Loam
The rain total is only the beginning
It is tempting to make a rule. A quarter inch is fine. An inch means stay home. The problem is that trails do not experience rain as a single number.
What matters is how much water reached the ground, how quickly it arrived, how wet the ground already was, and how easily that water can leave. A dry trail going into a storm has more room to absorb water than one that was already saturated.
The useful question
Do not ask only how much rain fell. Ask whether the trail has had enough time to shed the water it received.
A short storm can be worse than a long one
Two storms can leave the same rainfall total and produce very different trail conditions. A fast downpour can put a lot of water on the surface before the ground has time to take it in. A slower period of rain spreads that water over more time.
The same goes for the days before the storm. If the trail has already been wet for several days, another round of rain has nowhere to go. If the ground has been dry and the weather is warm and breezy afterward, recovery can be much faster.
Where the water goes matters
Drainage
A trail with good drainage can move water away from the riding surface quickly. Flat sections, dips, and places where water collects tend to stay wet longer.
Soil
Different soils hold different amounts of water. Fine, compacted soil can stay soft near the surface, while rockier ground often sheds water more readily.
Shade
Sun and wind help a wet surface dry. Dense canopy can slow that process, especially when the weather after the storm is cool.
Trail shape
A trail that sheds water across its surface usually recovers differently from one with ruts, flat turns, or low spots where water can sit.
Do not confuse wet with damaged
Some trails are designed to handle wet weather better than others. That does not mean every wet trail is ready to ride. The important distinction is whether tires can pass over the tread without deforming it.
If your tires sink into the surface, leave a groove, or pick up a layer of mud, the trail is telling you something. Turn around. Riding through it can turn a temporary wet spell into a longer repair problem.
How Loam approaches the question
Loam does not use a single rainfall cutoff. It looks at recent weather alongside the physical traits of the network, then gives you a condition estimate. That is more useful than pretending every trail needs the same number of dry hours.
Check the official closure first. Then check the network condition. If the ground looks soft when you arrive, trust the ground.
The science behind the estimate
Loam starts with a simple physical fact: a trail does not dry just because the rain stops. Water has to enter the ground, move through or across the soil, and leave the trail environment. How quickly that happens depends on the soil, terrain, vegetation, recent weather, and how wet the ground already was.
Soil controls infiltration
Soils do not accept water at the same rate. USDA hydrologic soil groups range from high-infiltration soils such as deep sands and gravels to very slow-infiltration soils associated with clay, high water tables, or restrictive layers. Typical infiltration-rate ranges used in the hydrologic-group framework run from more than 0.30 in/hr for Group A to less than 0.05 in/hr for Group D when thoroughly wet.
Previous rain still matters
A storm does not start with an empty soil profile. USGS notes that soil already saturated from previous rainfall cannot absorb much more, so a larger share of the next storm becomes runoff. Recent weather therefore matters even when the latest storm was not especially large.
Terrain changes the water path
Slope changes how quickly water can move away from a surface. Low spots, drainage features, and trail geometry can change where water collects or leaves the tread. Soil classification and slope are separate pieces of the landscape, which is why both matter to a trail-condition model.
Drying is a water budget
After rainfall, water can remain in the soil, move downward or sideways, run off, or return to the atmosphere through evaporation and plant transpiration. Soil-water-balance models use these processes to estimate changing soil moisture and net infiltration over time.
Why Loam is not a rain timer
There is no useful rule that says every trail becomes rideable after the same number of dry hours. Starting moisture, infiltration behavior, terrain, canopy, and weather after the storm all change the answer.
That is the problem Loam is designed to estimate. The model combines public soil and terrain information with recent weather and network characteristics to estimate how conditions are changing. It is a model of likely trail conditions, not a sensor embedded in the dirt, and it never overrides an official closure.
What the research says
USDA and USGS hydrology work treats infiltration, soil moisture, runoff, canopy, land cover, slope, and evapotranspiration as interacting parts of the water cycle. Recent trail research adds an important piece: rainfall intensity and accumulated rainfall can strongly affect runoff and sediment generation on recreational trails, while wet conditions make trail surfaces more vulnerable to degradation.
USDA NRCS: Hydrologic Soil Groups
Soils are classified by infiltration and runoff behavior when thoroughly wet. The framework distinguishes four main groups and dual drained/undrained classes.
NRCS National Engineering Handbook →USGS: Infiltration and the Water Cycle
Explains how soil characteristics, saturation, land cover, slope, and evapotranspiration affect where precipitation goes.
USGS Water Science School →USGS: Soil-Water-Balance
A published water-budget model that estimates soil moisture, net infiltration, evapotranspiration, and canopy interception from gridded environmental data.
USGS SWB Version 2.0 →NRCS: RUSLE2
A USDA model for estimating soil loss caused by rainfall and associated overland flow, connecting rainfall and runoff to erosion risk.
USDA NRCS RUSLE2 →Fang & Ng, Journal of Environmental Management, 2026
A year-long field study found cumulative rainfall and maximum daily rainfall predicted runoff and sediment yield on recreational trails, with maximum daily rainfall the stronger predictor in that study.
Read the research →These sources describe the physical processes and research Loam draws from. They do not describe Loam's proprietary model or disclose its weights and thresholds.