Drainage engineering

Agricultural Engineering (B.E./B.Tech) · Soil and water · free study notes for exams, placements and interviews

In short

Drainage engineering is the design and management of systems that remove excess water from the land surface and from the crop root zone, and that carry away the salts dissolved in that water. In India, canal irrigation without drainage has waterlogged and salinised large areas of the Indo-Gangetic plain, Punjab, Haryana, Rajasthan's Chambal and Indira Gandhi Nahar command, and the deltas of Andhra Pradesh and Tamil Nadu, so drainage is now treated as an inseparable half of irrigation. It is examined in GATE Agricultural Engineering (Soil and Water Conservation Engineering section), Civil Engineering irrigation papers, and in ICAR/ASRB JRF-SRF and state agricultural-engineering service exams. Employers who ask about it include CADA and state irrigation/water-resources departments, WAPCOS, NABARD-funded command-area projects, ICAR institutes (ICAR-CSSRI Karnal, ICAR-IISWC Dehradun), and pipe and geotextile manufacturers.

Explained

Why drainage is needed: waterlogging and salinity

Waterlogging - the water table rises into or near the root zone so that soil air is displaced; roots, being aerobic, suffer oxygen stress within 1-3 days for most field crops (rice is the exception).
Causes: over-irrigation, canal and field-channel seepage, poor natural outlet, flat topography, an impermeable subsoil layer (hardpan, clay lens), obstruction of natural drainage by roads, railways and embankments, and high rainfall on heavy soils.
Effects: fall in soil temperature, denitrification and nitrogen loss, build-up of CO₂ and reduced substances (Fe²⁺, Mn²⁺, H₂S), restricted root depth, delayed tillage, weed shift to sedges, and yield loss of 20-100%.
Secondary salinisation - capillary rise from a shallow water table evaporates at the surface and leaves salts behind. Rule of thumb: when the water table is within about 1.5-2 m of the surface in arid climates, capillary salinisation becomes significant.
Drainage therefore has two jobs: water removal (aeration, trafficability, flood relief) and salt removal (leaching with a downward flux and an outlet).
Area estimates for India vary between agencies and surveys; broadly, several million hectares are affected by waterlogging and salt-affected soils, with the largest canal-command problem areas in Punjab, Haryana, Uttar Pradesh, Rajasthan and Gujarat. Quote the current NRSC/ICAR-CSSRI figure rather than an older one if a precise number is required.

Surface drainage

Removes ponded water from the surface before it infiltrates; suited to flat lands, heavy soils of low permeability, and high-intensity-rainfall regions where subsurface drainage would be too slow.
Random field drainage - shallow ditches connecting scattered depressions; used in undulating land.
Parallel field drains - parallel shallow, broad, flat-bottomed drains across the slope, farmable over; standard for uniformly flat land.
Bedding system - land ploughed into raised beds ("lands") 8-30 m wide with dead furrows between them; the oldest method, used on very flat heavy soils.
Graded/interception drains - run on a slight grade across a slope to intercept overland flow and seepage from above.
Land grading and smoothing - reshaping the field to a continuous slope (typically 0.1-0.5%) so that no pockets hold water; measured and designed from a grid survey.
Open ditch (collector/outlet) drains - trapezoidal channels designed by Manning's equation; side slopes 1:1 to 2:1 (flatter in sandy soils), freeboard 0.3-0.5 m, non-scouring, non-silting velocity (roughly 0.4-1.5 m/s in earthen drains depending on soil).
Advantages: low initial cost, handles large storm flows, easy to inspect. Limitations: takes 5-10% of land out of cultivation, obstructs farm machinery, needs frequent desilting and weed clearance, banks erode.

