Water Tower of the Yellow River in a Changing Climate

Abstract

Climate change due to increasing greenhouse gas emissions is likely to alter the hydrological cycle resulting in large impacts on water resources worldwide. Mountain regions are important sources of freshwater for the entire globe, but their role in global water resources could be significantly altered by climate change. Mountains are expected to be more sensitive and vulnerable to global climate change than other land surface at the same latitude owing to the highly heterogeneous physiographic and climatic settings. Furthermore, there is also evidence from observational and modelling studies for an elevation-dependent warming within some mountain regions. With the increasing certainty of global climate change, it is important to understand how climate will change in the 21st century and how these changes will impact water resources in these mountain regions. Our understanding of climate change and the associated impacts on water availability in mountains is restricted due to inadequacies in observations and models. This is also the case in the Yellow River source region (YRSR). The YRSR is often referred to as the water tower of the Yellow River as it contributes about 35% of the total annual runoff of the entire Yellow River. Located in the northeast Tibetan Plateau, a “climate change hot-spot” and one of the most sensitive areas to greenhouse gas (GHG)-induced global warming, the potential impacts of climate change on water resources in this region could be significant with unknown consequences for water availability in the entire Yellow River basin. The YRSR is relatively undisturbed by anthropogenic influences such as abstractions and damming, which enables the characterization of largely natural, climate-driven changes. A growing number of studies suggest that the YRSR is experiencing warming and streamflow reduction in recent decades, which has drawn increasing attention about the future climate changes and their impacts on water availability. While most previous studies focused on historical changes in the mean values of hydroclimatic conditions, future climate change impacts were less explored. Additionally, compared to assessing the impact of a change in average hydroclimatic condition, changes in extremes were solely missing in this region in spite of high relevance of such events on our society. This study attempts to fill these research gaps by investigating the spatial and temporal variability of both recent and future climate change impacts with specific focus on extremes. An integrated approach is applied consisting of (i) statistical analysis of historic data, (ii) downscaling of large-scale climate projections and (iii) hydrological modelling. This study contributes towards an improved understanding of spatial and temporal variability of climate change impacts in the YRSR through four major topics. The first topic focuses on the assessment of recent climate change impacts in the YRSR. Historical trends in a number of temperature, rainfall and streamflow indices representing both mean values and extreme events are analyzed over the last 50 years. The linkages between hydrological and climatic variables are also explored to better understand the nature of recent observed changes in hydrological variables. Significant warming trends have been observed for the whole study region. This warming is mainly attributed to the increase in the minimum temperature as a result of the increase in magnitude and decrease in frequency of low temperature events. In contrast to the temperature indices, the trends in rainfall indices are less distinct. However, on a basin scale increasing trends are observed in winter and spring rainfall. Conversely, the frequency and contribution of moderately heavy rainfall events to total rainfall show a significant decreasing trend in summer. In general, the YRSR is characterized by an overall tendency towards decreasing water availability, which is shown by decreasing trends in a number of indices in the observed discharge at the outlet of basin over the period 1959–2008. The hydrological variables studied are closely related to precipitation in the wet season (June, July, August and September), indicating that the widespread decrease in wet season precipitation is expected to be associated with significant decrease in streamflow. To conclude, this study shows that over the past decades the YRSR has become warmer and experienced some seasonally varying changes in rainfall, which also supports an emerging global picture of warming and the prevailing positive trends in winter rainfall extremes over the mid-latitudinal land areas of the Northern Hemisphere. The decreasing precipitation, particularly in the wet season, along with increasing temperature can be associated with pronounced decrease in water resources, posing a significant challenge to downstream water uses. In the second topic, three statistical downscaling methods are compared with regard to their ability to downscale summer (June–September) daily precipitation to a network of 14 stations over the Yellow River source region from the NCEP/NCAR reanalysis data with the aim of constructing high-resolution regional precipitation scenarios for impact studies. The methods used are the Statistical Downscaling Model (SDSM), the Generalized LInear Model for daily CLIMate (GLIMCLIM) and the non-homogeneous Hidden Markov Model (NHMM). The methods are compared using several criteria, such as spatial dependence, wet and dry spell length distributions and inter-annual variability. In comparison with other two models, NHMM shows better performance in reproducing the spatial correlation structure, inter-annual variability and magnitude of the observed precipitation. But its performance is less satisfactory in reproducing observed wet and dry spell length distributions at some stations. SDSM and GLIMCLIM showed better performance in reproducing the temporal dependence than NHMM. These models are also applied to derive future scenarios for six precipitation indices for the period 2046-2065 using the predictors from two global climate models (GCMs; CGCM3 and ECHAM5) under the IPCC SRES A2, A1B and B1scenarios. There is a strong consensus among two GCMs, three downscaling methods and three emission scenarios in the precipitation change signal. Under the future climate scenarios considered, all parts of the study region would experience increases in rainfall totals and extremes that are statistically significant at most stations. The magnitude of the projected changes is more intense for the SDSM than for other two models, which indicates that climate projection based on results from only one downscaling method should be interpreted with caution. The increase in the magnitude of rainfall totals and extremes is also accompanied by an increase in their inter-annual variability. In the third topic, we investigate possible changes in mean and extreme temperature indices and their elevation dependency over the YRSR for the two future periods 2046–2065 and 2081–2100 using statistically downscaled outputs from two CGMs under three IPCC SRES emission scenarios (A2, A1B and B1). The projections show that by the middle and end of the 21st century all parts of the study region may experience increases in both mean and extreme temperature in all seasons, along with an increase in the frequency of hot days and warm nights and decrease in frost days. By the end of the 21st century, inter-annual variability increases in the frequency of hot days and warm nights in all seasons. The frost days show decreasing inter-annual variability in spring and increasing one in summer. Six out of eight temperature indices in autumn show significant increasing changes with elevation. The fourth topic presents a modelling study on the spatial and temporal variability of the future climate-induced hydrologic changes in the YRSR. A fully distributed, physically based hydrologic model (WaSiM) was employed to simulate baseline (1961-1990) and future (2046–2065 and 2081–2100) hydrologic regimes based on climate change scenarios. The climate chance scenarios are statistically downscaled from two GCM outputs under three emissions scenarios (B1, A1B and A2). All climate change projections used here show yearround increases in both precipitation and temperature, which result in significant increases in streamflow and evaporation on both annual and seasonal basis. High flow is expected to increase considerably in most projections, whereas low flow is expected to increase slightly. Snow storage is projected to considerably decrease while the peak flow is likely to occur later. We also observe a significant increase in soil moisture on annual basis owing to increased precipitation. Overall, the projected increases in all the hydro-climatic variables considered are greater for the mid of the century than for the end of the century. The magnitude of the projected changes varies across the subbasins, and is different under different emission scenarios and GCMs, indicating the uncertainty involved in the impact analysis. Inconsistency of observed streamflow trends with future projections indicates that the recently observed streamflow trends cannot be used as an illustration of plausible expected future changes in the YRSR. Such inconsistency calls for an urgent need for research aiming to reconcile the historical changes with future projections. This study has covered a wide range of topics and a number of relevant issues of hydrology, climate change and downscaling in mountain areas. The applied multidisciplinary approach has clearly added value and provided new insights (e.g. multisite downscaling in a mountainous catchment, climate-induced changes in extremes) and opened many new avenues for scientific research in the future to be explored including investigating the potential feedbacks between land cover change and climate change and reconciling the observed trends with future projections. In general, the knowledge generated in this study can be used as the basis of local scale adaptive water resources management in a changing climate.