tailieunhanh - REGULATING THE EVAPORATION

The demonstration in the last chapter that the water which falls as rain or snow may be stored in the soil for the use of plants is of first importance in dry-farming, for it makes the farmer independent, in a large measure, of the distribution of the rainfall. The dry-farmer who goes into the summer with a soil well stored with water cares little whether summer rains come or not, for he knows that his crops will mature in spite of external drouth. In fact, as will be shown later, in many dry-farm sections where the summer rains are. | REGULATING THE EVAPORATION The demonstration in the last chapter that the water which falls as rain or snow may be stored in the soil for the use of plants is of first importance in dry-farming for it makes the farmer independent in a large measure of the distribution of the rainfall. The dry-farmer who goes into the summer with a soil well stored with water cares little whether summer rains come or not for he knows that his crops will mature in spite of external drouth. In fact as will be shown later in many dry-farm sections where the summer rains are light they are a positive detriment to the farmer who by careful farming has stored his deep soil with an abundance of water. Storing the soil with water is however only the first step in making the rains of fall winter or the preceding year available for plant growth. As soon as warm growing weather comes water-dissipating forces come into play and water is lost by evaporation. The farmer must therefore use all precautions to keep the moisture in the soil until such time as the roots of the crop may draw it into the plants to be used in plant production. That is as far as possible direct evaporation of water from the soil must be prevented. Few farmers really realize the immense possible annual evaporation in the dry-farm territory. It is always much larger than the total annual rainfall. In fact an arid region may be defined as one in which under natural conditions several times more water evaporates annually from a free water surface than falls as rain and snow. For that reason many students of aridity pay little attention to temperature relative humidity or winds and simply measure the evaporation from a free water surface in the locality in question. In order to obtain a measure of the aridity MacDougal has constructed the following table showing the annual precipitation and the annual evaporation at several well-known localities in the dry-farm territory. True the localities included in the following table are

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