Irrigation System Design

Canal systems

A canal is an artificial channel that carries water from a river or reservoir to the fields.

Classification

Canal alignment

Principles of alignment: command the maximum area with the shortest length; avoid villages, cemeteries, temples, valuable property; minimise cross-drainage works and balance cutting and filling; avoid rocky and unstable strata; keep curves gentle; place the head where the river is stable.

Canal losses

• Evaporation — small (a few per cent) except in hot, dry, windy conditions.
• Seepage — the major loss: absorption (wetting soil above the water table) and percolation (flow reaching the water table). Depends on soil permeability, water table depth, wetted perimeter, canal age (silting reduces loss) and velocity.
• Seepage raises the water table in the command and is a main cause of waterlogging — lining reduces it (see Canal Lining).

Canal cross-section

• Side slopes — in cutting roughly 1:1 (depending on soil); in filling flatter, about 1.5:1 to 2:1.
• Freeboard — vertical distance between full supply level and top of bank; increases with discharge.
• Banks — retain water, carry service road and inspection path; top width depends on discharge and road requirement.
• Berms — horizontal strips at the full supply level or the natural ground level between the bank toe and water; strengthen banks, protect against erosion, allow future widening. In partial cutting, the berm width is chosen so that the silt deposited on the side slopes forms the stable regime slope (about ½ : 1).
• Service road / inspection road on the bank; dowla — small bund on the road side for safety.
• Spoil banks — excess excavated earth dumped along the canal when cutting exceeds filling.
• Borrow pits — pits from which earth is taken when filling exceeds cutting; dug inside the canal bed (preferably) or at a safe distance outside the bank.
• Balancing depth — the depth of cutting at which the earth from cutting exactly equals that required for the banks (economical section).

Sediment and regime

Alluvial canals carry silt. If velocity is too low, silt deposits (silting); if too high, the bed and banks erode (scouring). A canal is in regime when it neither silts nor scours over a hydrological cycle.

• Initial regime — only the bed slope and depth adjust; width fixed by banks.
• Final (true) regime — all variables (width, depth, slope, shape) adjusted to the discharge and silt.
• Permanent regime — protected (lined) banks and bed; no further adjustment.

Kennedy's silt theory (1895)

R.G. Kennedy's observations on the Upper Bari Doab Canal (Punjab): silt is kept in suspension by the vertical eddies generated from the bed; so the silt-supporting power depends on the depth of flow (not width).

Design procedure (for given $Q$, $n$, $m$, $S$): assume depth $y$ → $V_0$ from Kennedy → $A=Q/V_0$ → find bed width (with side slopes ½ : 1) → compute $R$ and the velocity from Kutter's formula → compare with $V_0$; repeat until they agree.

Limitations:

• Considers only eddies from the bed (ignores those from the sides).
• No equation for bed slope — slope taken from Kutter's formula, which itself has limitations.
• B/D ratio chosen arbitrarily; trial procedure lengthy.
• Silt charge and grade are expressed only through the arbitrary ratio $m$.
• Based on data from one canal system.

Lacey's regime theory (1930)

Gerald Lacey found that regime channels have definite relations among discharge, velocity, hydraulic radius and slope. Silt is supported by eddies generated over the whole wetted perimeter.

Regime conditions assumed: constant discharge; constant silt charge; channel flowing in incoherent alluvium of the same grade that it transports (scoured and deposited with equal ease); unlimited incoherent alluvium.

Design procedure: find $f$ → $V$ → $R$ and $A$ → $P$ → solve for bed width and depth with ½ : 1 slopes → bed slope $S$.

Drawbacks of Lacey's theory: true regime is rarely attained; the concept of incoherent alluvium is idealised; the equations do not treat silt charge and grade separately; the single silt factor cannot represent both; semi-circular shape idea and empirical constants based on limited data.

Kennedy versus Lacey

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".