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Rainfall Characteristics of Missouri in Relation to Runoff and Erosion
Author(s) -
Baver L. D.
Publication year - 1938
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1938.036159950002000c0084x
Subject(s) - relation (database) , citation , surface runoff , erosion , library science , hydrology (agriculture) , history , computer science , geology , database , geotechnical engineering , ecology , geomorphology , biology
The rainfall factor is one of the most important variables in the complex runoff and erosion problems. Under constant conditions of slope, soil, and vegetation, runoff and erosion would be expected to vary with differences in rainfall. It is usually taken for granted that the amount and intensity of the precipitation are the most important characteristics of rainfall which should be considered in runoff and erosion studies. There are, however, two other significant phases of rainfall which play important parts in the amount of runoff and erosion that occurs from a given rain. These are the moisture condition of the soil at the time the storm takes place and the distribution of storms in relation to the amount of protection afforded by crops. Horton (l) recognized the importance of the moisture content of the soil when he divided storms into two classes, A and B, upon the basis of the time of their occurrence in relation to previous rains. Runoff and soil losses on the erosion plots at the University of Missouri have always given perplexing results when analyzed solely from the point of view of the amount and intensity of rainfall. For example, there was always more runoff and erosion in April-than in May, although May had more rainfall and more intense storms than April. Moreover, more runoff and erosion occurred in September than in August, although the rains falling in August had a greater intensity. Since an analysis of the intensity and amount of precipitation did not permit a complete picture of the significance of rainfall characteristics in these experiments, an attempt was made to evaluate the importance of the effective moisture content of the soil during precipitation. Inasmuch as no continuous record of soil moisture was kept, it was impossible to directly calculate the effective moisture in the profile. In light of this fact, it was necessary to indirectly estimate the relative effective moisture content of the soil for the various months by using precipitation and temperature data and calculating the precipitation-evaporation ratio. The method of Thornthwaite (4) was first used because he had shown the value of using monthly data for interpreting the rainfall characteristics of a region. (The reader is referred to his paper for a discussion of climates from the point of view of precipitation and evaporation.) It was found that the calculated data agreed with the measured evaporation in the warmer months but deviated considerably from the actual during the cooler months. Since April was one of the key months in the problem, it was necessary to reexamine the meteorological data of precipitation, evaporation, and temperature. In the 'first place, the assumption was made that for a given temperature evaporation would vary as some function of the precipitation. With this assumption in mind, data from twentynine different meteorological stations and comprising 243 monthly records were analyzed. These stations were chosen to represent (l) warm, humid, and maritime climates (Porto Rico); (2) cool, humid climates (Ohio, New York, Maine, etc.); (3) semi-arid conditions (Texas, Kansas, Nebraska, etc.) and desert conditions (Nevada, Utah, New Mexico, etc.). Five degree Fahrenheit temperature intervals were chosen, beginning at the freezing point and ending at 92° (3S°-36.9°, 37°-41.9°,———, 87°-91.9°). The precipitation-evaporation ratios which were calculated from actual weather station measurements were plotted as a function of precipitatidn. Some very interesting correlations were found. The curves for two of these temperature intervals are shown in Figure 1 in order to illustrate the type of relationship which was observed. It is seen that P/E varies almost linearly with precipitation for rainfall amounts of one to ten inches per month. For smaller or larger rains this approximately linear relationship does not exist. In considering this approximate relationship, which will include most of the rains in the humid region, it was found that the slope of the straight portion of the curve was a function of temperature. These correlations can be expressed in simple mathematical terms as follows: