Water-sediment-carbon effects of ecological conservation in the Yellow River Basin

The Yellow River Basin (YRB) is one of the world’s most densely populated regions that has suffered greatly from water resources shortage, serious soil erosion and high sediment loads, over-cultivation and ecological degradation in the past centuries. Since 1960, especially in the last 20 years, the middle reaches of the Yellow River have experienced large-scale soil and water conservation projects, including terraces, check dams and ecological restoration. The soil-water conservation (SWC) could have multi-faceted impacts on water, sediment and carbon-related processes. It’s crucial to assess the potential impacts of these human-induced transitions to provide guidance for the sustainable development in the YRB, as well as in other river basins around the world.

We first assess the multi-faceted impacts of SWC measures using a distributed ecohydrological model, which accounts for both the effects of hillslope SWC (HSWC) (i.e., terracing, afforestation, etc.) and river-network SWC (i.e., check dams) explicitly. During the study period (1961-2024), the YRB was characterized by a mismatch between the source areas of runoff and sediment. While the magnitude of runoff showed a pattern of initial decrease followed by a subsequent increase, soil erosion intensity exhibited an overall significant reduction throughout the period. Quantitatively, the erosion intensity decreased by 48.9% during 1981-2000 and 71.8% during 2001-2024 relative to the 1961-1980 baseline. An exponentially decreasing relationship between soil erosion intensity and the area of hillslope conservation measures was also found. Check dams along the river channel further intercepted approximately 4.52 billion m³ of sediment. The combined effects of these intervention measures caused the magnitude of sediment reduction to significantly exceed that of runoff volume.

We also investigate the water-sediment-carbon changes in response to intensive ecological restoration in the middle Yellow River basin. According to the results, ecological restoration promoted synergies between carbon sequestration and sediment control and led to improved water use efficiency (WUE). The actual Leaf Area Index and Gross Primary Productivity (GPP) showed improvements in region-averaged values by +0.56 m² m^-2 yr^-1 (+7.4 %) and + 52 gC m^-2 yr^-1 (+10.9 %) compared to those under natural conditions. Furthermore, WUE changes indicated higher GPP gain per unit evapotranspiration. Meanwhile, trade-offs were also found when taking account of the water yield reduction. During 1982–2019, ecological restoration significantly increased actual evapotranspiration (+8.3 mm yr^-1; +2.2 %) and decreased runoff (-7.6 mm yr^-1; -12.7 %). Two indicators evaluating the cost-effectiveness of ecological restoration, i.e., carbon sequestration and sediment settlement at the cost of per unit runoff decline, remained positive with the average values of 6.12 kgC and 0.22 ton sediment load at the cost of per m³ water yield during 2000–2019, respectively. Nevertheless, both indicators showed downward trends, indicating decreasing marginal benefits brought by the ecological restoration measures which could have approached the optimal scale in the middle YRB. These results provide a scientific basis and quantitative indicators for sustainable water-carbon-sediment management.