Rainfall and Soil Moisture Considerations in Prescribed Burning

Underburning

Because rainfall affects both fuel moisture and soil moisture, the burn executor should have some idea of the amount of recent rainfall on the area to be burned. In winter, rainfall is fairly uniform throughout large regions of the South and rainfall data can be obtained from local weather stations. In summer, when shower activity prevails, rainfall at individual locations is much more variable. The only reliable method to determine the amount of precipitation that actually falls is to place an inexpensive rain gauge on the site.

The importance of adequate soil moisture cant be overemphasized. The preferred soil moisture is damp. Damp soil protects tree roots and microorganisms. Even when burning to expose a mineral soil seedbed it is desirable to leave a thin layer of organic material to protect the soil surface. Burning should cease during periods of prolonged drought and resume only after a soaking rain of at least 1 inch and a check for adequate soil moisture. As soil moisture conditions improve, less rain is needed before burning, but a site specific check, which includes areas adjacent to tree boles, is still needed to verify adequate soil moisture. If recent precipitation has been near average, 1/4 to 1/2 inch of rain followed by sunny skies, brisk winds, and low humidities will generally result in several days of good prescribed fire conditions with adequate soil protection.

On clay soils, such as are found in the Piedmont, much of the rainfall is lost through surface runoff, and duration is more important than amount. For example, 1 inch of rain occurring in 1/2 hour will not produce as large a moisture gain as 1/2 inch falling over a 2 hour period.

For background weather information on rain see: Clouds and Precipitation.

Debris Burning

Generally, rain has a much greater effect on fuel moisture in cleared areas than under a stand because none is intercepted by tree canopies. However, fuels also dry much faster in cleared areas because of increased sunlight and higher wind speeds. This differential drying can often be used to advantage from a fire-control standpoint. Burn the cleared area several days after a hard rain while fuels in the surrounding forest are still damp. Burning under these conditions assures good soil moisture. However, when burning cleared areas, soil damage is as much a function of fire intensity and duration as it is of soil moisture. Intense, long duration fires will bake the soil regardless of the moisture present. Both the chemical and phyical properties of the soil can be altered. This type of fire should be avoided, especially on clay soils and steep slopes. These undesirable fire effects are often produced when burning windrowed or piled debris, and are one reason piling or windrowing slash prior to burning are discouraged.

For background weather information on rain see: Clouds and Precipitation.