| Cascade hydropower development can effectively regulate the temporal andspatial distribution of water resources, which makes it the latest trend in the utilizationof water resources. Since the basin is functioned as a system with high correlation andstrong integrity，when achieving the goal of flood control, irrigation, power generationand other economic benefits，cascade hydropower development will also trigger aseries of biophysical cumulative effects through changing hydrological regimes，affecting the annual distribution and the interannual variation characteristics in runoff.The main purpose of this research was to study the cumulative effects of runoffvariations caused by cascade hydropower development in the upper reaches ofminjiang river, where the hydropower development activities are particularly intense.The characteristics and development degree of hydropower development in study areawere be comprehensively assessed. Different methods such as Mann-Kendall trendanalysis and rescaled range analysis were used to analyze the spatial-temporalvariability of annual streamflow distribution during1956to2013in the Zagunao river,a main tribulary of study area. Then determined the main driving forces on runoff indifferent periods according to the relationships between the variability of runoff andits’ main influencing factors, such as climate change, forest cover change, andhydropower development. Finally, choose the period when hydropower developmentact as the main influencing factor to study the impact mechanisms of differenthydropower development scenarios on runoff. The results were as follows.(1) The main type of hydropower plants in study area is not large dams but smalldiversion plants with high water head and low dam. Low gate diversion dam, bottomgrid fencing style and overflow dam are the main hydropower dam types of diversionplants in the upper Minjiang river basin. Hydropower development climax period wasfrom2000to2010, cascade development in study area has been expanded to the thirdlevel tributaries of Minjiang river.(2) Built three indexes namely hydropower development rate, hydropowerdevelopment density and hydropower development intensity to comprehensivelyassess the hydropower development degree in a region or watershed. The assessment results show that, the indexes of hydropower development rate and hydropowerdevelopment intensity in study area are much greater than that of other main rivers inChina. Wenchuan-Dujiangyan section is the highest developed section of main stream.Zagunao river is the highest developed tributary because the hydropower developmentrate, hydropower development density and hydropower development intensity arerespectively as high as99%,0.57and0.17.(3) The annual runoff of Zagunao river showed a decreasing trend for nearly60years. Compared to the annual average runoff in1960s,2000-2013has reduced by11%. The annual average runoff in1956-1965and1971-1991showed an increasingtrend, while1966-1970and1992-2013showed a significant decreasing trend. As faras the annual streamflow distribution in Zagunao river, the nonuniform coefficient inhistorical periods overall show a decreasing trend. The concentration of1970-1979and2000-2013are respectively the smallest and the penultimate small than that of anyother periods, and the occurrence time of maximum runoff during these two periodsshowed an obvious advancement. The amplitude of variation before2000showed adecreasing trend, while2000-2013showed a slightly increasing trend.(4)The main influencing factors of runoff in1950s~early1980s,mid-1980s~1990s, and2000~2013are respectively forest cover change, climatechange, and hydropower development. The results of runoff mutation test show that,climate change has induced one time annual runoff mutation, and forest vegetationchange has induced twice average monthly runoff mutations. Hydropowerdevelopment has not induced annual or monthly runoff mutation yet.(5) Hydropower development has significantly changed the annual streamflowcharacteristics during the period of2000-2013. The operation mode of single-pointhydropower plant reduced the nonuniform coefficient and concentration, andadvanced the occurrence time of maximum runoff. This may be due to the channel(ortunnel) of diversion type hydropower plants has similar hydrological effects with thestraightening of natural river, that is to accelerate the excretion of flood. The operationmode of cascades without first reservoir showed a cumulative influence feature onrunoff, which can be reflected by the significant reduction of the nonuniformcoefficient, the concentration, and the amplitude of variation. The operation mode ofcascades with first reservoir has minimazed the nonuniform coefficient and theconcentration, and delayed the occurrence time of maximum runoff for5days, whichnot only showed the great regulation ability of reservoir, but a more pronouncedcumulative effect caused by this cascade mode. (6) The main influencing factor on annual runoff was not hydropowerdevelopment but climate change. Hydropower development has showed a significantinfluence on monthly runoff variation. |
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