Calculating Daily Plant Water Needs (ETc)

Hello! Welcome to the next lesson in your irrigation design course.

So far, we have established two fundamental pillars of our design process. First, we learned how to determine the Reference Evapotranspiration (ET₀), which represents the maximum potential water loss driven by local climate. Second, we learned how to calculate the Crop Coefficient (Kc), which adjusts that potential based on the specific characteristics of the plants in each hydrozone—their species, density, and microclimate.

Today, we bring these two concepts together. The goal of this lesson is to calculate the daily water requirement (ETc) for a specific plant type using the formula you've seen: ETc = ET₀ × Kc.

This calculation is the final step in determining the depth of water (in millimeters) that each hydrozone needs per day during the peak season. It's the critical link between your landscape design and the hydraulic requirements of the irrigation system.

1. The Core Formula: Calculating Crop Evapotranspiration (ETc)

The water requirement of a plant is known as its Crop Evapotranspiration, or ETc. As a landscape architect, you determine the "what" and "where" of planting; this calculation defines the "how much water" each plant group needs.

The formula is elegantly simple:

E T_{c} = E T_{₀} \times K_{c}

Where:

ETc is the Crop Evapotranspiration—the actual water depth your specific plants need (mm/day).
ET₀ is the Reference Evapotranspiration for your location during the peak month (mm/day).
Kc is the Crop Coefficient for that specific hydrozone (dimensionless).

The resource below provides a clear, academic explanation of this formula, which it calls the "ETL formula" (Landscape Evapotranspiration), a term used interchangeably with ETc in landscape contexts.

WUCOLS 2000 - City of Healdsburg

To start, let's review the definition of the formula and its components. The WUCOLS manual presents this concept clearly.

Please read Chapter 1, which introduces the core formula ETc = Kc x ETo. Then, read the first part of Chapter 2, 'The Landscape Evapotranspiration Formula', which adapts it for landscape use. Focus on understanding the relationship between the three variables.

2. Putting the Formula into Practice: Worked Examples

The best way to understand this is to work through some practical calculations. For our examples, we will assume a peak summer ET₀ of 6.0 mm/day. This is a realistic value for many warm, sunny climates.

Example 1: A Lawn Hydrozone

Lawns are often the most water-demanding part of a landscape.

Given:
- ET₀ = 6.0 mm/day
- Plant type: Standard cool-season turfgrass lawn. From typical Kc tables, a value of Kc = 0.8 is appropriate for a well-maintained lawn in peak summer.
Calculation:
$E T_{c} = E T_{₀} \times K_{c}$ $E T_{c} = 6.0 mm/day \times 0.8 = 4.8 mm/day$
Result: This lawn hydrozone needs 4.8 mm of water applied to its root zone each day to stay healthy during the peak season.

Example 2: A Mixed Shrub Border

Let's use the scenario from our last lesson: a hydrozone of shrubs with average water needs, planted at average density, but in a high-stress microclimate (e.g., next to a sunny driveway).

Given:
- ET₀ = 6.0 mm/day
- In the previous lesson, we calculated the Kc for this zone using the three-factor method:
  - Kc = Species (0.5) × Density (1.0) × Microclimate (1.3) = 0.65
Calculation:
$E T_{c} = E T_{₀} \times K_{c}$ $E T_{c} = 6.0 mm/day \times 0.65 = 3.9 mm/day$
Result: This shrub zone requires 3.9 mm of water per day. Notice that even in a high-stress location, these shrubs are more water-efficient than the lawn. This is a perfect example of why separating hydrozones is crucial for saving water.

Test your understanding!

Let's use the other scenario from the previous lesson's exercise: a protected courtyard with sparsely planted, low-water-use ornamental trees. We calculated a Kc of 0.05 for this zone. Assuming the same ET₀ of 6.0 mm/day, what is the daily water requirement (ETc) for this hydrozone?

Show answer

Given:
- ET₀ = 6.0 mm/day
- Kc = 0.05
Calculation:
$E T_{c} = 6.0 mm/day \times 0.05 = 0.3 mm/day$

This extremely low value highlights the massive water savings possible with drought-tolerant plant selection and thoughtful placement in protected microclimates. This zone needs less than a tenth of the water required by the shrub zone!

3. A Note on Units and Resources

The calculation is straightforward, but you will find that different resources (especially those from the US) use different units. The Rain Bird manual, for example, uses inches and gallons. The concept and the formula remain identical.

The section below from the Rain Bird manual demonstrates the exact same calculation, but using inches. Their term "PET" (Potential Evapotranspiration) is the same as our ET₀.

Low-Volume Landscape Irrigation Design Manual

Let's look at another example from the Rain Bird design manual. This will reinforce the calculation, but notice that it uses imperial units (inches).

Please find the subsection titled 'Calculate Water Requirement for Dense Plantings' (page 24). Read the short explanation and the example provided. Note that their formula Water Requirement = Kc × PET is identical in principle to our ETc = Kc × ET₀.

While we will continue to work in metric units (mm, liters, meters), it is valuable for you to recognize the same principles at work regardless of the measurement system.

4. Designing for the Peak

You might wonder if ETc changes. It does—daily and seasonally. The ET₀ value changes with the weather, and the Kc value can change as a plant grows or goes dormant.

Crop Coefficient (Kc) Variation Over Growing Season — This graph illustrates how the Crop Coefficient (Kc) for an annual crop changes throughout its life cycle. It starts low, peaks during mid-season when the plant is largest and most active, and declines as it matures.

So, which value should you use for design?

For irrigation system design, we always calculate for the worst-case scenario: the peak of the hottest, driest month (e.g., July or August in the Northern Hemisphere). We use the highest average ET₀ for that month and the highest Kc for the plants.

This ensures the system you design—the pipes, valves, and water source—has the capacity to meet the landscape's needs under maximum stress. The daily and seasonal adjustments are then managed by the irrigation controller's schedule, which can be programmed to apply less water during cooler, less demanding months.

Conclusion

You have now mastered a cornerstone calculation in irrigation design. By combining climatic data with plant-specific information, you can determine the precise depth of water each part of your landscape needs.

Key Takeaways:

The daily water requirement (Crop Evapotranspiration, or ETc) is calculated using the formula: ETc = ET₀ × Kc.
This calculation determines the depth (in mm) of water needed to be replaced in the soil each day.
For design purposes, we always use the peak season values for both ET₀ and Kc to ensure the system can handle the highest demand.
Different hydrozones will have significantly different ETc values, reinforcing the importance of zoning for water efficiency.

In our next lesson, we will answer the final question for our water demand analysis: "How do we get from a depth of water to a total volume?" We will use the area of each hydrozone to convert the ETc (in mm/day) into a flow demand (in Liters/day and Liters/hour), a critical value needed for pipe sizing and sprinkler selection.

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