engineering irrigation
IRRIGATION TERMINOLOGY
Water Cycle concepts (page 5): Condensation, Precipitation, Transpiration, Percolation, Evaporation, leading to the broader water cycle that supports irrigation planning.
Irrigation is defined as artificial application of water to soil to support crop growth and compensate for insufficient rainfall.
Key terms (Tables 1–3) with definitions and notes:
Irrigation: Artificial application of water to soil to support crop growth; compensates for insufficient rainfall.
Evapotranspiration (ET): Combined water loss from soil (evaporation) and plants (transpiration). ETc = ETo \times Kc; determines crop water needs.
Potential ET (PET): ET from a fully watered crop under ideal conditions; max possible water use.
Actual ET (AET): ET under real field conditions; often < PET due to water stress.
Field Capacity (FC): Maximum water soil can hold after excess drains away; upper limit of available water.
Permanent Wilting Point (PWP): Moisture level at which plants cannot extract water; lower limit of available water.
Available Water Capacity (AWC): Water available for plants between FC and PWP. AWC = FC - PWP
Table 2 concepts:
Irrigation Efficiency: Ratio of water beneficially used to water applied. \text{Efficiency} = \frac{\text{Useful Water}}{\text{Water Applied}}; expressed as a percentage.
Application Efficiency: How well water is stored in the root zone; part of overall efficiency.
Conveyance Efficiency: Water loss during transport in canals/pipes (leaks, seepage, evaporation).
Irrigation Scheduling: Deciding when/how much water to apply; methods include Calendar, soil moisture, weather-based.
Irrigation Interval: Days between irrigations; depends on crop water use and soil type.
Irrigation Depth: Water amount applied per event (mm); note: 1 mm ≈ 10 m³/ha.
Net Irrigation Requirement (NIR): Water needed in the root zone for crop growth. NIR = ET_c - \text{Effective Rainfall}
Gross Irrigation Requirement (GIR): Total water to supply, accounting for losses. GIR = \frac{NIR}{\text{Application Efficiency}}
Table 3 concepts:
Effective Rainfall: Rainwater stored in root zone and usable by plants; reduces irrigation need.
Infiltration Rate: Speed water enters soil (mm/hr); affects irrigation time and runoff risk.
Leaching Requirement: Extra water to wash salts below root zone to prevent salinity damage.
Conveyance System: Structures moving water from source to field (canals, pipes, pumps).
Distribution Uniformity (DU): Measure of how evenly water is applied; high DU supports uniform crop growth.
Crop Coefficient (Kc): Links ETo to ETc for a crop; varies by crop stage.
Head (Pressure): Energy per unit weight of water; measured in m or kPa.
Irrigation Water Quality: Chemical/physical properties (EC, SAR, pH) affecting soil/crops.
What irrigation is and why it matters (pages 9–11): Irrigation as artificial water supply to soil for crop production; used to ensure soil moisture, support yields, and enable cropping in dry periods; science includes planning, design, construction, operation, and maintenance of irrigation works.
Global irrigation context (page 12): World irrigation distribution by region:
Asia ~ 68%
America ~ 17%
Africa ~ 5%
Oceania ~ 9%
Europe ~ 1%
Additional uses of irrigation (page 13): Frost protection and weed suppression in grain fields.
SPAC concept (page 86): Soil-Plant-Atmosphere Continuum; irrigation implications depend on soil moisture, root health, and atmospheric demand; water moves from soil → plant roots → stems/leaves → atmosphere via a water potential gradient.
Crop-water relations (pages 87–89): Definition of Crop Water Requirements (CWR); factors include crop type/variety, growth stage, climate, soil type; Kc varies with crop stage; ETo is the atmospheric evaporative demand.
Growth-stage sensitivity to water (pages 88–89):
Initial: low water demand but essential for germination.
Vegetative: increasing demand; mild stress may reduce leaf growth.
Flowering/fruit set: highly sensitive; water stress can severely reduce yield.
Maturity: slight stress may accelerate ripening in some crops but reduce quality.
