So What Constitutes An Effective Rain Event?

Larry Stein, Ph.D, Extension Horticulturist, Department of Horticultural Sciences, Texas AgriLife Extension Service

Mist, drizzle, fog; dreary, cloudy days, great for fruit tree chilling but does this wee bit of moisture associated with these events have any impact on agriculture? Well, it depends!; just what you wanted to hear!; as usual no cut and dried answer. The key to this dilemma lies in the amount of moisture which falls, the soil type and whether or not the soil is wet or dry. To answer this question we must examine the many, varied soil types.

Most soils are composed of varying amounts of sand, loam and clay. Soils with a lot of sand are called sandy soils and those with more clay are clay soils and of course there are many variations in between. The most ideal soil for most plants would be a sandy loam, as it takes and hold some water, but has ample oxygen as well. It is critical to remember that the only way plants can take up water is if there is oxygen present in the soil. Sands allows for rapid water infiltration, but they hold very little water. So typically sandy soils take in water, but hold very little and plants will dry out very quickly. Loams do not take water as fast as sands, but hold more water which can be used by plants for growth. Finally, clay soils take water slowly, so often a lot of water runs off in a heavy rain. Still even though such clay soils hold a lot of water, much of it cannot be extracted for plant growth.

The other soil factor which comes into play is the amount of rock as well as the depth of soil. Some of you have 6 inches of soil sitting on solid rock, others have less and then some have more. So if your soil is shallow, rocky and/or sandy, a small amount of water will penetrate further than on deeper soils.

To get a better understanding of water infiltration you have to know a little about soil physics. Soil physics tells one that soil must be totally wet before the water moves. So you cannot partially wet the soil, rather the water only penetrates as far as the water wets the soil. This should make sense because just a little bit of moisture (0.1 inches) will wet the soil on top and cause it to stick to your shoes, but go an inch deep and the soil will be bone dry.

The best place to see wildflowers during a drought is on the side of the highway.

Let’s examine some common infiltration rates for various soils.

Table 1 indicates that sands take in water faster than loams and clays, i.e., 2.0-6.0 vs 0.6-2.0 vs 0.2-0.6 inches per hour. This means that if the rainfall rate is 2 inches per hour, the sands will take in quite a bit of water whereas the loam and clay will not. So often times heavy, fast rain events are not effective at re-wetting the soil profile. In most cases much of this water runs off rather than in the soil. By the same token one should realize that when the rain does not infiltrate the soil and runs off, the place of “ponding” will experience a much larger rain event. Bottomland is a prime example. Farmers today actually put small berms or dikes in their fields to increase water infiltration so that rain events will more effectively wet their soils.

The other thing you have to remember is how far an inch of water (from rain or a sprinkler, etc.) will wet the soil. Table 1 shows that one inch of water that does not run off will wet the soil 12 inches deep; but it will take more water to wet a loam, 1.5 to 2.0 inches and even more, 2.5 inches to wet a clay; and again this is assuming that no water runs off.

The mist, drizzle and showers which often amount to a quarter of an inch or less obviously do not run off and so the water has wet into the soil. The only problem is that in most cases the soil is only wet a ½ to maybe an inch deep in most cases.

Since many areas have not received an inch of water in a long time, the soil profile has continued to dry. Those of you have that only have a little bit of soil, there is no question that your soil profile is dry. So if you have a foot of clay soil over rock, you will need to receive 2½ inches of rain where none of the water runs off to re-wet your soil. Some of you have 3 to 4 feet of soil. The effective root zone of most plants is 3 feet. So if you have 3 feet of sandy soil, you will need 3 inches of rain where no water runs off to wet the soil 3 feet deep.

The last factor to consider in water infiltration and an effective rain event is how wet the soil is. It should be obvious that a soil which has received a half inch irrigation is easier to wet than one which has not. A ½ inch watering will wet a sandy soil to about 6 inches. This means that only another ½ inch will be required to wet the soil to a foot. Calculations cannot be exact due to losses from evaporation. Still a soil which has been watered will benefit from a quarter to a half inch rain.

Lastly, the best time to water is actually during a rain event, of course if it is raining “cats and dogs” one would not want to water, because the water is already coming so fast that it will run off anyway. But mist and drizzle is a different story. No water will be evaporating since it is raining and the amount you apply along with the mist or drizzle will help wet the soil to a greater depth.

As many of the farmers have remarked, “I used to need a 2 inch rain, but 2 inches will no longer do it.” Rather we need a steady 4 inch rain where the water comes slowly to re-wet many of the different soils. If all we get is a cloud burst, the only places which will get any real benefit will be the places where the water ponds.

Grass is always greener closest to the highway because of water runoff from a hard surface.

Perhaps you have noticed how the grass or weeds right beside paved roads or the overhang of buildings greens up in times of severe drought. This is again due to “runoff” of the water. The water does not penetrate the road and hence runs off. This ponding of a tenth or two results in at least a quarter or half inch rain beside the road. As a result the grass and weeds are able to grow.

Notice that wildflowers come up and bloom ONLY
next to roadways during droughts.
Wildflowers can even get enough supplemental
moisture from roadways to actually produce seed.


The same is true of plants or trees beside the overhang of buildings. The water runs off the roof and is able to effectively wet soil sufficiently at the drip line of the building. So trees beside structures are able to survive better in their droughty times.

So it should be clear that the type and depth of soil along with the amount and speed of the rainfall event will determine just how effective the rain event is at re-wetting the soil profile. Mist and drizzle can be good at times, but in times of severe drought, it does very little.

Table 1. General soil water storage and depletion characteristics for three different soil types.
Soil Texture
Sands Loams Clays
Water infiltration rate (inches per hour) 2.0 – 6.0 0.6 – 2.0 0.2 – 0.6
Available water (inches per ft.) 1.0 – 1.5 1.5 – 2.5 2.5 – 4.0
Days to depletion when ET = .2 inches/day 5 – 7.5 7.5 – 12.5 12.5 – 20.0
Amount of water to wet to 12 in a dry soil (inches) 1.0 1.5 – 2.0 2.5

Publication updated April 2011

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