Designing a Drip Irrigation System

Why Choose Drip Irrigation

Drip irrigation delivers water directly to the plant root zone through a network of tubes and emitters. Compared to overhead sprinklers (60-75% efficiency) or flood irrigation (40-60% efficiency), drip systems achieve 90-95% application efficiency. This means significantly less water is lost to evaporation, runoff, and deep percolation.

Beyond water savings, drip irrigation reduces weed pressure (only the crop row is wetted), lowers disease risk (foliage stays dry), and enables precise fertigation (fertilizer injection through the system). For high-value crops like strawberries, tomatoes, and vineyard grapes, drip irrigation is the standard for good reason.

Key Design Concepts

Emitter flow rate is measured in gallons per hour (GPH) or liters per hour (L/hr). Common rates range from 0.25 GPH for nursery containers to 2.0 GPH for orchard trees. Most vegetable row crops use 0.5-1.0 GPH emitters.

Emitter spacing determines how many emitters are on each lateral line. For closely spaced crops like strawberries, 12-inch spacing is typical. For wider-spaced crops like tomatoes, 18-24 inch spacing works well. The goal is to create a continuous wetting strip along the row.

Operating pressure affects emitter performance. Most pressure-compensating emitters are rated for 10-45 PSI. Below the minimum pressure, flow drops; above the maximum, emitters may not regulate properly. Maintaining uniform pressure across all laterals is critical for even water distribution.

Lateral length limits depend on emitter flow rate, spacing, pipe diameter, and allowable pressure variation. Longer laterals lose more pressure to friction, causing emitters at the far end to deliver less water than those near the header.

Worked Example: Strawberry Field

You are designing a drip system for a strawberry field with 20 rows, each 200 feet long. Plants are spaced 12 inches apart in the row.

Step 1: Emitter Count and Flow

• Emitter spacing: 12 inches (1 foot)
• Emitters per row: 200 ft / 1 ft = 200 emitters
• Emitter flow rate: 0.5 GPH
• Flow per row: 200 × 0.5 = 100 GPH = 1.67 GPM
• Total system flow (20 rows): 20 × 1.67 = 33.3 GPM

Step 2: Lateral Pipe Sizing

For 200-foot laterals at 1.67 GPM, a 5/8-inch (16mm) drip tape or 3/4-inch polyethylene tubing is typically adequate. Check friction loss to confirm the pressure variation stays within 20% of the design pressure:

• Using Hazen-Williams with C=150 for polyethylene pipe
• Friction loss over 200 ft at 1.67 GPM in 3/4-inch pipe: approximately 3.2 PSI
• At 15 PSI operating pressure: 3.2 / 15 = 21% variation (slightly above target)
• Consider upgrading to 1-inch laterals or limiting run length to 175 feet

Step 3: Pump Selection

The pump must deliver 33.3 GPM at sufficient pressure to overcome friction losses in the mainline, sub-mains, laterals, and the filter system, plus the emitter operating pressure:

• Emitter operating pressure: 15 PSI
• Filter pressure drop: 5 PSI
• Mainline and sub-main friction: 8 PSI
• Lateral friction: 3 PSI
• Total dynamic head: 15 + 5 + 8 + 3 = 31 PSI (about 72 feet of head)

Tips and Common Mistakes

• Always install filtration. Drip emitters clog easily. Use a screen filter (120-200 mesh) for clean water sources or a sand media filter for surface water with sediment. Replace or clean filters regularly.
• Flush lateral lines monthly. Open the end caps and run water for 30-60 seconds to clear accumulated sediment and biological growth from the tubing.
• Check pressure uniformity. Measure pressure at the beginning and end of several laterals. If the difference exceeds 20% of design pressure, shorten the laterals or increase the pipe diameter.
• Plan for expansion. Size your mainline and pump with 10-20% extra capacity. Adding rows to a system that is already at capacity requires replacing the pump and possibly the mainline.