Practical irrigation management (page 92): match irrigation intervals and application rates to soil water holding capacity, crop rooting depth, and daily ET losses; avoid over-irrigation (waste, salinity, erosion) and under-irrigation (stress, yield loss).
Irrigation planning and evaluation (pages 67–71, 106–107): planning steps, water/resource assessment, system design, economic and environmental considerations, monitoring/evaluation methods (soil moisture, flow, field observation, crop response).
Irrigation water quality (page 85): parameters to monitor include Electrical Conductivity (EC), Sodium Adsorption Ratio (SAR), and pH; impacts soil structure and crop health.
Crop-water relationships and climate (pages 20–21): temperature, humidity, wind, sunshine influence ET and water needs; field capacity and PWP determine plant-available water.
In-field irrigation system layouts (page 103): Surface (basin, furrow, border), Sprinkler (rectangular, triangular, perimeter; center pivot; traveling gun), Drip/Microjet (single-line, double-line, microjet spots), Orchards/vegetables as typical crops for each layout.
Surface irrigation specifics (pages 27–33): Basin, Furrow, Border; advantages (low cost, low energy) and disadvantages (low efficiency, erosion, labor-intensive).
Sprinkler irrigation basics (pages 34–40): definitions and types (portable quick-coupling, centre pivot, linear move, solid set); advantages (uniform application, soil/crop versatility) and disadvantages (high initial cost, wind drift, energy use).
Drip/Micro/Trickle irrigation (pages 44–49, 54–59): delivers water at the root zone; high efficiency (≈ 90–95%); advantages (reduced weeds, water savings) and disadvantages (high installation/maintenance cost, emitter blockage risk); includes drip layout components (drip emitters, feeders, filters, manifolds).
Subsurface Drip Irrigation (SDI) (pages 55–58): water applied below soil surface; very high efficiency; advantages (reduced evaporation, weed suppression) and disadvantages (high initial cost, maintenance and reliability concerns); suitability for irregular fields and fertigation.
Micro-jet systems (pages 60–63): low pressure; high efficiency; advantages (localized application, reduced erosion) and disadvantages (clogging, wind drift, limited crop suitability).
In-field design steps (pages 65–71): planning survey, topography, soil characteristics, water resources, crop water requirements, method selection, system sizing, capital/operating costs, environmental and social considerations; create a water management plan and schedule.
Infiltration, DU, Kc, and head details (pages 81–85): DU = distribution uniformity; Kc linked to ET via ETc = ETo \times Kc; Head/Pressure as energy per unit weight; water quality impacts.
Calculation examples (pages 94–99): worked problems illustrating how to compute ETc, CWR, NIR, GIR, AWC, depletion p and irrigation interval.
Example 1: Given ETo = 5.20 mm/day, Kc = 1.05; ETc = ETc = ETo \times Kc = 5.20 \text{ mm/day} \times 1.05 = 5.46 \text{ mm/day}
Example 2 (10 days): CWR(10d) = ETc × days = 5.46 × 10 = 54.60 mm.
Example 3: If Pe = 18.0 mm; NIR = CWR − Pe = 54.60 − 18.00 = 36.60 mm.
Example 4: Ea = 0.75; GIR = NIR / Ea = 36.60 / 0.75 = 48.80 mm.
Example 5: Available Water Capacity calculation: FC = 0.28, PWP = 0.12, rooting depth = 0.60 m. Volumetric AWC = FC − PWP = 0.16 m³/m³. AWC(mm) = AWC(vol) × rooting depth × 1000 = 0.16 × 0.60 × 1000 = 96.0 mm.
Example 6: Depletion-based irrigation interval: AWC = 96.0 mm, p = 0.50, readily available water = 96.0 × 0.50 = 48.0 mm. Interval = readily available water / ETc = 48.0 / 5.46 ≈ 8.79 days; irrigate after about 8.8 days (~8–9 days).
Water sources (page 100): Surface water, groundwater, rainwater harvesting, non-conventional sources.
Pumps (page 101): Centrifugal, Turbine, Submersible, Positive displacement, Axial-flow pumps; suitability depends on lift, depth, and application.
Conveyance systems (page 102): Gravity/open channel vs pipe/closed; special structures for complex terrain; portable systems for smallholders.