Subsurface (sub-surface) drainage

Lowers and controls the water table by letting groundwater flow into buried conduits or deep open drains; the only method that reclaims saline-waterlogged land because it provides a salt outlet.
Horizontal subsurface drainage: buried pipes (laterals) discharging into a collector, then to a sump or gravity outlet.
- Pipe materials: corrugated PVC/HDPE perforated pipe (now standard in India; 60-110 mm laterals, 100-250 mm collectors), formerly clay/concrete tiles laid in short lengths with open joints.
- Depth: typically 0.9-1.8 m in India; deeper in saline areas (1.5-2.0 m) to control capillary rise, shallower in high-rainfall humid areas.
- Grade: 0.1-0.3% minimum to maintain self-cleaning velocity (~0.3-0.5 m/s).
- Layouts: *gridiron* (laterals on one side of the collector, for flat uniform land); *herringbone* (laterals on both sides, for a natural depression line); *interceptor* (a single line at the toe of a slope or along a canal to catch seepage); *random* (for isolated wet spots).
Mole drainage - unlined circular channels (75-100 mm) pulled by a mole plough at 0.4-0.6 m depth in stable clay soils (clay >35%, no sand/gravel); very cheap, life 2-5 years, used for temporary relief.
Vertical drainage - pumping from tube wells to lower the regional water table; viable where the aquifer is fresh and the water is reusable (Punjab/Haryana fresh-ground-water tracts). Limitation: pumping cost, and saline pumped water needs disposal.
Bio-drainage - transpiration by deep-rooted, high-water-use trees (*Eucalyptus*, *Populus*, *Acacia*, *Casuarina*) planted in strips along canals; supplementary, no salt outlet, so salts accumulate in the root zone over time.
Drainage filters/envelopes - gravel (graded sand-gravel) or synthetic (geotextile, pre-wrapped loose coconut/coir, PP fibre) surround to stop soil particles entering the pipe while keeping entry resistance low. Criterion of the gravel-envelope type: D₁₅(filter)/D₈₅(soil) < 4-5 and D₁₅(filter)/D₁₅(soil) > 4-5.
Outlet - gravity outlet into a drain/stream where levels allow; otherwise a sump and pump (common in the Haryana and Rajasthan schemes). Disposal of saline effluent is the hardest problem: evaporation ponds, dilution into drains, or reuse on salt-tolerant crops/aquaculture.

Drainage hydraulics and design theory

Flow to drains is saturated flow governed by Darcy's law; the water table between two drains forms a curved (roughly elliptical) surface with its highest point midway.
Two design states:
- Steady state - recharge equals discharge and the water table stays at a chosen midpoint height; used for humid areas with continuous recharge (Hooghoudt, Donnan, Ellipse equations).
- Unsteady (transient) state - a rise after irrigation/rain falls over time; design is "lower the water table from level h₀ to h₁ in t days" (Glover-Dumm equation). Suited to Indian irrigated arid areas where recharge is intermittent.
Hooghoudt's equation accounts for both horizontal and radial flow near the pipe through an equivalent depth d, which replaces the real depth D of the layer below drain level (d < D, because converging flow adds resistance).
Drainage coefficient (DC) - the design discharge per unit area, expressed in mm/day (or m³/s per 100 ha). Indian values broadly: 1-3 mm/day for salinity control in arid irrigated land; 5-15 mm/day for humid/high-rainfall areas; much higher for surface drainage design, where it comes from a rainfall-frequency and runoff analysis.
Drain spacing is the main design output; it increases with hydraulic conductivity K and drain depth, and decreases with recharge rate.
Site data needed: hydraulic conductivity (auger-hole method in the field, or constant/falling-head permeameter in the lab), drainable porosity (specific yield), depth to the impermeable barrier, water-table records (piezometers/observation wells), salinity profile of soil and groundwater, topography and outlet levels.
Leaching requirement for salinity control: the fraction of applied water that must pass below the root zone to keep root-zone salinity at the chosen level - the drainage system must be able to carry this away.
Drainage benefit/cost - judged by yield improvement, reclaimed area and cropping-intensity gain against pipe, installation, O&M and pumping costs; CSSRI-type Haryana projects report payback in roughly 3-6 years, but this depends strongly on current pipe prices and crop prices, so recompute rather than quote.

Construction, materials and Indian standards/practice

Installation: trenchless/trencher drainage machines lay corrugated pipe with envelope in one pass; laser or GPS grade control holds the designed slope. Manual trenching is still used in small schemes.
Appurtenances: manholes/inspection chambers at junctions and changes of grade, surface inlets with silt traps, outlet pipe with a flap/rodent guard, and non-return provision where the outlet can be submerged.
Maintenance: flushing of laterals (jetting), desilting collectors, mowing/weed removal in open drains, checking pump and sump, and record-keeping of water-table and salinity observations.
Codes and practice references (India):
- Bureau of Indian Standards has standards on *unplasticised PVC pipes*, *corrugated HDPE double-wall pipes*, *subsurface drainage practice for agricultural land*, *measurement of hydraulic conductivity* and *geotextiles for drainage filters*. IS numbers are revised and some old ones withdrawn - verify the current number and year on the BIS catalogue before citing in a report.
- Indian Standard for land drainage/subsurface drainage design guidance is supplemented by the CWC/Ministry of Jal Shakti guidelines, ICAR-CSSRI Karnal design manuals (the main Indian authority on subsurface drainage for salt-affected land) and ICID technical bulletins.
- Command Area Development and Water Management (CADWM) programme funds on-farm drainage and land levelling in canal commands.
- Environment (Protection) Act, 1986 and state pollution-control boards regulate the discharge of saline/agrochemical-laden drainage effluent into natural water bodies; the Water (Prevention and Control of Pollution) Act, 1974 applies to such disposal.
- The Wetlands (Conservation and Management) Rules, 2017 restrict draining of notified wetlands - relevant when a "waterlogged area" is ecologically a wetland.
- Highway and railway drainage of agricultural land crossings follows IRC and railway codes for cross-drainage works.
Drainage of urban/road cross-drainage, and conservation structures such as bunds and terraces, are handled by their own topics; here the focus is agricultural land drainage.

