“A good soaking rain”

by Dr. Tim Ball on October 18, 2012

in Atmosphere,Biology,Data,Forests,Land,Statistics,Theory,Water,Weather

I wrote the following article as a regular monthly column in the Canadian magazine called The Landowner. It is designed to provide general understanding of basic natural events; help people know how the world works; make better decisions in living and working on the land; and reduce potential for exploitation by environmentalists. The magazine is for rural people, but it is likely more valuable information for urbanites, who are increasingly detached from the land.

My Father loved his garden and spent much time there. Mother said it was because, with five children, it was quiet out there. Regardless, he beamed when there was a “good soaking rain.” He aways practiced moisture conservation and reduced watering to a minimum using procedures he learned during wartime restrictions in England. As I recall from working with him he developed a great sense of how much water to apply.

The amount of water people apply to their gardens, especially the lawn, varies because of factors most people don’t even think about. The major difference between rural and urban is because of the supply; those on municipal supply generally use more, but that also varies with pricing structure. If the water is priced with a flat rate they use too much. If metered they use too little.

There are limits to the amount of water you can use with a municipal supply. Only so much water can flow through as determined by the pressure and diameter of the pipe. So even if you leave it running all day there is a finite amount, although it is a surprising how much. This idea of size and pressure, like so much else in our water systems as words like aqueduct, spigot and riparian, are legacies of the Roman systems. A Roman paid for water by the size of the hole and its location on the side of the aqueduct. It cost more for a large hole at the bottom where pressure was highest. Other factors that determine the amount are whether you are using municipal or on site supply like a well, or if you have a septic field. There is also a larger regional variation depending on whether you’re in a wet region where they usually put on too much or a dry region, where they add too little.

It is not a simple issue, as irrigators know; a great deal of research has tried to solve some basic questions. For example, how long do you go in a dry spell before watering? Leave it too long and the plants suffer serious, possibly irreparable, damage. Too much water and you can ‘drown’ the plant. You don’t need all the science, but it helps to understand the basics.

About one percent of all clouds yield precipitation. Obviously that’s an average and it varies considerably from very high in the tropics to average in most of the middle latitudes and extremely low in the high latitude polar regions. The North Pole is one of the driest places on earth.

What happens when the raindrop leaves the cloud? Some of the raindrop evaporates as it falls. When really hot all of it can evaporate before reaching the ground. It looks like streaks or stripes stretching down from the cloud so it is called Virga after the Latin word for rod or stripe.

Figure 1: Virga: Rainfall evaporating before it reaches the ground.

Before the rain reaches the ground it’s intercepted by vegetation, much of which evaporates back into the atmosphere. The amount intercepted varies, but is high in forests, ranging from 25 to 75 percent. What’s not intercepted is called “throughfall “and moisture running down the trunk is called “stemflow”. Water is also shed out by the canopy and falls around the outer perimeter of the tree, which is where the water absorbing fibrous roots are located. It takes a long time for ground under the tree to get wet. Accurate measures of rainfall, above and under the forest, are difficult to obtain.

Grasses are very effective at interception and it takes considerable soaking for moisture to penetrate to the soil. Obviously reducing thatch, the accumulation of dead and dying vegetative material, makes a difference. Like the forest, the amount reaching the soil is reduced by interception and evaporation, but what gets there soaks in gradually. Figure 2 shows a soil cross-section down to the bedrock. Generally the water moves down the column pulled by gravity.

When water arrives faster than it can be absorbed the surface layer quickly saturates and forms an impervious surface so subsequent water runs off. You can see this with a very heavy rain and it can cause the greatest amount of soil erosion called sheet erosion. Steady rain at the absorption rate made my father smile. Obviously the rate of absorption varies with porosity of the soil. On average soil can hold approximately 100 mm of water but this varies considerably with the type of soil. Generally, a soil with a balance, of 40% sand, 40% silt, and 20% clay called loam, is the ideal mixture to retain nutrients, oxygen and maximum water. Plant’s can remove this porous water through it’s roots, but there is always a film of water that the plants can’t remove. At that level it is called the Wilting Point because the plants wilt. It amused my father to know scientists called this the wilting point because it was such a subjective eye ball measure, but he did what he could to prevent it from occurring in his soil.

Plants, including trees, take most of their moisture from the topsoil. However, they have remarkable abilities to seek water. For example, during the Prairie drought years of 1988/89 University of Manitoba researchers discovered wheat put roots down 3 m (9 feet).

Figure 2: Cross-section through upper soil layer. Regolith is rock broken down by water seeping down to break up the bedrock.

Like anyone who works with nature my Father learned to read the signs. He talked with and learned from older gardeners. He anticipated soaking rains because the backs of the leaves were visible as the wind came from the East. The high cloud had a distinctive pattern he knew as a mackerel sky (Figure 3) and the cloud base gradually lowered until the steady soaking rain began and a knowing smile appeared.

Figure 3: Mackerel sky foretells rain.

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