Yield of Sediment in Relation to Mean Annual Precipitation

Effective mean annual precipitation is related to sediment yield from drainage basins throughout the climatic regions of the United States. Sediment yield is a maximum at about 10 to 14 inches of precipitation, decreasing sharply on both sides of this maximum in one case owing to a deficiency of runoff and in the other to increased density of vegetation. Data are presented illustrating the increase in bulk density of vegetation with increased annual precipitation and the relation of relative erosion to vegetative density. It is suggested that the effect of a climatic change on sediment yield depends not only upon direction of climate change, but also on the climate before the change. Sediment concentra­ tion in runoff is shown to increase with decreased annual precipitation, suggesting further that a decrease in precipitation will cause stream channel aggradation. Introduction—The yield of sediment from a drainage basin is a complex process responding to all the variations that exist in precipitation, soils, vegetation, runoff, and land use. This study is aimed only toward a discernment of the gross variations in sediment yield that are associated with climate as defined by the annual precipita­ tion. Such a study may contribute to an under­ standing of the effects of climatic change on erosion and of the regional variations in sediment yield. Data on sediment yields are now available in sufficient number for this kind of study, al­ though still quite deficient in geographic coverage. Two major sources of sediment data exist. Records collected at about 170 gaging stations of the U. S. Geological Survey, where sediment transported by streams is measured, is one source of data; whereas, the other source of data is provided by the surveys of sediment trapped by reservoirs. Both kinds of data are used in this study. Precipitation data—Precipitation is used as the dominant climatic factor in the study of sediment yield, because it affects vegetation and runoff. However, the effectiveness of a given amount of annual precipitation is not everywhere the same. Variations in temperature, rainfall intensity, number of storms, and seasonal and areal dis­ tribution of precipitation can also affect the yield of sediment. For example, Leopold [1951] in an analysis of rainfall variation in New Mexico, found that despite the absence of any trend in annual rainfall, changes in the number of storms produced a significant influence upon erosion. Although analyses of these effects are beyond the scope of this study, the effect of temperature, which controls the loss of water by evapotranspira­ tion, can be readily taken into account. As is well known, the greater the temperature, the greater are the evapotranspiration demands upon soil moisture; hence, less moisture remains for runoff. More precipitation is required for a given amount of runoff in a warm climate than is a cool climate. Therefore, instead of using actual figures of annual precipitation, it is preferable to use figures of precipitation adjusted for the effect of annual temperature. However, in liei of carrying out these extended computations, it appears possible to use the data on annual runof which already reflect the influence of temperature. Annual runoff data are available for all the gaging station records and for most of the reservoir records. Because of the well-established relation­ ships between annual precipitation and runoff, it is readily possible to estimate precipitatiot from the runoff figures. We shall define effective precipitation as the amount of precipitation required to produce the known amount of runoff. Figure 1 shows a re­ lationship between precipitation and runol based on data given in Geological Survey Circuk 52 [Langbein, 1949]. This graph has been used to convert known values of annual runoff U effective precipitation, based on a reference temperature of 50°F. In a warm climate, will temperature greater than 50°, the precipitata so estimated would be less than the actual amount of precipitation; in a cool climate, the effects precipitation so estimated would be more to the actual amount. This is the desired relation­ ship. Sediment-station data—In recent years a num­ ber of records of sediment yield, as measured it sediment-gaging stations, have become available Annual loads were computed for about 100 sta­ tions giving preference to the smaller drainage areas in any region. All parts of the country, SEDIMENT YIELD AND ANNUAL PRECIPITATION 1077 41 1 1 Mil 1 1 1 1 II 1 -