7. Irrigation and Drainage (ACiE07)

7.1 Water Demand Estimation

Crop Water Requirement and Irrigation Water Requirement

Crop water requirement (CWR) is the total amount of water required by a crop for its growth and production, including evapotranspiration and water incorporated into plant tissues. Irrigation water requirement (IWR) is the portion of CWR that must be supplied by irrigation to meet the crop's needs, after accounting for effective rainfall and soil moisture contributions.

The primary method for estimating crop evapotranspiration (ETc) is:

ETc = Kc * ETo

Where:

ETc = Crop evapotranspiration (mm/day or cm/season)
Kc = Crop coefficient (dimensionless, specific to crop type and growth stage)
ETo = Reference crop evapotranspiration (mm/day or cm/season), typically calculated using Penman-Monteith equation for a reference crop like grass or alfalfa.

Water Availability for Irrigation

Water availability for irrigation considers the sources and quantities of water that can be utilized. This includes:

Inflow: Water entering the system from rivers, streams, or other natural sources.
Storage: Water stored in reservoirs, ponds, or groundwater aquifers, which can be drawn upon during periods of high demand or low natural inflow.

Accurate assessment of water availability is crucial for planning and designing irrigation schemes.

Command Areas

Command areas define the land that can be irrigated by a particular irrigation project.

Gross Command Area (GCA): The total area that can be physically commanded or irrigated by an irrigation system, including uncultivable land, villages, roads, etc.
Culturable Command Area (CCA): The portion of GCA that is suitable for cultivation. This excludes barren land, forests, habitations, etc.
Intensity of Irrigation: The percentage of the CCA that is actually irrigated in a particular season or year.

Irrigation Intensity (Annual, Seasonal)

Irrigation intensity expresses the proportion of the culturable command area that is irrigated in a given period.

Seasonal Irrigation Intensity: The percentage of CCA irrigated in a specific season (e.g., Rabi, Kharif).
Annual Irrigation Intensity: The sum of seasonal irrigation intensities, expressed as a percentage of CCA. It can exceed 100% if different crops are grown in different seasons on the same land.

Duty, Delta, and Their Relationship

Duty (D): The area of land that can be irrigated by a unit discharge of water (1 m³/s) flowing continuously for the entire base period of the crop. It is typically expressed in hectares per cumec (ha/cumec).
Delta (Δ): The total depth of water required by a crop during its entire base period (growth period). It is expressed in meters (m) or centimeters (cm).
Base Period (B): The total period in days during which a crop needs water for its growth, from the first watering to the last.

The relationship between Duty, Delta, and Base Period is given by:

D = 8.64 * B / Δ

Where:

D = Duty (ha/cumec)
B = Base period (days)
Δ = Delta (meters)
8.64 = Conversion factor (8.64 x 10⁴ seconds in a day / 10⁴ m²/ha)

Example: If a crop requires 120 cm of water (Δ = 1.2 m) over a base period of 100 days, the duty would be D = (8.64 * 100) / 1.2 = 720 ha/cumec.

Water Losses

Water is lost at various stages in an irrigation system, reducing the overall efficiency.

Evaporation: Water lost to the atmosphere from open water surfaces (canals, reservoirs) and moist soil.
Seepage: Water lost by percolation through the bed and sides of unlined canals into the surrounding soil.
Percolation: Downward movement of water through the soil profile beyond the root zone, often leading to groundwater recharge or waterlogging.
Conveyance Losses: Sum of evaporation and seepage losses occurring during the transport of water from the source to the field.

Irrigation Efficiencies

Irrigation efficiencies quantify how effectively water is utilized in an irrigation system.

Conveyance Efficiency (η_c): The ratio of water delivered to the field to the water diverted from the source. η_c = (Water delivered to field / Water diverted from source) * 100%
Application Efficiency (η_a): The ratio of water stored in the root zone to the water delivered to the field. η_a = (Water stored in root zone / Water delivered to field) * 100%
Overall Efficiency (η_o): The product of conveyance efficiency and application efficiency, representing the ratio of water stored in the root zone to the water diverted from the source. η_o = η_c * η_a

Effective Rainfall

Effective rainfall is the portion of total rainfall that is available for meeting the crop's evapotranspiration requirements and contributes to soil moisture storage in the root zone. It excludes runoff and deep percolation losses.

