Mingzhong Xiao

Copula-based risk evaluation of hydrological droughts in the East River basin, China

Probabilistic characteristics of hydrological droughts in a basin are closely related to the variability and availability of water resources of the basin. The East River basin in China is the main source for water supply for mega cities in the Pearl River Delta and cities in the vicinity of the Delta, such as Hong Kong. The water supply is subject to the vagaries of weather and water resources in the basin exhibit probabilistic characteristics. Using daily streamflow data for a period of 1975–2009 from 4 hydrological stations in the East River basin, this study attempts to determine probabilistic characteristics of hydrological droughts using copula functions. The bivariate quantile curves of the secondary return periods for hydrological drought of all the hydrological stations have been built and the results a higher risk of hydrological droughts in the upper East River basin. Furthermore, water resources should be managed by considering the entire East River basin in order to sustain the regional socio-economic development, and the extreme value copula has been used to describe the extreme drought events in the East River basin.

Regional Frequency Analysis of Droughts in China: A Multivariate Perspective

Joint probability behavior of droughts is important for China due to the fact that China is the agricultural country with the largest population in the world and it is particularly the case in the backdrop of intensifying weather extremes in a warming climate. In this case, regionalization of droughts is done using Fuzzy C- Means (FCM) clustering technique and also multivariate L-moment method. Besides, copula is used to estimate regional joint probability in terms of drought duration and severity. Evaluation of uncertainty in the joint probability curves is done using the Bootstrap resampling technique. The results indicate that: (1) five homogenous regions of droughts are subdivided. Regionalization in this study clarified the changing properties or nature of droughts, i.e., the blurred or ambiguous boundaries of the drought-impacted regions; (2) droughts in the northwest China are characterized by longer drought duration and larger drought severity, and the occurrence of the droughts in the northwest China is subject to be higher due to longer waiting time between drought events. Adverse is found for changes of droughts in the southeast China. The droughts in the north China are moderate in terms of drought duration and severity and also waiting time between drought events when compared to those in the northwest and southeast China; (3) the regional joint frequency curves are obtained with respect to drought duration and severity using the bivariate copula functions. Then the joint probabilities of droughts can be calculated using the regional probability curves and also results of mean drought duration, drought severity and waiting time between drought events. Furthermore, droughts in the regions without meteorological data can also be estimated in terms of joint probability using index-drought method proposed in this study. This study will provides theoretical and practical grounds for development and enhancement of human mitigation to drought hazards in China, and is of great importance in terms of planning and management of water resources and agricultural activities in the backdrop of intensifying weather extremes under the influences of warming climate.

Reservoir-induced hydrological alterations and environmental flow variation in the East River, the Pearl River basin, China

The East River basin is the major source of water supply for megacities in the Pearl River Delta and Hong Kong. Intensifying development of water resources and reservoir-induced hydrological alterations negatively affect ecological hydrological requirements. In this study, hydrological alterations and environmental flow variation are determined. Results indicate that: (1) multi-day maxima have reduced, while multi-day minima have increased, due to hydrological regulations of water reservoirs; (2) hydrological regimes of the East River have also been severely affected by hydropower generation, leading to a greater frequency of high and low pulses of lesser duration, and these effects are increasingly evident from the upper to lower East River basin; (3) owning to the water being released rapidly for hydropower generation or flood protection, the number of hydrologic reversals have increased after reservoir operations, also with increasing rise and fall rate; and (4) the alteration of three different types of environmental flow components have been shown in the study, which can be used to support the determination of environmental flow requirements in the East River basin.

Evaluation of risk of hydrological droughts by the trivariate Plackett copula in the East River basin (China)

East River, one of the major tributaries of Pearl River, is the major source of water supply for mega-cites within and in the vicinity of the Pearl River Delta, China. The availability and variability of water resources of the East River basin are therefore of practical importance. This study aims to investigate the probabilistic behavior of hydrological droughts in the East River basin using the trivariate Plackett copula. Daily streamflow data for the period of 1975–2009 from 3 hydrological stations in the East River basin are analyzed. Defining hydrological droughts by drought severity, duration, and minimum flow, secondary return periods are computed. Results show that the Plackett copula satisfactorily models bivariate and trivariate probability distributions of correlated drought variables. Results of risk evaluation show an increasing drought risk from the upper to the lower East River basin. This result is important for basin-scale water resources management in the East River basin.

Assessment of drought vulnerability of the Tarim River basin, Xinjiang, China

The Tarim River basin is dominated by arid climate, and agriculture plays a key role in the regional socioeconomic development. The basin is subjected to frequent droughts which are a common natural hazard. Based on Standardized Precipitation Index at 3-, 6-, and 12-month timescales, drought hazard index and composite drought vulnerability indices, drought risk and drought vulnerability are evaluated. Results indicate that (1) drought hazard is higher in northern and eastern parts of the Tarim River basin at 3- and 6-month timescales, while it is high in central and northwestern parts at a 12-month timescale and (2) drought vulnerability is higher in northwestern and southwestern parts of the basin, and the highest drought vulnerability is identified in the southwestern part. Results also indicate significant relations between drought vulnerability and the percentage of farmers, dependency ratio, and the percentage of drought-induced agricultural loss. Results of this study can be useful for drought hazard mitigation as well as for planning and management of agricultural activities and agricultural irrigation in the Tarim River basin.

