Infiltration versus runoff

Around here, people frequently make the following remark about rainfall:

“What we need is one of those steady rains that soaks in instead of just running off.”

I venture that comment bears similarity to many comments made at feed stores and on front porches worldwide. When the rainfall rate exceeds the infiltration rate (rate at which the ground absorbs water), runoff occurs. Lately, I have been reading a book called Rangeland Hydrology, a publication of the Society for Range Management. Several chapters review the extensive research regarding runoff and infiltration, and hint upon the complexity of factors that control whether rain soaks in or runs off.

The book mentions several determining factors regarding the amount of infiltration and runoff, including:

  • rainfall rate
  • soil temperature (frozen)
  • soil material (types of minerals, and amount of organic matter)
  • soil texture
  • soil structure (cracks, the size and amount and arrangement of pores)
  • vegetation
  • amount of leaf/plant litter on surface
  • soil bulk density
  • animal grazing and human activity such as plowing
  • animal burrowing activity

In fact, things are even more complicated than that. Some rainfall is intercepted by living and dead vegetation, and evaporates from the leaves before reaching the soil surface. And to complicate matters more, the soil infiltration rate changes based upon how wet the soil is. Infiltration rates are usually greater at the beginning of a storm, and decrease as soil becomes saturated.

So who cares? Farmers need to know the mot effective way to manage their soil in a way that provides adequate water to their crops. Range managers need to know the effects of grazing and the health of their range. Foresters want to know how rain storms effect forest health and fire hazards. Knowing the proportion of rainfall that runs off is important to flood planners, dam operators, and water providers. Quantifying infiltration is also key to understanding landslides and other geologic hazards.

It rained at my house last week. The rain was of low to moderate intensity, lasted off and on throughout the day, and totaled about 1/2 inch (a little over 1cm). While that seems like a low rainfall total for many areas, 1/2 an inch is a relatively high daily total for the Phoenix area. I observed no runoff except from impervious surfaces such as streets and parking lots. Using laymen’s terminology, this would be considered a “slow, steady rain that soaks in”.

While digging in my garden the following day, I noticed only the top 1-2 inches of soil was wet, beneath which was bone-dry. Obviously, not much of that 1/2 inch rainfall had infiltrated into the soil. Likely, much of that rainfall was intercepted, and evaporated from vegetation (in this case dead bermuda grass) before it ever reached the soil surface. The same is true for rooftops. Throughout the entire day, very little water ran from my gutter downspouts. Obviously, much of the rainfall evaporated from my rooftop before reaching the gutters. Despite a relatively high storm total for the Phoenix area, the rainfall rate was not high enough to produce runoff or cause significant infiltration.  In fact, my garden required irrigation two days later.

This illustrates that simply knowing how much rainfall an area receives is not sufficient for determination of drought, range, or forest moisture conditions. An area could receive average precipitation, but if the intensity is too low drought conditions will persist. Likewise, a wet year as measured by a rain gauge may not produce enough runoff to fill reservoirs. Simply knowing the rain gauge total reveals little about the effectiveness of the rainfall for the various things dependent upon it.

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