137Cs budget during the period of 1960s in a small drainage basin on the Loess Plateau of China

A sediment profile with a thickness of 28.12 m in a failed reservoir in a small catchment of the Yuntaishan Gully in the Loess Plateau of China consisted of 44 flood couplets deposited during the period from 1960 to 1970 with total volume of 2.36 × 106 m3. Specific sediment yields for a flood event varied from 300 t km−2 to 14,400 t km−2 and annual sediment yields varied from 2500 t km−2 in 1966 to 40,000 t km−2 in 1964 with a mean value of 12,700 t km−2 a−1 for the period. Average annual 137Cs concentrations of the sediments increased from 0.92 Bq kg−1 in 1960 to 4.82 Bq kg−1 in 1963, then decreased to 1.53 Bq kg−1 in 1970. The total 137Cs activity in the reservoir sediments was 9.22 × 109 Bq, which accounted for 31.9% of the total 137Cs fallout precipitation of 2.89 × 1010 Bq within the catchment during the period. The proportion of the 137Cs loss from the catchment to the 137Cs fallout precipitation within the catchment in a year varied between 8.01% and 66.8%, and it was 20.9% for the peak 137Cs deposition year of 1963 and 52.0% in 1964. By analysis of the 137Cs budget in the catchment for the 137Cs peak precipitation period from 1962 to 1964, the 137Cs surface enrichment coefficient Γ should be much less than 0.23. And for calculation of soil losses on the cultivated land in the inter-gully area by using the Mass Balance Model II, the value of Γ should be 0.05–0.1.