AGSY102 - Lecture 7: Irrigation

Lecture opened with respect to custodians of Aboriginal and Torres Strait Islander lands where University of New England (UNE) operates.
Artwork referenced: Warwick Keen, “Always was, always will be” (2008).

Irrigation = artificial application of water to land for crop/pasture growth.
Functions as a substitute for rainfall → improves flexibility, quality, reliability of production.
Core dilemma in Australia: balance healthy, sustainable ecosystems vs. supplying water for food, feed, fibre, livelihoods.
Class Padlet activity (not examinable): pros & cons brainstorming.

Identify trends in irrigated agricultural production (main areas + top-3 irrigated systems in AU).
Recall & describe features of the three dominant Australian irrigation methods.
Explain key production constraints in irrigated agriculture.

Most intensive/irrigated zones cluster along south-eastern seaboard & major river valleys.

• Total irrigated crops & pastures: 1.9\text{ M ha}

• Total to crops: 5.7 M ML (73 % of national irrigation)

Control
• Precise timing & frequency – supply water between field capacity & wilting point.
• Risk management: 30–40 % lower yield variability; greater climate resilience.
Productivity
• Extended seasons, higher diversity, yield & quality gains.
• Example yields: Cotton 7.5–12.5 bales ha⁻¹ (irrigated) vs 2.5–5 (dryland); Rice 10–12 t ha⁻¹ (requires irrigation); Dairy up to 3× milk/ha.
Economics
• 50–70 % higher returns ha⁻¹; better market access; enhanced quality premiums.

Reliability critical; common sources: rivers, reservoirs, lakes, groundwater.
Quality guidelines • Salinity: <700\ \mu\text{S cm}^{-1} • pH range: 5.5–8.5 • Sodium Adsorption Ratio (SAR):
- <3 → non-sodic (safe)
- 3!\text{–}!6 → medium risk
- >6 → high risk to soil structure.

Well-designed dams/tanks minimise evaporation & seepage.
System components: intakes → pumps → conveyance (pipes/channels) → distribution → field application → drainage.
Monitoring hardware: soil-moisture probes, weather stations, flow-meters; software (e.g. IrriSAT) for scheduling.

Infiltration (structure), water-holding capacity (texture) & drainage (texture + structure) dictate method suitability & scheduling.

Water Use Efficiency (WUE): WUE = \dfrac{\text{Yield (kg or t)}}{\text{Water used (ML or mm)}}
Typical WUE ranges by method
• Surface (flood/furrow): 60–90 %
• Overhead sprinkler/centre-pivot: 65–90 %
• Drip/subsurface drip: 75–95 %
Evapotranspiration & remote-sensing data refine schedules.

Flood (rice, some cotton) & furrow (cotton) → water flows overland.
Best on heavy/clay soils that slow infiltration.
WUE 60–90 % (lowest of three main methods).
Costs: Capital \$3{,}000!\text{–}!5{,}000 ha^{-1}; Operating \$200!\text{–}!400 ha^{-1}\,\text{yr}^{-1} (cheapest overall).

Pressurised pipelines deliver water through nozzles above canopy.
Centre-pivot: circular coverage; Linear-move: rectangular swath.
Ideal for lighter soils; also used for frost control & canopy cooling.
WUE 65–90 %.
Costs: Capital \$6{,}000!\text{–}!8{,}000 ha^{-1}; Operating \$300!\text{–}!500 ha^{-1}\,\text{yr}^{-1}.
Challenges: wind drift, distribution uniformity, disease-conducive humidity, higher pump energy.

Network of emitters delivers slow, precise water at soil surface or below.
Highly compatible with fertigation.
WUE 75–95 % (potentially highest).
Low pump pressure ⇒ energy savings.
Capital \$8{,}000!\text{–}!12{,}000 ha^{-1}; Operating \$400!\text{–}!600 ha^{-1}\,\text{yr}^{-1}.
Limitations: frequent irrigations, stringent filtration, minimal failure tolerance (crop stress quickly).

Constraint	Annual Cost	Yield-loss Potential	Key Issues	Management Highlights
Weeds	\$4.8\text{ billion}	20–40 %	Resource competition; seedbank buildup	Pre- & post-emergent herbicides; cultural tactics (row spacing, planting time); GM herbicide-tolerant crops; integrated weed management; rotate chemistries to avoid resistance
Diseases	\$2.7\text{ billion}	10–30 % (e.g. Stripe rust 60 %, Crown rot 90 %, Fusarium 30 %, Verticillium 25 %)	Foliar vs soil-borne pathogens	Crop rotation, residue/stubble management, resistant cultivars, fungicides, control volunteers/weeds, optimise irrigation & nutrition
Pests	\$1.5\text{ billion}	15–25 % (pre-Bt Helicoverpa in cotton \$100!\text{–}!200\text{ M} yr⁻¹)	Insects, nematodes, slugs/snails attack above & below ground	Insecticide timing, IPM to protect beneficials, GM Bt cotton, cultural & biological controls
Soil limitations	—	Indirect yield impacts	Poor water storage/drainage; erosion; nutrient depletion	Maintain structure & organic matter, manage compaction, balanced fertilisation, erosion controls

Challenges
• Climate variability: rainfall ±20–30 %; temp +1–3 °C by 2050.
• Water security & allocation reliability.
• Escalating costs (water, energy, labour).
Opportunities
• Precision agriculture, digital monitoring, automation, remote sensing.
• Improved crop genetics (drought- & salinity-tolerant, pest resistance, higher WUE).
• New market niches & value-adding for high-quality irrigated produce.
Sustainability Targets
• Maximise WUE, protect soil health, minimise environmental footprint (e.g. return flows, salinity, greenhouse gases).
• Close projected global food gap via efficient, resilient irrigation systems.

Prepare dot-point list covering:
- Importance of irrigation.
- Main irrigation areas.
- Top-3 irrigated land uses.
- Top-3 water-using crops.
Compile comparison table of the three irrigation methods (Surface, Overhead, Drip): description, suitable uses, WUE, costs.
Build constraint table summarising weeds, pests, diseases & soils: description, examples, management options.

Builds on Lecture 2 (Soils) → soil structure & texture drive irrigation suitability.
Extends Lecture 3 (Crops) → crop choice dictates water demand and susceptibility to pests/diseases.
Forms foundation for upcoming Lectures 5–6 (Pastures & Grazing) where irrigation underpins forage quality.
Practical implications: designing on-farm irrigation strategies; complying with MDBP rules; integrating technology for sustainable intensification.