Evaluation methods (pages 106–107): Soil moisture measurement, flow measurement, field observation, and crop response assessment to gauge irrigation effectiveness.
In-field crops and layouts (page 103): Typical crops for surface, sprinkler, drip/microjet systems and corresponding layouts and suitability.
Economic and environmental considerations (pages 69–71): Capital and operating costs, affordability, sustainability (salinity, waterlogging, over-abstraction), and downstream impacts.
IRRIGATION EFFICIENCY, SCHEDULING AND MANAGEMENT
Irrigation Efficiency concepts:
Overall efficiency combines application and conveyance efficiency; high efficiency means more of the applied water benefits the crop.
Application Efficiency focuses on how well water is stored in the root zone and used by the crop.
Conveyance Efficiency captures water losses during transport to the field.
Scheduling approaches:
Calendar-based: irrigation after a fixed number of days.
Soil-moisture-based: using tensiometers, neutron probes, capacitance sensors.
Plant-based: leaf water potential, canopy temperature.
Climate-based: using ETo and Kc values to estimate ETc.
Practical planning and monitoring:
Establish a water management plan including irrigation scheduling, maintenance, and data collection.
Monitor system performance and crop response to adjust schedules.
CROP-WATER- SOIL- CLIMATE RELATIONSHIPS AND CWR CALCULATIONS
CWR definition: total water needed for a crop to grow optimally; calculated via \text{ET}c = \text{ETo} \times Kc; crop coefficient changes with crop type and growth stage.
Influencing factors for CWR:
Crop type and variety
Growth stage (initial, vegetative, reproductive, maturity)
Climate variables (temperature, humidity, wind, solar radiation)
Soil type (texture, structure, infiltration rate)
Crop-water relationships: soil moisture supply governs ET; water potential gradient drives movement from soil to atmosphere (SPAC concept).
READY-TO-USE WORKED CALCULATIONS (EXAMPLES)
Given ETo = 5.20 mm/day, Kc = 1.05:
ETc = ETc = ETo \times Kc = 5.20 \text{ mm/day} \times 1.05 = 5.46 \text{ mm/day}
Over a 10-day period:
CWR(10d) = ETc × days = 5.46 × 10 = 54.60 mm
With effective rainfall Re = 18.0 mm:
NIR = ETc − Re = 54.60 − 18.00 = 36.60 mm
With application efficiency Ea = 0.75:
GIR = NIR / Ea = 36.60 ÷ 0.75 = 48.80 mm
Available Water Capacity (AWC) in a 0.60 m rooting depth:
FC = 0.28 m³/m³, PWP = 0.12 m³/m³
Volumetric AWC = FC − PWP = 0.16 m³/m³
AWC(mm) = AWC(vol) × rooting depth × 1000 = 0.16 × 0.60 × 1000 = 96.0 mm
Irrigation interval based on depletion and ETc:
Readily available water = AWC × p = 96.0 × 0.50 = 48.0 mm
Interval = readily available water / ETc = 48.0 ÷ 5.46 ≈ 8.79 days → about 8–9 days
CROP-WATER RELATIONSHIPS AND PLANNING GUIDELINES
IRRIGATION WATER REQUIREMENTS steps (summary):
Determine ET0 (reference evapotranspiration) and crop coefficient Kc.
Compute ETc = ETo × Kc.
Assess effective rainfall Re from precipitation and run-off; compute NIR = ETc − Re.
Determine application efficiency Ea for the chosen system; compute GIR = NIR / Ea.
Estimate available soil water (AWC) from FC and PWP and rooting depth; convert to mm if needed.
Schedule irrigation to replenish soil moisture up to FC minus allowable depletion to avoid over-watering.
Growth stage sensitivity emphasizes aligning irrigation with crop stage to maximize yield and quality.
IRRIGATION PLANNING, SYSTEM SELECTION, AND EVALUATION
Planning steps (containing core decision factors):
Survey/topography: map slopes/contours; assess elevation differences for water distribution; plan land leveling if required.
Study soil characteristics: type, infiltration rate, drainage; identify limitations (salinity, shallow depth, stones).