Drainage in special situations

Rice-based systems: rice needs ponding, so drainage must be *controlled* - shallow water-table control, drains closed during the rice season and opened before harvest and for the following upland crop. Controlled drainage structures (stop-logs in manholes) achieve this.
Coastal/deltaic areas (Kerala *kole* and *kuttanad*, Sundarbans, Godavari-Krishna delta): tidal influence means the outlet works only at low tide; sluice gates with tidal operation, ring bunds and pumped drainage are used; acid-sulphate soils and saline intrusion complicate design.
Canal seepage control: interceptor drains parallel to the canal, canal lining, and vertical drainage wells along the canal.
Drainage of orchard and plantation land: deeper drains (1.5-2 m) because the root zone is deeper; often individual tree-pit drainage in heavy soils.
Drainage congestion in flat flood plains (Bihar, eastern UP, Assam): needs regional main-drain networks and pumped outfalls rather than field drainage alone.
Protected cultivation and structures: drainage around polyhouses, farm ponds and buildings to protect foundations and avoid local waterlogging.

Key points

Drainage - removal of excess surface and subsurface water, and of the salts carried in it, from agricultural land.
Waterlogging - water table within the root zone so soil air is replaced; damaging for most crops within 1-3 days.
Secondary salinisation - salt accumulation at the surface due to capillary rise and evaporation from a shallow water table.
Surface drainage - removes ponded water; types are random, parallel field, bedding, graded/interception, plus land grading.
Subsurface drainage - lowers the water table through buried pipes or deep open drains; the only route that reclaims salt-affected land.
Layouts - gridiron (laterals one side), herringbone (both sides), interceptor (toe of slope/canal), random (isolated spots).
Drain depth and spacing - the two main design variables; deeper drains allow wider spacing for the same water-table control.
Drainage coefficient - design discharge per unit area in mm/day; ~1-3 mm/day for arid salinity control, 5-15 mm/day for humid areas.
Hydraulic conductivity K - measured in situ by the auger-hole method; the single most sensitive input to spacing.
Drainable porosity (specific yield) f - volume of water released per unit volume per unit fall of water table; needed for transient design.
Hooghoudt equation - steady-state spacing with an equivalent depth d representing radial flow resistance near the pipe.
Glover-Dumm equation - transient spacing: how fast the water table falls from h₀ to h₁ in t days.
Envelope/filter - gravel or synthetic wrap preventing soil entry; designed by D₁₅/D₈₅ grain-size criteria.
Mole drains - cheap unlined channels in stable clays, life 2-5 years; vertical drainage uses tube wells; bio-drainage uses trees but gives no salt outlet.
Leaching requirement - the extra water fraction that must drain below the root zone to control salinity.
Outlet - gravity where levels permit, otherwise sump and pump; disposal of saline effluent is regulated under the Water Act, 1974 and the Environment (Protection) Act, 1986.
Controlled drainage - drains throttled during the rice season and opened for upland crops; essential in rice-based rotations.
Open-drain design - trapezoidal section by Manning's equation, non-scouring velocity, 0.3-0.5 m freeboard, 1:1 to 2:1 side slopes.

Formulas and examples

Darcy's law: q = −K (dh/dl) · A — q = discharge (m³/day), K = hydraulic conductivity (m/day), dh/dl = hydraulic gradient (dimensionless), A = cross-sectional area (m²).

Donnan (steady state, drains on the barrier): S² = 4K (b² − a²) / q — S = drain spacing (m), K = m/day, b = height of water table above barrier at midpoint (m), a = height of water in drain above barrier (m), q = drainage coefficient (m/day).

