| Global population and economic growth are causing changes in landcover and the climate.Changes in the landcover and climate are expected to alter the regional hydrological balance.The Minjiang river watershed is the largest watershed in Fujian province of China.Since 1990s,the watershed has been suffering from massive environmental damage resulting from flooding,and soil erosion.The relative contributions of changes in climate parameters and landcover classes on the hydrological processes of the watershed needs to be assessed for improving soil and water conservation efforts and watershed management.In this study,hydrological model is used to evaluate the impacts of recent and projected future changes in the landcover and climate on the hydrological parameters of the watershed.First,the landcover and climate change occurred in the watershed during the recent past is examined.Then,the Soil and water assessment tool（SWAT）model is calibrated and used it to quantify changes in the hydrological parameters and attribute them to changes in different landcover classes and climate parameters during 1992-2016.Afterwards,projected future landcover and climate for 2026-2097 is used to predict the variations between historical and future hydrology.The main results are,（1）The landcover experienced 12.2%changes during 1992-2018.Needle-leaved evergreen forests covered 55%of the watershed area during 1992.It lost 10%of area to crop-woodland,croplands,and mixed forests.Urban areas increased from 399 to 1033 km2.（2）Temperature increased with annual increment of 0.01-0.1oC.The intense precipitation events increased after 1990 and the wet years became wetter and the dry years became drier in the watershed.（3）SWAT model showed a good performance in simulating the flow and sediment loadings of the watershed.Average annual water yield in the watershed is 1000-1100 mm and sediment yield is 14 t/ha.（4）Increasing cropland was the most influential landcover change,followed by increasing crop-woodland and urban areas during 1992-2016.Sediment yield increased by1.9 t/ha（14.31%）,surface runoff by 10.5 mm（5.49%）and evapotranspiration by 3.4 mm（0.49%）.Cropland,crop-woodland and urban areas increased surface runoff and sediment yield,while cropland and crop-woodland increased evapotranspiration,and urban areas decreased it.（5）The major climate change factor influencing the watershed hydrology during1992-2016 was precipitation.Water yield,surface runoff and sediment yield increased with annual increment of 4.74 mm,2 mm and 0.13 t/ha,due to increasing trend in precipitation.evapotranspiration showed significant increasing trend with annual increment of 3.53 mm due to increase in minimum temperature.（6）Among the landcover and climate change,climate change was the most influential factor for increasing water yield,surface runoff and evapotranspiration,while landcover was more influential in increasing sediment yield.（7）Temperature in the watershed will increase until 2050 under both RCPs（4.5,8.5）.After 2050 it will stabilize under RCP 4.5,keeping the rise limited to 2oC until the end of the 21st century,while it will continue rising under RCP 8.5,bringing a total increase of 4oC since 1979.Precipitation will increase in 2nd period（2062-2097）of RCP 4.5,while it will decrease during both periods of RCP 8.5 and 1st period（2026-2062）of RCP 4.5.Urban areas will expand at the expense of croplands and crop-woodlands in future.（8）Increase in temperature alone will increase evapotranspiration while decrease water yield,sediment yield and surface runoff.These effects will be countered by changes in CO2.Increase in temperature and CO2 together will increase water yield,sediment yield and surface runoff while decrease evapotranspiration.（9）RCP 8.5 will resemble RCP 4.5 in most of the hydrological parameters,i.e.,decrease in evapotranspiration and increase in surface runoff.Water yield will decrease in1st,while increase in 2nd periods.Sediment yield will increase in the 2nd period of RCP 8.5unlike the 1st period and both periods of RCP 4.5,where it will decrease.The influence of future landcover changes on the hydrological parameters will be negligible.（10）Monthly river flow will decrease during January-April while increase during August-December.Minimum and maximum flows will increase except minimum flows in1st period of RCP 8.5,which will decrease.Frequency of large floods will increase.Overall,Surface runoff,water yield and Sediment yield increased in the recent past mostly due to increase in precipitation and conversion of forests to croplands.The role of increasing temperature was not pronounced.In future precipitation and atmospheric CO2concentrations will be dominant factors.The role of increasing temperature will be pronounced in increasing the water and sediment yield during the long future of RCP 8.5only.The role of landcover changes will not be pronounced in future due to no transitions from forests to croplands.In future surface runoff and frequency of large floods will generally increase.Conversions of croplands to forest land could increase infiltration while decrease surface runoff and soil degradation,moreover it can enhance the decrease in sediment yield together with increasing atmospheric CO2 concentrations and may increase the soil moisture and water availability during dry seasons in the form of delayed groundwater flow.Soil support structures and trees on steep slope areas and river channel banks could decrease the soil loss due to landslides during large floods,moreover riparian tree species along the river banks specifically floodplains could protect the soil and biodiversity during heavy precipitation events.Changes in monthly precipitation patterns and river flow could alter the land and aquatic species life cycles and composition,therefore the impacts should be investigated to protect biodiversity.Further studies should be conducted to investigate the impacts of future climate change on sub watershed scale to identify the hotspots within the watershed. |
