carry away the dissolved salts; scraping the surface crust; growing salt-tolerant crops during reclamation.
• Sodic soils — sodium must first be replaced by calcium using chemical amendments: gypsum (calcium sulphate) — the most common; also pyrites, sulphur, press mud, calcium chloride, acids. Then leach the displaced sodium out with drainage. Green manuring (e.g. dhaincha), organic matter and salt-tolerant crops (e.g. rice, barley, berseem) help.
• Saline-sodic soils — apply amendments before leaching; leaching alone would convert them into sodic soils.

Land drainage

Drainage removes excess surface water and lowers the water table to keep the root zone aerated and to leach salts.

Surface drainage

Open channels (field drains, collector drains, main drains) that remove excess rainfall runoff and irrigation tail water. Designed by Manning's formula for a design runoff (e.g. from a design storm of a few days' duration) with gentle slopes, stable side slopes and velocities that neither silt nor scour. Land shaping and grading support surface drainage.

Subsurface drainage

• Drainage coefficient — the depth of water (mm or cm) to be removed from the drained area in 24 hours; it fixes the capacity of drains.
• Layouts: natural (following depressions), gridiron (parallel laterals on one side of a main), herringbone (laterals on both sides of a main at an angle), intercepting (drain across the path of seepage along the foot of a slope or along a canal).
• Depth commonly around 1–2 m (below the root zone); minimum slopes and velocities chosen to avoid silting; envelopes prevent entry of soil.

Drain spacing

Worked examples

Frequently tested points

• Waterlogging: saturated root zone; commonly water table within about 2 m.
• Main cause in canal commands: seepage from unlined canals; also over-irrigation, poor drainage.
• Effects: poor aeration, salinisation (efflorescence), weeds, malaria.
• Remedies: lining, intercepting drains, efficient irrigation, vertical drainage, bio-drainage, drainage systems.
• Saline: EC > 4, ESP < 15; sodic: EC < 4, ESP > 15, pH > 8.5; saline-sodic: both high.
• Saline → leaching + drainage; sodic → gypsum then leaching.
• $LR=E{C}_{iw}/E{C}_{dw}$; $D_i=D_c/(1-LR)$.
• Drainage coefficient = depth removed per 24 h; ellipse equation $S^2=4K(b^2-a^2)/q$.
• Layouts: natural, gridiron, herringbone, intercepting.