Drought Spatio-temporal Variation In China And Drought Forecast In A Typical Irrigation Area Of The Yellow River

Due to the specific geographical landscapes and monsoon climate, severe drought of high frequency is one of the most devastating natural disasters in China. Under the background of climate change, and with the effect of human activities, China is facing increasingly severe drought risks. The spatio-temporal variation trend of drought in the past could give some indication of what is likely to expect in the future, and drought forecast can be useful for implementing drought resistance activities as well as for drought disaster risk management. In this thesis, we conduct the analysis of drought spatio-temporal variation in China including a drought forecast method in a typical irrigation area of the Yellow River. Firstly, a spatio-temporal continuous drought identification method and a copula based multivariate drought frequency analysis model are proposed. On this basis, drought spatio-temporal variation during the past 50 years is analyzed. Finally, a drought forecast method for water diversion relied irrigated areas is developed by combining a long-term runoff prediction with a short-term soil moisture forecast.A drought event is a spatio-temporal continuous process, with long duration and large spatial extent. Conventionally, drought events are usually identified separately, either spatially or temporally. In this study, based on a spatio-temporal conductivity algorithm of drought voxels in the longitude-latitude-time space, a three dimensional drought identification method is proposed. Drought duration, area, severity, intensity, center position are then used to characterize the drought event, which fully depict its spatio-temporal. Copulas are used to construct the joint probability distribution of drought duration, area and severity. The three dimensional drought identification and frequency analysis method is used in Southwest China, based on SPI(Standardized Precipitation Index). Results show that the identified drought processes are almost the same as the historical records, proving the rationality and practicability of the method, and that the return period of the severe autumn-winter-spring drought in Southwest China that lasted form Aug. 2009 to Jun. 2010 is about 94 years.The variational trends of major drought influencing factors(including precipitation, air temperature, and evaporation) and SPI, RDI(Reconnaissance Drought Index), SPEI(Standardized Precipitation Evapotranspiration Index) are calculated based on Mann-Kendall trend test for the period 1961—2012. Based on SPEI, the drought event spatio-temporal variation over China is analyzed using the proposed three dimensional drought identification and frequency analysis method. Results show that Southwest China, the eastern part of Sichuan Province, and Loess Plateau suffered from significant drying trends; and Tibet Plateau and Xinjiang region suffered from significant wetting trends. A totally of 176 drought events are identified. Among them the Oct. 1962—Oct. 1963 drought was the most severe one, with a return period of 109 years. North China Plain to the downstream of Yangtze River was the drought center, which suffered from large scale droughts most frequently. Furthermore, 1960 s was the period with the worst drought incidences. Drought events were increasing since 1980, and they were more prone to be centered over South China, indicating a more severe drought risk.North China Plain is one of the major grain producing areas in China, which is also the drought center. Therefore, drought forecasting in this region is of great importance. Farmland irrigation in North China Plain mainly relies on the runoff of Yellow River, which means that drought of the irrigated areas is determined by the local precipitation and the incoming runoff of Yellow River. Weishan irrigated areas, which is the largest water diversion relied irrigated areas along the downstream of Yellow River is selected as the study region. The return period of the local precipitation and the natural runoff of Yellow River both below their 25% quantiles is about 10 years. CHMI(Combined Hydrological-Meteorological drought Index) based on the methodology of SPI and copulas is proposed to synthesize the effect of local precipitation and Yellow River runoff to the diversion relied irrigated areas. Random forest model is employed to predict the long-term runoff of Yellow River, and numerical weather prediction products of ECMWF(European Center of Medium-range Weather Forecasts) are adopted to drive HELP(Hydrologically-Enhanced Land Process) model to forecast the short-term soil moisture of the irrigated area. The final drought forecasts method is developed by combining the long-term runoff prediction and short-term soil moisture forecast.