| The wetland in the middle reaches of Heihe River is the core wetland of Zhangye Heihe Wetland National Nature Reserve. Furthermore, it has a great value for keeping the ecological balance and maintaining the sustainable development. In recent decades, more and more wetland had been reclaimed or occupied because of the rapid expansion of Heihe Oasis, leading the wetland area decreased rapidly and the function of wetland degradation significantly. Therefore, it is very urgent to research and protect the wetland.In this thesis, the middle reaches of Heihe River had been selected as study area. At first, the spatial distributions of the wetland for 1987a,2000a and 2014a had been rebuilt respectively, using three Landsat images which covered the study area as the main research data, based on the 3S technology. And then, the spatial-temporal changes of the wetland had been discussed by analyzing the changes of wetland area, spatial distributions, landscape patterns and deterioration types, respectively. Finally, some natural factors and human activities had been analyzed in order to explore the causes of the spatial-temporal changes for the wetland. Then on this basis, some protection strategies for the wetland had been proposed.The main conclusions are as follows:(1) The decision tree model which combined use of AWEI_sh/AWEI_nsh could extract the wetland effectively in study area. Furthermore, the results of accuracy assessment for 1987a,2000a and 2014a were 87.5%,83% and 93%. And the method could be used to research the wetland in a wide range. Moreover, feature variables of the decision tree model were AWEI_sh (reference values:-0.46,-0.55 and-1.1), AWEI_nsh(reference values:-0.1,-0.15 and -0.58), NDVI (reference values:0.3 and 0.46) and DEM (reference values:slope<10°).(2) The main type of the wetland in study area was natural wetland and the one of the natural wetland was river wetland. In 2014, the area of river wetland and artificial water were 148.38km2 and 29.83km2, and the percentages of the total area for them were 56% and 12% respectively. The body wetland was mainly along the Heihe River. Except for river wetland, the most types of wetland were mainly located in low-lying places or the area where closed to the river.(3) From 1987 to 2014, the change of total area showed a trend of rapid decrease. The total area of the wetland decreased 308.22km2 compared with 1987, the area of which was 554.95km2, with the reduction by 55.54%. For each type of the wetland, the area of marshes reduced greatly, which reduced 207.11km2 by 74.56% during the study period. The patterns of wetland spatial-temporal changes could be divided into stability, repeated changes, increase and reduction. From 1987 to 2014, stability and reduction were the main form of wetland spatial-temporal changes. And increase and repeated changes were the supplement. The proportions of area for the types of the wetland spatial-temporal changes were as follows:53.92%,26.18%,7.59% and 12.31%. Moreover, the stable wetland was mainly along the Heihe River, where supplied with stable water; reduction wetlands were mainly distributed in the central and southwest of the study area, where the agricultural cultivation was very common.(4)The result of landscape pattern change analysis showed that wetland patches tended to be more and more complex or broken. The changes of PD, PAFRAC and AI showed a trend of increase, COHESION, SHDI and SHEI showed a downward trend. The values which increased or decreased were 0.12,0.07,0.02,0.59,0.0085 and 0.0059, respectively. As a whole, the degree of patch combination and landscape diversity were declining, the disturbance of human activities on the wetland landscape was increasing.(5) The main type of wetland degradation was Human Activities Leading Type, and Natural Factors leading Type and the Combined Effects leading Type were the supplement. The degraded wetland of Human Activities Leading Type was mainly along the Heihe River or at the neighborhood of Yanchi. And the one of Natural Factors leading Type was mainly in the periphery of the oasis with the main type of mash or seasonal river wetland. The wetland of Wujiang Town, Heiquan Town and Luocheng Town could be the representative of the three deteriorations respectively. And the ratios of conversion or transformation were 99.86%,30.39% and 54.44% respectively.(6) The process of wetland spatial-temporal changes was mainly affected by human activities. In recent decades, mean annual temperature and precipitation were increased, the evaporation volume were reducing, and the changes of natural factors were conducive to the growth and development of wetlands. However, the interventions of human activities were so frequently, leading that the wetland area did not increase but decrease. In a way, the rapid rise of the total population and the region’s gross domestic product had accelerated the processes of wetland degradation. The wetland policies and the awareness of public participation on wetland conservation should be further improved or strengthened. |
PDF Full Text Request