USE OF RESERVOIR DEPOSITS AND CAESIUM-137 MEASUREMENTS TO INVESTIGATE THE EROSIONAL RESPONSE OF A SMALL DRAINAGE BASIN IN THE ROLLING LOESS PLATEAU REGION OF CHINA

Abstract

This paper reports the results of an investigation of the erosional response of the 3.86 km(2) Zhaojia Gully catchment in the rolling loess plateau region of Zichan County, Shaanxi Province, China . In the absence of direct measurements , information derived from reservoir deposits and from caesium - 137 measurements on both the catchment soils and reservoir deposits was used to obtain a retrospective assessment of the longer-term (ca. 30 year) erosional response of the catchment and of the relative contributions of the rolling plateau surface and the gully areas to the sediment output from the basin . Net erosion rates on cultivated land occupying the gentle crest slopes and steeper lower slopes of the rolling plateau and the steep gully slopes were estimated to be 4500 t km(-2) year(-1), 8584 t km(-2) year(-1) and 15851 t km(-2) year(-1) respectively. Estimates of annual sediment yield from the study catchment based on analysis of sediment deposits year in the two sediment-trap reservoirs ranged between 4627 and 32472 t km(-2) year(-1). Almost all the sediment transported from the catchment was contributed by 2-4 large floods each year. Measurements of the caesium - 137 content of recent sediment deposits in a sediment-trap reservoir allowed the relative contributions of the total sediment yield derived from the rolling plateau and gully areas of the catchment to be estimated at 23 and 77 per cent, respectively. Analysis of the sediment deposits dating from 1973-1977 in another sediment trap reservoir allowed individual flood event couplets to be identified and indicated that the sediment associated with the first one or two floods in a season, when the soils of the plateau area were relatively dry, was derived primarily from the gully areas. The cultivated soils of the rolling plateau contributed an increased proportion of the total sediment yield during the latter stages of the flood season when the soils were wetter, and surface runoff and erosion were more widespread. Based on analysis of the caesium - 137 content of the sediment deposited in this sediment-trap reservoir , the relative contributions of sediment from the rolling plateau and gully areas over the period 1973-1977 were estimated to be 21 and 79 per cent, respectively. The results obtained demonstrate the potential for using caesium - 137 measurements and analysis of reservoir deposits to document the erosional response of a drainage basin . (C) 1997 by John Wiley & Sons, Ltd.