Irrigation, Drainage, and Groundwater Development

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This section covers crop and irrigation water requirements, water availability, command areas, irrigation intensity, the duty-delta relationship, water losses/efficiencies, effective rainfall, the soil-moisture-irrigation relationship, and design discharge for canals.

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Crop Water and Irrigation Water Requirement Consumptive use (CU) / evapotranspiration (ET): water used by a crop for transpiration plus evaporation from the adjoining soil, expressed as depth per unit time (mm/day).

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Crop water requirement (CWR) = consumptive use + losses during application (percolation, evaporation from field).

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Net irrigation requirement (NIR) = CWR − effective rainfall − contribution from soil moisture/groundwater.

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Gross irrigation requirement (GIR) = NIR ÷ field application efficiency, further increased by conveyance losses to give the requirement at the head of the canal.

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Crop water requirement varies through the crop's growth stages (initial, development, mid-season, late- season), typically peaking at mid-season.

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Water Availability, Command Area and Irrigation Intensity Term Meaning Gross Command Area (GCA) Total area bounded by the irrigation project boundary, cultivable + uncultivable Culturable Command Area (CCA) Area within GCA that is culturable (fit for cultivation), whether currently irrigated or not Culturable Cultivated Area (CCA- cultivated) Portion of CCA actually under cultivation in a season Intensity of irrigation Percentage of CCA proposed to be irrigated in a season (Kharif/Rabi), may exceed 100% if double-cropped Duty, Delta and Base Period Delta (Δ): total depth of water required by a crop during its entire growing period (base period), expressed in depth units (e.g. cm or m).

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Base period (B): total time (days) from first watering (sowing) to the last watering before harvesting.

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Duty (D): area (in hectares) that can be irrigated with a unit discharge (1 cumec/m³/s) flowing continuously for the entire base period.

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Duty is a measure of water-use efficiency — a higher duty means less water is needed per unit area.

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Relationship: Δ = 8.64B/D (Δ in metres, B in days, D in hectares/cumec) — derived from equating the volume delivered (Q×B in seconds) to the volume represented by the area × delta.

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Duty is measured either at the field (outlet), at the head of the distributary/watercourse, or at the head of the main canal — duty at the head of canal is always less than duty at the field because of conveyance losses along the way.

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Water Losses, Irrigation Efficiencies and Effective Rainfall Efficiency Definition Conveyance efficiency Water delivered to the outlet / water diverted from the source (accounts for canal seepage and evaporation losses) Application (field) efficiency Water stored in the root zone / water delivered to the field (accounts for deep percolation, runoff losses) Storage/water-use efficiency Water stored in root zone / water actually needed (required) in the root zone Overall (project) efficiency Conveyance efficiency × application efficiency — water used beneficially / water diverted at source Effective rainfall: the portion of total rainfall that is actually available to meet crop water requirements (excludes runoff, deep percolation beyond root zone, and evaporation before infiltration).

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Design discharge for canals is fixed considering peak crop water demand period, culturable command area, intensity of irrigation, and duty — with an allowance for conveyance losses so the required discharge reaches the field.

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This section covers canal types, components, alignment, design of stable/alluvial/lined canals (Kennedy's and Lacey's theories), components of a weir/barrage, seepage theories (Bligh's, Lane's, Khosla's), silt control structures, and design of weir/barrage and energy dissipaters.

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Canal Types, Components and Alignment Canals are classified by function (main canal, branch canal, distributary, minor, watercourse — decreasing order of discharge) and by alignment (watershed/ridge canal, contour canal, side-slope canal).

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Watershed/ridge canal: runs along the ridge/watershed line, so it can irrigate on both sides by gravity without crossing any natural drainage — no cross-drainage works needed.

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Contour canal: runs roughly along a contour in hilly/undulating terrain, crosses natural drainages, so cross- drainage structures are required; can only irrigate on one side (the lower side).

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Side-slope canal: aligned along the side slope, roughly at right angles to contours, has a steep bed slope; often used as a link between a watershed canal and a contour/drainage channel.

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Canal network components: head regulator, cross regulators, escapes (surplus weirs), canal falls, and canal outlets, in addition to the canal prism itself (bed width, depth, side slopes, freeboard).

