Search

Although the Late Pleistocene–Holocene fluctuations in the Black Sea have been studied over many years, no overall picture of this difficult natural phenomenon has as yet been developed and described. Moreover, in the literature, many researchers have provided mutually exclusive opinions about many of the processes occurring within this time interval. The variation in conceptualizations about the filling of the Black Sea basin is explained by the following reasons. (1) Fluctuations of sea level in the basin were never properly studied as physical processes under conditions of increased freshwater runoff, change in Bosporus strait depth, increase in World Ocean level, etc. Therefore, some conclusions about sea-level changes were not verified by appropriate calculations and have not been physically substantiated. (2) The study of sea-level fluctuations are conducted according to modern sea depth markers and sediment age, but as the sediment was formed under varying depth conditions, and could have moved relative to modern markers by the agency of various geological processes, it is possible that that every core may give a different level curve. (3) Processes of sedimentation, erosion and redeposition of sediment occurring in the sea have been studied insufficiently. For this reason the same geological material has been interpreted differently by various researchers. Therefore, mathematical modeling of sea-level fluctuations based on various physical laws seems to be of great importance. In this paper, the value and sign for the Black Sea freshwater balance during the Upper Pleistocene–Holocene are discussed. The hypothesis suggests that during the specified period of time, the freshwater balance of the sea was always positive, the sea was filled with water up to the sill mark in the Bosporus strait, and a river flowed into the Sea of Marmara along the strait bottom. In the present paper, geologic evidence of the river flow in the strait during the glacial age and later is presented. Other points of view about the sea's freshwater balance change are considered. A mathematical model of the Black Sea basin filling with freshwater in the Upper Pleistocene–Holocene is suggested. It considers that change in water volume inflowing to the Black Sea to be a result of ablation, neotectonic processes in the strait, bottom erosion, and sediment accumulation. Black Sea level change calculations have shown the following. As a result of repeated increases in river runoff during glacial melting, the level of the Black Sea started to rise from −80 m, probably to −20/−30 m, making, in the process, numerous secondary fluctuations. The reason for this phenomenon was the circumstance that the water volume brought by the rivers could not flow to the Sea of Marmara through the narrow canyon of the Bosporus strait at the time. Therefore, water collected in the sea, raising its level. At approximately 12 ka BP, the World Ocean level rose to the river surface and began to increase its depth. As a result of the increase in strait depth, the accumulated water in the Black Sea flowed out, thereby lowering its level to a mark close to the Ocean level. According to calculations, this occurred ca. 11 ka BP. Thereafter, the Black Sea level rose together with the Ocean level. Water within the Sea of Marmara has flowed to the Black Sea since approximately 9 ka BP in the form of a bottom counterflow. With a small time delay, theoretical change in the Black Sea level practically corresponds to data obtained from geological investigations. The theory offered here of fluctuations in Black Sea level is self-sufficient; it explains all processes from the melting of the glaciers, by physical laws and does not demand the application of any additional hypotheses. In the conclusion of this paper, the debatable questions about the Black Sea's possible lowering and the subsequent flooding of the basin are considered.

Runoff has been in decline in most river basins in China during the last 50 years. The Weihe River, the largest tributary of the Yellow River, has experienced runoff declines as large as 35% in the last century. Little is known regarding the relative contributions of climate and human impact to the observed hydrological trends in the Weihe River Basin. In the present paper, data from seven hydrological stations and 21 meteorological stations are used to analyze the long-term trends of precipitation, temperature, and streamflow. Using the daily climatic data, a Variable Infiltration capacity (VIC) hydrological model is calibrated and verified to a baseline period from 1956 to 1970. Subsequently, natural runoff for the following years (1971–2006) is reconstructed using the VIC model without considering local human impacts. On the basis of observed meteorological data and runoff and the reconstructed runoff data from 1971 to 2006 in the Weihe River Basin, we quantified long-term trends and decadal and annual variations. The results showed that precipitation and runoff have decreased since the baseline decade. We further estimated the relative contributions of human activity and climate change to the hydrological response of the Weihe River Basin and determined that human activity (such as large-scale soil conservation practices and large irrigation areas) has a greater impact on basin runoff than do climate change factors. The percentages in change of runoff due to climate change (PC) are 36%, 28%, 53% and 10% in the 1970s, 1980s 1990s and 2000s, respectively. The percentages in change of runoff caused by human activity (PH) are 64%, 72%, 47%, and 90%, respectively.

Accurate estimates of the uncertainties associated with hydrological model are essential for better streamflow simulation. This paper explores the Ensemble Kalman Filter (EnKF), an ensemble data assimilation method, for semi-distributed hydrological model updating. The semi-distributed model is very practical and often used for moderate and large basin streamflow forecasting and water resources management. The studied area in this study is a large basin of Baohe, upper branch of Hanjiang River. The semi-distributed Xinanjiang model states are updated by assimilating several spatially distributed measurement points within the whole basin. The spatial pattern and ensemble of model states such as soil water content are derived. A lumped model updating case is taken for comparison. The results show that the semi-distributed model case does better in high flow simulation than the lumped case, with 16% and 25% improvement to the simulation performance at peak flow in two periods of heavy rain processes. The smaller streamflow uncertainty at main basin outlet is also found in the semi-distributed updating case.

Long monthly streamflow series of three control hydrological stations of the Pearl River basin were analyzed by using the scanning t-test and the scanning F-test. Possible implications of the changing properties of streamflow variations for the water resource management of the Pearl River Delta are also discussed. The results indicated that: 1) more complicated changes were observed in terms of the second center moment when compared to the first original moment. More significant abrupt changes of the second center moment imply more sensitive response of streamflow stability to climate changes and human activities; 2) abrupt behaviors of the first (second) center moment of the streamflow variations tend to be more sensitive to climate changes and/or human activities in the larger river basin when compared to those in the smaller river basin. These phenomena are attributed to buffering functions of more storage space of longer river channel, and more complicated and longer runoff yield and concentration processes in the river basin of larger drainage area; 3) annual minimum streamflow of the Pearl River basin tends to be increasing. This will be helpful for better human mitigation of the salinity intrusion in dry seasons across the Pearl River Delta. Annual maximum streamflow, when compared to annual minimum streamflow, shows larger-magnitude variability reflected by larger standard deviation, implying unfavorable conditions for flood mitigation in the Pearl River Delta. The results of this paper are of scientific and practical merits for water resource management and sound human mitigation to water hazards across the Pearl River Delta, and also are a good case study for similar researches in other river deltas in the world under the changing environment.