Drought Assessment At Basin Scale In The Context Of Climate Change

As a consequence of climatic anomalies, drought usually originates from precipitation deficit, propagates in hydrological systems with evaporation, soil water conservation and runoff generation, and to a large extent undermines crop growth and socioeconomic water use directly relating to hydrological conditions. Drought is more complicated and changeable under climate change background. On one hand, the total water resources available in a ceratin region is affected by the changes in the volume and variations of precipitation, and on the other hand, characteristics of hydrological cycle are also disturbed by changing climate. This study aims at investigating the impacts of climate change on drought variations and the behaviors of drought propagation in hydrological systems. Main contents and conclusions are as follows:(1) A scheme for meteorological drought risk assessment at various temporal and spatial scales based on a spatial Bayesian interpolation method is presented and used to analyze the spatial and temporal distributions of meteorological drought during the latest half century in the Huai River basin. First, risks of extreme, severe and moderate drought are evaluated at multiple scales based on observed precipitation data, and then the trend and spatial distribution of drought risks are analyzed. Results indicate:drought at seasonal scale was more threatening than the other two time scales with a larger number of observed drought events and more notable variation; an upward trend of drought risk in April and September reveals that drought condition may become more severe in spring and autumn in future; there were larger risks of extreme and severe drought in southern and northwestern parts of the basin while the northeastern areas tended to face larger risks of moderate drought.(2) Performance of three statistical downscaling tools, including SDSM, SVM and LARS-WG, on precipitation downscaling is compared by evaluating their capability of reproducing the mean behaviors, extremes and temporal distributions of precipitation series. Besides, a flexible framework of multi-criteria model averaging is established from the perspective of point forecast, and used to generate the model averaging ensembles of the three models. The usefulness and limitations of applying model averaging method in precipitation are discussed.(3) Precipitation and temperature series under SRES A2, A1B and B1scenarios in the upper part of the Huai River basin above Bengbu station are projected using the outputs of two GCMs, including CSIRO-MK3.5and CCCma-CGCM3.1. Then, future runoff and soil moisture values are simulated with Xinanjiang, HBV and SIMHYD hydrological models. Subsequently, precipitation, temperature, runoff and soil moisture responses to climate change are examined.(4) Based on the frequency analysis of precipitation, runoff and soil moisture series, drought is identified from meteorological, hydrological and agricultural perspectives respectively. Climate change impacts on drought characteristics of frequency, intensity and duration, as well as the propagation behaviors of drought in hydrological systems are investigated. Results reveal that drought frequency and duration tend to increase from A2to A1B to B1scenario, which suggests that a higher emission background may contribute to future drought severity. Hydrological drought agricultural drought show stronger sensitivity to climate change than meteorological drought. In general, future drought characteristics in terms of frequency, duration and intensity tend to be amplified significantly from meteorological drought to hydrological and agricultural droughts. Such greater sensitivity to climate change suggests that hydrological and agricultural drought may still pose larger threats to local water resources management with the increasing temperature and nonlinear propagation of water from rainfall to soil water and runoff, despite being under a more steady background to meteorological drought in future.(5) The uncertainties from different sources, including emission scenario, GCM structure and hydrological model structure, are quantified, and the importance and ordering of the uncertainty sources are compared. Results indicate that uncertainty arises in the propagation of drought from meteorological to hydrological and agricultural systems. The uncertainty in the hydrological model structure, as well as in the selections of GCMs and emission scenarios, are aggregated to the results and lead to wider variations in hydrological and agricultural drought characteristics. Moreover, the uncertainties in simulation of extremes, such as frequency of extreme drought and maximum drought duration, are much larger than that in other drought characteristics. The comparison of uncertainty ranges from different sources confirms that the ordering of uncertainty sources also varies with drought types. The ranges of emission scenario uncertainty and GCM uncertainty are generally more similar, between which the uncertainty from emission scenario is a larger source for meteorological drought and agricultural drought while the uncertainty from GCM shows more influence in hydrological drought. The selection of hydrological models is a major uncertainty source in both hydrological and agricultural drought simulations. Especially in the projections of extreme drought frequency and maximum duration, the uncertainty of hydrological model structure may even play a dominating role.