Ellipse (Hooghoudt form for drains above the impermeable layer, radial flow neglected): S² = 4K h (2D + h) / q — h = height of water table above drain level at midpoint (m), D = depth of permeable layer below drain level (m).

Hooghoudt (with equivalent depth): S² = (8 K₂ d h + 4 K₁ h²) / q — K₂ = conductivity below drain level (m/day), K₁ = conductivity above drain level (m/day), d = Hooghoudt equivalent depth (m), h = midpoint water-table height above drain (m). For a homogeneous soil K₁ = K₂ = K.

Equivalent depth (approximate, for D ≤ S/4): d ≈ D / [1 + (8D)/(πS) · ln(D/u)] — u = wetted perimeter of the drain (m) ≈ π r for a pipe of radius r flowing half full. Standard practice is to read d from Hooghoudt's tables; the equation is iterative because d depends on S.

Glover-Dumm (transient): S² = π² K d t / [f · ln(1.16 h₀/h_t)] — t = time (days), f = drainable porosity (fraction), h₀ = initial and h_t = final midpoint water-table height above drain level (m). Different textbooks write the leading constant differently (π²Kd/μ form); state the version used.

Manning's equation for open drains: V = (1/n) R^2/3 S₀^1/2, Q = A·V — V = velocity (m/s), n = roughness (0.025-0.040 for earthen farm drains), R = A/P hydraulic radius (m), S₀ = bed slope (m/m), Q = m³/s, A = flow area (m²).

Drainage coefficient: DC (mm/day) = Q (m³/day) × 1000 / Area (m²); design discharge of a lateral Q = DC × area served.

Leaching requirement: LR = EC_iw / (5·EC_e − EC_iw) (Rhoades), or LR = EC_iw / EC_dw — EC_iw = EC of irrigation water (dS/m), EC_e = target saturation-extract EC of the root zone (dS/m), EC_dw = EC of the drainage water (dS/m). Depth of water needed: D_iw = D_cw / (1 − LR).

Auger-hole method: K = C · (Δy/Δt) — C = a factor from standard charts depending on hole radius, depth of hole below water table and depth to the barrier; Δy/Δt = rate of rise of water in the hole (m/s or cm/s), K in the same length-time units.

Example 1 - Ellipse equation spacing. Drains are laid on top of the impermeable layer. K = 1.2 m/day, allowable midpoint water-table height above drain level h = 0.5 m, drainage coefficient q = 3 mm/day = 0.003 m/day, depth of permeable layer below drain level D = 0 (drains on the barrier).

With D = 0: S² = 4K h² / q = 4 × 1.2 × 0.25 / 0.003 = 1.2/0.003 = 400 → S = 20 m.
If instead the barrier is 2 m below drain level and we (crudely) use the ellipse form with D = 2 m: S² = 4 × 1.2 × 0.5 × (2×2 + 0.5)/0.003 = 2.4 × 4.5/0.003 = 10.8/0.003 = 3,600 → S = 60 m. The ellipse form ignores radial resistance and so over-estimates spacing; Hooghoudt's d corrects it (Example 2).

Example 2 - Hooghoudt with equivalent depth. Homogeneous soil, K = 1.2 m/day, h = 0.5 m, q = 0.003 m/day, depth to barrier below drains D = 2 m, and from Hooghoudt's tables the equivalent depth for this geometry is d = 1.6 m.

S² = (8 K d h + 4 K h²)/q = (8 × 1.2 × 1.6 × 0.5 + 4 × 1.2 × 0.25)/0.003
8 × 1.2 × 1.6 × 0.5 = 7.68; 4 × 1.2 × 0.25 = 1.20; sum = 8.88
S² = 8.88/0.003 = 2,960 → S ≈ 54.4 m, adopt 54 m (or a round 50 m for layout convenience).
Note the drop from 60 m to 54 m once radial resistance is counted; d itself depends on S, so one iteration with the new S is normal practice.

Example 3 - Transient (Glover-Dumm) design in an irrigated arid command. Requirement: after an irrigation the water table must fall from h₀ = 0.8 m to h_t = 0.3 m above drain level in t = 3 days. K = 0.8 m/day, d = 2.0 m, drainable porosity f = 0.05.

ln(1.16 h₀/h_t) = ln(1.16 × 0.8/0.3) = ln(3.093) = 1.129
Numerator: π² K d t = 9.8696 × 0.8 × 2.0 × 3 = 47.37
Denominator: f × 1.129 = 0.05 × 1.129 = 0.05645
S² = 47.37/0.05645 = 839 → S ≈ 29 m. Transient design gives a much closer spacing than steady-state design for the same soil, because it must handle a rapid post-irrigation recharge.

