黄土高原'退耕还林（草）'（Grain-for-Green）是世界上规模最大的植被恢复工程。植被恢复引起的土地利用变化（land use change）会改变区域的水碳循环过程，从而影响生态环境稳定和可持续发展。明确植被恢复的水碳效应是当前黄土高原生态水文研究的一个重要科学问题。本研究以黄土高原泾河流域为例，量化了植被恢复引起的水碳效应，并以'减沙、保水、固碳'为原则确定了流域尺度上退耕还林（草）的适宜区域。为此，本研究利用水文-生物地球化学循环耦合模型（SWAT-DayCent），通过设计多种基于植被恢复政策的土地利用转换情景，模拟了水碳循环关键要素的变化。结果表明，与基准情景（2000 年土地利用）相比，当以森林或草地代替坡耕地时，产沙量和产水量都大幅下降。当用森林植被替代坡耕地（坡度大于25°、15°和6°）（CTF）时，会增加土壤的固碳量且对土壤水分产生轻微影响（＜1.0%）；而当用草地替换坡耕地（CTG）时，会导致净初级生产力的下降和土壤含水量大幅增加（3.8%⁓14.9%）。与基准情景相比，土壤有机碳（SOC）在CTF情景下增加0.9%⁓3.2%，而在CTG情景下保持相对稳定。通过分析土地利用变化与水碳效应的关系，进一步确定了实施CTF和CTG的适宜区域，并能实现'减沙、保水、固碳'的效益最大化。本研究基于水碳耦合模拟提供了一个植被恢复工程实施的新视角，即在恰当区域进行退耕还林（草）可实现水碳耦合效益最大化，这可为区域未来生态恢复工程的优化提供科学支撑。

The 'Grain-for-Green' project on the Loess Plateau is the largest revegetation program in the world. However, revegetation-induced land use changes can influence both water and carbon cycles, and the diverse consequences were not well understood. Therefore, the reasonability and sustainability of revegetation measures are in question. This study quantifies the impacts of revegetation-induced land use conversions on the water and carbon cycles in a typical watershed on the Loess Plateau and identifies suitable areas where revegetation of forest or grassland could benefit both soil and water conservation and carbon sequestration. We used a coupled hydro-biogeochemical model to simulate the changes of a few key components in terms of water and carbon by designing a variety of hypothetical land use conversion scenarios derived from revegetation policy. Compared to the baseline condition (land use in 2000), both sediment yield and water yield decreased substantially when replacing steep cropland with forest or grassland. Converting cropland with slopes larger than 25°, 15°, and 6° to forest (CTF) would enhance the carbon sequestration with a negligible negative effect on soil water content, while replacing cropland with grassland (CTG) would result in a decline in net primary production but with a substantial increase in soil water content (3.8%–14.9%). Compared to the baseline, the soil organic carbon would increase by 0.9%–3.2% in CTF and keep relatively stable in CTG. Through testing a variety of hypothetical revegetation scenarios, we identified potential priority areas for CTF and CTG, where revegetation may be appropriate and potentially beneficial to conserving soil and water and enhancing carbon sequestration. Our study highlights the challenges in future water and carbon coupling management under revegetation policy, and our quantitative results and identification of potential areas for revegetation could provide information to policy makers for seeking optimal management on the Loess Plateau.