Impacts of climate change on surface hydrology in the source region of the Yellow River

Abstract: The source region of the Yellow River contributes about 35% of the total water yield in the Yellow River basin playing an important role for meeting the downstream water resources requirements. The declining water availability caused by climate change in the source region of the Yellow River is expected to have severe repercussions for the 110 million basin inhabitants in terms of water resources affecting agricultural productivity, municipal, and industrial water supply. Thus, this study investigated the impacts of climate change on surface hydrology in the source region of the Yellow River. The presented results have important implications for water resources management in the Yellow River. Hydroclimatic trend and periodicity during the last 50 years were investigated to identify significant changes in time and space over the study area. Results showed that mean annual temperature increased for all stations and it had an accelerated increasing trend during the last decade. Mean annual precipitation trends varied depending on station location; however, they were generally slightly decreasing. Annual streamflow decreased markedly especially from the 1990s but showed recovery during recent years. Statistically significant changes in trend occurred for temperature in 1998 and for streamflow in 1990. Based on the streamflow change point, seasonal analysis results showed that precipitation mainly decreased during the summer monsoon period (July-September) and temperature increased throughout the year. Corresponding to the weakened monsoon period the average runoff depth is decreasing by 0.74 mm/year over the whole area. Statistically significant 2 to 4-year periodicities for mean areal precipitation and temperature occurred over the area. For streamflow, an even stronger 8-year periodicity was revealed from the end of the 60s to the beginning of the 90s. Frequency analysis investigated the magnitudes of mean annual precipitation and discharge corresponding to a given frequency. Hydroclimatic trends and linkages at each sub-basin were investigated to further improve the understanding of observed streamflow changes. The summer precipitation (June-September) in the source region of the Yellow River accounts for approximately 70% of the annual total playing an important role for water availability, and its decreasing trend will cause water shortage in the whole river basin. Hence, summer precipitation trends and teleconnections with global sea surface temperature (SST) and Southern Oscillation Index (SOI) from 1961 to 2010 were investigated. Results show that the precipitation has a strongly decreasing gradient from southeast to northwest due to the weakening summer monsoon, and a division of the region into three homogeneous precipitation zones shows marked spatial variability. The northwest part (zone 1) had a non-significantly increasing trend, and the middle and southeast parts (zone 2 and 3) that receive the most precipitation displayed a statistically significant decreasing trend. The summer precipitation in the whole region shows statistically significant negative correlations with the central Pacific SST for 0-4 month lags and with the southern Indian and Atlantic Ocean SST for 5-8 month lags. Analyses of sub-regions reveal intricate and complex correlations with different SST areas that further explain the summer precipitation variability. The SOI had significant positive correlations mainly for 0-2 month lag with summer precipitation. It is seen that El Niño Southern Oscillation (ENSO) events have an influence on the summer precipitation, and the predominant negative correlations indicate that higher SST in equatorial Pacific areas corresponding to El Nino coincides with less summer precipitation in the source region of the Yellow River. The linkages between the precipitation and global teleconnection patterns were identified, and summer precipitation was predicted based on revealed teleconnections. It was found that precipitation in the study area is positively related to North Atlantic Oscillation, West Pacific Pattern and El Nino Southern Oscillation, and inversely related to Polar Eurasian pattern. Summer precipitation was overall well predicted using these significantly correlated climate indices, and the Pearson correlation coefficient between predicted and observed summer precipitation was in general larger than 0.6. The performance of the Xinanjiang model for daily rainfall-runoff simulation in the source region of the Yellow River was evaluated. The Blaney–Criddle method was used to calculate the potential evapotranspiration as model input due to data scarcity of this area. The Monte Carlo method was used to optimize the sensitive model parameters. The resulting Pearson correlation coefficient between observed and simulated runoff for the calibration period was up to 0.87, and 0.85 for the validation period. Accordingly, the Xinanjiang model simulated the daily runoff series well in general. Thus, the Xinanjiang model can be a proper tool for further water resources management involving runoff simulation and flood forecasting in the source region of the Yellow River.