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Research on the effects of grassland degradation on soil water conservation in the Sanjiangyuan region of China is essential as it can provide a scientific basis for evaluating and monitoring water conservation in this important water conservation area.Its regional strategic position is extremely important as 1.2% of the total water in the Yangtze River,40% of the total water in the Yellow River,and 15% of the total water in the Lancang River comes from this area.As a result of climate change and human activity,grassland degradation had been more severe in the Sanjiangyuan region since the 1970s.Rangeland degradation not only impacts vegetation,it can also have great effects on soil physical and chemical properties.Changes to the physical and chemical properties of soil can subsequently affect the efficiency of soil water conservation.The vegetation in this region is dominated by grassland.In grassland ecosystems 99% of water conservation capacity comes from the capacity of soil to retain water.While soil water conservation efficiency is influenced by many factors such as vegetation type,land use and land cover,studies have shown that water yield in this region has decreased year by year and that water conservation capacity has declined sharply.These findings show that further studies exploring the impacts of grassland degradation on the function of soil water conservation are of great significance.Field surveys and experimental analyses were applied to detect vegetation biomass,soil moisture and physical characteristics,and soil water conservation capacity of alpine meadows in different soil types in this area.Results showed that above-ground biomass,under-ground biomass,capillary porosity,total porosity,natural water-holding capacity,maximum water holding capacity,and soil water conservation capacity decreased significantly at severely degraded alpine meadows(P0.05).When an alpine meadow deteriorated from not degraded alpine meadow status to moderately degraded and severely degraded alpine meadows,soil bulk density increased gradually.Soil non-capillary porosity,minimum water-holding capacity and capillary water-holding capacity did not change dramatically.The ranges of soil water conservation capacity at the three degradation stages were 1884.32—1897.44t/hm2,1360.04—1707.79t/hm2 and 1082.38—1550.10t/hm2 respectively.Compared with not degraded alpine meadows with different soil types,soil water conservation capacity was reduced by 9.37%—10.35% at moderately degraded meadows,and by 18.31%—27.82% at severely degraded meadows.Reduced soil water conservation capacity was caused by diminished total porosity and capillary porosity and was also indirectly caused by a reduction of above-ground biomass and under-ground biomass associated with the degradation of the alpine meadows.The contribution of capillary porosity to water conservation should be taken into account when evaluating alpine meadow soil water conservation capacity in the Sanjiangyuan region.The results of the correlation analysis showed that soil water conservation capacity was positively correlated with above-ground biomass and under-ground biomass(P0.05).In addition to promoting the evaluation of water conservation function,our findings can be used to provide guidelines and methods for water conservation function monitoring in the Sanjiangyuan region by remote sensing.

最新发布的1981-2012年的AVHRR GIMMS NDVI3g数据为了解区域植被的近期变化状况提供了数据基础。深入理解该版本与老版本GIMMS NDVIg（1981-2006年）之间的关系，对于使用新数据时充分利用已有老版本的研究结果具有重要意义。以我国西北干旱区的典型区域——新疆为例，研究了两个数据集在反映生长季、春季、夏季和秋季植被现状，植被变化趋势及其对气候变化响应方面的异同。研究结果表明：两个数据集在描述植被活动空间分布、变化趋势及其与气候的相关性方面大体相似，但在数值、动态变化率及其对气候变化响应强度等方面存在的差异也不容忽略。NDVI3g数据生长季和各季节NDVI数值多大于NDVIg，尤其是在夏季和在植被覆盖较好的区域。区域尺度，NDVI3g所反映的植被变化趋势更为平稳，尤其是在夏季和较长的时段，这可能与像元尺度NDVI3g显著增加范围小于NDVIg，而显著减少范围多于NDVIg有关。两个数据集对气温、降水量、潜在蒸散发和湿润指数的响应具有大体一致的空间格局，但对气候因子变化的敏感性存在差异，哪一个数据集更为灵敏依赖于不同的气候因子和时段。一般规律是NDVI3g与热量因子显著正相关的区域小于NDVIg，而与水分因子显著正相关的区域则大于NDVIg。利用长期的生态数据集，尽快理清两个数据集在表征植被变化之间的异同并建立两者的转换关系，对于合理开展植被变化、碳平衡、生态系统服务功能评估等广泛利用NDVI数据的相关研究十分重要。

The accurate identification of areas of ecosystem degradation and an objective evaluation of the effects of ecological restoration projects is necessary for ecological management and planning at the landscape scale.Ecological degradation and ecological restoration are dynamic equilibrium processes,while degradation must be considered relative to the pristine state of the ecosystem,and need to be analyzed from a time series perspective.Satellite remote sensing is widely used in ecological monitoring and assessment,as it provides extensive coverage,good spatial and temporal continuity and a long time-series dataset.This paper proposes a method for the identification of areas of ecosystem degradation and ecosystem recovery using long time-series satellite remote sensing and precipitation data,which are integrated into trend analyses of the precipitation use efficiency(PUE) and a normalized difference vegetation index(NDVI).The PUE is calculated as the ratio of cumulative NDVI to precipitation.Significantly reduced PUE and NDVI values indicate that ecological degradation is occurring,while increasing PUE and NDVI values indicate ecological recovery.This method was applied to the headwater catchment of the Yellow River basin in China using Advanced Very High Resolution Radiometer NDVI data from 1982 to 2006 and meteorological data from the same time period.The annual PUE value was calculated using the cumulative NDVI and precipitation in the growing season for each pixel.A comparative analysis of the trends in PUE and NDVI was undertaken to detect areas of ecosystem degradation and restoration.The results show that:(i) The average PUE and NDVI values across the region increased slightly,and the area undergoing ecosystem rehabilitation during 1982—2006 comprised more than 80% of the study area.(ii) The degraded area,with significantly negative trends in the PUE and NDVI values,mainly occurred in the region surrounding Maduo County,Zaling and Eling lakes and Longyangxia Reservoir.Significant ecosystem recovery was found in southeast part of the study area,where strong positive trends were identified in the PUE and NDVI values.(iii) The areas with ecosystem degradation and restoration detected in this study were consistent with the current grassland degradation situation in the headwater catchment of the Yellow River basin,indicating that this method is reliable and could be applied to the evaluation of the regional ecosystem.(iv) The PUE value is partially dependent on changes in the precipitation,so false ecosystem degradation or ecosystem recovery trends may be identified caused by rapid increases and decreases,respectively,in the precipitation rate.For example,a decrease PUE in Xining City was partly caused by increased precipitation.Evaluating the trends in both PUE and NDVI may avoid the errors caused by using PUE as the only factor when evaluating ecosystem degradation and recovery.This method is simple,feasible,usable and practical,and the required data are easily accessible.This method allows ecosystem changes to be studied at the landscape scale,and can be used for a preliminary investigation prior to a large scale survey of ecological degradation or restoration.The results of this study suggest that evaluating the temporal change in PUE and NDVI may provide a useful large-scale indicator of landscape degradation or recovery at decadal time intervals.We recommend the broad-scale application of this method,using temporal trends in the PUE and NDVI.This study provides a scientific basis for the planning and regulation of ecological reconstruction in the headwater catchment of the Yellow River basin,China.