Suitable Method: Various empirical methods exist, often based on monthly rainfall data and crop characteristics.
Rational Method: While primarily for runoff estimation, its principles can be adapted to consider rainfall intensity and duration in determining what portion infiltrates and becomes effective. More advanced methods like USDA SCS method or FAO methods are commonly used for effective rainfall estimation.

Soil-Moisture-Irrigation Relationship

Understanding soil moisture characteristics is fundamental for efficient irrigation scheduling.

Field Capacity (FC): The maximum amount of water that a soil can hold against the force of gravity after excess water has drained away.
Permanent Wilting Point (PWP): The soil moisture content at which plants can no longer extract sufficient water from the soil to prevent permanent wilting.
Available Moisture (AM): The difference between Field Capacity and Permanent Wilting Point. This is the water available for plant uptake. AM = FC - PWP
Readily Available Moisture (RAM): The portion of available moisture that crops can extract without experiencing significant stress, typically 50-70% of AM.

Depth and Frequency of Irrigation

Net Irrigation Requirement (NIR): The actual amount of water required to bring the soil moisture in the root zone back to field capacity. NIR = (FC - Current Moisture Content) * Bulk Density * Root Depth / 100 (in cm)
Gross Irrigation Requirement (GIR): The total water to be diverted from the source, accounting for application and conveyance losses. GIR = NIR / (η_a * η_c)
Frequency of Irrigation: The number of days between two successive irrigations, determined by the rate of water depletion from the root zone by evapotranspiration. Frequency = (Readily Available Moisture / Daily ETc)

Design Discharge for Canals

The design discharge for a canal is the maximum flow rate the canal is expected to carry to meet the irrigation requirements of its command area.

Q = A / D

Where:

Q = Design discharge (m³/s or cumecs)
A = Culturable Command Area (CCA) to be irrigated during the peak demand period (ha)
D = Design Duty (ha/cumec) for the most demanding crop/season.

Alternatively, Q = (Area * Δ) / (8.64 * B) considering losses.

7.2 Design of Canals

Canal Types

Canals are classified based on their capacity and position in the irrigation network.

Main Canal: Takes off directly from the river or reservoir, carries a large discharge, and feeds branch canals. It does not directly irrigate land.
Branch Canal: Takes off from the main canal, carries a substantial discharge, and feeds distributaries. It may irrigate a small area directly.
Distributary Canal: Takes off from a branch canal (or main canal), carries a smaller discharge, and feeds minor canals and water courses. It directly irrigates agricultural fields.
Minor Canal: Takes off from a distributary, carries a small discharge, and feeds water courses.
Water Course (Field Channel): The smallest channel, taking off from a minor or distributary, delivering water directly to individual farm fields.

Canal Network and Alignment

Canal alignment involves planning the path of the canal to efficiently serve the command area while minimizing costs and environmental impact.

General Slope: Canals are generally aligned along contours or ridges to minimize cross-drainage structures and maximize command area by gravity flow.
Command Area Consideration: The alignment should ensure maximum culturable command area is served by gravity.
Types of Alignment: Ridge line (watershed) canals, contour canals, side-slope canals. Ridge line canals are preferred as they command areas on both sides and avoid cross-drainage works.

Tractive Force Approach of Canal Design

The tractive force approach designs stable channels by ensuring that the shear stress (tractive force) exerted by flowing water on the channel bed and sides does not exceed the permissible shear stress for the soil material, preventing erosion.

Tractive Force (τ): Shear stress acting on the wetted perimeter of the channel, τ = γ * R * S, where γ is specific weight of water, R is hydraulic radius, and S is bed slope.
Shields Diagram: A graphical representation used to determine the critical shear stress for initiation of particle motion for different sediment sizes and specific gravities.
Tractive Force Ratio: The ratio of permissible tractive force on the side slopes to that on the bed, used to adjust design for side slope stability.

