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The erosion, transport and redeposition of sediments shape the Earth's surface, and a ect the structure and function of ecosystems and society 1,2 . The Yellow River was once the world's largest carrier of fluvial sediment, but its sediment load has decreased by approximately 90% over the past 60 years 3 . The decline in sediment load is due to changes in water discharge and sediment concentration, which are both influenced by regional climate change and human activities. Here we use an attribution approach to analyse 60 years of runo and sediment load observations from the traverse of the Yellow River over China's Loess Plateau -the source of nearly 90% of its sediment load. We find that landscape engineering, terracing and the construction of check dams and reservoirs were the primary factors driving reduction in sediment load from the 1970s to 1990s, but large-scale vegetation restoration projects have also reduced soil erosion from the 1990s onwards. We suggest that, as the ability of existing dams and reservoirs to trap sediments declines in the future, erosion rates on the Loess Plateau will increasingly control the Yellow River's sediment load.Change of soil erosion and the resulting river sediment transport are important components of global change, so understanding the mechanisms behind such change is crucial to developing strategic plans for the sustainable management of catchments 4,5 . In recent decades, significant decreasing trends in river sediment loads have been observed in approximately 50% of the world's rivers 6,7 . The benefits and risks of the change in river sediment load largely depend on the baseline load and the scale of the change 8,9 . Hence, it is important to quantify the change of river sediment loads through time, and to understand the drivers and mechanisms behind them 2,5 .The Huang He, or Yellow River (YR) (Fig. 1), was the most sediment-laden river in the world, but its annual sediment load has continually decreased since the 1950s (refs 10-13). The yearly sediment loads at the main gauging stations along the YR, all show significant decreasing trends (p < 0.01) over the past six decades (Fig. 1b). Sediment load increases most suddenly in the middle reach of the river, when crossing the Loess Plateau (LP), between the Toudaoguai gauging station (TDG) (0.07 Gt yr −1 ) and the Tongguan station (TG) (0.63 Gt yr −1 ), and then gradually declines in the lower reach (Fig. 1b, top right inset). The LP is thus the largest sediment source, nearly 90% (refs 3,11) for the YR, and we therefore focus on this part of the river's catchment. A mass budget over the middle reach of the YR can be obtained from the difference of measured sediment flux and water discharge at TG and TDG (Fig. 1). Both the river discharge and sediment load across the LP show significant decreasing trends (−0.25 km 3 yr −2 , p < 0.001; and −0.02 Gt yr −2 , p < 0.001, respectively) over the past six decades, whereas precipitation decreased slightly (−1.2 mm yr −2 , p = 0.015). As Fig. 2a shows two abrupt falls in sed...

a b s t r a c tSpatial stratified heterogeneity, referring to the within-strata variance less than the between stratavariance, is ubiquitous in ecological phenomena, such as ecological zones and many ecological variables. Spatial stratified heterogeneity reflects the essence of nature, implies potential distinct mechanisms by strata, suggests possible determinants of the observed process, allows the representativeness of observations of the earth, and enforces the applicability of statistical inferences. In this paper, we propose a q-statistic method to measure the degree of spatial stratified heterogeneity and to test its significance. The q value is within [0,1] (0 if a spatial stratification of heterogeneity is not significant, and 1 if there is a perfect spatial stratification of heterogeneity). The exact probability density function is derived. The q-statistic is illustrated by two examples, wherein we assess the spatial stratified heterogeneities of a hand map and the distribution of the annual NDVI in China.

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China's Loess Plateau is both the largest and deepest loess deposit in the world, and it has long been one of the most severely eroded areas on Earth. Since the 1970s, numerous soil- and water-conservation practices have been implemented: terracing, planting of vegetation, natural vegetation rehabilitation, and check-dam construction. With the implementation of the Grain-for-Green Project in 1999, the Loess Plateau has become the most successful ecological restoration zone in China. However, these large-scale restoration measures and drought have significantly reduced both runoff and sediment from the Loess Plateau. This situation has both advantages and disadvantages for the lower Yellow River. Some local soil erosion has been successfully controlled, but the whole regional ecosystem remains very fragile. Therefore, it is necessary to balance each ecosystem service, for example, by determining the region's vegetation capacity and its spatial distribution for the sustainable development of the socioecological system of the Loess Plateau.

As one of the key tools for regulating human-ecosystem relations, environmental conservation policies can promote ecological rehabilitation across a variety of spatiotemporal scales. However, quantifying the ecological effects of such policies at the regional level is difficult. A case study was conducted at the regional level in the ecologically vulnerable region of the Loess Plateau, China, through the use of several methods including the Universal Soil Loss Equation (USLE), hydrological modeling and multivariate analysis. An assessment of the changes over the period of 2000–2008 in four key ecosystem services was undertaken to determine the effects of the Chinese government's ecological rehabilitation initiatives implemented in 1999. These ecosystem services included water regulation, soil conservation, carbon sequestration and grain production. Significant conversions of farmland to woodland and grassland were found to have resulted in enhanced soil conservation and carbon sequestration, but decreased regional water yield under a warming and drying climate trend. The total grain production increased in spite of a significant decline in farmland acreage. These trends have been attributed to the strong socioeconomic incentives embedded in the ecological rehabilitation policy. Although some positive policy results have been achieved over the last decade, large uncertainty remains regarding long-term policy effects on the sustainability of ecological rehabilitation performance and ecosystem service enhancement. To reduce such uncertainty, this study calls for an adaptive management approach to regional ecological rehabilitation policy to be adopted, with a focus on the dynamic interactions between people and their environments in a changing world.

The Loess Plateau, China, has long been suffering from serious soil erosion. About 2000 years ago, larger areas were used for grain production and soil erosion was thus becoming severe with increase in human activity. Severe soil and water loss led to widespread land degradation. During the past decades, great efforts were made in vegetation restoration to reduce soil erosion. However, the efficiency of vegetation restoration was not as satisfactory as expected due to water shortage. China initiated another state-funded scheme, the `Grain-for-Green' project in 1999, on the Loess Plateau to reduce soil erosion and improve land quality. However, the control of soil erosion effectively by land-use modification raised problems. In this paper, the lessons and experiences regarding soil and water conservation in the Loess Plateau in the past decades are analysed first. Urgent problems are then elaborated, such as the contradiction between land resource and human population, shortage of water both in amount and tempospatial distribution for vegetation growth, weak awareness of the problems of soil conservation by local officials, and poor public participation in soil and water conservation. Finally, suggestions regarding soil and water conservation in the Loess Plateau are given. In order to control soil erosion and improve vegetation, a scientific and detailed land-use plan for the Loess Plateau has to be made, in the first instance, and then planning for wise use of water resources should be undertaken to control mass movement effectively and to improve land productivity. Methods of improving public awareness of environmental conservation and public involvement in vegetation rehabilitation are also important.