River network system was heavily damaged during the rapid urbanization in the Taihu Lake Basin in recent decades. A large number of rivers were buried and disappearing, which changed the hydrological process of the basin. Regulation capacity of river networks is an important factor for flood disaster mitigation. Based on river networks data derived from the topographic map in the 1960s, 1980s and 2010s at a scale of 1:500000 and water level data of three hydrological stations from 1960 to 2010, we selected a list of indices, such as drainage density (R_d), water surface ratio (W_p), box dimension (D) and Hurst index (H) to analyze the changes of river networks and its regulation capacity in Wuchengxiyu region during the past 50 years. What's more, the relationship was established between the regulation capacity of river networks and the indices of stream structure using the Hurst index and the principles of Representative Elementary Watershed (REW) Model. Results showed that: (1) the R_d, W_p, and D decreased by 18.87%, 24.59% and 7.60% in the past 50 years respectively due to the process of urbanization, and the reduction of three stream structure indices from the 1980s to 2010s was higher than that during the 1960s to 1980s. (2) The Hurst index of water level was used to describe the regulation capacity of river networks and was calculated through the R/S analysis of the time series. Decreasing trends of the Hurst index were detected during annual and wet season, which indicated that the regulation capacity of river networks was gradually reduced due to urbanization. (3) The relationship between the indices of river networks and its regulation capacity was established through the concept of REW and Hurst index. The calibration and validation was carried out for this relationship with relevant data. This work will provide a simple and practical method to assess regulation capacity of river networks.

Field investigation was carried out in the Badain Jaran Desert, where develop numerous megadunes with the largest height difference in the world. Infiltration-excess runoff, slow seepage runoff, erosion features of runoff caused by atmospheric precipitation, as well as mixed physical sediment of runoff-aeolian sand and chemical deposition (secondary salts) caused by ground runoff converted into surface runoff, were discovered in the slope surfaces of megadunes. These natural features are rare in arid desert area. According to the data of electron microscopy observation, energy spectrum analysis, chemical analysis and particle-size analysis and others, some issues are discussed, namely, mineral and chemical composition of secondary salts, particle-size composition of aeolian sand and mixed physical sediment of runoff-aeolian sand, runoff type, water balance of megadune area and mechanism of precipitation recharging groundwater and lake water. The emergence of infiltration-excess runoff means that the strong effective precipitation can be used as a source of groundwater, although there is less rainfall in this region. Various runoff and runoff depositions on a deep level sufficiently prove that precipitation via infiltration reaches the bottom of megadune and constitutes effective recharge to groundwater. The fine layer with relative impermeability is the reason why underground runoff is exposed to Earth's surface. The positive water balance is surprising in the megadune area. Precipitation, after evaporation and transpiration, and other consumption, could provide about 134648.4 tons of water per square kilometer per year to recharge groundwater and lake water, which has become one of the important recharge sources of lake water. Positive water balance and precipitation can recharge groundwater in the megadune area and mainly depend on four conditions, namely, the presence of effective precipitation, scarce vegetation and less transpiration, a high infiltration rate and a small evaporation depth of sand layer. These factors are also the priority for numerous lakes developed in the megadune area of the Badain Jaran Desert.

Based on 6-hourly (02:00, 08:00, 14:00 and 20:00 Bejing time (BJT)) air temperature data of 38 meteorological stations over Tibet from 1981 to 2014, the spatial-temporal distribution and climate abrupt characteristics of air temperature are analyzed by using the methods including linear regression and Mann-Kendall test. Also, the correlation between the change rates of surface air temperature and latitude (longitude, and altitude) is discussed. The results showed that, the seasonal air temperature in Tibet exhibits unanimously increasing trend with a rate of 0.14-0.80 ℃/10a during the past 34 years, and the most significant increase occurred in winter. In terms of the rate per decade for the 6-hourly air temperature observations, 08:00 BJT during summer experienced the highest increasing rate, while 14:00 BJT showed the highest values for the other three seasons . The maximum rate for the increasing air temperature ranges from 0.36 ℃/10a (P < 0.001) to 0.94 ℃/10a (P < 0.001). Among all the 38 stations, there were only 32% (about 12) showing the peak time with the highest rate of changes at 08:00 BJT air temperature, which are predominantly located in much of Qamdo, Ngari prefecture and at weather stations such as Nagqu, Lhasa and Xigazê, while the rest of weather stations showed the highest increasing rate at 14:00 BJT. In spring and autumn, as the increasing rate was related to longitude, it has a larger rate in western than that in eastern Tibet. In winter, the highest increasing rate of air temperature occurred in the regions with higher altitudes and latitudes, and the higher increasing rate of air temperature was observed at higher latitudes in summer. As for the decadal characteristics of 6-hourly air temperature, the 1980s experienced negative anomalies, compared with positive anomalies in the first decade of the 21st century. Additionally, it was found with abrupt change test that at the annual and seasonal scales most of hourly air temperatures have abrupt change. For instance, the abrupt change of all four hourly air temperatures in summer occurred in the first decade of the 21st century. In winter, the abrupt change of air temperature at 02:00 and 08:00 BJT occurred in the late 1990s, while that at 14:00 and 20:00 BJT was found in the first 10 years of the 21st century. As can be seen in the article, many factors such as topography, various meteorological elements in the plateau and the atmospheric circulation play important roles in the surface air temperature change in Tibet.