ABSTRACT
It is necessary to assess the non-stationarity of a hydrological series under changing environments. This study aimed to determine the validity of the stationarity of low flow series in terms of trends and possible change points, as well as the time-scale that is responsible for the production of trends and change points in low flow series. Further, we investigated how climatic variables affect low flow variations by studying their scale-dependent relationships. The modified Mann-Kendall trend test, heuristic segmentation method, discrete wavelet transform, and Pearson correlation coefficient were co-utilized to achieve these objectives. The Wei River Basin (WRB), a typical Loess Plateau region in China, was selected as the case study. Results showed significantly decreasing trends and change points in the low flow series, indicating that its stationarity assumption is invalid. The 2-year and 4-year events were the most important time-scales contributing to the trend of the original low flow series, and the 8-year periodic scale was the most influential frequency component for change point generation. Additionally, the strongest scale-dependent relationships among high frequency components (2-year and 4-year scales) of the low flow series and climatic variables (precipitation, potential evaporation, and soil moisture) demonstrated the importance of climatic factors for driving the trends of a low flow series. In contrast, human activities, including water withdrawals and water and soil conservation projects showed strong influences on the non-stationarity of low flows via affecting the low frequency component (8-year frequency and approximate components). These findings contribute to a better understanding temporal variations of low flow and their responses to changing environments, and the results also would be helpful for local water resources management as well as agricultural and ecological sustainable development.
ABSTRACT
Projection of future drought is often involved large uncertainties from climate models, emission scenarios as well as drought definitions. In this study, we investigate changes in future droughts in the conterminous United States based on 97 1/8 degree hydro-climate model projections. Instead of focusing on a specific drought type, we investigate changes in meteorological, agricultural, and hydrological drought as well as the concurrences. Agricultural and hydrological droughts are projected to become more frequent with increase in global mean temperature, while less meteorological drought is expected. Changes in drought intensity scale linearly with global temperature rises under RCP8.5 scenario, indicating the potential feasibility to derive future drought severity given certain global warming amount under this scenario. Changing pattern of concurrent droughts generally follows that of agricultural and hydrological droughts. Under the 1.5 °C warming target as advocated in recent Paris agreement, several hot spot regions experiencing highest droughts are identified. Extreme droughts show similar patterns but with much larger magnitude than the climatology. This study highlights the distinct response of droughts of various types to global warming and the asymmetric impact of global warming on drought distribution resulting in a much stronger influence on extreme drought than on mean drought.