Design of Stable Canals (Manning's Formula)

For non-erodible or lined canals, Manning's formula is widely used to determine flow velocity and channel dimensions.

V = (1/n) * R^(2/3) * S^(1/2)

Where:

V = Mean flow velocity (m/s)
n = Manning's roughness coefficient (dimensionless, depends on channel material)
R = Hydraulic radius (m) = (Wetted Area / Wetted Perimeter)
S = Bed slope (m/m)

For design, given discharge (Q) and desired velocity (V), the required cross-sectional area (A = Q/V) is determined, and then channel dimensions are selected to satisfy A and R, while keeping V within permissible limits.

Design of Alluvial Canals

Alluvial canals are constructed in erodible soil, and their design must account for sediment transport and channel stability (regime conditions).

Kennedy's Theory

Kennedy proposed that a channel is in regime if there is no silting or scouring. He related critical velocity to depth:

V₀ = 0.55 * m * y^0.64

Where:

V₀ = Critical velocity (m/s)
m = Critical velocity ratio (dimensionless, depends on silt grade; 1.0 for standard silt, >1 for coarser, <1 for finer)
y = Depth of flow (m)

Design involves trial and error to satisfy continuity equation (Q = A*V) and critical velocity criteria.

Lacey's Theory

Lacey's theory defines a "regime channel" as one that flows in incoherent alluvium and has attained a stable state regarding its cross-section and slope after carrying a constant discharge and silt charge for a long time.

Silt Factor (f): f = 1.76 * sqrt(d_mm), where d_mm is the mean diameter of silt particles in mm.
Regime Width (P): P = 4.75 * sqrt(Q) (m)
Regime Velocity (V): V = (Q * f² / (140 * R))^(1/6) or V = (Q * f² / 140)^(1/3) if R approx. y
Regime Hydraulic Radius (R): R = (Q * f / 140)^(1/3)
Regime Slope (S): S = f^(5/3) / (3340 * Q^(1/6))

Lacey's theory provides direct formulas for channel dimensions and slope once Q and f are known.

Design of Lined Canals

Lined canals have an impervious layer (concrete, geomembrane) on their bed and sides to reduce seepage losses and allow for higher velocities without erosion.

Concrete Lining: Durable, high velocity, low seepage. Design involves determining economic cross-section using Manning's formula and providing structural stability for lining.
Geomembrane Lining: Flexible, cost-effective, excellent seepage control. Design considers material properties, anchoring, and protection from damage.

Advantages include reduced seepage, increased carrying capacity for a given cross-section (due to smoother surface and higher permissible velocity), reduced maintenance, and prevention of waterlogging.

7.3 Diversion Headworks

Components of Headwork

A diversion headwork is a structure built across a river to raise the water level and divert it into an off-taking canal.

Weir/Barrage: A barrier constructed across the river to raise water level. A weir has an ungated crest, while a barrage has gates on its crest for better flow control.
Guide Bank: Earthen embankments provided on one or both banks upstream and downstream of the weir/barrage to guide the river flow smoothly through the structure and protect the river banks.
Afflux Bund: Earthen embankments extending from the guide banks to the high ground, protecting the surrounding area from submergence due to afflux (rise in water level) caused by the weir.
Fish Ladder: A passage provided in the weir/barrage to allow fish to migrate upstream and downstream, maintaining aquatic ecology.
Canal Head Regulator: A structure at the head of the off-taking canal to control the entry of water into the canal and regulate the flow.
Sill: A raised portion at the entrance of the canal head regulator to prevent entry of bed silt into the canal.

Seepage Theories

These theories are used to analyze and design the impervious floor and cutoffs of hydraulic structures on permeable foundations to prevent uplift pressure and piping.

Bligh's Theory

Bligh's theory assumes that seepage water follows the path of contact between the base of the structure and the subsoil (creep path). Head loss is proportional to the length of creep path.

L = H / C

Where:

L = Total creep length (sum of horizontal and vertical creep)
H = Total head loss across the structure
C = Bligh's creep coefficient (depends on soil type)

Limitations: Does not differentiate between horizontal and vertical creep, ignores exit gradient.

