How Does Soil Surface Evaporation Shift Between Two Stages?
As an Amazon Associate we earn from qualifying purchases.
The question of how does soil surface evaporation shift between two stages comes up whenever bare soil dries out faster than expected. The answer comes from laboratory and modeling research on drying soil. Water leaves the surface quickly while the soil is wet, then the process slows and the location of evaporation moves downward. The sources below describe that sequence, the role of a dry surface layer, and the effect of wind. Quoted wording is copied from the fetched pages listed at the end.
Quick answer
- Drying happens in stages. The first stage has “a relatively high evaporation rate,” and later stages have a lower rate.
- The shift starts when “the surface moisture is depleted,” and then “the location of evaporation shifts from surface to subsurface.”
- A “dry surface layer (DSL)” forms above the evaporation zone, and it slows loss further.
- Wind mainly affects the first stage, according to a modeling study.
Why soil drying is described in stages
Soil evaporation is a physical process that matters across many fields. The laboratory study notes that “Evaporation from a porous medium plays a key role in various fields, including hydrological, agricultural, environmental, and engineering applications, such as water evaporation from soil surfaces.” The second paper describes soil evaporation as part of a larger cycle: “A critical component of the water cycle at local, regional, and global scales is evaporation.”
Researchers describe drying with a set of named phases. The laboratory paper says that “Soil drying by evaporation has been often separated into three or two stages: the high rate (stage 1), the falling rate (stage 2), and/or the low and constant rate stages (stage 3).” The phrase “two stages” in the question usually refers to the switch from the high-rate stage to the falling-rate stage, with a low, steady stage possibly following.
Stage 1: the surface supplies water
While the surface is wet, water reaches it from below and leaves at a fast rate. The laboratory study reports that the process begins “with a relatively high evaporation rate during stage 1.” The study measured a sequence of phases in drying soil, and stage 1 is the phase where the surface is still fed by water moving up through the soil.
The modeling study confirms that the first stage has its own rate. It says that “Increasing the wind speed increases the first stage evaporation rate.” The rate in stage 1 therefore responds to air movement at the surface.
The transition: when the surface runs out
The switch comes when the upper soil can no longer keep the surface wet enough. The laboratory paper states that “With the progressive drying, when the surface moisture is depleted, the evaporation rate drops below the potential rate; then stage 2 or falling rate evaporation begins.”
The modeling study describes this handoff as a change in control. It says that the transition is between “soil water flow to vapor diffusion controlled” stages. In the first stage, the liquid water moving through the soil sets the pace. In the later stage, vapor has to move through the dry soil, which slows the process.
Stage 2: evaporation moves below the surface
Stage 2 is the falling-rate stage, and the defining change is where the water leaves. The laboratory paper says that “During this stage, the location of evaporation shifts from surface to subsurface, resulting in formation of a dry surface layer (DSL).” Evaporation no longer happens at the top of the soil. It happens at a level below the top.
The paper describes the end of this stage with the same term. “When the evaporation zone totally moved into the subsurface, a dry surface layer (DSL) formed above the evaporation zone at the end of stage 2.” The dry surface layer is therefore the physical sign that the shift is complete.
The modeling study notes that wind has less influence here. It states that “On the contrary, the impact of wind speed on second stage evaporation (diffusion-dominant stage) is not significant.” In other words, once the process is limited by diffusion through dry soil, the surface wind matters much less than it did at the start.
Stage 3: a low, steady rate
After the falling-rate phase, the laboratory study describes a last phase. Its wording is that stage 3 is “maintaining a very low and constant rate.” The study’s summary reads: “Results show that the evaporative drying process could be divided into three stages, beginning with a relatively high evaporation rate during stage 1, followed by a lower rate during transient stage and stage 2, and finally maintaining a very low and constant rate during stage 3.”
This means the two-stage picture is a simplification. The laboratory paper itself reports three stages, with a transient period between the first and second. The question’s wording about “two stages” fits the main switch from fast surface loss to slower subsurface loss, while the third stage describes the late, very slow end of drying.
Why wind changes the first stage
The modeling study tested how wind speed changes the process. It reports that “Increasing the wind speed increases the first stage evaporation rate and decreases the transition time between two evaporative stages” when velocities are low. The same paper adds that “at high wind speeds the evaporation rate becomes less dependent on the wind speed.” Wind therefore speeds the early phase and shortens the time before the switch, but its effect levels off at higher speeds.
The study also reports that the coupled model could predict the stages. It states that “Results demonstrated that, by using this coupling approach, it is possible to predict the different stages of the drying process with good accuracy.” That is a modeling result, not a field measurement in a garden bed.
Connecting the stages to the garden
Garden soil is more varied than the laboratory columns and modeled soil in these papers. The sources do not give a rule for when a particular garden bed will switch stages. To check moisture at depth rather than only at the top, see how to check soil moisture.
For a watering routine built around deeper soil moisture, see how to deep water plants.
How we chose the sources
Both sources are peer-reviewed research articles hosted on PubMed Central, a non-retail archive. The first is a laboratory and numerical study of soil water evaporation during drying. The second is a study of how wind speed affects evaporation from soil, using an integrated model. Both pages returned HTTP 200 when fetched directly, and every quote here was copied from the fetched text.
What we could not verify
The sources study soil columns and a modeled soil profile. They do not measure a planted garden, so claims about lawns, raised beds, or container mixes are not made here.
What the sources do not say
The laboratory paper describes the number of stages as “three or two,” which means the count depends on how the drying is divided. The modeling study does not give a time for the switch in any real soil. Neither source gives a watering schedule, so this explainer does not provide one.
Frequently asked questions
What is stage 1 of soil drying?
Stage 1 is the high-rate phase. The laboratory study says it begins “with a relatively high evaporation rate during stage 1,” while the surface is still supplied with water from below.
What causes the switch to a slower stage?
The surface moisture runs out. The study says that “when the surface moisture is depleted, the evaporation rate drops below the potential rate.”
What is a dry surface layer?
It is a layer that forms above the place where evaporation happens. The paper defines it as “a dry surface layer (DSL)” that “formed above the evaporation zone at the end of stage 2.”
Does wind speed matter after the switch?
Much less. The modeling study reports that “the impact of wind speed on second stage evaporation (diffusion-dominant stage) is not significant.”
Are there always exactly two stages?
No. The laboratory study reports three stages and states that drying “has been often separated into three or two stages.” The count depends on how the drying is divided.
Sources
- “Dynamics of Soil Water Evaporation during Soil Drying: Laboratory Experiment and Numerical Analysis.” PubMed Central article. https://pmc.ncbi.nlm.nih.gov/articles/PMC3892940/ (HTTP 200, checked 2026-10-28). Quote: “When the evaporation zone totally moved into the subsurface, a dry surface layer (DSL) formed above the evaporation zone at the end of stage 2.”
- “Study of the effect of wind speed on evaporation from soil through integrated modeling of the atmospheric boundary layer and shallow subsurface.” PubMed Central article. https://pmc.ncbi.nlm.nih.gov/articles/PMC4171757/ (HTTP 200, checked 2026-10-28). Quote: “Increasing the wind speed increases the first stage evaporation rate and decreases the transition time between two evaporative stages.”
