| Vegetation is the backbone that connects the atmosphere,the hydrosphere,and the biosphere.It plays an important role in the heat and moisture exchange of the ground-gas system.At the same time,it plays an irreplaceable role in maintaining regional ecological balance and affecting global ecological environment changes.Therefore,it is of great practical significance to study the dynamic changes of regional vegetation in depth,predict its future development trend,and comprehensively evaluate its ecological security status.Based on the MODIS NDVI remote sensing data,meteorological data,DEM data and land use type data,the dynamic changes of NDVI in the Weihe Watershed from 2000 to 2017 were studied by trend analysis.The correlation between NDVI and environmental factors was analyzed by correlation analysis.Then the CA_Markov model was used to predict the spatial distribution pattern of NDVI in the Weihe Watershed in 2020 and 2025.Finally,the landscape ecological security evaluation model was used to evaluate the vegetation ecological security.The dynamic changes were analyzed.The main conclusions are as follows:(1)From 2000 to 2017,the NDVI of the Weihe Watershed showed an increasing trend,but there was a certain spatial difference.The time and space of temperature and precipitation in the same period were significantly different.In time,the annual average NDVI increase rate is about 0.051/10 a.Among them,the increase of NDVI in summer,autumn and growing season is the most significant,indicating that the vegetation coverage has improved.In space,the area with a significant increase in NDVI accounted for 80.61%,of which the area above the Zhangjia Mountain in the Jinghe River and the area above the head of the Beiluo River were the most significant.During the 18 years,the interannual changes of temperature and precipitation in the Weihe Watershed fluctuated greatly.In space,about 78.3% of the region's temperature is increasing,and 88.6% of the region's precipitation is increasing.The temperature in the western part of the watershed is increasing and the precipitation is stronger than the east.(2)There is a certain difference in the correlation between NDVI and environmental factors.In terms of climate,the correlation between NDVI and temperature in the Weihe Watershed is higher than that in precipitation,indicating that the dynamic change of vegetation is more susceptible to temperature;the NDVI in the northwest of the watershed is significantly positively correlated with precipitation,mainly due to the relative climate of the area.In the drought,the vegetation growth is restricted by water.The NDVI in the southwest of the watershed is significantly positively correlated with the temperature,indicating that the temperature rise in this area provides more heat for vegetation growth,thus promoting the growth of vegetation.In terms of terrain,NDVI shows a certain regularity with changes in altitude,slope and aspect.The mean NDVI showed a decrease-increasing-decreasing fluctuation characteristic with the increase of altitude.NDVI mainly increased with the increase of slope,which was mainly caused by the change of vegetation cover type with altitude and slope.The change of mean NDVI with slope direction is mainly due to the fact that the mean value of NDVI on the west slope is greater than that on the east slope,and the mean value of NDVI on the north slope is larger than that on the south slope,which is mainly caused by the difference of hydrothermal conditions in each slope direction.Human activities have a significant impact on the spatial distribution and trends of NDVI.Humans have improved the coverage of regional vegetation through agricultural cultivation,and the interannual variation trend of NDVI in the farming area is relatively stable.The engineering afforestation measures have a great positive effect on the restoration of vegetation and the increase of vegetation cover.The development of urbanization has transformed the land use types in the surrounding areas,the surface vegetation has been destroyed,and the NDVI has been decreasing.At the same time,the vegetation coverage has been improved through urban greening projects and the NDVI has increased.(3)Based on the CA_Markov model,the spatial distribution pattern of NDVI in the Weihe Watershed was predicted in 2020 and 2025.The results show that the vegetation coverage in the Weihe Watershed will be further improved.According to the prediction results,in 2020,the area of high vegetation coverage and higher vegetation coverage will increase by 6.52% and 2.08%,respectively.The area of low vegetation coverage,lower vegetation coverage and moderate vegetation coverage will be respectively reduced by 4.45%,3.92%,and 0.23%.In 2025,the area of high vegetation coverage and higher vegetation coverage will increase by 4.14% and 3.25%,respectively.The area of low vegetation coverage,lower vegetation coverage and moderate vegetation coverage decreased by 1.84%,3.86%,and 1.69%,respectively.(4)The ecological security status of vegetation in the Weihe Watershed has been continuously improved.The ecological safety assessment results of landscape scales indicate that the ecological security level of each vegetation cover landscape is generally medium and good,and the better the vegetation coverage,the higher the ecological safety level.From 2000 to 2015,the ecological safety index of each vegetation cover landscape showed an upward trend.The spatial pattern of landscape ecological security shows that the mountainous areas with high vegetation cover landscapes have the highest level of ecological security,followed by valleys and plains,and the ecological security of the Loess Plateau in the northern part of the Watershed is relatively poor.From 2000 to 2015,the landscape ecological security level was poor,the area of the poor area was reduced,and the space moved northward;the level of medium,good and safe areas increased,and the space expanded.According to the prediction results of NDVI and land use type,by 2020 and 2025,the ecological security status of vegetation will be further improved,and the ecological security level of each vegetation cover landscape will reach a good level. |
PDF Full Text Request