Characteristics Of PUE And Soil Erosion During Vegetation Restoration In Northern Shaanxi And Their Relationship

The northern Shaanxi is located in Loess Plateau and adjacent to the Yellow River. The ecological problem of soil erosion is outstanding. Under the influence of human activity and global climate of warming and drying, ecological degradation in northern Shaanxi is serious. In order to improve the level of ecological environment and implement the China Western Development, the northern Shaanxi started the "Grain-for-Green Project" since 1991. Under the policy of "Grain-for-Green Project", there can provide a basis for the development of effective macro-control measures for the ecological restoration in the northern Shaanxi by quantificationally grasping the ecological restoration's characteristics of "Process, Structure, Effect and Service".For the ecological issues of dominant and the ecological purposes of "Grain-for-Green Project" in northern Shaanxi, in this paper, basing on remote sensing and mathematical model, we combined "Grain for Green" with variation of vegetation fraction, evaluated the change of vegetation restoration and estimated PUE and soil erosion in the northern Shaanxi. We applied multiple models and multiple data sources to verify the results we estimated, and ensured the accuracy and reliability of the data in this paper. According to the measures of trend analysis and spatial autocorrelation, explored the ecological restoration/degradation of PUE and soil erosion, and then used general QQ-Plot distribution Pearson correlation and Johansen co-integration to probe the relationship between PUE and soil erosion in the context of vegetation restoration. The main conclusions are as follows:(1) The calculation result of PUE basing on NPP was superior than basing on NDVI; the calculation result of the amounts of soil erosion basing on the model of RUSLE was superior. This paper further verified some parameters of model, in the northern Shaanxi, the results showed that the estimation of NPP basing on CASA is more accurate than the estimation of MODIS-NPP, the measured data and calculation results of other models; Corrected spatial interpolation of precipitation and temperature was better than Uncorrected spatial interpolation.(2) In the context of "Grain for Green", the effects of ecological restoration in the northern Shaanxi showed significantly spatial differentiation. Vegetation fraction increased significantly from 2000 to 2014, it reflected that the efficiency of vegetation restoration was well in northern Shaanxi. In the process of vegetation restoration, PUE increased significantly, and the soil erosion showed a downward trend except for some special year, it reflected that the "Grain for Green" policies had a positive effect on PUE and soil erosion. The effects of Ecological restoration showed significantly spatial differentiation, this spatial differentiation pattern had strong spatial correlation with natural or geographical factors. The spatial pattern of PUE was obviously influenced by hydrothermal pattern and presented "the north is higher than the south"; soil erosion was serious in the undulating terrain and some areas along the river.(3) During the period of vegetation restoration or consolidation, PUE and soil erosion showed significantly spatial differentiation. In the period of vegetation restoration, PUE was increasing obviously, but the area of PUE passing the M-K significant test was little, which the area of improvement is 3.5% and degradation region was 0.5%; the declining area of soil erosion was 59.1% of the total in northern Shaanxi. Then we found that area of the improved soil erosion was 7.1% of the total by M-K test and located in North Central of the northern Shaanxi. In the period of vegetation consolidation, the improved efficiency of PUE was lower and the declining area was increasing, the declining region concentrated in the South Central of the northern Shaanxi. Compared with the period of vegetation restoration, the area of improved soil erosion was increased 0.21% and degradation of soil erosion was decreasing in the period of vegetation consolidation, it reflected that the improved effect of soil erosion was increasing significantly.(4) During 2000-2014, clustering pattern of PUE and soil erosion showed a significant spatial differentiation, but clustering pattern of variety had a similar trend. The spatial concentration pattern of PUE was influenced by hydrothermal significantly. The hot zone mainly concentrated in Ziwuling and Huanglong Mountain, and the cold zone mainly located in north of the Great Wall of northern Shaanxi. The hot zone of soil erosion concentrated in rivers and the region of undulating terrain, and the cold zone located in Mu Us desert of northern Shaanxi. In process of vegetation restoration, the growth region of PUE and soil erosion mainly located in North Central of the northern Shaanxi, and the declining region mainly located in the south of northern Shaanxi.(5) At the level of vegetation restoration, the trend of PUE and soil erosion had significant differences. The maximum value of the standardized PUE was Significant improvement (0.67), then slight improvement (0.21), obvious improvement (0.19), obvious degradation (-0.14), slight degradation (-0.78) significantly degradation (-0.91). The maximum value of standardized soil erosion was significant degradation (0.97), then slight degradation (0.49), obvious improvement (-0.09), obvious degradation (-0.11), slight improvement (-0.53), significantly improvement (-0.62). It indicated that vegetation promotes precipitation use efficiency and suppresses the growth of soil erosion.(6) Vegetation fraction, PUE and soil erosion showed significant correlation. Vegetation fraction and PUE remained a growth of synchronization, the relationship between PUE and soil erosion overall showed an inversely proportional relationship, so did the relationship between vegetation fraction and soil erosion. In Ecoregions of northern Shaanxi, a long-term equilibrium relationship between vegetation fraction and PUE located in the south of study area. In most area of northern Shaanxi, the relationship between PUE and soil erosion remained long-term equilibrium, so did the relationship between vegetation fraction and soil erosion.