Hydrological Regime Alteration And Wetland Landscape Pattern Change In Upper Heihe River Basin

Wetlands,the transitional zones between terrestrial ecosystem and aquatic ecosystem,are one of the most important ecosystems on earth.They play an irreplaceable role in biodiversity conservation,economic and social sustainable development and human well-being preservation.Hydrological regime occupies a most important position in evolution of the wetland ecosystem.However,hydrological regime has altered significantly due to climate change and human activities,especial in arid and semi-arid regions suffering from water scarcity.Research on hydrological regime alteration in arid and semi-arid regions and its driving force,as well as its impact on changes of wetland area and spatial pattern is of great theoretical and practical significance in wetlands conservation and restorationThis research was conducted in Upper Heihe River basin,the second largest inland river basin located in northwest of China.Firstly,a basin hydrological model was improved by incorporating topography information and adjusitn model structure.The model was carefully calibrated as well as validated to ensure its reliability.Secondly,Mann-Kendall test was carried out to identify the catastrophe point of flow regime,which divided the hydrological time series into two periods.Indicators of hydrologic alteration(IHA)and range of variability approach(RVA)were applied to quantify the degree of alteration of flow regime before and after the catastrophe point.Then the precipitation and evapotranspiration in two periods were compared.After that,various scenarios were set to analyze the driving force of the hydrological regime alteration.Finally,changes of wetland area and spatial patterns were quantified and discussion about possible hydrological reasons for the changes was conducted.Conclusions were drew as follows:(1)The improved basin hydrological model obtained better simulation and proved reliable.The model was based on three simulation units,i.e.upland,slop and lowland,which that were obtained by using topography information.Three runoff generation mechanisms were applied to simulate the hydrological processes in these three units.Objective function values of the pareto front of the improved model was smaller than the original model,indicating better simulation effect.The Nash-Sutcliffe efficiency coefficients of calibration period(1990?2002)and validation period(2003?2015)were respectively 0.85 and 0.82,indicating the reliability and accuracy of the improved model.(2)The flow regime of Upper Heihe River basin has highly altered.2002 was identified as the catastrophe point in period of 1960?2015,thus the hydrological time series was divided into 1960?2002 and 2003?2015.Compared to 1960?2002,indicators of hydrologic alteration changed significantly.The median values of each calendar months increased,especially in August,September and October;the annual minima 1-day mean,3-day means,7-day means and annual maxima 7-day means decreased while the annual maxima 1-day mean,3-day means,7-day means 30-days means and 90-day means increased to a various extent;the timing of annual 1-day maximum and 1-day minimum was respectively delayed for 32 days and 7days;high pulses frequency increased by 1 time but its duration decreased by 1.5 days,low pulses frequency and duration increased respectively by 1 time and 1.5 days;rise rate and fall rate respectively increased by 0.8 m3/(s d)and 1.1 m3/(s d),the number of hydrologic reversal increased by 7 times.Taking the hydrologic year types alteration into consideration,the degree of flow regime alteration was 74.62%,indicating high alteration.(3)The precipitation and evapotranspiration increased to different extent.Based on the simulation results,annual precipitations inand 2004?2015 were respectively 443.85 mm and 501.56 mm,and the evapotranspiration in these two periods were 273.88 mm and 303.18 mm;compared with 1960?2002,precipitation decreased slightly in March,April,November and December while increased in other month with a maximum 21.97 mm in 2003?2015;as for evapotranspiration,it increased in all months,especially in summer and autumn.(4)Climatic factors resulted in different impact on evapotranspiration and flwo regimes.Increase in precipitation and temperature together caused increase of 10.41 m3/s in average annual flow rate in which precipitation and temperature respectively resulted in the 12.83 m3/s and-2.43 m3/s.Rise in precipitation frequency decreased average annual flow rate by 1.34 m3/s.Increase in precipitation and temperature together caused average annual evapotranspiration rising 26.97 mm,in which precipitation and temperature respectively resulted in 18.80 mm and 8.17 mm.Rise in precipitation frequency increased average annual evapotranspiration by 3.19 mm.(5)Wetlands area decreased and wetland landscape pattern did not change significantly.After the hydrological regime alteration,wetland area decreased by 1973.59 hm2 in total.The number of the patch did not change much.The shape of the wetland landscape tended to be irregular.The classes of patch aggregated slightly.Landscape diversity increased slightly and the landscape evenness did not change significantly.Riverine wetland and marsh are the main wetland types.Riverine wetland area and patch number increased slightly,and its shape tended to be irregular,its aggregation and connectivity did change significantly.Marsh area decreased by 2435.07 hm2,its patch number decreased and its shap tended to be regular,its aggregation and connectivity did change significantly.The results showed that the hydrologic regime and wetland of upper Heihe River basin changed to various extent,providing helpful information for water management and wetland conservation and restoration.Increase in runoff facilitates the restoration and conservation of wetlands and oasis in middle and downstream of Heihe River basin.Attention should be paid to mash because it is more sensitive to the alteration of hydrologic regime.