Evaluate water resources: availability, reliability, quality; test water salinity, pH, contaminants; estimate total volume available.
Assess crop water requirements: crop types, water needs, rooting depth, peak irrigation demand.
Choose irrigation method based on crop type/spacing, soil characteristics, water supply, cost and labor, climate.
Design system: size mainlines/submains/laterals; select pumps, sprinklers/emitters/valves; determine application rate and system efficiency; ensure uniform distribution.
Economic and environmental considerations: capital/operating costs, sustainability (salinity, waterlogging, over-abstraction), downstream impacts.
Implementation plan: construction schedule, equipment procurement, training; develop water management plan (scheduling, maintenance).
Monitoring and evaluation: continually monitor water use efficiency and crop performance; adjust scheduling based on weather/soil/crop stage.
Evaluation methods (for existing systems):
Soil moisture measurements (tensiometers, probes, gravimetric methods).
Flow measurements (inflow to the field, using flow meters).
Field observation (ponding, dry spots, uniform wetting).
Crop response assessment (growth, yield, stress symptoms).
IRRIGATION SYSTEMS: TYPES, ADVANTAGES, DISADVANTAGES
SURFACE IRRIGATION (definition and examples):
Water flows over the soil surface by gravity.
Examples: Basin irrigation (rice, other crops), Furrow irrigation (between crop rows), Border irrigation.
Advantages: Low cost, low energy use, simple operation.
Disadvantages: Low water use efficiency (often 40–60%), evaporation and percolation losses, soil erosion, uneven distribution, labor-intensive.
SPRINKLER IRRIGATION:
Definition: Water sprayed into the air and falls like rainfall.
Types: Portable quick-coupling, Centre pivot, Linear move, Solid set.
Advantages: Suitable for most soils/crops; relatively uniform application.
Disadvantages: High initial cost; wind drift and energy use; potential for uneven application if not managed.
DRIP / MICRO / TRICKLE IRRIGATION:
Definition: Water delivered directly to plant root zone via emitters.
Advantages: High efficiency (≈ 90–95%), weed suppression, suitable for water-scarce areas.
Disadvantages: High installation and maintenance costs; emitter blockage risk; limited crop suitability.
SUBSURFACE DRIP IRRIGATION (SDI):
Definition: Water applied below soil surface via buried pipes or capillary action.
Advantages: Minimizes evaporation losses; high water use efficiency.
Disadvantages: High initial cost; complex maintenance; risk of clogging; water quality dependence.
MICRO-JET SYSTEMS:
Definition: Low-pressure irrigation system delivering water with small droplets near the plant.
Advantages: Efficient water use; low pressure; localized application; adaptable to uneven terrain.
Disadvantages: Clogging risk; wind drift losses; limited crop suitability; potential for root zone restrictions if overused.
CENTRE PIVOT AND TRAVELLING GUNS:
Centre pivot: Large-area, automated, high efficiency; typically 40–200+ ha per pivot; uniform distribution; adaptable to soil types; long lifespan if maintained.
Travelling guns: Large-area, adjustable; high pumping pressure (6–10 bar) required; higher energy costs; potential soil compaction and wind drift issues; not ideal on very delicate crops.
GENERAL ADVANTAGES ACROSS SYSTEMS:
Uniform water distribution, potential for fertigation/chemigation, adaptation to diverse crops and soils, ability to automate and monitor.
GENERAL DISADVANTAGES ACROSS SYSTEMS:
High capital costs for most pressurized systems, ongoing maintenance, energy costs, risk of water losses (evaporation, drift, infiltration), and in some cases soil compaction or crop damage.
IN-FIELD IRRIGATION SYSTEMS AND LAYOUTS
Common in-field layouts by irrigation type:
Surface: Basin, Furrow, Border; crops include cereals, vegetables, orchards.
Sprinkler: Rectangular, triangular, perimeter; crops include maize, wheat, pastures, vegetables.
Drip/Microjet: Single-line, double-line, microjet spots; crops include orchards, vineyards, vegetables, high-value crops.
Practical implications: Layouts influence uniformity, land preparation, labor needs, and crop suitability.
