6.3 Cycle-and-Soak & Soil Water Management

6.3 Cycle-and-Soak & Soil Water Management

Quick Answer: Cycle-and-soak scheduling divides total required irrigation run time into multiple shorter watering cycles separated by soak intervals. This prevents surface runoff when sprinkler precipitation rate (PR) exceeds soil infiltration rate. Maximum allowable cycle runtime before runoff is $\text{Max Cycle (min)} = \frac{\text{Allowable Storage Depth (in)} \times 60}{\text{PR (in/hr)} - \text{Infiltration Rate (in/hr)}}$. Fine clay soils require short cycles (3 to 5 minutes) and long soak periods (30 to 60 minutes).

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Soil Infiltration Rates by Texture

Soil Infiltration Rate (or intake rate) is the speed at which soil absorbs water at the surface, measured in inches per hour (in/hr). Infiltration is controlled by soil texture, structure, compaction, and pore size distribution.

Basic Soil Texture Intake Rates

• Coarse Sand / Sandy Loam: $1.0 - 2.0 \text{ in/hr}$ (Large macropores allow rapid gravitational drainage, but low water holding capacity).
• Loam / Silt Loam: $0.5 - 1.0 \text{ in/hr}$ (Balanced mixture of sand, silt, and clay; ideal agricultural pore matrix).
• Clay / Clay Loam: $0.1 - 0.2 \text{ in/hr}$ (Dense micropores with high water retention capacity, but extremely slow intake rate).

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Slope Modifiers & Compaction

When irrigation is applied on sloping terrain, gravity causes surface water to flow downhill before it can infiltrate vertically. Topographic slope significantly reduces the effective soil intake rate.

Soil Compaction Impact

Foot traffic, heavy riding mowers, and construction equipment collapse soil macro-pores. Compacted clay turf soils often exhibit intake rates as low as $0.02 - 0.05 \text{ in/hr}$, causing immediate runoff even under low-precipitation rotors.

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The Runoff Problem: $\text{PR} > \text{Infiltration Rate}$

Standard landscape spray heads apply water at $1.2 - 2.0 \text{ in/hr}$. If a spray head applying $1.50 \text{ in/hr}$ operates continuously on a clay soil with an intake rate of $0.15 \text{ in/hr}$, water accumulates on the surface at a rate of $1.35 \text{ in/hr}$ ($1.50 - 0.15$). Once soil micro-topography fills, excess water runs off onto hardscapes, causing erosion, non-point source pollution, and severe water waste.

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Cycle-and-Soak Scheduling Physics

To apply the full required water volume without runoff, irrigation controllers must be programmed using Cycle-and-Soak (or Cycle/Soak). The system runs for a short cycle, pauses to allow water to soak into the root zone, and repeats until the total daily run time is achieved.

Formula for Maximum Allowable Cycle Runtime

$\text{Max Cycle Duration (min)} = \frac{\text{Allowable Surface Storage Depth (in)} \times 60}{\text{PR (in/hr)} - \text{Infiltration Rate (in/hr)}}$

Where:

• Allowable Surface Storage Depth: The depth of water that surface micro-depressions can temporarily pond before runoff begins (typically $0.10 \text{ in}$ for flat turf, reduced to $0.05 \text{ in}$ or $0.08 \text{ in}$ on slopes).
• $\text{PR}$: Gross zone precipitation rate (in/hr).
• Infiltration Rate: Effective soil water intake rate (in/hr).

Worked Example:

A spray zone with $\text{PR} = 1.40 \text{ in/hr}$ on a 6% gentle clay slope (effective intake $= 0.12 \text{ in/hr}$) has an allowable storage depth of $0.08 \text{ in}$. $\text{Max Cycle} = \frac{0.08 \times 60}{1.40 - 0.12} = \frac{4.80}{1.28} = 3.75 \text{ minutes}$ Conclusion: The controller cycle run time must not exceed 3.75 minutes per cycle. If total required daily run time is $15 \text{ minutes}$, the controller must be programmed for 4 cycles of 3.75 minutes each.

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Soak Duration Guidelines

The soak interval between cycles must be long enough to allow all ponded surface water to drain completely into the soil matrix.

• Clay Soils: Minimum soak time of 30 to 60 minutes between active cycles.
• Loam Soils: Minimum soak time of 15 to 30 minutes.
• Sandy Soils: Minimal soak required (5 to 10 minutes, though cycle-and-soak is rarely needed on sand).

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Evapotranspiration (ET$_0$) & Plant Water Requirements

Water management requires calculating net crop water demand based on weather conditions:

1. Reference Evapotranspiration (ET$_0$): The total water lost through evaporation from soil surfaces and transpiration from a standardized cool-season grass reference crop ($4 \text{ in}$ height), measured in inches per day or week.
2. Plant Coefficient ($K_c$): A multiplier adjusting ET$_0$ for specific plant species:
   • Cool-Season Turf (Tall Fescue, Kentucky Bluegrass): $K_c = 0.80 - 0.85$
   • Warm-Season Turf (Bermudagrass, Zoysia, St. Augustine): $K_c = 0.60 - 0.70$
   • Low-Water Native Shrubs: $K_c = 0.20 - 0.40$

$\text{Net Water Requirement (ET}_c\text{)} = \text{ET}_0 \times K_c$

Reference Table: Soil Intake Rates, Slope Modifiers & Max Cycle Times

The table below summarizes standard soil intake rates, slope adjustments, and maximum recommended initial cycle run times for typical spray head ($1.50 \text{ in/hr}$) and rotor ($0.50 \text{ in/hr}$) zones assuming a $0.10 \text{ in}$ surface storage depth.