Entropy-Based Research on Precipitation Variability in the Source Region of China's Yellow River

The headwater regions in the Tibetan Plateau play an essential role in the hydrological cycle, however the variation characteristics in the long-term precipitation and throughout-the-year apportionment remain ambiguous. To investigate the spatio-temporal variability of precipitation in the source region of the Yellow River (SRYR), different time scale data during 1979-2015 were studied based on Shannon entropy theory. Long-term marginal disorder index (LMDI) was defined to evaluate the inter-annual hydrologic budget for annual (AP) and monthly precipitation (MP), and annual marginal disorder index (AMDI) to measure intra-annual moisture supply disorderliness for daily precipitation (DP). Results reveal that the AP over the SRYR exhibits remarkable variation, with an inclination rate of 2.7 mm/year, and a significant increasing trend. The climatic trend reversed from warm-dry to warm-wet around the turn of this century. The start of the wet season has advanced from May instead of June, supported by the proportion of MP in AP and the LMDI for May are both comparable with the values during June-September. May contributes the main changes in AP, as it is the only month in the wet season which shows a significant increasing trend during 1979-2015, and has a value in the LMDI that divides the basin in half spatially, the same as AP, with a high value in the northwest and low in the southeast. The AMDI roughly rises with latitude in spatial distribution, with wetlands and glaciers disturbing the continuity of the pattern for a relatively perennial moisture supply. AP has increased on northwest high-altitude areas first and then the southern corner since the beginning of this century. Wetting is mainly attributed to the enhanced southwest monsoon and the warming-induced freeze-thaw process. Meanwhile, AMDI variation concentrated on the Zoige Plateau Wetland, the headwater corner, the summit and part of the North Slope in the Bayan Har Mountain, as a result of a single or combined effect of global climate change and human protection.