The driving factors of runoff changes can be divided into precipitation factor and non-precipitation factor, and they can also be divided into natural factor and human activity factor. In this paper, the ways and methods of these driving factors impacting on runoff changes are analyzed at first, and then according to the relationship between precipitation and runoff, the analytical method about impacts of precipitation and non-precipitation factors on basin's natural runoff is derived. The amount and contribution rates of the two factors impacting on natural runoff between every two adjacent decades during 1956-1998 are calculated in the Yellow River Basin (YRB). The results show that the amount and contribution rate of the two factors impacting on natural runoff are different in different periods and regions. For the YRB, the non-precipitation impact is preponderant for natural runoff reduction after the 1970s. Finally, by choosing main factors impacting on the natural runoff, one error back-propagation (BP) artificial neural network (ANN) model has been set up, and the impact of human activities on natural runoff reduction in the YRB is simulated. The result shows that the human activities could cause a 77

A series of annual natural runoff, recorded at eight hydrological stations in the Yellow River basin (YRB) and distributed at the main channel and two sub-basins (Weihe basin and Fenhe basin) are studied during the period from 1951 to 1998. The trends and the beginning points (or abrupt changes) of these series are detected with Mann-Kendall test (M-K test) and its progressive application. The results show that there are similar trends for different stations. The annual natural runoff series, except for stations of Lanzhou, Hekouzhen, and Longmen, have a downward tendency at some stations whose beginning points of the trend or abrupt changes occurred in the early or mid 1990s. However, their initial years of decrease are different. The decreasing trend of runoff has led to drying-up in lower reaches of the YRB, a water resources scarcity crisis, and a deterioration of the ecosystem. The reasons for the trends have been analyzed in this paper. The trends of precipitation in the corresponding regions are similar to those of annual natural runoff. It is an important aspect for runoff decrease. In addition, human activities have changed the hydrological cycle and lead to a reduction in runoff to a certain degree. Finally, two countermeasures for the runoff decrease are put forward. One is to develop water savings in irrigation regions, and the other is to plan South-to-North water transfer projects for the YRB. However, the conflict between supply and demand will still exist for a long time because of the water scarcity in the future.

根据水量平衡原理建立干流区间河道水量平衡关系，得到干流河道水量自然补损计算方程。利用1956～1979年干流区间自产天然径流量与区间河道上下游控制站天然径流量、区间降水量系列之间的线性相关性，延长得到黄河流域1951～1998年龙羊峡以下6个干流区间逐月自产天然径流量，并计算分析各干流区间的干流河道自然补损水量。结果表明：多年平均河口镇-龙门干流区间和三门峡-花园口干流区间河道水量补充大于损失，其余区间干流河道水量为损失大于补充。总体上，龙羊峡以下区间干流河道净自然损失水量多年平均为35.43×108m3，而且呈增大趋势，20世纪90年代达到最高，平均为44.16×108m3/年。

NDVI是表征区域植被覆盖的重要指标。本文把黄河流域划分为16个区域，利用Pathfinder(探路者)的AVHRR-NDVI资料，得到各区域1982～1998年NDVI统计序列，详细分析了各区域NDVI时空变化特征。研究表明，17年来黄河流域各分区年平均NDVI都呈增加趋势，说明植被覆盖增加，生态环境好转，但是8月份龙羊峡以上和兰河干流区间NDVI则有所下降，生态环境有局部恶化趋势。最后研究各区域NDVI与降水、径流和径流系数的年内、年际相关关系，结果表明：从年内变化看，黄河流域各区域NDVI变化与降水、径流变化呈明显的正相关；从年际变化看，黄河流域各区域NDVI波动趋势与降水具有一定的相关性，但与径流、径流系数变化的关系相对复杂，不具有明显的相关性。

水资源具有自然再生和社会再生、水量再生和水质恢复等四种类型。本文根据水资源可再生性基本涵义建立了黄河流域地表水资源可再生性评价一般指标体系，并运用TOPSIS法进行评价，结果表明：黄河流域主要产水区域如龙羊峡以上、湟水流域、洮河流域和渭河流域等都是水资源可再生性相对最强或较强的区域，北洛河流域是最弱的区域，其余属于中等或者较弱区域。