What Constitutes an Effective Rain Event?

March 31, 2007 Article

by David Rodriguez

Mist, drizzle, fog and dreary, cloudy days are great for fruit tree chilling, but does this wee bit of moisture associated with these events have any impact on agriculture? Well, it depends! As usual there is 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 various soil types.

Most soils are composed of varying amounts of sand, loam and clay. Soils containing a lot of sand are called sandy soils, and those with more clay content are clay soils. Of course there are many variations in between. The most ideal soil for most plants would be a sandy loam, as it takes and holds some water, but contains 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 allow 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. Even though such clay soils hold a lot of water, much of this moisture 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 six inches of soil sitting on solid rock, while others have less, and 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 dig an inch deep, and the soil will be bone dry. 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 two inches per hour, the sands will take in quite a bit of water whereas the loam and clay will not. Therefore, much of the time heavy, fast rain events are not effective at re-wetting the soil profile. Much of this water runs off rather than being absorbed into the soil. 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 what depth one 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 soil (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, allowing the water to moisten the soil. The only problem is that in most cases, the soil has only been moistened one-half inch to one inch deep.

Since many areas have not received an inch of water in a long time, the soil profile has continued to dry. For those of you 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, to effectively re-wet your soil, you will need to receive 22 inches of rain where none of the water runs off. Some of you have three to four feet of soil. The effective root zone of most plants is three feet. So if you have three feet of sandy soil, in order to wet the soil to a depth of three feet, you will need three inches of rain where no water runs off.

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 one-half inch irrigation is easier to wet than one which has not. A one-half inch watering will wet a sandy soil to a depth of about six inches. This means that only another one-half 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 one-quarter inch to one-half inch of 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 two inch rain, but two inches will no longer do it.” Rather we need a steady four 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.

Perhaps you have noticed how the grass or weeds right beside paved roads or the overhang of buildings green 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.

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. Therefore, trees planted beside structures are able to survive better in drought times.
It should be clear that the type and depth of soil, along with the amount and speed of rainfall events, 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, they do 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 foot) 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 needed to wet 12 inches in a dry soil 1.0 1.5 – 2.0 2.5

Remember, Learn and Have Fun!

David Rodriguez is the County Extension Agent-Horticulture for Bexar County. For more information, call the Master Gardener ‘Hotline’ at (210) 467-6575 or visit our County Extension website at: https://bexar-tx.tamu.edu and click on Horticulture and Gardening.

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