Example 4 - Lateral discharge, pipe size check and collector load. A field of 10 ha is drained by laterals at 50 m spacing, 200 m long, with DC = 2 mm/day.

Total design flow for 10 ha = 0.002 m/day × 100,000 m² = 200 m³/day = 2.31 L/s.
Area per lateral = 50 m × 200 m = 10,000 m² = 1 ha → Q per lateral = 0.002 × 10,000 = 20 m³/day = 0.231 L/s, so 10 laterals.
A corrugated 80 mm perforated lateral at 0.15% grade carries several litres per second, so it is comfortably adequate; laterals are usually sized by minimum self-cleaning diameter and by the strength/handling requirement, not by this small flow.
The collector must carry the full 2.31 L/s plus a safety margin; a 150 mm collector at 0.1% grade is ample. If the outlet needs pumping, a sump pump of about 5 L/s with a 0.75-1.0 kW motor covers the design flow with margin.

Example 5 - Leaching requirement and extra water depth. Irrigation water EC_iw = 1.5 dS/m; the crop (wheat) tolerates a root-zone saturation-extract EC_e of 6 dS/m; consumptive use for the season D_cw = 400 mm.

LR = 1.5 / (5 × 6 − 1.5) = 1.5/28.5 = 0.0526 (5.3%)
Water to be applied D_iw = 400/(1 − 0.0526) = 400/0.9474 = 422 mm → extra 22 mm must pass the root zone and be carried away.
Over 1 ha, 22 mm = 220 m³ per season of drainage water needing an outlet - trivial for a pipe system, but it is salt-laden and its *disposal* is the real constraint.

Feature	Surface drainage	Subsurface pipe drainage	Vertical (tube-well) drainage	Mole drainage
Removes	Ponded surface water	Groundwater + salts	Regional groundwater	Perched water in clay
Best soil	Heavy, low-K	Medium to high K	Permeable aquifer below	Stable clay >35% clay
Typical cost	Low	High (capital-intensive)	Medium (plus pumping)	Very low
Salt reclamation	No	Yes	Yes, if water usable	Limited
Land loss	5-10%	Nil	Negligible	Nil
Life / O&M	Needs regular desilting	20-30 years, flushing	Pump O&M, energy cost	2-5 years, re-pull

Steady state	Transient (unsteady) state
Recharge = discharge, fixed water-table height	Water table falls with time after recharge
Donnan, Ellipse, Hooghoudt	Glover-Dumm, Kraijenhoff van de Leur
Inputs: K, D, h, q	Inputs: K, d, f, h₀, h_t, t
Suits humid areas, continuous recharge	Suits irrigated arid areas, intermittent irrigation
Wider spacing for the same soil	Closer spacing (more demanding)

Where it is used

State irrigation/water-resources and CAD authorities - design and execution of main drains, link drains and on-farm subsurface drainage in canal commands (Haryana's Rohtak-Jhajjar tracts, Rajasthan's Chambal and IGNP commands, Punjab's Muktsar belt, Andhra Pradesh's Krishna-Godavari delta).
Consultancies and PSUs - WAPCOS, NWDA and private firms prepare drainage DPRs with hydraulic conductivity surveys, spacing design and cost-benefit analysis for externally aided projects (World Bank, NABARD-RIDF, CADWM).
Pipe, geotextile and machinery industry - manufacturers of corrugated PVC/HDPE perforated pipe, coir and PP envelopes, and drainage trenchers/laser-graders; roles in product design, grade specification and field commissioning.
ICAR system and SAU research - ICAR-CSSRI Karnal (subsurface drainage and salt-affected soil reclamation), ICAR-IISWC Dehradun, ICAR-IARI and state agricultural universities; AICRP on Management of Salt Affected Soils and Use of Saline Water.
Job roles - agricultural engineer / drainage design engineer, irrigation & drainage consultant, CAD field engineer, GIS-remote-sensing analyst mapping waterlogged area, agri-input and precision-irrigation companies offering drainage with land levelling.
Allied applications - drainage for sports fields, polyhouses, airstrips and highway subgrades; dewatering and seepage control for farm ponds, embankments and construction sites.

