| Climate change, especially the precipitation change, intensify the terrestrial hydrological cycle, and alter water resources regime. Disasters associated with heavy precipitation, such as floods, landslides, and mud-rock flows, affect directly on the natural ecological and social economic systems. The continuous rainstorm process is the main flash-flood-producing rainstorm on basin-scale, and the extreme continuous rainstorm is more likely to cause catastrophic floods. On the other hand, human activities, especially the land use/cover change, such as urbanization and agricultural land diminish, impact the rainstorm runoff process directly and profoundly. Climate change and land use/cover change will continue to develop in the 21st century for majority basins in the world. Therefore, it is important to research the responses of the extreme consecutive precipitation to climate change and the impacts of the changes in the extreme consecutive precipitation on the extreme runoff or flood, and further investigate the joint influence of climate change and land use/cover change on the extreme runoff or flood, especially for understanding the occurrence and evolution in a specific basin and formulating the adaptation strategies to climate change.Huaihe River Basin is located in East Asian monsoon climate region, in the climate transition zone from the humid southern region to the semi-humid region of the north, with diverse climate types and much heavy rain. The heavy rain in Huaihe River Basin is characterized by large rain area, high rain intensity and occurrence in everywhere in the whole basin due to the climate and physical geographical conditions. The researches about observed and projected precipitation change indicated that the extreme precipitation in flood season show a significant upward trend in Huaihe River Basin, which must be relevant to the increased precipitation in the continuous rainfall process due to plum rains. Therefore, it is necessary to investigate the spatial-temporal characteristics of the extreme consecutive precipitation in flood season and their responses to climate change, and further understand their impacts on the runoff in Huaihe River Basin.Increasingly intensified human activities have profoundly affect the surface hydrological processes in Huaihe River Basin. Unreasonable human activities, such as land use/cover change, reduce effects of the engineer system for flood control and mitigation, and aggravate the flood disaster in the basin. Therefore, it is necessary to investigate the hydrological effect of the human activities, especially their impacts on the extreme runoff in Huaihe River Basin.Applied statistical analysis, climate model, hydrological simulation and other methods, we investigated the characteristics of the climate change, the hydrological effect of the climate change or land use/cover change and their joint influence on the runoff and flood in the upper and middle reaches of Huaihe River Basin.(1) We analyzed the main features of the observed and projected climate change in Huaihe River Basin, based on observed climate data during the period from 1958 to 2009 and the output of RegCM3 under Series A2 Greenhouse Gas Emission Scenarios from 2071 to 2100. In term of the observed climate change, during the period from 1958 to 2009, annual mean temperature showed a significant upward trend, especially in the central basin (the middle and lower reaches of Wohe River and Yinghe River Basin) and the southern basin of the lower reaches Huaihe River Basin. The temperature showed an upward trend in spring and autumn while showed a downward trend in summer, annual temperature range increased. Annual precipitation showed a non-significant upward trend, especially in the lower reaches of Yinghe River Basin and the area closed to the mainstream around Bengbu hydrological gauge. On the other hand, annual precipitation in the southern basin showed a non-significant downward trend. The precipitation showed an upward trend in summer while showed a downward trend in autumn, seasonal contrast of the precipitation increased in the later years of the past 50 years. The increase in the minimum temperature in night and in winter led to the mean temperature increase significantly in Huaihe River Basin. Consecutive dry days showed a significant downward trend while maxiumm 1-day precipitation amount showed a significant upward trend. The precipitation frequency increased in dry season, precipitation intensified in wet season and seasonal contrast of the precipitation increased. The increased precipitation in summer is mainly caused by the higher precipitation intensity while the increased precipitation in winter is mainly caused by the higher precipitation frequency. As for the projected climate change, under Series A2 Greenhouse Gas Emission Scenarios, during the period from 2071 to 2100, RegCM3 predicted that annual mean temperature rise, especially in summer, and annual precipitation total increase, especially in autumn.At the same time, the extreme consecutive precipitation in flood season increased, especially in the southern basin of the lower reaches Huaihe River.(2) We calculated several precipitation indices and streamflow indices based on the observed daily precipitation and streamflow data from 1958 to 2009 and investigated the relationship between the precipitation and streamflow in Huaihe River Basin.Furthermore, we studied the influences of the precipitation change on the runoff and extreme runoff, especially in flood season. During the period from 1958 to 2009, streamflow and extreme streamflow showed a non-significant upward trend, especially for extreme streamflow. In flood season, streamflow and extreme streamflow increased more remarkable. The increased streamflow in flood season and the decreased streamflow in dry season led to seasonal contrast of streamflow increase, along with the risk of flood, drought and sudden turn of drought and flood increasing. The precipitation from higher percentile or continuous rain process had much impacts on the streamflow