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Tractive Force Approach and Stable/Alluvial Canal Design Tractive force (unit tractive force): the drag force exerted by flowing water per unit wetted area of the channel boundary, approximated as τ=γRS (γ=unit weight of water, R=hydraulic radius, S=bed slope).

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Channel is stable when the tractive force does not exceed the critical (permissible) tractive force of the boundary material.

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Kennedy's theory: based on the concept of critical velocity Vo, the velocity that just keeps silt in suspension without scouring or silting;

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Vo=0.55my0.64 (y=depth of flow, m=critical velocity ratio, a factor for silt grade).

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Assumes a fixed, empirically-based silt factor and rough-and-ready roughness (Kutter's formula) — does not consider silt concentration or channel shape rationally, and does not directly give channel bed slope.

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Lacey's theory: based on the concept of a channel in regime (a dynamic equilibrium state where the channel neither silts nor scours over time, on average).

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Key relations: silt factor f=1.76√dm (dm=mean particle size, mm); velocity V=(Qf²/140)1/6; hydraulic radius/mean depth and wetted perimeter P=4.75√Q; bed slope S=f5/3/(3340Q1/6).

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Lacey's theory is considered more rational/comprehensive than Kennedy's, as it directly relates all regime channel dimensions and slope to discharge and silt factor.

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Lined canals: lined with concrete, brick, or other impermeable material to reduce seepage losses, permit higher permissible velocity (steeper gradient, smaller cross-section, hence cost savings on excavation despite lining cost), reduce weed growth, and reduce maintenance.

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Economic analysis of lining compares capital + maintenance cost of lining against value of water saved and other benefits over the project life.

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Components of a Weir / Barrage Component Function Impervious floor (apron) Provides a impermeable barrier to control seepage/uplift beneath the structure and dissipate energy Crest / weir wall Raises upstream water level to the required pond level for diversion into the canal Piers and gates Divide the waterway into bays and regulate flow, allow flushing of silt/flood discharge Divide wall Separates the undersluice portion (silt-excluding) from the main weir/barrage portion Fish ladder Allows fish passage past the barrage Guide banks Confine and guide the river flow towards the barrage, preventing outflanking Seepage Theories, Silt Control and Design Bligh's creep theory: assumes seepage travels along the contact profile of the base of the structure (the 'creep length' L, sum of horizontal and vertical creep) at a uniform loss of head per unit length (H/L, the coefficient of Bligh's percolation);

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HL/L must not exceed a safe value 1/C (C=Bligh's coefficient, depends on soil type) to prevent piping.

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A simple but conservative theory that does not distinguish between horizontal and vertical creep, nor account for the exit gradient concentration.

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Lane's weighted creep theory: refines Bligh's theory by weighting vertical creep three times more effective than horizontal creep in reducing uplift (since seepage more readily follows steep/vertical paths), giving a weighted creep length Lw=Σ(Vertical)+Σ(Horizontal)/3.

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Khosla's theory (method of independent variables): the most rigorous of the three, based on potential flow theory; treats the seepage flow net beneath a composite profile as a combination of elementary standard forms (straight floor with sheet piles at ends/intermediate points) analysed independently, then corrected for floor thickness, interference between piles, and the slope of the floor.

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Also identifies the critical exit gradient (hydraulic gradient at the downstream end where seepage emerges) — the actual exit gradient must remain below the critical value to prevent piping/undermining.

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Silt-control structures: silt excluders are located in the river bed upstream of the undersluices, excluding coarser silt from entering the canal by diverting bed-layer flow away from the head regulator; silt ejectors are located in the canal itself, downstream of the head regulator, removing silt that has already entered the canal by creating a rapid vortex/tunnel flow that ejects silt-laden water back to the river; settling basins allow silt to settle out of slow-moving water before it enters the main canal system.

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Design of weir/barrage includes fixing the crest level (from required pond level and afflux), waterway/length (from Lacey's regime perimeter), and the thickness of the impervious floor at each section (from the uplift pressure computed by Khosla's/Bligh's theory, providing adequate weight of floor plus any anchoring to resist uplift).

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Energy dissipaters (e.g. a hydraulic-jump stilling basin downstream of the glacis) dissipate the high kinetic energy of water passing over/under the weir, preventing scour of the downstream bed and protecting the structure; friction blocks, chute blocks and end sills are commonly used within the stilling basin to help stabilize and shorten the jump.

