Moisture

Conserving Soil Moisture

Chester E. Evans and Edgar R. Lemon.

Conserving the soil moisture is mostly a matter of preventing drought. But drought, like so many other natural phenomena, is a subtle process, a stubborn problem for those who seek to understand it, an occurrence that baffles exact definition.

Farmers and agronomists, to be sure, have devised methods of combating it. Even the obvious solution of adding irrigation water is empirical, for it is based more on observation and experience than on science, and the theories differ as to when and how much water should be used to prevent drought.

We all associate a deficiency of rainfall with poor crops. Some men have defined drought in terms of the number of days without rain. But 10 days without rain (or irrigation) in the humid East is not the same as 10 days without rain in the Great Plains.

Carrying the definition of drought one step farther, some have emphasized the available water in the soil and used it as a yardstick. Yet here again the amount of available water in the soil for one crop at a given location does not have the same effect as an equal amount of available water upon the same crop or a different one in another place.

We really need to ask the crop how it is feeling! Is it taking up water fast enough from the soil by its roots to satisfy the demands for water in the leaves, from which it passes into the atmosphere?

If the demand for water in the leaves is greater than the supply from the roots, then basically we have drought. The extent of the lag between supply and demand within the plant is the true yardstick of drought. It is this lag that determines the plant's efficiency in manufacturing food. Plants differ in their sensitivity to drought, but still, for any given crop, drought is basically the lag between the water uptake from the soil and evaporation of water from the leaves. We can conclude that too little soil moisture invariably prevents plants from realizing full efficiency in their growth.

The subject of conserving soil moisture must necessarily concern itself therefore with the accumulation and saving of soil moisture for efficient crop production. And to understand soil moisture conservation as a means of minimizing drought, one needs to look into the distribution and transport of water the hydrologic cycle.

A portion of the total precipitation that reaches the ground is returned to the oceans as runoff. The remainder is returned to the atmosphere by evaporation, either directly from the earth's surface or through plant transpiration.

The soil acts as a reservoir; at any given time some water is in storage in the soil. Considerable time may elapse before this water in storage flows underground to the streams or is returned to the atmosphere by evaporation. Eventually, however, the water in storage in the soil enters the hydrologic cycle even though the amount of water in storage increases with precipitation and decreases with evaporation and transpiration. Thus conservation Of soil moisture deals with the balance among precipitation, runoff, and evaporation.

IN A CASUAL OBSERVATION of the average annual rainfall distribution over the United States, one notices a gradual decrease from east to west until the Rocky Mountains are reached. Westward, a complex of humid mountain and and intermountain patterns extends almost to the west coast. A narrow region of high rainfall is encountered along the west coast.

Nearly all of the rainfall east of the Rockies is caused by the interaction between the great cold airmasses arising in the vast Canadian Arctic and the warm airmasses arising over the Gulf of Mexico. When the Arctic air, moving southeastwardly, meets the maritime Gulf air, the cold dry air forces the warm moist air to rise and yield some of its moisture as precipitation.

The Gulf air characteristically takes a semicircular path across the Gulf up through the Mississippi Valley and out through the St. Lawrence Valley, covering the eastern United States but falling short of the Great Plains.

The dry tropical airmasses that normally flow northward across the Great Plains arise in Mexico and yield little water when they meet the Arctic air. Occasionally, however, an especially strong Gulf mass veers westward into the Great Plains, bringing brief, violent rainstorms.

These incursions are rare and erratic. Their frequency nonetheless seems to be associated with long-term variations in solar activity. People living in the Great Plains know about the higher rainfall periods of the early 1900's, the 1920's, and the 1940's and the droughts in the 1930's and 1950's. Meteorologists have observed that longtime, large-scale migrations of rainfall into the Great Plains occur during periods of high solar radiation and that migrations to the East occur when radiation is low.

Precipitation west of the Rocky Mountains usually is the result of moist Pacific air masses rising and cooling over mountain ranges. Between the mountain ranges, however, precipitation generally is low, and irrigation is necessary to produce crops.

Average rainfall figures, we know, are deceptive. Year-to-year and month-to-month variations from the normal are the rule, not the exception. Nevertheless, normal figures have to be used as a yardstick to tell us in general what type of climate a region has. We need also to know the variations that are included in the normal.

Probably the relative variations from normal rainfall determine to a large degree the importance of soil moisture conservation. In the arid West, for instance, the farmer knows what to expect and plans accordingly by developing irrigation. In the humid East, variation in rainfall is relatively small.

Here, too, the farmer can plan with fair assurance and can even use irrigation. Farmers of the subhumid and the semiarid regions, however, must expect wide departures from the "normal" rainfall. It is in this region of erratic rainfall that conservation of soil moisture often means the difference between a crop and a crop failure.

1. The average annual precipitation over the United States. The numbers denote inches.

OF THE RAINFALL that reaches the ground, the amount that enters the soil is important. We are still limited in our ability to modify rainfall. We still have to take it as it comes. Infiltration and runoff are highly responsive to management.

For runoff to occur, rainfall must exceed in intensity and volume what the soil, vegetation, and land surface will absorb and retain. The slope, the nature of the soil, its depth and condition, the nature of the cover, the nature of the storm, and the season in which it occurs determine the amount of water taken into the soil.

The soil characteristics that play a major role in infiltration are the size of pores, the total volume of space unoccupied by water, and the stability of the pores during a storm. The size of the pores largely depends on the size and arrangement of the particles that make up the soil. The size and arrangement of the soil particles we call structure. Soils with large pores and good structure have a rapid infiltration rate if they are not filled with water.

A soil that during the early stages of a storm has large pores and a favorable structure for rapid infiltration may lose its favorable structure during the course of the storm by the melting or slaking down of the particles. Closing the pores during this process obviously does not favor infiltration. One has to take into account the prior history of tillage and cropping systems in order to evaluate the infiltration response in terms of stability of soil structure. The cover that is on the land also has a marked influence on the deterioration of soil structure and infiltration.

The characteristics of the individual storm and the season in which it occurs influence infiltration and runoff. Certain combinations of intensity and amounts of rainfall are important in this respect. The size of the raindrops and their impact upon the ground affect infiltration and runoff. The form of precipitation, either as rain, snow, or hail, is important, too.