Water-carbon-sediment synergies and trade-offs: multi-faceted impacts of large-scale ecological restoration in the Middle Yellow River Basin

Ecological Restoration (ER) measures can achieve considerable carbon benefits and reduce sediment loads, concurrently resulting in unintended hydrological consequences. The Middle Yellow River Basin (MYRB), with intensive large-scale ER implementation during the past decades, serves as an excellent case to investigate the concomitant water-carbon-sediment synergies and trade-offs. This study combined a vegetation dynamics simulation scheme and a distributed hydrological model with explicit ER representation to investigate the water-sediment-carbon changes in response to ER in the MYRB. According to the results, ER 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. In the Toudaoguai-Tongguan section which suffered the most serious soil erosion, ER decreased the sediment loads by 11.3×10⁸ ton yr^-1 (71.1% of the natural level). 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, ER led to significant increases in actual evapotranspiration (+8.3 mm yr^-1; +2.2%) and decreases in runoff (-7.6 mm yr^-1; -12.7%). Two indicators evaluating the cost-effectiveness of ER, 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 m^-3H₂O yr^-1 and 0.22 ton m^-3H₂O yr^-1 during 2000-2019, respectively. Nevertheless, both indicators showed downward trends, indicating decreasing marginal benefits brought by ER measures which could have approached the optimal scale in the MYRB.