and extreme streamflow, which is the main reason for floods occurring frequently in Huaihe River Basin. The greater precipitation anomaly in the upper and central basin was a major reason for floods and droughts occurred frequently in the whole basin. Heavy precipitation process that last a week to ten days directly caused severe flooding occurring. The extreme consecutive precipitation in flood season show a non-significant upward trend in Huaihe River Basin. In the upper reaches of Huaihe River Basin, the upper reaches of Wohe River Basin and the southern basin of the lower reaches of Huaihe River, the extreme consecutive precipitation in flood season increased more remarkable. The perennial average value of the extreme consecutive precipitation in flood season was higher in the southern basin and the western basin. However, in the southern basin and the western basin, storm runoff discharged quickly due to the hilliness and mountainous landform. On the other hand, storm runoff discharged slowly in the middle and lower reaches of Huaihe River Basin due to the flat terrain. Therefore, the spatial patterns of the extreme consecutive precipitation in flood season caused the flood and waterlogging in the middle reaches of Huaihe River Basin. The probability of the extreme consecutive precipitation in flood season was sensitive to the changes in the extreme consecutive precipitation, especially in the northern basin and the lower reaches of Huaihe River Basin. The frequency of the extreme consecutive precipitation in flood season in the lower reaches of Huaihe River Basin was much higher and more concentrated, which led to much higher flood risk in the middle and lower basin than the upper basin.(3) We constructed the SWAT model for Huaihe River Basin and used the outputs of RegCM3 under the reference period from 1961 to 1990 and the scenarios period from 2071 to 2100 to drive the model. Based on the SWAT simulation results, we further studied the impacts of climate change on runoff in Huaihe River Basin. The constructed SWAT model simulated reasonably the monthly streamflow and the maximum 9-day mean streamflow in flood season in Huaihe River Basin. Under the climate change scenarios characteristized by warming, climate change led to an increase in mean annual streamflow, increased more than 10% on average. In August and September, streamflow increased more remarkable, and the streamflow seasonal contrast increased, along with the risk of flood, drought and sudden turn of drought and flood increasing. The maximum 9-day mean streamflow in flood season increased more remarkable, increased more than 20% on average. The probability distribution of the extreme streamflow in flood season also changed and the frequency of the extreme streamflow was much higher and more concentrated. On the other hand, under the climate change scenarios characteristized by warming, the relationship between precipitation and runoff is much closer, especially the relationship between the extreme consecutive precipitation and the extreme streamflow in flood season.(4) We obtained the land use and coverage actuality map updated to 2000 for the study basin by interpreting the MrSID images. Based on the land use and coverage actuality map, we constructed land use/cover change scenarios characterized by urbanization and agricultural land diminish according to three driving forces:urbanization, ecological restoration and agricultural structure adjustment. Used the observed climate data during the period from 1958 to 2009, we run the constructed SWAT model and investigated the runoff effects of the land use/cover change in Huaihe River Basin. The results indicated that, annual streamflow increased slightly due to the land use/cover change, increased about 2.76%o on average.Annual streamflow increased greatly due to urban expansion. Land use/cover change contributed to the increase in the seasonal difference of the streamflow, especially for urban expansion, which further increased the flood risks in Huaihe River Basin. On the other hand, land use/cover change had great impact the extreme streamflow in flood season. Under the land use/cover change scenarios, the maximum 9-day mean streamflow in flood season increased about 41.22%o on average. The probability distribution of the extreme streamflow in flood season also changed and the frequency of the extreme streamflow in flood season is much higher and more concentrated, which increased the severe flood risks in Huaihe River Basin.(5) We analyzed the difference in the runoff responses to land use/cover change under different climate change scenarios and the difference in the runoff responses to climate change under different land use/cover change scenarios, and further investigated the joint influence of climate change and land use/cover change on runoff or flood in Huaihe River Basin. The results indicated that, climate change mitigated the increase in annual mean streamflow due to land use/cover change but amplified the increase in extreme streamflow due to land use/cover change. Climate change also amplified the impacts of land use/cover change on the probability distribution of the extreme streamflow in flood season, which led to the recurrence level of the extreme streamflow at a given return period increase remarkable, especially for longer return periods. On the other hand, land use/cover change mitigated the increase in annual mean streamflow due to climate change but amplified the increase in extreme streamflow due to climate change. Land use/cover change also amplified impacts of climate change on the probability distribution of the extreme streamflow in flood season, which led to the extreme streamflow more extreme. The joint influence of climate change and land use/cover change caused the annual mean streamflow and the extreme streamflow further increase and the probability distribution parameters of the extreme streamflow changed, which led to the extreme streamflow become more extreme and the flood risks increased. |
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