Characteristics And Driving Factors Of Runoff Change In The Wanqun River Upstream Forest Basin

Wanquan River Basin(WRB)has a tropical monsoon and marine climate and is endowed with rich tropical forest resources.It therefore plays a pivotal role in conserving water resources.Meanwhile,the region is often plagued by natural disasters,particularly by the tropical cyclones and human activities,offering its water resources both unique and vulnerable.Climate change and human activities have significantly altered the runoff generation and concentration as well as hydrological regime in the WRB.As a result,it is of great value to study the hydrological response of climate change and land use change in the upstream WRB so as to shine light on the impacts of changing environment on change in runoff across different regions.This paper took the upstream WRB as the research subject and conducted the following study.First,mathematical statistics was employed to analyze the characteristics of climate change,and remote sensing images were processed to examine those of land use change.Next,the Indicators of Hydrological Alteration(IHA),the Range of Variability Approach(RVA),the Histogram Matching Approach(HMA)were adopted to probe into the change of alteration.Finally,the SWAT model was applied to simulate runoff,and the separation method was explored to quantitative analyze the impact of climate change and human activities on runoff.In the meantime,the hydrological responses under various climates and land use scenarios were also simulated.The main research findings are as follows:(1)From 1967 to 2014,the annual precipitation showed a slightly upward trend;the average annual precipitation was 2,462.22 mm;and the inter-annual precipitation fluctuated sharply.The inter-annual precipitation distribution was uneven and unimodal,where the rainy season lasted from May to November,representing 80%-90%of the total annual precipitation.The average maximum and minimum temperatures were 28.7°C and21.6°C,respectively,both presenting a moderately upward trend.The annual average evaporation was 1,083.0 mm,displaying an increasing trend and with considerable fluctuations in inter-annual evaporation.The increase of evaporation primarily resulted from the destruction of natural forests and the growing of myriad planted forests(i.e.rubber forests and other economic forests).(2)It was found by comparing the land use maps of 1988 and 2014 that the principal land use types were natural forests,rubber forests and other economic forests,and that the forest cover in the upstream WRB was high,making up 89%of the total area.According to the land use change,the areas of rubber forests,waters and settlements were on the rise.To be specific,that of rubber forests skyrocketed by 54.90%,whereas that of natural forests plunged by 29.54%.Regarding the change to land-use structure,land use within the WRB was inter-replaceable.Cultivated land was most likely converted to planted forests(i.e.rubber forests and other economic forests).(3)Based on daily runoff data from 1957 to 2014,the annual runoff was decreasing.The change-point detection method revealed that the abrupt change point in the runoff of the upstream WRB occurred in 1990.After the abrupt change point,the runoff sequence changed substantially,so did a number of hydrological indicators.In particular,reversals shot up by 28.81%;the degree of hydrological alteration by RVA(D_R)was high at 89.85%,while that by HMA(D_H)was medium at 66.34%.Directly affected by precipitation,inter-annual distribution of runoff was uneven,where the runoff concentration took place around October before and after the abrupt change point.By comparison,water conservancy projects and land use change reduced the non-uniform coefficient(Cn),the complete accommodation coefficient(Cc)and the concentration degree(Cd)after the abrupt change point,demonstrating that human activities made the runoff distribution more even.(4)A SWAT model was developed following the analysis of the hydrological and meteorological characteristics as well as the changes in land use/cover change.Next,the SWAT?CUP software was employed to perform parameter sensitivity analysis,calibration and validation.The results of parameter sensitivity analysis indicate that the most sensitive seven parameters for runoff simulation are available water capacity of soil layer(SOL?AWC),groundwater re-evaporation coefficient(GW?REVAP),baseflow Alpha factor(ALPHA?BF),soil evaporation compensation factor(ESCO),threshold depth of water in the shallow aquifer required for return flow to occur(GWQMN),soil conservation service(SCS)curve number(CN2),and groundwater delay time(GW?DELAY).The SWAT model can properly simulate the upstream WRB before the abrupt change point.Specifically,the coefficients of determination(R~2)for the calibration period(1967-1978)and the validation period(1989-1990)was 0.84 and 0.91,respectively,while the Nash-Sutcliffe model efficiency coefficients(Ens)for these two periods were 0.82 and0.84,respectively.(5)Human activity is the dominant factor for runoff reduction.On top of this,climate change also exerts an important impact on runoff,the change in precipitation in particular.Climate change,land use change and human activity account for 44.05%,10.83%and45.12%,respectively,of the runoff reduction.(6)An array of climate change and land use change scenarios were simulated based upon the analysis of such changes.The scenarios of climate change suggest that the increase of precipitation leads to that of runoff,where a 10%increase in annual precipitation causes a 23.9%increase in annual runoff.Hence,precipitation is positively correlated to runoff and significantly influences runoff.By contrast,runoff falls as temperature increases,where a 1°C and 2°C increase in temperature could reduce annual runoff by 0.52%and 0.92%,respectively.Therefore,temperature is negatively correlated to runoff.With respect to the scenarios of land use/cover change,the runoff in cultivated land is the highest,indicating that cultivated land can boost runoff.On the contrary,the runoff in rubber forests is the lowest,followed by that in other economic forests and natural forests.The results show that forest land can well conserve and store water,so in the development of tourism in the WRB,priority should be given to protecting local natural forests,enhancing forest quality and preserving planted forests.Additionally,the forest cover is relatively high in the study area during the base period,so there is no significant gap in the impacts of the three forest scenarios(i.e.rubber forests,natural forests and other economic forests)on runoff...