| Climatic variability and forest disturbance are commonly recognized as two major drivers influencing streamflow change in large scale forested watersheds.However,these perturbations are further exacerbated in a warming climate due to widespread permafrost thaw.The Da Hinggan Mountains is located on the the southeastern margin of the Eurasian Cryolithozone.It is the one of the permafrost distributions those have been greatly affected by the climate wanning in China,even on a global scale.The forest in this region experienced a dynamic changes with a prolonged timber harvesting period after the 1960s and a forest recovery period after 2000.Thus,the scientific question of the impacts of changes in forest,climate and permafrost on local hydrological processes and water resources should be revealed.In this study,the forested watersheds(66-6581 km2)of Da Hinggan Mountains were used to determine the trends in long-term hydrometeorological data,the relationships between hydrological variables and the changes in forest,climate and permafrost,and the relative contributions of forest change,climate variability and permafrost thaw to the long-term annual streamflow changes.The results can provide theoretical basis and scientific guiding for the forest and water resources integrated management and sustainable development in the context of globle climate change.(1)There was no significant trend in annual streamflow of Upper Tahe River watershed during the past 40 years f-rom 1973 to 2012 with a changing rate of 0.35 mm/year.However,it showed a obviously phase shifts including a positive trend from 1973 to late 1980s aind a negative trend from late 1980s to 2012.There was a slight positive trend in annual precipitation during the study period,but no significance was found(p>0.05).The annual potential evapotranspiration(PET),mean annual air temperature(MAAT),mean annual minimum air temperature(Tmin)and mean annual maximum air temperature(Tmax)all showed significant positive trends(p<0.05)with the increasing slope of 1.38 mm/year,0.04 ?/year,0.04 ?/year and 0.03 ?/year.This results indicated that there was a closed relationship between annual streamflow and precipitation.However,there was no significant effect of the changes in air temperature on long-term trend in annual stremflow.(2)Annaul precipitation is the major driver regularing annual streamflow.There was a significantly positive linear relationship between annual streamflow and precipitation(R2=0.738,P<0.01).There was no significant relationship between annual streamflow and MAAT.However,the annual streamflow decreased associated with the increasing potential evopatranspiration.From the period of 1993-1987 to the period of 1988-2001,the average annual streamflow signigicantly increased by 67.25 mm(3.47 mm/year).However,the average annual streamflow only increased by 54.30 mm(2.16 mm/year).Additionaly,the potential evopatranspiration increased by 1.69 mm/year due to the significant increase of MAAT(1.20?,0.06 ?/year).From the period of 1988-2001 to the period of 2002-2012,the average annual streamflow signigicantly decreased by 60.21 mm(4.18 mm/year).However,the precipitation only showed a reduction of 29.66 mm(2.35 mm/year).Also,the air termperature showed no significant change.Thus,the changes of precipitation between different periods can not fully explain the significant changes in annual streamflow,although the precipitation is the major driver for the streamflow.(3)The annual streamflow and forest cover showed oppsite changing trends during the study period.The forest cover can explain about 30%of the trend in annual streamflow.,although the linear relationship was not significant between them.The runoff ratio in the forest stability phase(1988-2001)was 12.7%and 14.9%higher than those in the forest harvesting phase(1973-1987)and the forest recovery phase(2002-2012)associated with the reduction(5.5%)and increasing(10.0)in forest cover,which indicated that the dynamic changes in long-term annual streamflow were not only related to the changes in annual precipitation,but also closely related to the forest cover changes.The deforestation have the negative impacts on the fanction of water conservation of the forest.The afforestation can strengthen the hydrological function of forest,improve the structure of soil,and enhance the infiltration,which all have the positive impacts on the regional water resources.(4)In the paired small watersheds study,the results showed that low flow regimes were more sensitive than high flow regimes to forest disturbance.The average magnitude(0.47 mm)of low flows was lower by 26.6%and the average variability(0.39)of low flows was higher by 36.8%than those of low flows(0.60 mm and 0.28,respectively)in the control watershed(Xiaobeigou watershed).Besides,the differences in low flow magnitude and variability between the paired watersheds were statistically significant(p<0.01).In contrast,the magnitude,duration,and variability of high flows in the disturbed watershed were greater than those in the control watershed,but no statistical significance was found,suggesting that the small forest disturbance did not significantly affect the flow regimes of high flows in the study region.These results indicate that the natural flow regimes of small forested watersheds in northern Daxing'anling are sensitive to forest disturbance.Since small-scale disturbance in the study area can lead to significant changes in flow regimes,more focus should be applied to forest disturbance and water resources in future management.(5)Long-term data(40 years)from two large permafrost watersheds in northeastern China,the Tahe River and Upper Duobukuer River watersheds,indicate that winter baseflows are characterized by significant positive trends(p<0.01)of 1.7%/year(0.09 mm/year)and 2.5%/year(0.16 mm/year),respectively.The indicators of permafrost degradation,MAAT and the thawing index also showed significant positive trends(p<0.01).Winter baseflows exhibited statistically significant positive correlations with mean annual air temperature and the thawing index,for both watersheds as well as the increasing annual rainfall fraction of precipitation for the Duobukuer River watershed.The statistical analyses suggest that the increases in winter baseflow are likely related to permafrost thaw and are potentially also due to an increase in the wet season rainfall.It implyed that there was a sinificnat effect of climate warming induced permafrost thaw on hydrological processes in northeastern China.Permafrost degradation increases infiltration,enlarges the groundwater reservoir and leads to a higher winter baseflow.(6)The results of quantifying the relative hydrologic effects of climate variability,forest cover change and permafrost thaw on average annual streamflow indicated that the relative contribution of combined effect of forest harvesting and permafrost thaw on long-term annual streamflow change was 70.0%(+ 47.0 mm)and 63.1%(-38.0 mm)during the forest harvesting period(1973-2001)and the forest recovery period(1988-2012).However,the relative contribution of climate variability was lower(-30%)than those of combined effect of forest harvesting and permafrost thaw.A simple method including calculation of permafrost thaw-induced meltwater and increased winter baseflow was used to separate the distinct impacts of forest cover change and permafrost thaw.The results showed that the relative contributions of permafrost thaw and forest cover change was 15.8%and-3.3%,and 54.1%and 66.4%during the two periods,respectively,which indicated that forests play an important role in regulating water in Da Hinggan Mountains.The results also highlight the need to consider multiple streamflow drivers of forest,climate and permafrost in researches on the hydrological processes in the cold forested watersheds in the context of future climate warming. |
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