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This section covers river stages, the need for river training, design of guide bunds, launching aprons and spurs, and the causes, effects, and prevention of water logging, including design of surface and sub-surface drainage systems.

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River Stages and Need for River Training Rivers pass through three broad stages: upper (torrential/hill) stage — steep slope, high velocity, boulders/coarse material, degradation (down-cutting) dominant; middle (semi-alluvial) stage — moderate slope, mixed erosion/deposition, meandering begins; lower (alluvial/deltaic) stage — flat slope, fine silt, heavy meandering/braiding, aggradation (deposition) dominant.

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Need for river training: to confine the river within a defined channel and prevent bank erosion/outflanking (protecting nearby structures like bridges/barrages), to align river flow properly through a barrage/bridge waterway, to prevent formation of new channels that could bypass a structure, to stabilize an unstable/meandering channel, and to improve navigation.

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Guide Bunds and Launching Aprons Guide bunds are earthen embankments constructed on either flank of a barrage/bridge to guide and confine the river flow smoothly through the waterway, preventing outflanking of the structure by the river shifting its course.

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Typically curved (elliptical or bell-mouthed shape) at the upstream end (divergent) and short/straight at the downstream end (convergent), with the upstream curved 'head' extending well beyond the structure to smoothly guide floodwaters.

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Guide bund length and radius of curvature are typically determined empirically relative to the waterway (Lacey's regime perimeter), with the upstream curved head length commonly about 1.0-1.5 times the total guide bund length and radius about 0.4-0.5 times the waterway.

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Launching apron: a flexible apron of loose stone (boulders in wire crates/gabions, or cement concrete blocks) laid on the riverbed at the toe of the guide bund/approach embankment; as the riverbed scours during floods, this loose apron progressively 'launches' (slides down) at roughly the natural angle of repose to form a continuous flexible protective cover over the scoured slope, preventing undermining of the guide bund/embankment.

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Apron thickness and quantity of stone are designed based on the anticipated maximum scour depth (estimated via Lacey's scour-depth formula) below the deepest anticipated bed level.

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Spurs (Groynes) Spurs (groynes) are structures projecting from the river bank into the flow, used to deflect the current away from an eroding bank, induce silting in the region they protect, and train the river along a desired alignment by a series of spurs rather than a continuous guide bund.

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Types by alignment: attracting spur (points downstream, attracts/pulls flow towards the bank it protects, used with care since it can cause local scour), repelling spur (points upstream, repels/pushes flow away from the bank), deflecting spur (aligned roughly normal to the bank, simply deflects flow without strongly attracting or repelling).

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Types by function: impermeable (solid) spurs obstruct flow completely, causing more pronounced local scour at the nose; permeable spurs (e.g. built of piles/porcupines) reduce flow velocity gradually and encourage silting with less severe local scour.

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Water Logging and Drainage Water logging: a condition where the water table rises close to the ground surface, saturating the crop root zone and impairing aeration, reducing crop yield or rendering land unfit for cultivation.

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Causes: over-irrigation/seepage from unlined canals and fields, poor natural/artificial drainage, impervious sub-soil strata obstructing percolation, flat topography with inadequate outfall, and rise in river/canal water levels relative to adjoining land.

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Effects: reduced soil aeration harming root respiration, salinity/alkalinity build-up (through capillary rise and evaporation of saline groundwater), reduced crop yield, and difficulty in farm operations (soil remains too wet/soft).

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Preventive measures: lining of canals/watercourses to reduce seepage, adopting efficient (need-based, not excess) irrigation practices, providing adequate natural or constructed surface/sub-surface drainage, and maintaining outfall channels.

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Surface drainage systems remove excess water from the land surface via a network of graded open drains discharging to a natural outfall; design considers drainage coefficient (design rate of removal), land slope, and drain spacing/capacity.

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Sub-surface drainage systems (e.g. buried perforated pipe drains or tile drains) lower the water table by intercepting and removing excess water from below the root zone; design considers depth and spacing of drains (based on soil hydraulic conductivity, desired water-table depth, and drainage coefficient), drain gradient, and pipe/tile size.

