Irrigation Runtime Calculation for Water Requirements

1. The Core Principle of Runtime Calculation

At its heart, calculating runtime is straightforward. The basic formula is:

$\text{Runtime}=\frac{\text{Amount of Water Needed}}{\text{Rate of Water Applied}}$

However, the way we measure "Amount Needed" and "Rate Applied" differs between sprinkler and drip systems. Let's break down each method.

2. Runtime for Sprinkler Zones (e.g., Lawns)

For sprinkler systems, we think in terms of applying a uniform depth of water (in millimeters) over the entire zone, similar to rainfall.

1. Water Needed (Depth): In a previous module, we calculated the daily water requirement for our plants, called the Crop Evapotranspiration (ETc), in mm/day. For example, a lawn in summer might need an ETc of 5 mm/day.

2. Water Applied (Rate): This is the Precipitation Rate (PR) of the zone, measured in mm/hour. It tells you how quickly your sprinkler setup applies water over its area. You might recall this from an earlier lesson. The formula is:

   $\text{Precipitation Rate (mm/hr)}=\frac{\text{Total Zone Flow (Liters/hr)}}{{\text{Zone Area (m}}^2\text{)}}$

   Remember: 1 Liter applied over 1 square meter results in a water depth of 1 mm.

3. Runtime Calculation: With these two values, the runtime calculation is simple:

   $\text{Runtime (hours)}=\frac{\text{Water Needed (mm)}}{\text{Precipitation Rate (mm/hr)}}$

Worked Example: Villa Lawn Zone

Let's use our capstone villa project's lawn zone.

• Water Needed: Let's assume after checking local climate data, the peak summer ETc for our turfgrass is 5 mm/day.
• Zone Area: The lawn area is 250 m².
• Total Zone Flow: From our hydraulic design, the total flow for this zone is 30 GPM. We need to convert this to Liters/hour:
  • 30 GPM × 3.785 L/GPM = 113.55 Liters/minute
  • 113.55 L/min × 60 min/hr = 6813 L/hr
• Calculate Precipitation Rate (PR):
  $PR=\frac{6813\text{ L/hr}}{250{\text{ m}}^2}=27.25\text{ mm/hr}$
• Calculate Daily Runtime:
  $\text{Runtime (hours)}=\frac{5\text{ mm}}{27.25\text{ mm/hr}}=0.183\text{ hours}$
  $\text{Runtime (minutes)}=0.183\text{ hours}\times 60\text{ min/hr}\approx 11\text{ minutes per day}$

So, to replace the 5 mm of water lost each day, you would need to run this sprinkler zone for about 11 minutes.

3. Runtime for Drip Irrigation Zones

For drip irrigation, it's more intuitive to think in terms of total volume. We need to replace a certain volume of water (Liters), and we know the total volume per hour our drip system delivers.

As the guide explains, the process is:

1. Calculate Required Liters per day: Area (m²) × ETc (mm/day) = Liters/day
2. Calculate System Output (L/h): Number of Emitters × Emitter Flow Rate (L/h)
3. (Optional) Convert to L/m: System Output (L/h) ÷ 60
4. Calculate Run Time: Required Liters per day ÷ System Output (L/h or L/m)

Worked Example: Villa Shrub Zone

Let's apply this to our project's shrub zone.

• Water Needed: The ETc for established shrubs is lower, let's say 3 mm/day. The zone area is 120 m².
  • Required Liters = 120 m² × 3 mm = 360 Liters/day
• System Output: Our design for this zone uses an inline dripline with emitters spaced every 0.5m over a total length of 200m. The emitters are rated at 2 L/h.
  • Number of Emitters = 200m length / 0.5m spacing = 400 emitters.
  • System Output = 400 emitters × 2 L/h = 800 L/h
• Calculate Daily Runtime:
  $\text{Runtime (hours)}=\frac{\text{Required Liters (L/day)}}{\text{System Output (L/h)}}=\frac{360\text{ L}}{800\text{ L/h}}=0.45\text{ hours/day}$
  $\text{Runtime (minutes)}=0.45\text{ hours}\times 60\text{ min/hr}=27\text{ minutes per day}$

This zone would need to run for 27 minutes each day to meet the plants' water needs.

4. Smart Scheduling: Watering Frequency and the Water Budget Method

While you can run your system for the calculated time every day, it's often better for the plants and more water-efficient to water less frequently but for longer periods. This encourages deeper root growth, making plants more resilient.

This approach is called the Water Budget Method. Think of the soil's root zone as a bank account for water:

• Balance: The amount of water currently in the soil.
• Withdrawal: Daily water use by the plants (ETc).
• Deposit: Irrigation or rainfall.

You don't need to make a deposit every day. You can let the plant "withdraw" water for a few days until the "account balance" hits a minimum level, and then you make one larger "deposit" (irrigation) to refill it. This minimum level is known as the Maximum Allowable Depletion (MAD).

A professional way to track this is with a simple spreadsheet.

Applying the Concept to Our Lawn Zone

• Our lawn needs 5 mm/day.
• For lawns, a typical MAD is 50%. Let's say our soil can hold 30 mm of available water in the root zone. This means we can let the plants use up to 15 mm before we must irrigate.
• At a rate of 5 mm/day, it will take 3 days to use 15 mm of water (15 mm ÷ 5 mm/day = 3 days).
• So, instead of watering for 11 minutes every day, a better schedule would be to water for 33 minutes (3 days × 11 minutes/day) once every three days.

This approach saves water by reducing evaporation from constantly wet soil surfaces and builds stronger, deeper-rooted turf.

Conclusion

You can now calculate the precise runtime for any irrigation zone you design, whether it uses sprinklers or drip. This is the final piece of quantitative analysis required for our project.

Key Takeaways:

• Sprinkler Runtime is based on replenishing a depth of water (mm). It's calculated by dividing the required depth (ETc) by the system's Precipitation Rate (mm/hr).
• Drip Runtime is based on replenishing a volume of water (Liters). It's calculated by dividing the required volume by the system's total Flow Rate (L/h).
• Watering deeply and infrequently (e.g., every 2-3 days) is generally better for plant health than shallow, daily watering. The Water Budget Method provides a professional framework for implementing this strategy.

In our next lesson, we will put all of this together. We will use these runtimes and watering frequencies to develop a complete weekly schedule for an automatic controller. After that, we will be ready to produce the final project deliverables: the fully annotated irrigation plan and the Bill of Quantities (BOQ).