西苕溪流域土地利用对氮素输出影响研究

Effects of Land Use on Nitrogen Export in Xitiaoxi Watershed

以位于太湖地区的西苕溪流域为研究区,利用Arcview3.2的水文模拟和空间分析扩展模块进行子流域划分和典型小流域选择,通过对2000年TM/ETM遥感影像解译获得流域土地利用数据.在2004年分3次(7月、9月、12月)监测了各个小流域出口的总氮浓度,用以表征流域氮素输出强度.基于以上数据,分析了小流域氮素输出的时空变化差异以及土地利用结构对氮素输出影响.结果表明:各个小流域氮素输出差异很大,空间上有从上游到下游逐渐增大的趋势,时间变化上,西苕溪流域中下游的小流域氮素输出7月>9月>12月,上游地区的小流域则与之相反,这与流域的降雨径流分布和土地利用方式差异有关.多个小流域的对比分析表明,流域土地利用结构对氮素输出的影响更大.流域内林地面积比例越大,氮输出强度越小;相反,与人类活动有关的土地利用类型如耕地和居民地比例增大将带来氮素输出增大的后果,而氮素输出的强度似乎与流域面积大小无关.

The hydrological modeling and spatial analysis extensions of Arcview3.2 was used to delineate and select 11 representative sub-watersheds of Xitiaoxi River, a branch of Taihu Lake in China. Land use data derived from TM/ETM satellite images in 2000. Total nitrogen （TN） concentrations were measured 3 times in Jul. , Sep. and Dec. of 2004 within these sub-watersheds, to analyses temporal-spatial characteristics of nitrogen concentrations and the effects of land use on nitrogen export, Results show TN concentration increase from upriver to downriver sub-watersheds and generally rising from Jul. to Sep. to Dec., except upriver sub- watersheds have a contrary trend. It is due to TN concentrations vary greatly depending upon seasonal precipitation conditions and land use type and management across a watershed. Land use composition is a principal factor in controlling the amount of nitrogen exported from a watershed. Nitrogen export decrease with a higher percentage of forest land area, and the percentage of humaninduced land use type such as arable land and residential area enlarge can increase the nitrogen export, whereas no relation was found between the size of sub-watersheds and TN concentrations.