图4. 过去2500年黄河中游地区垦殖强度及土壤侵蚀与下游河道来沙及沉积速率变化
（a）黄土高原地区及其中丘陵沟壑区的人口密度与丘陵沟壑区人口占整个黄土高原人口的百分比^[60]。（b）渭河上游高山湖泊沉积物中的孢粉记录（木本孢粉比例下降指示垦殖对自然植被的影响增强,葎草及谷物孢粉比例增加指示垦殖强度加大）^[77]。（c）黄土高原不同时期的侵蚀率^[7,15]与渭河上游高山湖泊的沉积物磁化率（高值指示侵蚀强度大）^[78]。（d）黄河下游河段不同时期的年均来沙量^[79]及河道沉积速率^[80]

Figure 4. Changes of reclamation intensity, soil erosion in the middle reaches, and sediment load and sedimentation rates in the lower reaches of the Yellow River for the past 2500 years.
(a) Population density of the whole Loess Plateau (gray bar), as well as population density (star-solid line) and population percentage of its mid-hill gully region (dot-dash line)^[60]. (b) Pollen records in alpine lake sediments in the Upper Reaches of the Weihe River, in which the decreasing of tree pollen (solid line) indicates great impact of reclamation on natural vegetation, and the increasing Humulus-type (gray shading) and Cereal-type pollen (black shading) indicate large reclamation intensity^[77]. (c) Erosion rates (star-solid line) of the Loess Plateau^[7,15] and magnetic susceptibility (gray shading) of the alpine lake sediments (high values indicate great erosion intensity) in the Upper Reaches of the Weihe River^[78] in different periods. (d) The mean of annual sediment load^[79] (dot-dash line) and sedimentation rates^[80] (star-solid line) in the Lower Reaches of the Yellow River in different periods