REPORTS 2094 Chinese Science Bulletin Vol. 48 No. 19 October 2003 Chinese Science Bulletin 2003 Vol. 48 No. 19 2094 2101 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 FENG Mingyi 1,4 , D.E. Walling 3 , ZHANG Xinbao 1,2 & WEN Anbang 1 1. Chengdu Institute of Mountain Hazards and Environment, Chinese Academy of Sciences (CAS), Chengdu 610041, China; 2. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, CAS, Xi’an 710075, China; 3. Department of Geography and Archaeology, Exeter University, Exeter EX4 4RJ, UK; 4. Sichuan Teachers’ College, Nanchong 637002, China Correspondence should be addressed to Feng Mingyi (mingyifeng@ sina.com) Abstract By comparison of volumes and 137 Cs 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 clos- ing 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 re- sulted 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 de- creased 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 contribu- tion 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 in- crease of the total sediment yield from the catchment. Keywords: Loess Plateau closing cultivation, gully ara, inter-gully area, relative sediment contribution, 137 Cs. DOI: 10.1360/02wd0589 The Rolling Loess Plateau in northern Shaanxi and western Shanxi is one of the severest eroded regions in the Loess Plateau, China, where specific sediment yields are mostly greater than 1.0 10 4 t km 2 a 1 . The rolling plateau can be divided into two geomorphologic units: the gully area (gully itself) and the inter-gully area (rolling plateau surface). Sheet and rill erosion are widespread on the inter-gully area, where land has been extensively cul- tivated, while gully erosion and mass movements are predominant in the gully areas. Relative sediment contri- butions from the inter-gully area and from the gully area are about 30% and 70%, respectively [1,2] . Since the “clos- ing cultivation for rehabilitation” policy was announced in the 1990s, cultivation has been closed on sloping culti- vated land for revegetation on a large scale over the Loess Plateau. Reliable assessment of the responses of soil ero- sion and sediment yield to closing cultivation on sloping land is significant for understanding geomorphologic evolution processes as well as for predicting sediment yield changes after the closing cultivation campaign in the rolling plateau. 137 Cs is an artificial radionuclide with a half-life of 30.17 years which released into the environment as a re- sult of atmospheric testing of thermo-nuclear weapons primarily during the period of 1950s 1970s. 137 Cs fallout is strongly and rapidly adsorbed by fine particles in the surface horizons of the soil, when it falls down on the ground mostly with precipitation. Subsequent redistribu- tion of the nuclide is associated with movements of the soil or sediment particles, because it resists downward leaching and plant absorption [3] . The 137 Cs nuclide tracing technique has been successfully used for soil erosion and sedimentation studies since the 1970s, for example, as- sessment of soil erosion or sediment deposition rates and investigation of sediment sources [4 12] . Relative sediment contributions from different sources in a catchment can be estimated by comparing 137 Cs contents between delivering sediments and source soils. Landuse changes in a catch- ment often result in changes of relative sediment contri- butions from different sources, which can be assessed from 137 Cs content changes of the delivering sediments. Therefore, responses of soil erosion and sediment yield to landuse changes in a catchment can be evaluated from the 137 Cs content changes of the delivering sediments (or the deposited sediments in the reservoir) in a catchment. Our previous study in 1993 used the 137 Cs technique to assess erosion rates of sloping cultivated land and rela- tive sediment contributions from the inter-gully area and from the gully area in the Zhaojia Gully catchment [13] . Cultivation has been gradually closed on most of culti- vated land in the catchment since the 1990s. To study the responses of soil erosion and sediment yield to closing cultivation on sloping land, we revisited the catchment to collect the deposited sediments from 1993 to 1996 for 137 Cs measurements in 2001. By comparison of the depos-