Quantitative evaluation of the effect of climate variability and human activities on runoff is of great importance for water resources planning and management in terms of maintaining the ecosystem integrity and sustaining the society development. In this paper, hydro-climatic data from four catchments (i.e. Luanhe River catchment, Chaohe River catchment, Hutuo River catchment and Zhanghe River catchment) in the Haihe River basin from 1957 to 2000 were used to quantitatively attribute the hydrological response (i.e. runoff) to climate change and human activities separately. To separate the attributes, the temporal trends of annual precipitation, potential evapotranspiration (PET) and runoff during 1957-2000 were first explored by the Mann-Kendall test. Despite that only Hutuo River catchment was dominated by a significant negative trend in annual precipitation, all four catchments presented significant negative trend in annual runoff varying from À0.859 (Chaohe River) to À1.996 mm a À1 (Zhanghe River). Change points in 1977 and 1979 are detected by precipitation-runoff double cumulative curves method and Pettitt's test for Zhanghe River and the other three rivers, respectively, and are adopted to divide data set into two study periods as the pre-change period and post-change period. Three methods including hydrological model method, hydrological sensitivity analysis method and climate elasticity method were calibrated with the hydro-climatic data during the pre-change period. Then, hydrological runoff response to climate variability and human activities was quantitatively evaluated with the help of the three methods and based on the assumption that climate and human activities are the only drivers for streamflow and are independent of each other. Similar estimates of anthropogenic and climatic effects on runoff for catchments considered can be obtained from the three methods. We found that human activities were the main driving factors for the decline in annual runoff in Luanhe River catchment, Chaohe River catchment and Zhanghe River catchment, accounting for over 50% of runoff reduction. However, climate variability should be responsible for the decrease in annual runoff in the Hutuo River catchment.

[1] As an indicator of atmospheric evaporating capability over a hypothetical reference surface, reference evapotranspiration (ET 0 ) is the most important hydrological and meteorological variable to reflect climate change. This is particularly true for the Yellow River Basin, which faces serious water shortages and is vulnerable to climate change. In this study, the ET 0 at 80 sites during 1957-2008 in the Yellow River Basin was calculated using the Penman-Monteith method with the calibrated Angstrom coefficients. Spatial and seasonal patterns of changes in ET 0 as well as the concerned climatic variables are specially focused on using advanced statistical tests and GIS method. The entire Yellow River Basin is characterized by complicated spatial variability in the change of ET 0 . Significant negative trends are mainly distributed in the southeast corner, northern side, and midwest of the Yellow River Basin, while significant increases of ET 0 mainly occur in the middle part and southwest corner of the basin. Still, no coherent spatial patterns in ET 0 trends are seen in any season. The dominance of warming trends in temperature and decreasing trends in wind speed and sunshine duration can be found in the basin. Relative humidity presents insignificant or weak trends at many sites but with a mixed spatial structure of positive and negative trends at both annual and seasonal scales. The combined effects of climatic variables to ET 0 changes and their spatial and seasonal variability are revealed by further analysis of sensitivity of ET 0 to climatic variables and the contribution of climatic variables to ET 0 changes over six homogenous regions identified by a rotated empirical orthogonal function (REOF) clustering method on annual and seasonal scales. The decline of surface wind speed offsets the increasing effect of the temperature increase and is mainly responsible for the ET 0 reduction in the west and north of the Loess Plateau. The reduced sunshine duration is the leading factor for ET 0 decrease in the middle-lower Yellow River Plain, especially during the summer time. The increasing mean temperature plays the most important role in the ET 0 increase in the source area of the Yellow River Basin. Furthermore, regional actual evapotranspiration and ET 0 present complementary behavior, but does not accurately fall in the 1:1 complementary relationship of the Bouchet's hypothesis, especially for the high elevation subregions. In addition, although precipitation changes are the main driving factors for drought variation, increasing ET 0 intensified the drought in middle regions.

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