Infiltration rates by soil texture
A soil's infiltration rate is how fast water soaks into its surface, measured as a depth per hour. It's highest when the soil is dry, then slows to a steadier rate as the soil wets up. For planning irrigation, the steady rate is the one that matters, because that's what the soil can keep absorbing through a long watering.
At a glance: USDA NRCS steady rates
| Soil type | in/hr | mm/hr |
|---|---|---|
| Sands | over 0.8 | over 20 |
| Loams | 0.2–0.4 | 5–10 |
| Clays | 0.04–0.2 | 1–5 |
By texture: University of Maryland Extension
| Soil texture | in/hr | mm/hr |
|---|---|---|
| Coarse sand | 0.75–1.00 | 19–25 |
| Fine sand | 0.50–0.75 | 13–19 |
| Fine sandy loam | 0.35–0.50 | 9–13 |
| Silt loam | 0.25–0.40 | 6–10 |
| Clay loam | 0.10–0.30 | 3–8 |
Basic infiltration rates: FAO
| Soil type | mm/hr | in/hr |
|---|---|---|
| Sandy loam | 20–30 | 0.8–1.2 |
| Loam | 10–20 | 0.4–0.8 |
| Clay loam | 5–10 | 0.2–0.4 |
| Clay | 1–5 | 0.04–0.2 |
FAO's table also has a row for sand, but prints it as "less than 30" mm/hr, which conflicts with every other row and with the NRCS and Maryland figures that show sand is the fastest texture. We've left that row out rather than guess what was intended.
FAO's training manual also groups rates broadly: low below 15 mm/hr (about 0.6 in/hr), medium 15–50 mm/hr, and high above 50 mm/hr (about 2 in/hr).
The sources don't agree exactly, because infiltration depends on structure, compaction and moisture as well as texture. Treat any table value as a starting point and measure your own soil where it matters.
Maximum sprinkler precipitation rates by soil and slope
For sprinklers, the practical question is how fast you can apply water before it runs off. These maximum rates, suggested by the US Department of Agriculture and published in Hunter Industries' irrigation design training, account for soil texture, subsoil, slope and whether the soil has plant cover.
Inches per hour
| Soil | 0–5% slope | 5–8% slope | 8–12% slope | Over 12% slope | ||||
|---|---|---|---|---|---|---|---|---|
| Cover | Bare | Cover | Bare | Cover | Bare | Cover | Bare | |
| Coarse sandy soils | 2.00 | 2.00 | 2.00 | 1.50 | 1.50 | 1.00 | 1.00 | 0.50 |
| Coarse sandy soils over compact subsoils | 1.75 | 1.50 | 1.25 | 1.00 | 1.00 | 0.75 | 0.75 | 0.40 |
| Uniform light sandy loams | 1.75 | 1.00 | 1.25 | 0.80 | 1.00 | 0.60 | 0.75 | 0.40 |
| Light sandy loams over compact subsoils | 1.25 | 0.75 | 1.00 | 0.50 | 0.75 | 0.40 | 0.50 | 0.30 |
| Uniform silt loams | 1.00 | 0.50 | 0.80 | 0.40 | 0.60 | 0.30 | 0.40 | 0.20 |
| Silt loams over compact subsoil | 0.60 | 0.30 | 0.50 | 0.25 | 0.40 | 0.15 | 0.30 | 0.10 |
| Heavy clay or clay loam | 0.20 | 0.15 | 0.15 | 0.10 | 0.12 | 0.08 | 0.10 | 0.06 |
Millimeters per hour
| Soil | 0–5% slope | 5–8% slope | 8–12% slope | Over 12% slope | ||||
|---|---|---|---|---|---|---|---|---|
| Cover | Bare | Cover | Bare | Cover | Bare | Cover | Bare | |
| Coarse sandy soils | 51 | 51 | 51 | 38 | 38 | 25 | 25 | 13 |
| Coarse sandy soils over compact subsoils | 44 | 38 | 32 | 25 | 25 | 19 | 19 | 10 |
| Uniform light sandy loams | 44 | 25 | 32 | 20 | 25 | 15 | 19 | 10 |
| Light sandy loams over compact subsoils | 32 | 19 | 25 | 13 | 19 | 10 | 13 | 8 |
| Uniform silt loams | 25 | 13 | 20 | 10 | 15 | 8 | 10 | 5 |
| Silt loams over compact subsoil | 15 | 8 | 13 | 6 | 10 | 4 | 8 | 3 |
| Heavy clay or clay loam | 5 | 4 | 4 | 3 | 3 | 2 | 3 | 2 |
Two patterns stand out. Plant cover lets most soils take noticeably more water than bare ground, up to twice as much on silt loams, and every step up in slope lowers the limit. Heavy clay on a slope over 12% should receive no more than 0.06 in/hr, a rate that practically every sprinkler exceeds.
Comparing your soil with your sprinklers
Runoff happens when sprinklers apply water faster than the soil takes it in. Colorado State University Extension gives typical rates of 1 to 2½ in/hr for pop-up spray heads and ¼ to ¾ in/hr for rotors. Compare those with the tables above:
- Spray heads at 1.5 in/hr exceed the maximum rate for uniform silt loam with turf cover on flat ground (1.00 in/hr), so water will pond or run off during a long run.