Interview questions

Q: Why is drainage called the other half of irrigation? A: Irrigation adds water and salts to the land; without a drainage outlet the water table rises and the salts accumulate, so an irrigation command without drainage eventually waterlogs and salinises itself.
Q: What is the difference between surface and subsurface drainage, and when do you choose each? A: Surface drainage removes ponded water through graded channels and land shaping and suits heavy, low-permeability soils and high-intensity rainfall; subsurface drainage lowers the water table through buried pipes and is chosen where the problem is a high water table and salinity, and the soil is permeable enough to yield water.
Q: What are the two design variables in subsurface drainage and how do they trade off? A: Drain depth and drain spacing - for the same water-table control, a deeper drain allows a wider spacing, so deeper installation reduces pipe length but increases trenching cost and risks reaching an unsuitable layer or outlet limit.
Q: Why does Hooghoudt use an "equivalent depth"? A: Flow converges radially into a pipe, adding resistance that simple horizontal-flow theory ignores; replacing the actual depth D of the layer below drains with a smaller equivalent depth d accounts for that extra resistance and gives a more realistic (closer) spacing.
Q: When would you use a transient design instead of a steady-state one? A: In irrigated arid areas where recharge arrives in bursts after each irrigation - the requirement is a specified fall of the water table within a few days, which the Glover-Dumm transient equation handles; steady-state formulae suit continuous recharge in humid areas.
Q: How is hydraulic conductivity measured for a drainage design? A: In the field by the auger-hole method (measuring the rate of rise of water in a bored hole below the water table and applying a chart factor), supplemented by piezometer or ring-permeameter tests; laboratory permeameter values on disturbed samples are unreliable for design.
Q: What is a drainage envelope and why is grain-size criteria important? A: A gravel or synthetic wrap around the pipe that stops fine soil entering while keeping entry resistance low; the D₁₅(filter)/D₈₅(soil) < 4-5 and D₁₅(filter)/D₁₅(soil) > 4-5 criteria balance filtration against permeability, and a wrong envelope either silts the pipe or chokes inflow.
Q: A subsurface drainage system has been laid in a saline area but salinity has not improved. What would you check? A: Whether the system actually discharges (blocked laterals, silted envelope, submerged outlet or idle pump), whether enough leaching water is being applied above the consumptive use, and whether the drains are deep enough to intercept capillary rise rather than just controlling the top of the water table.
Q: How do you drain a field that grows rice in kharif and wheat in rabi? A: Use controlled drainage - stop-logs or gates in the manholes keep the water table high during ponded rice, and are opened before harvest and through rabi so the wheat root zone drains and leaching takes place.
Q: What is bio-drainage and what is its main limitation? A: Planting high-transpiration trees such as Eucalyptus or Poplar to pump water out of the soil by transpiration; because trees take water and leave the salts, there is no salt outlet and the root zone gradually salinises, so it is a supplement, not a substitute for a drainage outlet.
Q: Where does the drainage effluent go, and what law governs it? A: To a natural drain, evaporation pond, or reuse on salt-tolerant crops; discharge of saline or agrochemical-laden effluent into water bodies falls under the Water (Prevention and Control of Pollution) Act, 1974 and the Environment (Protection) Act, 1986, with state pollution-control board consent, and draining notified wetlands is restricted by the Wetlands (Conservation and Management) Rules, 2017.
Q: How do you fix a drainage coefficient for a project? A: From the cause of the problem - for salinity control in arid irrigated land it is the deep-percolation plus leaching flux (about 1-3 mm/day), while in humid areas it comes from a rainfall-frequency analysis of the design storm and the allowable period of high water table (commonly 5-15 mm/day).

Common mistakes

Mixing units in spacing formulae - putting q in mm/day instead of m/day, which makes S wrong by a factor of about 31.6 (√1000).
Using the ellipse/Donnan form where the barrier is deep, and so over-estimating spacing; Hooghoudt's equivalent depth must be used, and it needs iteration because d depends on S.
Treating laboratory K on a disturbed core as the design K instead of a field auger-hole value, and ignoring the large spatial variation of K across a field.
Designing for water removal only and forgetting the salt balance - no leaching allowance, so salinity persists even though the water table has fallen.
Forgetting that the outlet controls everything: a beautifully designed network discharging into a drain whose water level is above the pipe invert simply will not work.
Confusing drainable porosity (specific yield, typically 0.02-0.10) with total porosity (0.35-0.55) in transient calculations.
Assuming subsurface drainage suits every soil - in very heavy clays with K below about 0.05 m/day spacings become uneconomically close, and surface drainage or mole drainage is the right answer.