Water Basics and Irrigation Fundamentals

Water Basics and Irrigation Fundamentals

Understanding how water behaves in a landscape system is the foundation of every irrigation decision you will make in the field. This lesson covers the water cycle, evapotranspiration, and the physical relationships among pressure, flow rate, and pipe size.

The Water Cycle in a Landscape Context

The hydrologic cycle describes how water moves continuously through the environment: precipitation falls, soaks into the soil (infiltration) or runs off the surface, is taken up by plants, evaporates from soil and leaf surfaces, and returns to the atmosphere. As a landscape irrigator, you work at the point where that cycle meets managed plant communities. Your goal is to replace the water that plants and soil lose, and nothing more. Overwatering wastes a limited resource, encourages disease, and can violate local water-use ordinances.

Evapotranspiration (ET)

Evapotranspiration (ET) is the combined loss of water from a landscape through two pathways: evaporation from soil and hard surfaces, and transpiration through plant leaves. ET is measured in inches per day or per week and varies with temperature, humidity, wind speed, solar radiation, and plant type. Weather stations and local cooperative extension offices publish reference ET (ETo) values for your region.

Smart irrigation controllers use real-time or historical ET data to schedule run times automatically. When you design or audit a system, you match the system's output (in inches per hour, or precipitation rate) to the crop's ET demand. Applying less than ET causes drought stress; applying more than ET wastes water and can cause runoff or leaching of nutrients.

Pressure: PSI and Feet of Head

Water pressure is the force that pushes water through pipes and out of sprinkler heads. In the United States, pressure is expressed in pounds per square inch (PSI). It can also be expressed as feet of head, a unit that describes how high a column of water the pressure could support. The conversion factor is:

• 1 PSI = 2.31 feet of head
• 1 foot of head = 0.433 PSI

Elevation changes directly affect pressure. Every foot you pump water uphill, you lose approximately 0.433 PSI. Every foot the water flows downhill, you gain approximately 0.433 PSI. This is critical when designing zones on sloped properties.

Most residential and light-commercial irrigation systems operate between 40 and 80 PSI at the water meter. Sprinkler heads and drip emitters are rated to perform correctly within a specific pressure range. Operating below that range reduces coverage; operating above it causes misting (fogging), uneven distribution, and accelerated wear. Pressure regulators are installed to protect sensitive components when supply pressure is high.

Flow Rate: GPM and GPH

Flow rate measures how much water moves through the system over time. Two units are common:

• Gallons per minute (GPM) — used for sprinkler heads, valves, meters, and main lines.
• Gallons per hour (GPH) — used for drip emitters and micro-irrigation.

Flow rate through a pipe or orifice increases as pressure increases. Flow rate also depends on pipe diameter: a larger pipe carries more water at the same pressure with less friction loss. Exceeding the recommended flow rate for a given pipe size causes velocity-related friction loss, pressure drop at heads, and can lead to water hammer — a damaging pressure surge when a valve closes suddenly.

Pipe Diameter and Velocity

Pipe diameter controls how fast water moves (velocity) for a given flow rate. High velocity (generally above 5 feet per second in lateral lines) creates excessive friction loss and noise. Designers select pipe size so that velocity stays within acceptable limits and so that each zone delivers adequate pressure to every head. As a rule, larger diameter pipe is used on mainlines and smaller diameter pipe on lateral lines serving individual heads.

Types of Irrigation Systems

Spray heads deliver water in a fixed arc pattern. They have high precipitation rates (often 1.5 inches per hour or more) and are suited to turf areas and low-growing groundcovers. They require consistent spacing and matched precipitation rates across a zone.

Rotor heads rotate slowly and cover larger areas at lower precipitation rates (typically 0.5 to 1.0 inch per hour). They are suited to large turf areas and slopes where slower application allows better infiltration.

Drip (micro) irrigation delivers water slowly at or below the soil surface directly to the root zone, minimizing evaporation and overspray. GPH ratings are used. Drip is best for shrub beds, trees, vegetables, and slopes. It requires filtration and pressure regulation to prevent clogging and emitter damage.

Never mix spray heads and rotor heads on the same zone. Their precipitation rates differ significantly, so one type will over- or underwater while the other is correctly watered.

Safety Points

• Trenching hazard: Before any digging, call 811 (USA national one-call system) to have underground utilities marked. Striking a gas or electrical line is life-threatening.
• Hydraulic pressure: Never open a pressurized line without confirming the system is depressurized or you have control of the shutoff. Use appropriate eye protection when inspecting fittings under pressure.
• Slips and trips: Wet soil and open trenches are fall hazards. Wear slip-resistant boots and barricade open excavations.
• Chemical interaction: If working near chemigation (fertilizer or pesticide injection into irrigation lines), review the relevant safety data sheets and wear appropriate PPE. Backflow prevention is legally required to protect potable water supplies.