| Songhua River Basin(SRB)is located at the highest latitude region in China.Like other high-latitude river basins,the SRB is significantly affected by climate change.Over the past half century,the SRB has experienced a highlighted trend of warm and dry trend.The hydrological cycle,sediment and nutrients fluxes have been shaped as a result.Annual runoff of the SRB showed a significant decreasing trend,particularly since the 1990s.Whereas the expansion of farmland,accompanied with the development of economy,has pressed increasing water demand for irrigation and other agricultural activities.The water resources deficit has led to severe over-exploitation of groundwater resources in the SRB,which has drawn wide attention of stake-holders in water resources sector and hydrologists.In the meantime,a widely-noticed trend of water erosion problem is also spreading in the SRB.Large amount of sediment and nutrients are transported to receiving water body via rainfall-runoff processes,which placed great risks to ecosystems and urban livelihood in the downstream.The impacts of climate change to water resources have been widely discussed,while little attention has been paid to the water quality part.Therefore,the aims of this study are to address the following questions:How will climate change affect the hydrological processes and water quality of the SRB?Will there be enough water resources to meet the future socio-economic demand?To what extent will the water erosion and nutrient loading reach in the setting of future climate scenarios?This study is conducted based on theories of multi-disciplines,including hydrology and water resources,environmental science,soil science,atmosphere science and geomorphology,etc.Headwater watershed of Yinma River Basin was selected as the study area,which is a typical headwater watershed located in high-latitude regions.Databases including hydrological,meteorological and geographical information have used in this study.Field-scale experiments were carried out to quantify the mechanism of sediment and nutrients export driven by rainfall-runoff events.Key processes of discharge and nutrients were analyzed.What's more,the physically-based hydrological model was used to simulate the fluctuation of discharge and sediment yield of the headwater watershed.Changes of discharge,sediment and nutrients have been projected in future climate scenarios.Major findings and conclusions are shown below.(i)Responsing mechanisms of the export of dissolved and particulate phosphorus,i.e.DP and PP,to runoff have been revealed.In this study,field-scale experiment and long-term data analysis have been conducted in a typical high-latitude headwater watershed.Results showed that PP was the main form in total phosphorus loss from the experiment plot,which accounted for 80 to 99%of the total amount.DP consitituted only less than 20%of the total phosphorus loss.In this respect,we mainly focused on studying the loss of PP and its relationships between runoff.Given the fact that the lack of PP measurement in high-latitude regions and its significant connection between sediment loss,we chose sediment loss as a proxy to PP loss in data-scarced regions,especially high-latitude regions.We assumed that the responsing mechanisms of sediment yield to runoff could reasonably reflect the relationshiips between PP loss and runoff.Results showed that cumulative runoff and suspended sediment yield consisted of 99.4%and 98.6%of the total amount when discharge was greater than intermediate flow.We also found that the loss of suspended sediment was significantly linearly-correlated with runoff at logarithmic scale under the condition of peak flow,high flow and intermediate flow(p<0.05).Most of the sediment yield was transported to downstreams during flood events.In this study,89 to 99%of the suspended sediment yield was delivered during the 82 flood events identified during the period of 2006 to 2014.It should be noted that significant linear correlation at logarithmic scale was also found between suspended sediment yield and runoff amount during flood events(R2=0.809,p<0.05).Similar results were obtained from the field-scale rainfall-runoff experiments(R2=0.836,p<0.05).This indicated that either PP or suspended sediment loss were tiedly connected together,but displayed a logarithmic linear relationship.Apart from those researches on PP,responses of DP loss to runoff were also studied via field-scale rainfall-runoff experiments.Results showed that average export rate of DP was 0.41 g/hm2,ranging from 0?1.74g/hm2.The fluctutation of DP and runoff amount showed strong agreement during the rainfall-runoff events monitored.Significant positive relationship was observed between DP and runoff(R2=0.635,p<0.05).What's more,tillage effects on phosphorus loss were investigated.Results showed that conservational tillage and no-till practices could significantly prevent PP and sediment export from soil to water,while could greatly improve the loss of DP,which became the main form in phosphorus loss.(?)Characteristics of climate change and impacts on runoff in the headwater watershed of Yinma River Basin have been assessed.A warming trend was observed in the watershed during the past five decades.Particularly,significant increasing in mean temperature was found since 1990s.Abrupt change point in mean temperature was found in 1982 according to Mann-kendall test results.Strong seasonal variation was detected in the warming trend.It was shown that the warming trend in spring and witner was more significant than that in summer and autumn.There existed a slight increasing trend in annual precipitation.Rising in seasonal precipitation was found in spring,summer and winter,while dropping in autumn.Under this circumustance,an increasing of 34%in annual runoff was detected.Since 1982,annual runoff experienced an decreasing period until 2005,followed by a monotonic increasing trend in the latest decade.This was attributed to the increasing in spring and summer runoff.Particularly,largest increment in monthly runoff was withnessed in July,being 20.34mm,by a percentage of 63%.Autumn and winter runoff tended to decrease in the past 30 years.Greater inter-and intra-annual variation were found in runoff under climate change.What's more,changes in flow regimes were also shown under climate change.Runoff under peak flow,high flow and intermediate flow after 1982 were larger than that before,while less were found under active flow and low flow.This could induce more erosion and phosphorus loss from headwater watershed,with greater temperal variation among different forms of phosphorus loading.(iii)Impacts of climate change on runoff,sediment and nutrient loading were projected.Results showed that a significant increasing trend of runoff,sediment and nutrient export from the headwater watershed.Annual runoff in the projected period were estimated to be 9?23%larger than that of the baseline period.Strong seasonal variations were also detected.Larger discharge was witnessed in summer months,while spring runoff was found to decline based on simulation results.This indicated that the Yinma River Basin may experience a larger spring water use deficit in spring,while higher flood risks in summer.An increase of 19?50%of water erosion rate was simulated in 2021 to 2050 compared with that in 1981 to 2010.This makes the water erosion rate to reach up to 6.2?7.8 t ha-1 yr-1 under future scenarios,which are greatly higher than the worldly-accepted tolerate erosion rate,i.e.1 t ha-1 yr-1.The phosphorus export loading of the headwater watershed was estimated to grow up to 28.5?35.7 kg/(km2*yr),which is as severe as those highly polluted regions in developed countries.These findings suggest that conservational practices,e.g.conservational tillage or no-till practices,should be implemented to for flood and pollution control in summer times,while in the meantime,water saving practices and diversion projects are expected to release the increasing spring water demand,given that less water yield would occur under future climate scenarios.(iv)Parameter uncertainty and implications in hydroclimatic projections in high-latitude headwater watershed have been studied.These uncertainties could range from differences in GCMs database and structure,downscaling methods,parameterization processes and RCPs scenarios.Particularly,impacts of parameter uncertainty in discharge and phosphorus loading estimation were examined.We found that mean annual runoff were projected to increase at a rate of 0-34%,while maximum and minimum annual runoff showed greater uncertainty ranges,i.e.2-38%and 17-72%,respectively.This indicated that it would be more uncertain to quantify the increment of annual runoff in dry years.Uncertainty range of monthly runoff was 0.1-39.4mm based on projection results,in which the largest uncertainty space was found in August.Besides,higher annual and monthly uncertainties were shown under RCP8.5 scenario than that under RCP4.5 scenario. |
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