Sediment source identification by using ^(137)_Cs and ^(210)Pb radionuclides in a small catchment of the Hilly Sichuan Basin, China

Double radionuclide tracers of 137Cs and 210Pb were used to investigate sediment sources in the Wujia Gully, a small catchment in the Hilly Sichuam Basin in 2002. Aver-age 137Cs and 210Pb concentrations in the source soils of the steep forest slopes, gentle cultivated terraces and bare slopes were 7.15±0.40 and 162.01±3.86 Bqkg-1, 4.01±0.31 and 70.96±2.65 Bq·kg-1, and 0 and 15.12±1.22 Bq·kg-1, re-spectively, while those concentrations in the recently depos-ited sediments in the reservoir were 3.06±0.23 and 72.66±1.61 Bq·kg-1, respectively. By using the mixing model, the relative sediment contributions from steep forest slopes, gen-tle cultivated terraces and bare slopes (including channel banks) were estimated to be 18%, 46% and 36%, respec-tively. Cultivated terraces and bare slopes (including channel banks) were the first and the second important sediment sources in catchment. Specific sediment yield in the catch-ment was 642 t·km-2·a-1 from the deposited sediment volume in the reservoir since 1956. Soil erosion rates for the forest slopes and cultivated terraces, which accounted for 2/3 and 1/3 of the drainage area in the catchment, were esti-mated to be 173 and 886 t·km-2·a-1, respectively.

210Pb(ex) depth distribution in soil and calibration models for assessment of soil erosion rates from 210Pb(ex) measurements

Three soil cores collected from China and UK show the 210Pbex and 137Cs depth distribution on uncultivated and cultivated land. Depth distributions of the two nuclides are very similar. Those nuclide concentrations are higher in the top horizon and decrease exponentially with depth on uncultivated land while the concentrations are almost uni-form throughout the plough layer as a result of mixing asso-ciated with cultivation on cultivated land. 210Pbex reference inventory in the Loess Plateau is 573 mBq·cm-2, a little higher than 520.0 mBq·cm-2 in UK. Distribution of artificial fallout 137Cs, produced during the nuclear testing period of the late 1950s and 1960s, is in an unsteady state. In contrast, distribution of natural fallout 210Pbex is in a steady state un-der constant conditions of land environment and erosion processes for a long term, e.g. >100 years. Therefore, mass balances of 137Cs and 210Pbex in soil are different. According to its processes of continues deposition from the atmosphere, nature decay and losses with soil losses, 210Pbex steady state mass balance models for cultivated land as well as for uncul-tivated land are developed to estimate soil erosion rates in this paper. Besides, the proportion of freshly deposited 210Pbex fallout removed by erosion on cultivated land can also be calculated from 210Pbex depth distribution at a reference site of uneroded permanent grass land.

A study on responses of soil erosion and sediment yield to closing cultivation on sloping land in a small catchment using (137)Cs technique in the Rolling Loess Plateau, China

Abstract By comparison of volumes and 137Cs contents of the deposited sediments before and after 1993, changes of specific sediment yields and relative sediment contributions from the gully area and from the inter gully area after closing cultivation on the later area on a small catchment of Zhaojia Gully, in the Rolling Loess Plateau, are analyzed in this paper. Closing cultivation in a large scale has not resulted in decrease but increase of specific sediment yield of the catchment, in sharp decrease of the sediment yield and the relative sediment contribution of the inter-gully area, and in increase of the sediment yield and the contribution of the gully area, for a short term. The mean specific sediment yield of 29650 t km-2 a-1 of 1994—1996 in the catchment was 2.2 times the average value of 13413 t km-2 a-1 for a long term. The specific sediment yield of the inter-gully area decreased from the 14335 t km-2 a-1 in 1994 to 7034 t km-2 a-1 in 1995 and 3517 t km-2 a-1 in 1996, while the yield of the gully area varied between 44944 and 62136 t km-2 a-1 during the period of 1994—1996 which was much greater than the value of 21118 t km-2 a-1 before 1993. The relative sediment contribution from the inter-gully area decreased from 23% in 1994 to 15% in 1995 and 6% in 1996, while the contribution from the gully area increased from 77% in 1994 to 85% in 1995 and 94% in 1996. It is suggested that compacting of ploughed soils resulted in increasing of the erosion resistance but in decreasing of the precipitation infiltration, therefore, the soil erosion reduced but the runoff amount increased on the inter-gully area. Increase of delivering runoff from the inter-gully area to the gully area should result in activeness of gully erosion and mass movements, consequently, in increase of the total sediment yield from the catchment.