| Water yield and purification services are important components of the water-related ecosystem services,which have significant impacts on the changes of water quantity and water quality in watersheds.They are also related to the sustainable development of water resources and human society and economy.Water-related ecosystem services are affected by both climate and land use changes.Due to recent global warming,the evapotranspiration of surface and vegetation has increased,while precipitation has decreased in many watersheds around the world.As the purifying capacity of the water-related ecosystem is weakened,the contradiction between human development and the ecosystem is becoming more and more obvious.Inorder to alleviate this crisis,it has become a current research hotspot to explore the historical temporal and spatial evolution of water-related ecosystems and their response under future climate/land use changes.Actually,there are large differences in climate,geography,and socioeconomics among different watersheds in China.The responses of water yield and water purification services under climate and land use changes has significant spatial heterogeneity.Inorder to quantify this spatial heterogeneity and its driving factors,we try to identify the areas with weakened water-related ecosystem services,and achieve rational utilization and allocation of water resources within and between different regions.Inthis paper,we selected 17 typical watersheds from south to north across the country,and assessed their changes in water yield services and water purification services under historical(1990 to 2020)and future scenarios(2020 to 2100).As a result,we analyzed the response of water-related ecosystem services to climate and land use change and the general rules of water-related ecosystem services in different watersheds.Finally,the water yield services of typical watersheds in the future period were predicted by reasonable assumptions under future climate and land use.Based on this research framework,this paper mainly includes the following four major contents and achievements:(1)We analyzed the characteristics of climate and land use change at each watershed in the historical period and future scenarios.Inthe historical period,the precipitation,air temperature and potential evapotranspiration differed significantly between watersheds.The maximum difference in precipitation between typical watersheds was 10-40 times.The maximum difference in evapotranspiration and temperature was 300-400 mm and about 25°C,respectively.The trend of climate factors also had spatial differences.There was a significant increase in some watersheds in the northwest inland and the southeast coast of China.The main characteristics of land use in the watersheds were:other types of land were converted to cropland,barren was converted to grassland,and cropland was converted to residential land,especially in the Nen,Longyangxia and Min-tuo River watersheds.Under the sustainable development path SSP1-2.6 scenario and the intermediate path SSP2-4.5 scenario,the temperature of typical watersheds increased slowly.Inthe regional competition path SSP3-7.0 scenario and traditional fossil fuels dominate path SSP5-8.5 scenario,the watersheds temperature increased and evapotranspiration decreases significantly.Among the two future land use change scenarios,there were more natural land in ecological environment protection scenario,such as forest and grassland,while land types such as cropland and residential land was reduced.(2)The Water Yield module of the InVEST model was used to evaluate the water yield of 17typical watersheds.The results show that the water yield of different watersheds varied with time and space.From 1990 to 2020,the total water yield of about one-third of typical watersheds showed a decreasing trend,while the reduction of watersheds water yield was 10-30%.Secondly,the annual total water yield and watershed water-yielding capacity varied greatly among typical watersheds.The smallest total watershed water yield and water-yielding capacity was only about 0.1%of the largest.There were differences in the response of water-yielding capacity to climate and land use changes in different watersheds.On the one hand,the response relationship of water yield to precipitation changes was basically similar in each watershed.When reducing by 100 mm,the water-yielding capacity of the watersheds would increase or decrease by about 10~4 m~3/km~2.Water yield in watersheds had the same response to the common land-use change,but the magnitude of this response varied among watersheds.For example,converting cropland into residential land would increase water yield in the watershed,and it would increase more in wet areas.This was due to the water-yielding capacity of specific land-use types in watersheds with different geographic and climatic characteristics was different.(3)The water purification module based on the InVEST model evaluated the total nitrogen output of typical watersheds.The results showed that the total nitrogen output in different watersheds had different temporal trends and spatial distribution patterns.From 1990 to 2020,the total nitrogen output of more than two-thirds of the watersheds showed an increasing trend over time,with a maximum increase of 137%.Second,there are spatial differences in total nitrogen output and output intensity among different watersheds,with an 80-fold difference between the maximum and minimum nitrogen output of typical watersheds.There were differences in the response of nitrogen output intensity to climate and land use changes in different typical watersheds.Precipitation was closely related to the nitrogen output in watersheds,but this correlation varied among watersheds.More than two-thirds of the typical watersheds had a significant positive correlation between nitrogen output and precipitation(r?0.91).The nitrogen output intensity in a small part of the watersheds was significantly negatively correlated with precipitation(r?-0.94).Inaddition,there was spatial heterogeneity in the response of watershed nitrogen output to land use change.Inthe case of the same land use change,the nitrogen output of all watersheds would increase or decrease,while the magnitude of this increase or decrease varied from watershed to watershed.This effect was more significant in arid regions.(4)Based on the set future climate and land use change scenarios,this dissertation evaluated the total water yield and total nitrogen output of typical watersheds in the future period.According to the SSP1-2.6 scenario,the total water yield of all watersheds would decrease in the next 80 years,with a maximum reduction of 10~9m~3.The total nitrogen output of the watersheds would also decrease,with a decrease of about 1-280 tons.According to the SSP3-7.0 or SSP5-8.5 scenarios,the total water yield of the watersheds would increase in the next 80 years,with a maximum increase of about 2×10~9m~3.The total nitrogen output value would increase under the SSP3-7.0 scenario,while decrease under the SSP5-8.5 scenario,with an increase or decrease of about 0-300 tons.The contributions of climate and land use changes to the total water yield and total nitrogen output of typical watersheds had spatial differences.Taking all typical watersheds as an example,climate change was the main factor affecting the water yield the watershed.Inmost typical watersheds,the contribution of climate change to total water yield basically reached more than 90%.Insome watersheds,the land use type of the watershed had a large degree of change,including the conversion of a large area of cropland to residential land or the conversion of forest or grassland to cropland in the future period.Inthese watersheds,land use change had a large contribution to the change of the total water yield and total nitrogen output of the watershed,which the contribution rate could reach 20-90%. |
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