A Simple Moisture Index Based Upon a Primary Law of Evaporation

A moisture index is a device which allows better comparison of natural landscapes than can be gained from the distribution of simple rainfall amounts. Such an index is then a more refined expression of the moisture factor in climate than "straight" rainfall, an advantage claimed for the simple indices proposed by De Martonne and others, and demonstrated clearly by K6ppen in the various editions of his classification of climate. In recent years C. W. Thornthwaite has been chiefly responsible for sustaining interest along the same lines. By his method of computing an index of "effective precipitation," and later a moisture index from a "water balance", Thornthwaite has provided terminology and certain concepts which have entered widely into hydrologic literature as well as geographic writing.' Thornthwaite and all his predecessors have had to face a common problem in formulating a moisture index: how should the effects of evaporation be simulated? The entire reason for the greater meaning claimed for moisture indices, compared to rain gage data, rests upon a term that recognizes water loss as well as the water gain measured by the rain gage. It is clear, of course, that the catch of a rain gage artificially arrests the hydrologic cycle. Water after striking natural surfaces undergoes complex movements and transformations unrepresented in the isolated enclosure of a rain gage, changes which have the net effect of reducing the amount of rainfall to a lesser effective amount. Evaporation from moist surfaces is an important agent in that reduction, and so becomes identified with all moisture indices, implicitly or explicitly. In Nature, evaporation is not a process which can be accounted for by climatic factors alone. The case of free water surfaces is nearly ideal for climatic analysis, but terrestrial surfaces offer varying proportions of rainfall for evaporation, proportions that change according to the nature of the aerial surfaces that intercept precipitation, and according to the freedom with which water moves through the soil to depths beyond the reach of local evaporation. If evaporation were known from measurements, those complications would be integrated painlessly in the results, but the necessary measurements are not