2001-2013年川西高原旱情监测及其地形分异

Drought monitoring of west Sichuan Plateau and its topographic heterogeneity during 2001 and 2013

利用2001-2013年EOS-MODIS归一化植被指数产品数据,应用距平植被指数法(AVI)对川西高原进行了干旱监测,并分析年平均干旱发生频率与海拔高度、坡度和坡向地形因子的关系,进一步结合研究区土地利用数据,分析了不同地形因子干旱频率差异的原因。研究表明:(1)受积雪覆盖的影响,川西地区干旱监测只在一定区域内适用,该适宜区为川西海拔4 300 m以下区域。(2)川西高原干旱发生频率同各地形因子都具有较强的相关性,其中,高程影响最为复杂,与不同高度带地表覆被类型的差异性有关;坡向影响非常明确,主要受稳定水汽来源输入的影响;坡度的影响具有较强的局域性,与不同坡度的保水蓄水能力及覆被类型有关。

On the base of EOS MODIS data of 16-Day ND VI production from 2001 to 2013, the Anomaly Vege- tation Index （AVI） was calculated and was used to get the drought frequency, which was the main data in the study of the drought topographic heterogeneity in West Sichuan Plateau, China. What＇ s more, the DEM and land use/land cover data were got. The former was used to draw the slope and aspect information, which were used as the index of the topography variations to analyze the drought topographic differentiation. The latter was mostly used to do some explanations. The results show as follows： （1）In western Sichuan Plateau, the A VI index method which was frequently used in drought monitoring on the support of Remote Sensing, was also suitable for the drought monitoring in these areas with the altitude below 4 300 m. While in more higher regions, it was not to be the optimum one because of the snow covering disturbances; （2） In the study area, the spatial distribution of drought showed obviously topographic heterogeneity properties, which could be seen from the good linear rela- tionships between the drought frequency and the elevation, so as to the slope and the aspect; （3）The relationship between the drought frequency and the elevation was so complicated, mostly because of the vertical zonality of the land use/land cover variations, besides, the interannual fluctuations of snow line was the other factor of great importance. In relatively lower regions with the elevation below 3 500 m, the variations of vegetation cover played the most important role in the spatial distribution of draught frequency, because different vegetation types had dif-ferent draught resistance, with the forest land in lower elevation regions taking the f＇trst, the wetland and grass- land in relatively higher regions followed. While in the regions with the elevation above 3 500 m, the draught fre- quency was dominated by the interannual fluctuations of the snow line. The elevation of 4 300 m was the average altitude that the influence of seasonal snow cover began to appear. The elevation of 4 716 m was the average ele- vation that the land cover and seasonal snow cover changed the most dramatically in the annual scale; （4）In dif- ferent slope gradient, there were good linear relationships between the slope and the draught frequency, with the linear goodness of fit reaching more than 0.9. But in different slope gradients, the linear trend between the slope and the draught varied greatly owing to the different categories of land covers in different slope gradients. The grassland showed highly sensitivity to the draught with the growing of slope, while the forest land was not the case, it showed some stable properties; （5） In different aspects, the draught frequency of the north〉northwest〉 northeast〉east〉southeast〉west〉south〉southwest. It was mainly due to the stable water vapor source from the south and west of Tibet, the different amount of precipitation in different aspects dominated by the topography, and the difference of ice-snow melt water caused by the uneven distribution of solar radiation. This study had caught some innovations. First, on the point of the drought in high altitude areas, the drought frequency method was firstly used in drought monitoring studies. And moreover, the drought topographic heterogeneity was dis- cussed in the macro scale, further, the reason for the heterogeneities were thoroughly explored.