Irrigation Runoff Calculator
Will your sprinklers cause runoff? This free tool simulates how your soil absorbs water over time using a modified Horton infiltration model. Enter your soil type, sprinkler application rate, slope, and duration to see exactly when runoff begins, how much water is wasted, and whether you need cycle-soak scheduling. The chart shows infiltration capacity decreasing as the soil wets up, and highlights the excess water that becomes runoff.
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Understanding Your Results
Understanding Irrigation Runoff
Runoff occurs when water is applied faster than the soil can absorb it. The infiltration capacity starts high when soil is dry and decreases as the soil wets up. When the application rate exceeds the infiltration capacity, water pools on the surface (ponding). On slopes, ponded water runs off instead of infiltrating.
This wastes water, causes erosion, and can carry fertilizers into waterways. Understanding your soil's infiltration capacity helps you choose the right sprinkler type and schedule.
The Cycle-Soak Method
If this calculator shows runoff, the solution is cycle-soak scheduling: split your irrigation into multiple short cycles with rest periods between them. For example, instead of running 30 minutes straight on clay soil, run 3 cycles of 10 minutes with 20-minute breaks. During the breaks, ponded water infiltrates and the soil's absorption capacity partially recovers.
Our AI optimizer automatically calculates optimal cycle-soak schedules based on real-time soil moisture data.
Related tools: Soil Moisture Interpreter · Water Savings Calculator
Soil Infiltration Rates
Reference table of typical steady-state infiltration rates and recommended sprinkler types for common soil textures.
| Soil Type | Steady Infiltration Rate | Best Sprinkler Type |
|---|---|---|
| Sand | >2.0 in/hr | Any type works |
| Sandy Loam | 0.6 - 2.0 in/hr | Rotors, spray heads |
| Loam | 0.3 - 0.6 in/hr | Rotors preferred |
| Silt Loam | 0.15 - 0.3 in/hr | Rotors, low-flow |
| Clay Loam | 0.05 - 0.15 in/hr | Low-flow rotors, drip |
| Clay | <0.05 in/hr | Drip or micro-spray |
Steady infiltration rate is the long-term rate after the soil has wetted up. Initial rates are typically 2-5x higher. Values are approximate and vary with soil structure, organic matter, and compaction.
Frequently Asked Questions
How do I know if my irrigation is causing runoff?
Look for water flowing off landscaped areas, pooling on sidewalks or driveways, or erosion channels in soil. If you see water running within 10 minutes of starting irrigation, your application rate exceeds your soil's infiltration capacity. This calculator helps identify the threshold by simulating exactly when excess water overwhelms your soil's absorption capacity and begins to run off.
What application rate do my sprinklers produce?
Check the manufacturer's spec sheet for precipitation rate. Typical values: drip systems 0.1-0.3 in/hr, rotary nozzles 0.4-0.6 in/hr, standard rotors 0.5-1.0 in/hr, fixed spray heads 1.2-2.0 in/hr. You can also do a catch-cup test: place several flat-bottomed containers in your yard, run sprinklers for 15 minutes, measure the average depth of water collected, and multiply by 4 to get your rate in inches per hour.
Does slope affect infiltration?
Yes, significantly. On slopes, water that would pond on flat ground instead runs downhill. Steeper slopes allow less ponding time and more runoff. As a rule of thumb, slopes over 10% almost always need cycle-soak scheduling, and slopes over 20% should use drip irrigation. This calculator models the slope effect by reducing both the effective infiltration capacity and the maximum ponding depth on steeper terrain.
What is cycle-soak scheduling?
Cycle-soak divides a single long irrigation run into multiple shorter cycles with rest periods. The total water applied is the same, but the rest periods allow surface water to infiltrate before the next cycle starts. This dramatically reduces runoff on clay soils and slopes. For example, three 10-minute cycles with 20-minute breaks can eliminate runoff that would occur during a single 30-minute run. Most smart controllers support this; our optimizer calculates it automatically based on real-time soil conditions.