Lane's Theory

Lane modified Bligh's theory by assigning different weights to horizontal and vertical creep paths, recognizing that vertical creep is more effective in reducing uplift.

L_w = (1/3) * L_h + L_v

Where:

L_w = Weighted creep length
L_h = Sum of horizontal creep lengths
L_v = Sum of vertical creep lengths (depths of pile walls)

The permissible weighted creep ratio (L_w/H) is used for design.

Khosla's Theory

Khosla's theory is based on the theory of flow nets and provides a more accurate assessment of uplift pressure and exit gradient. It considers individual elementary profiles (e.g., floor with a pile, floor with two piles) and combines their effects.

Uplift Pressure: Calculated at various points along the impervious floor using complex potential functions or flow net analysis.
Exit Gradient (G_E): The hydraulic gradient at the downstream end of the impervious floor, crucial for checking safety against piping. G_E = H / (d * π * sqrt(λ)) (for a simple case of a sheet pile) Where d is the depth of the sheet pile and λ is a factor related to geometry.

Khosla's theory is more rigorous and widely used for modern designs.

Design of Silt Control Structures

Silt entry into canals reduces their carrying capacity and requires frequent dredging.

Silt Excluder: Structures constructed in the river bed upstream of the head regulator to exclude silt from entering the canal. They typically consist of tunnels or undersluices that carry the bottom layer of silt-laden water back to the river.
Silt Ejector (Silt Extractor): Structures constructed in the canal bed, a short distance downstream of the head regulator, to extract silt that has already entered the canal. They work by creating a depression and utilizing a portion of the canal discharge to flush the silt back into the river or waste channel.
Settling Basin: A widened section of the canal where flow velocity is reduced, allowing silt particles to settle out by gravity. The settled silt is periodically removed.

Design of Weir/Barrage

Design involves determining various structural dimensions and levels.

Crest Level: Set to achieve the desired pond level for diverting water into the canal, considering afflux and flood levels.
Length of Weir/Barrage: Determined by river width, flood discharge, and permissible afflux.
Thickness of Impervious Floor: Designed to withstand uplift pressure using Bligh's, Lane's, or Khosla's theory (hydraulic gradient method). The floor must be thick enough to counteract the uplift pressure with a suitable factor of safety.

Design of Energy Dissipaters

Energy dissipaters are structures designed to reduce the high kinetic energy of water flowing over a weir or barrage, preventing scour downstream.

Stilling Basin: A concrete basin constructed downstream of the weir, where a hydraulic jump is forced to form, dissipating energy through turbulence. Design involves determining the length and depth of the basin based on the Froude number of the incoming flow. Fr = V / sqrt(g * y) Where Fr is Froude number, V is velocity, g is acceleration due to gravity, and y is flow depth.
Bucket Type Dissipaters: Include solid rollers or ski-jump buckets, which deflect the high-velocity jet either upwards into the air (ski-jump) or along the bed (solid roller), causing energy dissipation away from the structure.

7.4 River Training Works

River Stages

Rivers exhibit different characteristics at various flow levels.

Low Water Stage: Minimum flow, confined to the deepest part of the channel.
Bankful Stage: Flow just fills the channel without overflowing the banks. This is often associated with the dominant discharge that shapes the river morphology.
Flood Stage: Water level exceeds bankful, inundating floodplains.
Ordinary Flood: Frequent flood events with moderate intensity.
Maximum Flood: The highest recorded or design flood, often associated with a very long return period.

Need of River Training

River training works are constructed to improve the river's behavior for various purposes.

Protecting Banks: Preventing erosion and avulsion (sudden change in river course) of valuable land and infrastructure.
Training Flow: Guiding the river flow along a desired alignment, especially near bridges, barrages, or intake structures.
Preventing Avulsion: Stabilizing the river course to prevent it from abandoning its existing channel.
Flood Control: Reducing flood damage by confining floodwaters.
Navigation: Maintaining sufficient depth and width for navigation.

