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Hu Y.,Yellow River Conservancy Commission | Hu Y.,UNESCO-IHE Institute for Water Education | Maskey S.,UNESCO-IHE Institute for Water Education | Uhlenbrook S.,UNESCO-IHE Institute for Water Education | Uhlenbrook S.,Technical University of Delft
Climatic Change | Year: 2012

Spatial and temporal changes in daily temperature and rainfall indices are analyzed for the source region of Yellow River. Three periods are examined: 1960-1990, 1960-2000 and 1960-2006. Significant warming trends have been observed for the whole study region over all the three periods, particularly over the period 1960-2006. This warming is mainly attributed to a significant increase in the minimum temperature, and characterized by pronounced changes in the low temperature events composing a significant increase in the magnitude and a significant decrease in the frequency. In contrast to the temperature indices, no significant changes have been observed in the rainfall indices at the majority of stations. However, the rainfall shows noticeable increasing trends during winter and spring from a basin-wide point of view. Conversely, the frequency and contribution of moderately heavy rainfall events to total rainfall show a significant decreasing trend in summer. To conclude, this study shows that over the past 40-45 years the source region of the Yellow River has become warmer and experienced some seasonally varying changes in rainfall, which also supports an emerging global picture of warming and the prevailing positive trends in winter rainfall extremes over the mid-latitudinal land areas of the Northern Hemisphere. © 2011 Springer Science+Business Media B.V.

Liu X.,Yellow River Conservancy Commission | Yang S.,Beijing Normal University | Dang S.,Yellow River Institute of Hydraulic Research | Luo Y.,Beijing Normal University | And 2 more authors.
Science China Technological Sciences | Year: 2014

The impact of vegetation coverage on erosion and sediment yield in the Loess Plateau has been extensively studied, but the research has been primarily based on observations from slope runoff plots or secondary forest regions; the scaling method remains unresolved when it is applied at a large spatial scale, and it is difficult to apply to regions with severe soil and water loss given the predominance of herbs and shrubs. To date, there is little data on the quantitative impact of changes to vegetation on sediment concentration at a large spatial scale. This paper is based on vegetation information from remote sensing images, measured rainfall and sediment data over nearly 60 years, and results from previous runoff and sediment variation research on the Yellow River. We introduce the concepts of a sediment yield coefficient and the percentage of effective vegetation and erodible area, analyze the impact of different vegetation conditions on the flood sediment concentration and sediment yield, and evaluate the effect of rainfall intensity on sediment yield under different vegetation conditions at the watershed scale. We propose models to evaluate the impact of vegetation on sediment yield in the loess gully hilly region, which are based on remote sensing data and support an application at a large spatial scale. The models can be used to assess sediment reduction that results from the current significant improvement of vegetation in the Loess Plateau. © 2014 Science China Press and Springer-Verlag Berlin Heidelberg.

Qin D.,CAS Institute of Geology and Geophysics | Qian Y.,Yellow River Conservancy Commission | Han L.,International Atomic Energy Agency | Wang Z.,CAS Institute of Geology and Geophysics | And 2 more authors.
Journal of Hydrology | Year: 2011

Environmental tracers CFCs, 18O, 2H and tritium were used to determine the natural groundwater recharge and the impact of irrigation activity on the groundwater system in the semi-arid Zhangye Basin of China. Groundwaters in the irrigated areas have been identified as mixtures containing fractions recharged in different periods of time. The CFC and 3H data show that the oldest fraction in the groundwater was recharged before 1950, whereas the younger fractions were recharged in different periods of time since 1950. Stable isotope (18O, 2H), CFC and electrical conductivity data show that most of the samples can be regarded as binary mixtures with the river/irrigation water presents the younger fraction and the regional groundwater presents the older fraction. Binary mixing model is used to estimate the age and fraction of the younger component. Most of the younger fraction was recharged after 1980s, in response to the increasing irrigation activities. Compared to local precipitation surface water plays a major role in recharging the aquifer in the irrigated area. The irrigation activity had more impact on the aquifer under thin unsaturated zone (<10m), due to short travel times and high amounts of recharge, whereas it had less impact on the aquifer under thick unsaturated zone (tens of meters). CFCs are useful in identifying regions of different impact of irrigation return flow. The positive correlation between nitrate and CFC data show that contaminants are transported to the saturated zone by irrigation water. This study shows that in this semi-arid basin due to strong evaporation of infiltrating surface water and regional groundwater, δ18O and EC values, in contrast to CFCs, do not show simple relationship with NO3- concentration in groundwater. Combined with a proper mixing model, however, they can provide evidences that the CFCs found in groundwater were introduced by infiltrating irrigation return flow and, therefore, reveal that human activities can produce a much localized water circulation and influence groundwater vulnerability. © 2011 Elsevier B.V.

