流域土地利用变化对不同重现期洪水的影响——以奉化江皎口水库流域为例

Impacts of land-use change on flood process and frequency of various return periods: a case study of Jiaokou Reservoir watershed in Fenghua River

土地利用变化对流域洪水过程产生显著影响,并导致设计洪水发生变化。为进一步指导流域防洪及水库洪水设计,以浙东沿海奉化江皎口水库流域为例,应用HEC-HMS水文模型模拟分析土地利用/覆被变化(LUCC)对不同重现期暴雨洪水事件的影响。结果表明,流域内1985—2003年土地利用变化引起不同重现期洪水过程与洪水量级发生改变,其中,洪量和洪峰均增加,洪量较洪峰变化明显。LUCC对小洪水过程影响更明显,5年一遇以上洪水的洪峰和洪量分别平均增加3%和7.6%,而小于2年一遇洪水的洪峰和洪量分别平均增加5.41%和11.91%。同时,LUCC使100年、50年和25年一遇洪水重现期分别提前了15、6a和2a,即其对量级最高的特大洪水的重现期影响最大。此外,不同的土地利用转变方式对洪水的影响程度不一,其中,林地向裸地转变对洪水影响最大,林地向灌草地转变次之,林地向耕地变化对洪水影响最小,且这种差异性在低重现期洪水表现更明显。

The effects of land-use / land-cover change（ LUCC） on the watershed flood process have become one of the major hydrological concerns in the world. A change in the flood process leads to the alteration in flood design and control system,which has caused serious problems to human life and urban security. In this study,we take Jiaokou Reservoir watershed as a sample area to analyze the LUCC effects on storm floods of different return periods based on the hydrological model（ HECHMS） and GIS-related support. Jiaokou Reservior is located in the west of Ningbo city,Zhejiang Province,and is controlled by the plum rains and typhoon storms with various return periods during rainy seasons. Due to rapid economic development and intensive human activity,the land utilization pattern has changed in recent years. The spatial data of land use of the study area during 1985—2003 was obtained from Landsat TM（ Thematic Mapper） image data,and the images from different periods were interpreted and overlapped to setup the transfer matrix of the land-use types. The land-use types were dominated by forest land,bush grassland,and cultivated land,which covered 92% of the watershed. The main LUCC in this area is the conversion of forest to bare land,grassland,and agricultural land. The analysis of the LUCC impacts on flood shows that（ 1） during 1985—2003,3% —30% of the forest land was changed to bare land,bush grass,or agricultural land,and the LUCC has different effects on floods of different return periods. Both flood runoff and peak increased for all flood magnitudes,but the runoff changed at a higher range as compared to the flood peak.（ 2） Low magnitude flood was found to be more sensitive to the LUCC,the increase of the total runoff and flood peak of a 5-year flood increased by 3% and 7. 6%,respectively,while those of less than 2-year return period floods increased by 5. 41% and11. 91%,respectively. From the perceptive of flood classification,the flood return periods of 100,50,and 25 years were reduced by 15,6,and 2 years,respectively,during the study period.（ 3） Different land-use change patterns lead to flood variations. Along with the forest being characterized by a change ratio of 10% —50% since 1985,forest-bare land-use change pattern impacts flooding most intensively,and the forest-grassland change pattern has stronger effects than the forestagricultural change pattern. Compared with a simulated flood in 1985,when forest was converted into bare land with a ratio of 10% —50%,the flood peak increased by 0. 88% —4. 3% and the total runoff increased by 1. 61% —7. 91% in the 100-year-return-period flood. While,the widened range of the flood peak and total runoff under the same condition were2. 18% —10. 67% and 4. 15% —20. 62%,respectively,in the 5-year-return-period flood. As the change ratio gets bigger,the difference in impacts of different patterns of forest transformation to other lands on floods became more distinct,especially to the low-magnitude flood. The results of this study can provide scientific supports for the flood control and water resource management.