Changes in Precipitation and Air Temperature and Its Impacts on Runoff in the Nujiang River basin
怒江流域降水与气温变化及其对跨境径流的影响分析

Climate change is likely to produce significant effects on the hydrological cycle through altering snow and ice cover,patterns of precipitation,soil moisture,and runoff.In this study,trends in precipitation,mean air temperature,and runoff were detected by the non-parametric Mann-Kendall test in the Nujiang River basin.Nujiang River is one of the important international rivers in China.Impacts of changes in precipitation and air temperature on runoff were quantified based on the temporal and spatial distributions of these hydrometeorological variables.The non-parametric Mann-Kendall test and Morlet wavelet were used to detect trends and periodicity of major hydrometeorological variables respectively.Results show that both precipitation and air temperature exhibited increasing trends during the period 1958-2009.Although an increasing trend in precipitation was observed,it exhibited a markedly decreasing trend over the last 20 years.The magnitude of the increasing trends in air temperature increased,with a value of 0.5℃/10a during the last 20 years.Precipitation of the basin showed an increasing trend,with a magnitude of 6.0 mm/10a.It,however,tended to decrease over the past 20 years.Runoff at the Daojieba station showed a statistically significant increasing trend,with increasing magnitudes,e.g.,87×103m3/10a, 121×103m3/10a,and 440×103m3/10a in the periods of 1958-1979,1970-1990,and 1980-2000, respectively.All of monthly runoff also experienced increasing trends except for August.Changes in precipitation were considered to play a positive effect on the increasing runoff at the Daojieba station.In terms of hydrologic budget calculations,soil storage changes could be negligible at interannual scales.Therefore,runoff changes appear to be caused by changes in precipitation, glacier and snow melt,and evapotranspiration within the basin.Results show that impacts of changes in precipitation and air temperature on runoff were affected primarily by runoff sources and corresponding ratios.For example,increasing runoff at the Jiayuqiao and Daojieba stations were affected by increasing precipitation and increasing snow-melt runoff caused by rising air temperature,while impacts of increasing air temperature on runoff were consistent with the ratio of snow-melt runoff and total runoff.It was concluded that impacts of increasing precipitation on increasing runoff were at most 2.4 mm/a for the Jiayuqiao station located in the upper reaches of the Nujiang River,whereas increasing glacier and snow melting runoff caused by rising air temperature on runoff was at least 38.5%.As for the Daojieba station located in the lower reaches of the study region,impacts of increasing precipitation on increasing runoff were estimated to be 94.1%at most.