Agricultural Water Supply
Wells, pumps, and systems for irrigation — the largest use of groundwater worldwide.
Agriculture's Water Footprint
Agriculture consumes 70% of global freshwater withdrawals and 90% of consumptive use. Groundwater supplies approximately 43% of all irrigation water worldwide.
Irrigation Well Design
Yield Requirements
- Crop water demand: Varies by crop, climate, and growth stage — typically 4-12 mm/day
- Irrigation efficiency: Flood: 40-60% | Sprinkler: 60-80% | Drip: 80-95%
- Peak demand: Design for 1.5-2× average demand — hottest/driest periods
- Operating hours: 12-24 hours/day during peak season — continuous operation
Irrigation Water Requirement:
IWR = ET_crop / η_irrigation
IWR = irrigation water requirement (mm/day)
ET_crop = crop evapotranspiration (mm/day)
η = irrigation system efficiency
Well Yield Calculation:
Q = (A × IWR) / (T × 3600)
Q = required well yield (L/s), A = irrigated area (m²), T = daily pumping hours
IWR = ET_crop / η_irrigation
IWR = irrigation water requirement (mm/day)
ET_crop = crop evapotranspiration (mm/day)
η = irrigation system efficiency
Well Yield Calculation:
Q = (A × IWR) / (T × 3600)
Q = required well yield (L/s), A = irrigated area (m²), T = daily pumping hours
Well Sizing
| Farm Size | Area | Peak Demand | Well Yield | Well Count |
|---|---|---|---|---|
| Smallholding | 1-5 ha | 5-25 L/s | 5-15 L/s | 1 well |
| Medium farm | 5-50 ha | 25-100 L/s | 15-40 L/s | 1-3 wells |
| Large farm | 50-500 ha | 100-500 L/s | 40-100 L/s | 2-5 wells |
| Plantation | 500+ ha | 500-2000 L/s | 100-500 L/s | 5-15 wells |
Irrigation System Selection
Surface Irrigation
- Flood/border: Simple, low cost, 40-60% efficient — suitable for flat terrain, permeable soils
- Furrow: 50-70% efficient — moderate labor, works on slight slopes
- Basin: 60-80% efficient — paddy rice, orchards on flat land
Pressurized Irrigation
- Sprinkler (center pivot): 70-85% efficient — large areas, uniform application, automated
- Traveling gun: 60-75% efficient — flexible, moderate capital cost
- Micro-sprinkler: 80-90% efficient — orchards, vineyards, row crops
- Drip irrigation: 90-95% efficient — highest efficiency, targeted delivery, lowest labor
| System | Efficiency | Capital Cost | O&M Cost | Best For |
|---|---|---|---|---|
| Flood | 40-60% | Very low | High (labor) | Flat, permeable soil |
| Sprinkler | 70-85% | Medium | Medium | Large, uniform fields |
| Drip | 90-95% | High | Low | High-value crops |
| Micro-sprinkler | 80-90% | Medium-High | Low-Medium | Orchards, vineyards |
Crop Water Requirements
| Crop | Season Length | Peak ET (mm/day) | Season Total (mm) | Sensitivity to Salinity |
|---|---|---|---|---|
| Rice (paddy) | 120-150 days | 8-10 | 800-1200 | Low |
| Corn/maize | 90-120 days | 7-9 | 500-800 | Medium |
| Cotton | 150-180 days | 6-8 | 700-1000 | High |
| Sugarcane | 300-365 days | 5-7 | 1500-2500 | Medium |
| Wheat | 120-150 days | 4-6 | 350-500 | Medium |
| Tomatoes | 90-120 days | 5-7 | 400-600 | Low-Medium |
| Orchards (citrus) | Perennial | 4-6 | 900-1400 | Low |
Salinity Management
Irrigation water containing salts can damage soil and reduce crop yields:
- Salt accumulation: Irrigation water evaporates, salts remain — builds up over time
- Leaching requirement: Extra water to flush salts below root zone — typically 10-30% of irrigation
- Salt tolerance: Varies by crop — rice is salt-tolerant; citrus and beans are salt-sensitive
- EC threshold: Electrical conductivity (dS/m) — most crops tolerate <2 dS/m; some tolerate >8 dS/m
Leaching Requirement:
LR = EC_irrigation / (5 × EC_threshold)
LR = leaching fraction (decimal)
EC_irrigation = salinity of irrigation water (dS/m)
EC_threshold = crop salt tolerance threshold (dS/m)
LR = EC_irrigation / (5 × EC_threshold)
LR = leaching fraction (decimal)
EC_irrigation = salinity of irrigation water (dS/m)
EC_threshold = crop salt tolerance threshold (dS/m)
Agricultural Water Regulations
- Water rights: Prior appropriation (western US) vs riparian (eastern US) vs regulated permits
- Extraction limits: Many regions cap groundwater pumping — meters required
- Minimum flow requirements: Rivers must maintain ecological flows — limits pumping near rivers
- Water quality standards: Irrigation water quality criteria (US Bureau of Reclamation)
- Effluent reuse: Treated wastewater for irrigation — growing practice in water-scarce regions
Aquifer Sustainability for Agriculture
- Safe yield: Extraction must not exceed long-term recharge — otherwise aquifer depletes
- Ogallala Aquifer (US): Depleting 3× faster than recharge — some areas may exhaust in 50 years
- Groundwater banking: Store surplus surface water in aquifers during wet years — withdraw in dry years
- Deficit irrigation: Apply less water than full ET — reduces yield but preserves aquifer
- Crop switching: Shift to less water-intensive crops — reduces irrigation demand
Global Groundwater depletion hotspots
North China Plain, Indo-Gangetic Basin (India/Pakistan), Central Valley (California), Middle East (Saudi Arabia, Iran), North Africa. These regions extract groundwater 10-100× faster than natural recharge — unsustainable without intervention.
Solar-Powered Agricultural Pumping
Solar pumping is increasingly cost-competitive with diesel for agricultural irrigation:
- System design: PV array sized 1.3-1.5× motor rating for temperature and orientation losses
- Water storage: Elevated tank preferred over battery — stores water, not electricity
- Operating hours: 6-10 peak sun hours/day — sufficient for many irrigation schedules
- ROI: 3-7 year payback vs diesel — fuel savings dominate economics
- Maintenance: Minimal — panels last 25+ years, pump motors 15-20 years