Design of River Training Works

Guide Bunds

Guide bunds are earthen embankments with stone pitching, constructed upstream of hydraulic structures (like bridges or barrages) to guide the river flow centrally and smoothly through the structure, preventing outflanking and scour.

Launching Aprons: Flexible stone pitching provided at the toe of the guide bunds, designed to "launch" or settle down as scour occurs, protecting the foundation.
Positioning: Designed with a suitable radius of curvature and length to effectively guide the flow.

Levees/Flood Embankments

Levees are earthen embankments constructed parallel to the river banks to confine floodwaters within a desired channel, protecting adjacent land from inundation.

Free Board: The vertical distance between the top of the levee and the design flood level, providing a safety margin.
Slope Protection: The slopes of levees are protected with grassing, riprap, or other materials to prevent erosion due to wave action or rainfall.

Spurs/River Training Walls (Groynes)

Spurs are structures projecting from the bank into the river, designed to deflect current away from the bank, induce silting in desired areas, or scour a channel.

Permeable Spurs: Allow water to pass through but reduce velocity, promoting silting (e.g., timber piles, brushwood).
Impermeable Spurs: Solid structures that completely obstruct flow, causing deflection and scour (e.g., stone, concrete).
Classification: Repelling (deflects flow away), Attracting (attracts flow towards it), Deflecting (changes direction of flow).

Watershed Management

A holistic approach to managing land and water resources within a watershed to prevent degradation and enhance productivity.

Soil Conservation: Measures like contour plowing, terracing, strip cropping, and afforestation to reduce soil erosion.
Afforestation: Planting trees to increase vegetative cover, reduce runoff, and prevent soil erosion.
Check Dams: Small, temporary or permanent dams built across small channels or gullies to reduce flow velocity, trap sediment, and promote infiltration.

7.5 Regulating and Cross-Drainage Structures

Functions of Various Types of Regulators

Regulators control the flow of water in a canal system.

Head Regulator: Located at the take-off point of a canal from a river/reservoir, controls water entry and regulates discharge into the main canal.
Cross Regulator: Built across a main canal, upstream of an off-taking distributary, to control the water level in the main canal and divert flow into the distributary.
Tail Regulator: Located at the tail end of a canal, regulates the water level and discharge and prevents erosion at the canal's end.
Canal Escape: A structure that allows excess water in a canal to be discharged safely back into a river or natural drain, preventing overtopping of canal banks.

Design of Regulators and Escapes

Design principles are similar to weirs/barrages for their hydraulic and structural stability.

Crest Level: Determined by the required full supply level (FSL) in the canal and the head needed for flow.
Length of Weir Floor: Designed for stability against uplift and piping using Bligh's or Lane's theory.
Thickness of Floor: Calculated based on uplift pressure and material strength.
Gates: Designed to control flow efficiently and withstand hydraulic forces.

Design of Pipe Outlet

Outlets are structures that release water from a canal into a watercourse or directly to fields.

Free Outlet: Discharges water freely into the atmosphere or a channel where the water level does not affect the discharge. Q = C_d * A * sqrt(2gH) Where Q is discharge, C_d is coefficient of discharge, A is area of opening, g is acceleration due to gravity, and H is head.
Submerged Outlet: Discharges water into a channel where the downstream water level is above the outlet's crest, affecting discharge. Q = C_d * A * sqrt(2gH) (where H is the difference in water levels)

Design of Vertical Drop

A vertical drop is a structure used to lower the water level in a canal where there is a sudden change in ground elevation. It dissipates excess energy.

Crest Level: Set to achieve the desired upstream water level.
Length of Impervious Floor: Designed to ensure stability against uplift and piping, similar to weirs.
Stilling Basin: Essential downstream of a vertical drop to dissipate the energy of the falling water and prevent scour. Design involves determining its dimensions based on hydraulic jump characteristics.

Design of Cross-Drainage Structures

These structures are built where a canal crosses a natural drain or river.

