In recent years, geographers have focused attention upon the water balance as a basis for a climate classification. The soil-moisture balance, particularly, has been favored as the foundation of a climate system because it represents availability of water for plants, as well as an assessment of the availability of surplus water to supply stream flow and ground water,
The important concept involved is that precipitation by itself does not indicate the amount of water actually available to plants. Evapotranspiration must be subtracted from precipitation to reveal the net quantity of water as a surplus or a deficit.
Climate classification based upon the soil moisture balance uses two variables:
(1) precipitation, and
(2) evapotranspiration. Temperature is by no means disregarded in this classification, for temperature enters into the calculation of evapotranspiration. Insolation is also used in calculating evapotranspiration.
It is interesting to note that both precipitation and evapotranspiration are measured in the same quantities, namely, water depth or its equivalent per month or per year.
Warren Thornthwaite, the climatologist who developed the concepts of the soil-moisture balance also devised a climate classification system using precipitation and evapotranspiration. The Thornthwaite system uses an elaborate code to designate climate types.
Vegetation and soils as bases of climate classifications
With each plant species is associated a particular combination of climatic factors most favorable to its growth, as well as certain extremes of heal, cold, or drought beyond which it cannot survive. Plants tend to adapt their physical forms to meet the stresses of climate, hence we find that there is a wide range of forms taken by assemblages of the dominant plant species, and that the overall form patterns or habits closely reflect climate.
There is much to recommend a climate classification based on the different forms taken by groups of plants. On the other hand, such plant forms represent responses to climate, rather that cause of global climate variation. A fundamental principle of scientific classification is that the setting up of classes of things is better done according to the causes of the class differences than according to the effects that differences produce.
Since the late nineteenth century, soil scientists have reorganized the several fundamental classes of mature soils, seeing on a world-wide scope, are more strongly controlled by climate elements than by any other single factor. But plants also contribute to determining the properties of the very soils upon which they depend. Thus, as in the case of vegetation, soils reflect differences in climate but do not cause those differences.
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