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This section covers the functions of various regulators, design of regulators and escapes, pipe outlets (free and submerged), vertical drops, and cross-drainage structures where a canal intersects a natural drainage.

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Regulators and Escapes Head regulator: controls the discharge admitted from the parent channel (river/main canal) into the off- taking canal/branch, and helps exclude silt (often combined with an undersluice/silt excluder arrangement).

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Cross regulator: constructed across the main/parent canal, typically at (or just downstream of) an off-take point, to raise/control the upstream water level so that adequate head is available for the off-taking channel, and to regulate discharge downstream in the parent canal.

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Canal escape: a structure that allows surplus/excess water (from heavy rainfall, sudden closure of outlets, or operational needs) to be safely disposed of from the canal into a natural drain or depression, preventing overtopping/breaching of the canal banks.

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Design of regulators/escapes fixes the crest level, waterway (length of the structure/number of bays), and the thickness of the impervious floor at each point — following the same principles as weir/barrage design (uplift pressure via Khosla's/Bligh's theory, adequate floor weight/anchorage).

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Pipe Outlets A pipe outlet is a small structure (often a simple pipe) that delivers water from a distributary/minor into a watercourse serving individual fields; it should be simple, cheap, and require minimal maintenance while delivering a fairly consistent, proportionate discharge.

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Free (or non-submerged) outlet: discharge depends only on the head in the distributary (upstream water level), not on the water level in the watercourse downstream (i.e. flows like a free-discharging orifice/pipe into open air or shallow water level).

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Submerged outlet: discharge depends on the difference in water level between the distributary (upstream) and the watercourse (downstream), since the downstream end is submerged — the outlet behaves like a submerged orifice, so the discharge is influenced by any change of water level on either side.

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Vertical Drops (Falls) A canal fall (drop) is provided where the natural ground slope is steeper than the design canal bed slope, to safely lower the water surface by an abrupt vertical (or near-vertical) drop while dissipating the excess energy, preventing erosive high velocities in the canal downstream.

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Design of a vertical drop structure includes fixing the crest level and length (waterway) to pass the design discharge at permissible head over the crest, and the thickness of the impervious floor (upstream and downstream) based on uplift pressures — the same general principles as for a weir, applied at a smaller scale, with a stilling basin/cistern downstream to dissipate the energy of the falling jet.

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Cross-Drainage Structures Cross-drainage structures are required wherever a canal alignment (e.g. a contour canal) intersects a natural drainage (stream/nala), to safely pass both the canal water and the drainage flood water without mixing or obstructing either.

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Aqueduct: canal is carried in a trough (flume) over the drainage, which passes underneath through the natural bed/openings — used when the drainage HFL (high flood level) is well below the canal bed level.

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Syphon aqueduct: canal is carried over the drainage as in an aqueduct, but the drainage is made to flow under pressure (syphon) through barrels beneath the canal trough — used when the drainage HFL is higher, so the drainage would otherwise submerge/interfere with the canal.

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Super-passage: reverse of an aqueduct — the natural drainage is carried over the canal in a trough, while the canal flows underneath (open, not pressurized) — used when canal FSL (full supply level) is well below the drainage bed.

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Canal syphon (syphon super-passage): the drainage is carried over the canal as in a super-passage, but the canal is made to flow under pressure (syphon) beneath the drainage trough — used when canal FSL is close to or above the drainage bed level.

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Level crossing: used where the canal and drainage cross at essentially the same level with comparable discharges/flood levels, mixing the flows via a regulated crossing, with regulators to separate them again afterward — adopted only where an aqueduct/super-passage is impractical.

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This section covers aquifer characteristics and groundwater yield, groundwater exploration methods, design/selection of wells and tube wells, and working principles, design considerations, and selection of water- lifting devices and pumps.