Aqueduct: The canal passes over the drainage channel. Suitable when the canal bed level is higher than the highest flood level (HFL) of the drain.
Siphon Aqueduct: The canal passes over the drainage channel, but the drainage channel flows under pressure (siphon action) due to its HFL being higher than the canal bed.
Level Crossing: The canal and the drainage channel intersect at the same level. This requires regulatory gates on both to manage flow, often used when discharge in both is comparable.
Superpassage: The drainage channel passes over the canal. Suitable when the drainage bed level is higher than the canal's full supply level.
Canal Siphon: The canal passes under the drainage channel, flowing under pressure.

Design considerations include hydraulic capacity, structural stability, and minimizing head losses.

7.6 Water Logging and Drainage

Causes of Water Logging

Waterlogging occurs when the water table rises to or near the ground surface, saturating the root zone of crops.

Seepage from Canals: Unlined canals lose water through seepage, contributing to the rise of the water table.
Over-irrigation: Excessive application of irrigation water beyond crop needs and soil's holding capacity leads to deep percolation and groundwater recharge.
High Water Table: Naturally high groundwater table or inadequate natural drainage.
Obstruction to Natural Drainage: Roads, railway embankments, or other structures can block natural surface and subsurface drainage paths.
Inadequate Field Drainage: Poorly maintained or insufficient on-farm drainage systems.

Effects of Water Logging

Waterlogging has severe negative impacts on agriculture and the environment.

Reduced Crop Yield: Lack of oxygen in the root zone suffocates roots, impairs nutrient uptake, and promotes anaerobic conditions, leading to stunted growth and reduced yields.
Salinization: As water evaporates from the saturated soil surface, salts are left behind, accumulating in the root zone and making the soil saline and unproductive.
Health Hazards: Stagnant water creates breeding grounds for mosquitoes and other disease vectors, increasing the incidence of water-borne diseases.
Reduced Soil Fertility: Leaching of nutrients and changes in soil chemistry.

Preventive Measures

Mitigating waterlogging requires a combination of engineering and management practices.

Lined Canals: Lining canals with impervious materials (concrete, geomembranes) significantly reduces seepage losses.
Controlled Irrigation: Implementing efficient irrigation methods (e.g., drip, sprinkler) and scheduling based on actual crop water requirements to avoid over-irrigation.
Drainage: Providing adequate surface and/or subsurface drainage systems to remove excess water.
Conjunctively Use of Water: Using groundwater in conjunction with surface water to lower the water table.
Crop Selection: Cultivating water-tolerant crops in affected areas.

Design of Surface Drainage Systems

Surface drainage removes excess water from the land surface before it can infiltrate and raise the water table.

Open Drains (Field Drains, Collector Drains, Main Drains): Channels constructed to collect and convey excess surface runoff. Design involves determining appropriate cross-sections, slopes, and alignment based on expected runoff volumes and topography.
Land Grading/Levelling: Shaping the land surface to eliminate depressions and provide a uniform slope, facilitating smooth runoff.
Slope: Designed to ensure adequate velocity for water removal without causing erosion.

Design of Subsurface Drainage Systems

Subsurface drainage removes excess water from below the ground surface, lowering the water table.

Horizontal Drains (Tile Drains, Perforated Pipe Drains): Buried perforated pipes laid at a specific depth and spacing to collect groundwater and convey it to an outlet.
Vertical Drains (Drainage Wells): Wells drilled into an aquifer to pump out groundwater, lowering the water table over a larger area.
Tile Drains: Traditionally made of clay tiles, now often PVC or corrugated plastic pipes. Spacing and depth are critical design parameters.
Spacing Formulas: Empirical and theoretical formulas are used to determine the optimal spacing of drains to achieve a desired water table depth.
Hooghoudt Equation: A widely used analytical formula for calculating drain spacing in homogeneous soils, considering the depth of the impervious layer and hydraulic conductivity. S² = 8 * K * h * d / q + 4 * K * h² / q (for steady state) Where:
- S = Drain spacing (m)
- K = Hydraulic conductivity of the soil (m/day)
- h = Depth of water table midway between drains above the drain level (m)
- d = Equivalent depth to the impervious layer (m, adjusted for convergence near drains)
- q = Drainage rate (m/day)
This equation is often used with modifications for transient conditions and layered soils.