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黄河变清调查:绿色巨变藏风险 小水小沙隐患重

2017-09-25 11:27:36   瞭望

《黄河流域综合规划》认为,目前的水利水保措施年均减沙4亿吨左右,到2030年适宜治理水土流失区得到初步治理后,每年可减少入黄泥沙6亿~6.5亿吨。届时,入黄泥沙可减少至9.5亿~10亿吨,远期可减少至8亿吨。

黄河勘测规划设计公司董事长张金良发表论文指出,人类短期有限的人工干预,不会对来水来沙这种自然事件产生长远影响,未来黄河年度来沙量大致为8亿~17.5亿吨,多年平均来沙量不少于10亿吨。

历史上黄河来沙是否出现过类似变化?一些研究成果表明,黄土高原土地利用和植被的变化对黄河输沙有决定性的影响。历史上在植被较好的情况下,入黄泥沙量年均在6亿~11亿吨。也有专家认为在北宋以前人类活动影响较小,黄河年输沙量为2亿吨。

值得注意的是,观测河流泥沙的一个重要指标——来沙系数却在发出和现状不一致的反向信号。来沙系数是含沙量与流量的比值,是水沙搭配关系的一种量化表示。选取1919~1959年、1960~1986年、1987~2015年系列长度超过20年的时段对比发现,潼关水文站来沙系数分别为0.028、0.024、0.027,呈现先减小后增大的变化特点。

4月30日,游客在黄河壶口瀑布景区游览?饶北成摄

一些专家指出,近期入黄泥沙锐减只能反映一个特定时期内降雨和水利水保措施作用,不能用以代表今后一个较长时期内入黄泥沙的变化趋势,对未来来沙量估计不宜过于乐观。

  “绿色巨变”潜藏风险

1922~1932年,黄河曾经历过一个枯水枯沙时段,1928年径流量仅199亿立方米,输沙量只有4.83亿吨。大旱之后必有大涝,1933年,黄河出现特大暴雨洪水,输沙量高达39亿吨,造成黄河下游决口50余处,洪水席卷冀鲁豫苏4省67个县,导致300多万人受灾。

《瞭望》新闻周刊记者在陕北采访,但见昔日像一笼笼蒸熟的黄馍的山丘,被一片片油松、刺槐、林果盖得严严实实,“高土高坡”的印象已被绿色所颠覆。在惊叹这一巨变的同时,专家提醒,对减少黄河来沙而言,生态水保措施并不是万能的,要警惕被绿色掩藏的风险。

流经南泥湾的汾川河,流域植被覆盖率达85%。2013年7月,汾川河发生超过多年均值3.9倍的高强度降雨,径流量和输沙量分别为多年均值的17倍和15.5倍。流域内建于1958年的胜利水库,之前55年淤积了300多万立方米泥沙,可暴雨后淤沙陡增200多万立方米。据了解,汾川河流域当时共发生6次降雨,由于植被条件好,前4次没产流,大部分雨水下渗;第5次降雨产生小洪峰,证明下垫面饱和;到第6次降雨,生态“绿网”被撕开,出现大产流、大产沙情况。

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