So What Constitutes An Effective Rain
Larry A. Stein
Professor and Extension Horticulturist
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
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.
|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 - 2.0
|Amount of water to wet to 12 in a dry soil (inches)
||1.5 - 2.0