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Aquifer Characteristics and Groundwater Yield Term Meaning Aquifer A saturated permeable geological formation (sand, gravel, fractured rock) capable of transmitting and yielding significant quantities of water to a well Confined (artesian) aquifer Aquifer bounded above and below by impermeable layers, water under pressure — water level in a well rises above the top of the aquifer (piezometric surface) Unconfined (water- table) aquifer Aquifer with a free water table as its upper boundary, directly connected to the atmosphere through overlying permeable material Specific yield Volume of water an unconfined aquifer releases from storage per unit surface area per unit decline in water table (drainable porosity) Storage coefficient (storativity) Volume of water a confined aquifer releases from storage per unit surface area per unit decline in piezometric head Transmissivity (T) Rate at which water is transmitted through the full saturated thickness of an aquifer per unit hydraulic gradient (T = K × b, K=hydraulic conductivity, b=aquifer thickness) Safe yield The quantity of groundwater that can be withdrawn from an aquifer over the long term without causing progressive depletion, adverse water-quality changes, or other undesirable effects Groundwater Exploration Methods Surface geophysical methods: electrical resistivity survey (measures subsurface resistivity contrasts to infer aquifer depth/extent) and seismic refraction survey (uses travel time of induced seismic waves to infer subsurface layering) — both non-invasive, used for preliminary siting.

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Test/exploratory drilling: drilling test bores to directly confirm subsurface strata, aquifer thickness, and water-bearing zones, followed by collection of samples/geophysical logging (e.g. electrical resistivity logging of the borehole) to characterize the aquifer in detail.

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Pumping tests: conducted on a completed test well to determine aquifer parameters (transmissivity, storage coefficient) and to estimate a well's safe/sustainable yield, by observing drawdown response in the pumped well and nearby observation wells over time.

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Design and Selection of Wells;

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Tube Well Drilling Well types: dug wells (large diameter, shallow, hand/mechanically excavated, suited to shallow water tables and modest yields), tube wells (bore wells) (small diameter, drilled deep, fitted with a screen/strainer opposite the aquifer, suited to larger, more reliable yields from deeper/confined aquifers), and infiltration galleries (horizontal collector pipes, used near riverbeds).

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Tube well components: casing pipe (supports the borehole wall above the aquifer), screen/strainer (slotted pipe opposite the aquifer, allows water in while restricting sand entry), gravel pack (placed around the screen in fine-grained aquifers to prevent sand pumping and increase effective well diameter), and a sump/blind pipe at the bottom (settling chamber for any fine sand).

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Tube-well drilling methods: percussion (cable-tool) drilling (repeated lifting/dropping of a heavy bit to crush/loosen material, suited to boulder/hard formations), rotary drilling (rotating bit with circulating drilling fluid/mud to remove cuttings and stabilize the borehole, faster in soft/medium formations), and reverse (rotary) circulation drilling for very large-diameter wells.

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Selection of well type and drilling method depends on the depth/nature of the aquifer, formation material (soft alluvium vs. hard rock/boulders), required yield, and available drilling equipment/budget.

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Water-Lifting Devices and Pumps Device/Pump Working Principle / Typical Use Reciprocating (piston) pump Positive-displacement; a piston moving in a cylinder with suction/delivery valves draws and forces water; suited to high head, low discharge, and can handle a fairly constant discharge irrespective of moderate head variation Centrifugal pump Rotodynamic; an impeller imparts kinetic energy to water which is converted to pressure head in a volute casing; suited to moderate head and large discharge, widely used for irrigation lift from open wells/canals Submersible pump A close-coupled centrifugal pump-motor unit submerged directly in the water (borewell/tube well), eliminating the need for a long suction line and priming, suited to deep tube wells Propeller (axial- flow) pump Rotodynamic; an axial propeller imparts mainly axial velocity to water; suited to very low head and very large discharge (e.g. low-lift drainage/irrigation pumping) Hydraulic ram (hydram) Uses the energy of a larger flow of water falling through a small head (water hammer effect) to lift a small fraction of that water to a much greater height, without external power — suited to remote sites with a perennial stream/spring at a suitable fall Treadle pump A manually (foot) operated reciprocating/suction pump, low-cost and human-powered, suited to smallholder irrigation from shallow water tables Selection of a water-lifting device/pump considers the required head and discharge, depth to water, power availability (electricity/diesel/manual/renewable), cost (capital and operating), and the extent of aquifer/well yield (should not exceed the safe yield of the well).

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Installation considerations include correct pump setting depth (submergence for submersible pumps, suction lift limits for centrifugal/reciprocating pumps), foot valves/strainers to prevent sand ingress and pump damage, and proper alignment/foundation to minimize vibration and wear.