6.2 Distribution Uniformity (DUlq) & Catch-Can Audits

6.2 Distribution Uniformity ($DU_{lq}$) & Catch-Can Audits

Quick Answer: Lower-Quarter Distribution Uniformity ($\text{DU}{lq}$) measures application uniformity across an irrigation zone using catch cans. The formula is $\text{DU}{lq} = \left(\frac{\text{Average depth of lowest 25% catch cans}}{\text{Overall average depth of all catch cans}}\right) \times 100$. Excellent systems achieve $\text{DU}{lq} > 0.75$ (75%), while systems below 55% require head or nozzle maintenance. To compensate for non-uniformity without under-watering the dry spots, managers multiply base run time by the Scheduling Multiplier: $\text{SM} = \frac{1}{0.4 + 0.6 \times \text{DU}{lq}}$.

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Concept of Lower-Quarter Distribution Uniformity ($\text{DU}_{lq}$)

Even perfectly designed irrigation systems do not deposit water completely evenly across a lawn. Factors such as wind drift, head tilt, mismatched nozzles, pressure fluctuations, and improper spacing create wetter and drier patches. If an irrigation manager programmes run times based purely on average precipitation rate, half of the landscape will receive less than the targeted water volume—and the driest 25% of the zone will suffer severe drought stress, browning, or turf death.

To quantify irrigation uniformity, the Irrigation Association standardizes Lower-Quarter Distribution Uniformity ($\text{DU}_{lq}$). $\text{DU}_{lq}$ focuses specifically on the driest quarter of the audited zone:

$\text{DU}_{lq} = \frac{\bar{D}_{lq}}{\bar{D}_{all}} \times 100$

Where:

• $\bar{D}_{lq}$ = Mean catch depth (or volume) of the lowest 25% of catch devices in the test grid
• $\bar{D}_{all}$ = Mean catch depth (or volume) of all catch devices in the test grid

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Catch-Can Audit Field Protocol

Performing a standardized catch-can audit requires strict adherence to field protocols established by the Irrigation Association Smart Water Application Technologies (SWAT) and Certified Landscape Irrigation Auditor (CLIA) standards.

1. Equipment & Catch Can Specifications

• Use identical catch containers across the entire grid. Standard IA audit catch devices feature a sharp-edged plastic throat opening (typically $16.0 \text{ in}^2$ or $103.2 \text{ cm}^2$) to eliminate splash-out.
• Graduated cylinders calibrated in milliliters (mL) or liquid depth indicators marked directly in inches or millimeters.

2. Grid Layout Procedure

• Position catch cans in a uniform geometric grid across the target zone.
• Place cans 2 to 3 feet away from sprinkler heads to capture perimeter throw while avoiding head blind spots.
• Space remaining cans evenly between heads, ensuring grid spacing does not exceed 10 to 15 feet apart.
• For statistical validity, a minimum of 24 catch cans per irrigated zone is recommended.

3. Audit Test Duration

• Spray Head Zones: Run system for 10 to 15 minutes (spray heads apply high volumes quickly).
• Rotor Head Zones: Run system for 30 to 60 minutes (rotors apply water more slowly as arcs rotate).
• Measure dynamic operating pressure at the first, middle, and last heads during the run using a Pitot tube or pressure gauge cap.
• Audit must be halted if wind speeds exceed 5 mph ($8 \text{ km/h}$), as wind significantly skews catch distribution.

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Volume-to-Depth Calculations

If catch cans measure liquid volume in milliliters (mL), technicians must convert volume into linear depth in inches using the container throat area:

$1 \text{ mL} = 0.0610237 \text{ cubic inches (in}^3\text{)}$ $\text{Depth (in)} = \frac{\text{Volume (mL)} \times 0.0610237}{\text{Catch Can Throat Area (in}^2\text{)}}$

For a standard catch can with a $16.0 \text{ in}^2$ throat area: $\text{Depth (in)} = \frac{\text{Volume (mL)} \times 0.0610237}{16.0} = \text{Volume (mL)} \times 0.003814$

For example, a caught volume of $40 \text{ mL}$ in a $16.0 \text{ in}^2$ catch can equals: $\text{Depth} = 40 \times 0.003814 = 0.1526 \text{ inches}$

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Interpreting $\text{DU}_{lq}$ Performance Ratings

The Irrigation Association establishes clear benchmark performance ratings for overhead sprinkler systems:

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The Scheduling Multiplier (SM)

To ensure the lowest quarter of the turf area receives the target net depth of water without under-watering, the base run time must be increased using the Scheduling Multiplier (SM):

$\text{SM} = \frac{1}{0.4 + 0.6 \times \text{DU}_{lq}}$

Where $\text{DU}_{lq}$ is expressed as a decimal (e.g., $70% = 0.70$).

Operational Impact of SM:

• If $\text{DU}_{lq} = 0.80$ (80% excellent uniformity): $\text{SM} = \frac{1}{0.4 + 0.6(0.80)} = \frac{1}{0.88} = 1.136$ Run time is increased by only 13.6%.

• If $\text{DU}_{lq} = 0.50$ (50% poor uniformity): $\text{SM} = \frac{1}{0.4 + 0.6(0.50)} = \frac{1}{0.70} = 1.428$ Run time must be increased by 42.8% to adequately water the driest quarter, resulting in substantial over-watering in wetter areas.

Worked Catch-Can Audit Dataset & $\text{DU}_{lq}$ Calculation

The table below presents complete field audit data collected from 24 catch cans during a 15-minute spray zone test. Cans are sorted in ascending order by measured catch volume.