Ziqiang Xia

Hydrological Changes of the Irtysh River and the Possible Causes

Hydrological changes of the Irtysh River were analyzed concerning the changes of annual runoff and its distribution features within a year measured by coefficient of variation and concentration degree. Abrupt changes were detected by the heuristic segmentation method. Possible causes of the hydrological changes were investigated considering climate changes and human activities (especially the reservoir operation). The Mann-Kendall method was applied to estimate whether the temperature and precipitation was changed. The increased precipitation in winter may increase the runoff of April. The increased temperature and the decreased precipitation in the flood season may decrease the runoff. At the middle reaches, the impact of the reservoirs at the upper reaches is significant and may be the main factor leading to the abrupt decreases in annual runoff and its intra-annual variability and concentration. The increased water surface area of the reservoirs aggravates the evaporation and leads to annual runoff reduction. The reservoirs regulate runoff by storing water in the flood season and releasing water in the dry season. While at the lower reaches, the annual runoff remained steady and its intra-annual variation and concentration were reduced gradually because the impact of the reservoirs is relative small and the climatic impact may be more relevant.

Climate change detection and annual extreme temperature analysis of the Irtysh Basin

As the largest tributary of the Ob River, the Irtysh River is an international river partially joining the territories of China, Kazakhstan, and Russia. Four meteorological stations in the Irtysh Basin were selected and the long-term observed daily temperature data were collected. The extreme temperature change was analyzed considering climate change. Detected by the heuristic segmentation by histogram comparison approach, climate was changed during the first half of the 1970s in terms of the increased mean value and decreased standard deviation of the daily temperature series. The noticeable change of the monthly mean temperature was the warmer winter. After climate change, the annual maximum temperature was little changed and its series was not segmented. However, the annual minimum temperature was significantly changed in terms of the increased mean value by more than 2°C, so its series was segmented to the pre- and post-change point subseries. The generalized extreme value distribution was fitted to the annual extreme temperature and the parameters were estimated by the maximum likelihood method. The return levels for 10/50/100-year return periods estimated by the profile likelihood method were obtained for the annual extreme temperature. The probability of occurrence of the cold extremes was decreased after 1970s.

Reservoir-Induced Changes in Flow Fluctuations at Monthly and Hourly Scales: Case Study of the Qingyi River, China

AbstractReservoirs play an important role in modifying the natural hydrological regime. Reservoir-induced changes in flow fluctuations at monthly and hourly scales were analyzed through a case study of a tributary of the Yangtze River (China), the Qingyi River, where the Chencun reservoir is located at the upper reaches. Monthly and hourly flow data were collected from two flow gauging stations monitoring the inflow and outflow regimes, respectively. The flow fluctuations were characterized by coefficient of variation (CV) and concentration degree (Cd). Continuous Morlet wavelet transform was applied to detect diurnal period. The influence of the reservoir was revealed comparing the postdam with the predam conditions by and comparing the affected with the natural conditions. The results indicate two opposite change patterns. Reservoir impoundment may mitigate flow fluctuations at both of the monthly and the hourly scales. At a monthly scale, reservoirs usually impound water in the flood season for the dry...

Hydrological Changes of the Ili River in Kazakhstan and the Possible Causes

AbstractHydrological changes of the Ili River were analyzed concerning the changes of annual runoff and its distribution features within a year measured by coefficient of variation and concentration degree. Abrupt changes were detected by the heuristic segmentation method. Possible causes of the hydrological changes were investigated considering climate changes and human activities (especially the reservoir operation and irrigation extension). The Mann-Kendall method was applied to estimate whether the temperature and precipitation were changed. The increased temperature and the decreased precipitation in the flood season may decrease the runoff. At the middle reaches, the impact of the reservoir is significant and may be the main factor leading to the abrupt decreases in annual runoff and its intra-annual variability and concentration. The increased water surface area of the reservoir aggravates the evaporation and leads to annual runoff reduction. The reservoir regulates runoff by storing water in the f...

Research on calculation methods of instream ecological flows

In this paper, through review analyses of the existing methods calculating instream ecological flow, the monthly minimum flow calculation method and monthly frequency calculation method were applied to the Yi-Luo River, located in Northern China; whats more, some proposals were made to improve ecological flow calculation for rivers of middle-size in Northern China: the variable continuous period method and the monthly second-minimum flow method to calculate the minimum ecological flow; the monthly second-maximum flow method and the upper bound reference method to calculate the maximum ecological flow, and the synthesis method of monthly frequency calculation to determine the optimal ecological flow. Results show that the ecological flow calculated by these improved methods is favorable to the ecological protection of rivers; the output will be helpful for water resource protection and river management.