CONVEYANCE, PUMPS, AND WATER SUPPLY
Conveyance systems:
Gravity/open channel: cheap but inefficient due to losses.
Pipe/closed: more efficient; supports pressurized systems (sprinkler, drip).
Special structures (aqueducts, tunnels, flumes) for large schemes in complex terrain.
Portable systems useful for smallholders but labor-intensive.
Pumps: types and typical applications (page 101):
Centrifugal pumps: shallow water lifting.
Turbine pumps: deep wells and rivers.
Submersible pumps: boreholes.
Positive displacement pumps: drip and fertigation systems.
Axial-flow pumps: high-volume, low-lift irrigation.
WATER QUALITY, DU, Kc, HEAD AND SYSTEM PERFORMANCE
Distribution Uniformity (DU): A measure of how evenly water is applied over an area; high DU supports uniform crop growth.
Crop Coefficient (Kc): Relates ETo to ETc; varies with crop type and growth stage; used to compute ETc.
Typical relationship: \text{ET}c = \text{ETo} \times Kc; Kc changes over time (initial, development, mid-season, late-season).
Head/Pressure: Energy per unit weight of water; important for pressurized systems to ensure proper emitter/ sprinkler performance.
Water quality parameters (EC, SAR, pH): influence soil structure, salinity risk, and crop tolerance; poor water quality can reduce yield and soil health.
PRACTICAL IRRIGATION PLANNING AND ETHICS
Economic and sustainability considerations:
Capital cost versus expected yield benefits; operation costs (energy, labor, maintenance, water tariffs);
Environmental concerns: avoid salinity buildup, waterlogging, or over-abstraction; consider downstream impacts and social implications.
System selection should balance crop needs, soil characteristics, water availability, and farmer skills.
SUMMARY REAL-WORLD CONNECTIONS AND IMPLICATIONS
Efficient irrigation planning integrates soil science, plant physiology, climate, and crop growth stages to improve productivity and sustainability.
Technologies (drip, subsurface drip, center-pivot, sprinkler systems) offer different trade-offs in cost, efficiency, and suitability for crops and terrains.
Proper monitoring (soil moisture, flow, crop performance) and adaptive management are essential to maximize water use efficiency and minimize environmental impacts.
KEY FORMULAE RECAP (in LaTeX)
Crop evapotranspiration: \text{ET}c = \text{ETo} \times Kc
Net irrigation requirement: \text{NIR} = \text{ET}c - Re
Gross irrigation requirement: \text{GIR} = \frac{\text{NIR}}{E_a}
Available water capacity: \text{AWC} = FC - PWP
Available water capacity in depth (mm): \text{AWC(mm)} = (FC - PWP) \times \text{rooting depth} \times 1000
Example ETc: \text{ET}_c = 5.20 \text{ mm/day} \times 1.05 = 5.46 \text{ mm/day}
Example GIR: \text{GIR} = \frac{36.60}{0.75} = 48.80 \text{ mm}
Example AWC(mm): \text{AWC(mm)} = (0.28 - 0.12) \times 0.60 \times 1000 = 96.0 \text{ mm}
Depletion interval example: Readily available water = 96.0 × 0.50 = 48.0 mm; interval ≈ 48.0 ÷ 5.46 ≈ 8.79 days
NOTES ON TERMINOLOGY AND UNITS
1 mm of irrigation depth ≈ 10 m³/ha.
1 ha = 10,000 m²; depth in mm over 1 ha corresponds to volume in m³ via 1 mm depth over 1 ha = 10 m³.
ETo and ETc are typically reported in mm/day; Kc is dimensionless; Re is rainfall depth in mm.
FC and PWP are soil moisture contents expressed as a volumetric fraction (m³/m³).
Root depth is used to convert volumetric water content to an application depth in mm.
LIMITS AND ASSUMPTIONS
Calculations assume uniform soil properties within the field; real fields may have spatial variability.
Effective Rainfall is treated as rainfall contributing to root-zone moisture; rainfall losses (runoff, deep percolation) are not counted.
Efficiency values (Ea, Application Efficiency) depend on system design, operation, and management practices; real-world values may vary.