Analysis Of Changes In Precipitation Patterns And Effects On Runoff In The Nenjiang River Basin

Precipitation is one of the most important components of the water cycle.According to its forms,it is mainly divided into rainfall and snowfall.Different forms of precipitation have completely different effects on runoff,energy flow and material circulation.Under the background of global climate change,the change of precipitation pattern from snowfall to rainfall in high latitude cold regions caused by temperature rise and the advancement of snowmelt time have become one of the most prominent water cycle problems that may cause widespread impact.This paper takes the Nenjiang River Basin as an example to study the influence of the threshold temperature of precipitation pattern separation in cold regions and the change of precipitation pattern on runoff.In this paper,we collect meteorological data of 17 meteorological stations in the Nenjiang River Basin from 1951 to 2009 and hydrological data of the Dalai Hydrological Station.The objectives of this study are to 1)separate the total precipitation into different forms in the Nenjiang River Basin,2)analyze the temporal and spatial changes of precipitation patterns in the Nenjiang River Basin,and 3)quantify the contributions of precipitation patterns to runoff changes in the basin.This study will provide new ideas for the study of the impact of climate change on water resources in a river basin,and enrich the theories and contents of hydrological and water resources research in cold regions under the background of climate change.The study can also make contributions both to the scientific management and reasonable configuration of water resources in the Nenjiang River Basin.The research has achieved the following results:(1)Optimal method to separate precipitation into different forms in Nenjiang River BasinBased on the meteorological data of 17 meteorological stations in the Nenjiang River Basin,two temperature expressions(i.e.air temperature and wet bulb temperature)and three corresponding separation methods(i.e.probability guarantee method,graphic method and frequency intersection method),were selected to determine the temperature threshold of different precipitation forms(rainfall and snowfall)for each station in the Nenjiang River Basin.The temperature thresholds i obtained by the six methods are evaluated by compared the snowfall that calculated from these thresholds to the observed values with four criterions(i.e.Re,R~2,NSE and Ep).It is found that the temperature thresholds obtained by the frequency intersection method using air temperature is generally the best to separate precipitation into different forms at various meteorological stations comparing with other methods.(2)Empirical formula for separation threshold temperature of precipitation pattern in Nenjiang River BasinBased on the relationships between the optimal threshold temperature of precipitation patterns and meteorological factors,as well as local geographic information,the empirical formulas of threshold temperature are established.The results show that simulated threshold temperatures from the geographic empirical formula match better with the optimal threshold temperatures than those simulated by the meteorological empirical formula.The geographic empirical formula can be used as a simplified method to determine the threshold temperature of precipitation patterns in the Nenjiang River Basin,especially when meteorological data is not avaliable.(3)Temporal and spatial variation of precipitation characteristics in Nenjiang River BasinBased on Mann-Kendall test,the Nenjiang River Basin's total precipitation,precipitation days,snowfall,snowfall days,snow onset date,snow ending date,snow duration days,and snow rate are analyzed as 8 indicators that reflect precipitation patterns,and the results show that both precipitation and precipitation days in the Nenjiang River Basin show a downward trend in the past 49 years;although snowfall days show a downward trend during the study period,both snowfall and snow rate show significant upward trends.Spatially,precipitation,precipitation days,snowfall,snowfall days and snow rate all show the distribution characteristics of gradually decreasing from upstream to downstream,with some specific differences.On the whole,the onset date of snowfall in the Nenjiang River Basin has been advanced,while the ending date has been postponed,resulting in an overall extension of the snowfall duration days.(4)Quantification of impacts of precipitation pattern changes on runoffThe runoff in the Nenjiang River Basin showed an abrupt change in 1979,hence the study period is divided into the base period(1964-1979)and the human activity period(1980-2009).The water-heat balance equation considering the snow rate is used to quantify the impacts of precipitation pattern changes on runoff in the Nenjiang River Basin.The results show that runoff in the Nenjiang River Basin is most sensitive to changes in precipitation,followed by underlying surface parameters,potential evapotranspiration,and snow rate.The contribution of each element to the change of runoff varies with time and space.During 1980-1989 and 1990-1999,the increase in precipitation was the leading factor in the runoff increase,and contributes 83.67%and59.56%to runoff change,respectively.Change in underlying surface is the main factor leading to the decrease in runoff from 2000 to 2009;although runoff is less sensitive to change in snow rate,snow rate played an important role in changes in runoff from 2000to 2009,causing 56.79%increase(7.4 mm)in runoff.