- Rotors at 0.5 in/hr are below that limit on flat ground, but above it on silt loam with a slope over 12% (0.40 in/hr).
- On heavy clay or clay loam, even rotors exceed the maximum rate (0.20 in/hr on flat ground with cover).
When the sprinkler rate is higher than the soil can take, the fix is usually not a different sprinkler but a different schedule: split the run time into shorter cycles with a soak period between them. The cycle and soak calculator works out the split, and the sprinkler precipitation rate chart lists rates by sprinkler type.
What changes a soil's infiltration rate
| Factor | Effect on infiltration |
|---|---|
| Texture | Large pores in sandy soil let water in faster than the small pores of clay. |
| Soil moisture | Dry soil takes water fastest. As it wets, the rate slows to the rate of the most restrictive layer. |
| Structure | Granular soil takes water quickly; massive or compacted soil takes it very slowly. |
| Compaction | Foot, vehicle and livestock traffic, especially on wet soil, reduces pore space and infiltration. |
| Surface crusting | Rain or spray hitting bare soil dislodges particles that seal surface pores. |
| Organic matter | Builds stable aggregates and supports earthworms, both of which increase infiltration. |
| Shrinking clays | Some clays crack as they dry, letting water in quickly at first until the cracks swell shut. |
Unlike texture, structure and compaction can be changed. The NRCS lists practices that keep soil covered, add organic matter and reduce compaction as the long-term way to improve infiltration.
Measuring your own soil's infiltration rate
A ring test gives a far better number than any table. Both the NRCS and FAO describe versions of the same idea:
- Set a ring into the soil. Drive a metal ring or open-ended can a few inches into the ground so water can't escape sideways. FAO's field method uses a 30 cm ring driven at least 15 cm deep; the NRCS test uses a 6-inch ring.
- Add a known depth of water. The NRCS test adds a 1-inch layer of water inside the ring.
- Time how long it takes to soak in. Then repeat. Later readings on wetter soil are closer to the steady rate you need for irrigation.
- Test more than one spot. FAO recommends at least two tests at a site to be sure the results are reliable.
The NRCS recommends testing when the soil is near field capacity rather than bone-dry or saturated, so different spots can be compared fairly. The soil infiltration rate calculator turns your readings into a rate.
Frequently asked questions
What is soil infiltration rate?
Infiltration rate is how fast water soaks into the soil surface, measured as a depth per hour, in inches or millimeters. An infiltration rate of 15 mm per hour means a 15 mm layer of water on the surface takes one hour to soak in. It's highest when the soil is dry and slows to a steady rate as the soil wets up.
What is the infiltration rate of clay soil?
Clay soils are the slowest. The USDA Natural Resources Conservation Service gives a steady infiltration rate of 0.04 to 0.2 inch per hour for clays in deeply wetted soil, and FAO lists 1 to 5 mm per hour for clay. Some clays crack as they dry, which lets water in faster at first until the cracks close.
What is the infiltration rate of sandy soil?
Sandy soils are the fastest. The USDA Natural Resources Conservation Service gives a steady rate of more than 0.8 inch per hour for sands, and University of Maryland Extension lists 0.75 to 1.00 inch per hour for coarse sand and 0.50 to 0.75 inch per hour for fine sand.
Why does water run off my lawn when the sprinklers are on?
Most often the sprinklers apply water faster than the soil can absorb it. Pop-up spray heads typically apply 1 to 2.5 inches per hour, while loam soils steadily absorb only about 0.2 to 0.4 inch per hour. Splitting the run time into shorter cycles with a soak period between them lets the water soak in instead of running off.
How does slope affect how fast I can water?
The steeper the slope, the lower the rate the soil can take before water runs off. In the USDA-suggested values on this page, uniform silt loam with plant cover can take up to 1.00 inch per hour on a 0 to 5% slope, but only 0.40 inch per hour on a slope over 12%.
Does a soil's infiltration rate change over time?
Yes. Water enters dry soil quickly, then slows to a steady basic infiltration rate as the soil wets. Over longer periods, compaction from traffic, surface crusting on bare soil and loss of organic matter reduce infiltration, while plant cover, organic matter and good soil structure improve it.
Who built and checked this chart
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- Every rate transcribed from the NRCS, University of Maryland, FAO and USDA/Hunter sources and checked against them.
- Metric and inch conversions recalculated (1 in = 25.4 mm).
- Conflicting source values flagged on the page rather than silently corrected.
Sources
- USDA Natural Resources Conservation Service. Soil Quality Indicators: Infiltration.
- University of Maryland Extension. Table C-3: Soil Infiltration Rates Based on Soil Texture.
- FAO Irrigation Water Management training manual. Annex 2: Infiltration rate and infiltration test.
- FAO Irrigation Water Management training manual. Chapter 2: Soil and water.
- Hunter Industries (2005). Precipitation Rates and Sprinkler Irrigation, Irrigation Design Education Module. Maximum precipitation rates as suggested by the USDA.
- Rain Bird. Irrigation Design Tip: How Soil Type Affects Irrigation.
- Colorado State University Extension, Colorado Master Gardener program. Sprinkler Types Used for Lawn Irrigation.