Hu Y.,Yellow River Conservancy Commission | Hu Y.,UNESCO-IHE Institute for Water Education | Maskey S.,UNESCO-IHE Institute for Water Education | Uhlenbrook S.,UNESCO-IHE Institute for Water Education | Uhlenbrook S.,Technical University of Delft
Theoretical and Applied Climatology | Year: 2013

Three statistical downscaling methods are compared with regard to their ability to downscale summer (June-September) daily precipitation at a network of 14 stations over the Yellow River source region from the NCEP/NCAR reanalysis data with the aim of constructing high-resolution regional precipitation scenarios for impact studies. The methods used are the Statistical Downscaling Model (SDSM), the Generalized LInear Model for daily CLIMate (GLIMCLIM), and the non-homogeneous Hidden Markov Model (NHMM). The methods are compared in terms of several statistics including spatial dependence, wet- and dry spell length distributions and inter-annual variability. In comparison with other two models, NHMM shows better performance in reproducing the spatial correlation structure, inter-annual variability and magnitude of the observed precipitation. However, it shows difficulty in reproducing observed wet- and dry spell length distributions at some stations. SDSM and GLIMCLIM showed better performance in reproducing the temporal dependence than NHMM. These models are also applied to derive future scenarios for six precipitation indices for the period 2046-2065 using the predictors from two global climate models (GCMs; CGCM3 and ECHAM5) under the IPCC SRES A2, A1B and B1scenarios. There is a strong consensus among two GCMs, three downscaling methods and three emission scenarios in the precipitation change signal. Under the future climate scenarios considered, all parts of the study region would experience increases in rainfall totals and extremes that are statistically significant at most stations. The magnitude of the projected changes is more intense for the SDSM than for other two models, which indicates that climate projection based on results from only one downscaling method should be interpreted with caution. The increase in the magnitude of rainfall totals and extremes is also accompanied by an increase in their inter-annual variability. © 2012 Springer-Verlag.

Hu Y.,Yellow River Conservancy Commission | Hu Y.,UNESCO-IHE Institute for Water Education | Maskey S.,UNESCO-IHE Institute for Water Education | Uhlenbrook S.,UNESCO-IHE Institute for Water Education | And 2 more authors.
Hydrological Processes | Year: 2011

Much of the discussion on hydrological trends and variability in the source region of the Yellow River centres on the mean values of the mainstream flows. Changes in hydrological extremes in the mainstream as well as in the tributary flows are largely unexplored. Although decreasing water availability has been noted, the nature of those changes is less explored. This article investigates trends and variability in the hydrological regimes (both mean values and extreme events) and their links with the local climate in the source region of the Yellow River over the last 50 years (1959-2008). This large catchment is relatively undisturbed by anthropogenic influences such as abstraction and impoundments, enabling the characterization of widely natural, climate-driven trends. A total of 27 hydrological variables were used as indicators for the analysis. Streamflow records from six major headwater catchments and climatic data from seven stations were studied. The trend results vary considerably from one river basin to another, and become more accentuated with longer time period. Overall, the source region of the Yellow River is characterized by an overall tendency towards decreasing water availability. Noteworthy are strong decreasing trends in the winter (dry season) monthly flows of January to March and September as well as in annual mean flow, annual 1-, 3-, 7-, 30- and 90-day maxima and minima flows for Maqu and Tangnag catchments over the period 1959-2008. The hydrological variables studied are closely related to precipitation in the wet season (June, July, August and September), indicating that the widespread decrease in wet season precipitation is expected to be associated with significant decrease in streamflow. To conclude, decreasing precipitation, particularly in the wet season, along with increasing temperature can be associated with pronounced decrease in water resources, posing a significant challenge to downstream water uses. © 2011 John Wiley & Sons, Ltd.

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