| The characteristics of the loess platform tableland are water shortage,terrain fragmentation,vegetation degradation,and soil and water loss.Hydrological and ecological problems occur frequently in this region.Climate change and increasing human activities have brought great impact to the originally fragile loess tableland.Under the changing conditions,revealing the evolution mechanism of hydro-ecological processes from the perspective of representative small watershed plays an important role in supporting the comprehensive utilization of water resources,the formulation and decision-making of soil and water conservation and ecological construction measures,the establishment of a harmonious ecological environment system between human and water,and the sustainable development of regional economy.In this thesis,We take Youfanggou watershed—a typical small watershed in the loess platform tableland as the research object,and analyzed the climate and land use change rules and their future change trends.Besides,the change rules of hydrological and ecological processes in the corresponding period were also analyzed.On this basis,in combination with the actual evaporation model and water balance method,the variation coefficient method,hydrothermal coupling balance equation and potential NPP(PNPP)and actual NPP(ANPP)differential method were used to reveal the response of hydro-ecological processes to climate and land use change,and quantitatively evaluate the contribution of the two factors.At the same time,the regional hydrological and ecological model(RHESSys)was used to simulate the hydrological and ecological processes of the basin,and predicte the hydrological and ecological processes of the basin under different climate scenarios in the future.The main conclusions are as follows:(1)During 1985?2015,the average annual precipitation in the basin was 528.7mm,and there was no significant change trend and no abrupt change;the annual mean daily average temperature was 12.8?,showing a significant uptrend.The average potential evapotranspiration was 887.6mm,and there was no obvious change trend.Under the RCP2.6and RCP8.5 scenarios,the precipitation and temperature would show an upward trend.(2)During the whole study period,farmland was the main land use type in the basin,accounting for more than 60%,followed by forest land,grassland and residential site.The farmland area decreased year by year,while the area of remaining types increased.The transfer characteristics of land use mainly are the inward and outward transfer between farmland and woodland,grassland and residential sites.Before 2000,the change of land use was relatively single,while after 2000 the change tended to be obvious.The main driving factors of land use change are human activities such as forest rehabilitation from slope agriculture and watershed management under policy guidance.Based on the CA-Markov model,the land use composition in the future is similar to that of the present.Farmland is still the main land use type in the basin,whose area is still decreasing,and the area of remaining types is increasing.(3)The average annual runoff depth during 1985?2015 was 35.8mm,showing an extremely significant decreasing trend(P<0.01).The annual average NPP of vegetation was326.62 g C·m-2·a-1,showing an extremely significant increasing trend(P<0.01).There were significant differences(P<0.01)in annual runoff depth and annual vegetation NPP before and after 2000.The annual runoff is positively correlated with precipitation.Annual NPP is positively correlated with air temperature,while the precipitation is positively correlated with vegetation NPP lagging one year.The land use change significantly reduced the annual runoff,and the effects of climate and surface changes on the decline in streamflow were-98.1%and198.1%.The influence of land use change on vegetation NPP was completely contrary before and after 2000:the former could cause the decrease of vegetation NPP while the latter could cause the increase of vegetation NPP,and the impact of land use change on NPP after 2000was higher than that in before 2000.The contribution rates of climate change and land use change to NPP change are 32.7%and 67.3%,respectively.Land use change has dominant influence in the change of runoff and vegetation NPP.(4)On the whole,RHESSys can simulate the change of monthly runoff well.The average relative error and Nash efficiency coefficient of monthly runoff are-10.05%and 0.76in the calibration period,and-18.83%and 0.70 in the validation period.The RHESSys model can simulate the water cycle process well in Youfanggou watershed.The simulated vegetation NPP is basically consistent with the MODIS NPP,and the Nash efficiency coefficient is 0.60.The simulated LAI is also spatially in line with existing research results.RHESSys model can also simulate the ecological process of vegetation well in Youfanggou watershed.During the study period,the average annual vegetation ecological water consumption in the watershed was 456.0mm,showing a significant increasing trend(P<0.05);the proportion of annual ecological water for vegetation in precipitation was 0.865,which showed an extremely significant increasing trend(P<0.01)and was mutated in 2003.The ecological restoration measures bring about the decrease of runoff(offsite effect)and the increase of vegetation water consumption(local effect).The ecological environment of the basin improved gradually.The scenario setting simulation analysis results showed that the contribution of climate change and land use change to runoff reduction and vegetation NPP change were-89.4%,189.4%and 29.8%,70.2%,respectively.Land use change is the main influencing factor in runoff and vegetation NPP change.(5)The annual runoff of the Youfanggou watershed would decrease in all years except for 2040s under RCP8.5 scenario.Under the two climate scenarios,the annual vegetation NPP has no obvious trend in the future.The watershed management is becoming more and more scientific,and the vegetation restoration is gradually reaching a stable state,so the vegetation NPP in the watershed presents a stable change state under the interaction of climate and land use change.Under the two climate scenarios,the rainwater utilization rate(RUE)for vegetation in the basin shows an insignificant trend of decrease,with the trend rate of-0.0049/a and-0.0040/a,respectively.The RUE in the future would have a downward trend,which is mainly related to the fact that the current vegetation restoration had reached a relatively high level.Under RCP2.6 and RCP8.5 scenarios,the future annual average potential evapotranspiration are 868.22mm and 938.54mm,respectively.The change trend is not obvious under RCP2.6 scenario,while obvious under RCP8.5 scenario with a change rate of 9.48mm/a;the dryness index wavelike rises in the next 30 years,indicating that the drought situation of Youfanggou basin would intensify in the future. |
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