Regionalization-based spatiotemporal variations of precipitation regimes across China

Daily precipitation data from 595 stations are analyzed based on regionalization to investigate changing properties of precipitation regimes across the entire China. The results indicate that the northwestern China is characterized by increasing monthly precipitation. The abrupt increase of precipitation is after early 1980s and early 1990s in the western arid zone and Qinghai-Tibet plateau, respectively. Other climate zones are dominated by decreasing precipitation regimes in autumn and increasing precipitation regimes in winter, and it is particularly true in the southwestern, southern and central China, showing seasonal shifts of precipitation changes. Besides, weak precipitation regimes are decreasing and strong precipitation regimes are increasing, and it is particularly the case in the southwestern, southern and central China, implying intensifying hydrological cycle reflected by precipitation changes in these regions. This study steps further into different hydrological responses within different regions of China to climate changes and will be relevant in regional management of agriculture development and water resources.

Observational evidence of summer precipitation deficit-temperature coupling in China

Partition of the energy between sensible heat and latent heat indicates that surface temperatures are affected by soil moisture deficits. Since transpiration by plants is the largest contributor to the lands total latent heat, the coupling of temperature and soil moisture will depend on the response of vegetation to soil moisture deficit and those are influenced by the soil moisture regimes. Utilizing daily precipitation and temperature data from China for a period of 1961–2010, this study computes average annual climatic water balance (AACWB) for defining soil moisture regimes and then quantitatively investigates the summer soil moisture-temperature coupling. With precipitation deficits (indicated by standardized precipitation index with the selected optimal timescale of 3 months) as proxy of soil moisture deficits, results indicate that the relationship between summer precipitation deficits and hot extremes tends to be enhanced when the negative AACWB draws closer toward zero while tends to be weakened with the increase of positive AACWB. For the region with the negative AACWB closing zero, the enhanced relationship should be attributed to the increase of the proportion of latent heat compared to the absorbed total energy. However, the weakened relationship with the increase of positive AACWB may be owing to the different responses of vegetation to precipitation deficit that the transpiration in the region with lower positive AACWB is less when responding to precipitation deficit. However, the physiological mechanisms behind vegetation response to soil moisture deficits still need to be further analyzed. By quantifying relevant biological and hydrological processes and their interaction, it is expected that the uncertainties in future climate scenarios be reduced, which would then allow the development of early warning and adaptation measures prior to the occurrence of hot extremes. Further, the summer precipitation deficit-temperature coupling is strongest along the strip stretching from southwest to northeast in China.

Nonstationarity-based evaluation of flood risk in the Pearl River basin: changing patterns, causes and implications

ABSTRACT Nonstationary GEV-CDN models considering time as a covariate are built for evaluating the flood risk and failure risk of the major flood-control infrastructure in the Pearl River basin, China. The results indicate: (1) increasing peak flood flow is observed in the mainstream of the West River and North River basins and decreasing peak flood flow is observed in the East River basin; in particular, increasing peak flood flow is detected in the mainstream of the lower Pearl River basin and also in the Pearl River Delta region, the most densely populated region of the Pearl River basin; (2) differences in return periods analysed under stationarity and nonstationarity assumptions are found mainly for floods with return periods longer than 50 years; and (3) the failure risks of flood-control infrastructure based on failure risk analysis are higher under the nonstationarity assumption than under the stationarity assumption. The flood-control infrastructure is at higher risk of flood and failure under the influence of climate change and human activities in the middle and lower parts of Pearl River basin. EDITOR D. Koutsoyiannis ASSOCIATE EDITOR G. Thirel

General correlation analysis: a new algorithm and application

Correlation associations have been detected using Pearson’s r which aims to analyze linear correlation between two variables. It should be noted here that associations between hydro-meteorological variables are usually nonlinear. In this sense, the classical correlation analysis method cannot truly reflect the inherent associations between variables characterized by nonlinear associations. In this case, a new algorithm has been proposed by using the ideas of local correlation, detrended cross-correlation analysis and multifractals, and this novel algorithm is called as the general detrended correlation analysis. The newly-proposed algorithm was evaluated for the validity with numerically-generated time series and the real world hydrological series. The results indicate that the newly-proposed algorithm can well reflect the nonlinear and non stationary associations between two hydrological series when compared to the classical relation detection method such as the Pearson correlation analysis method, and it is particularly the case under the condition that hydrological abrupt changes of the hydrological processes occur where the classical association analysis is not appropriate.

Transitional variations and risk of hydro-meteorological droughts in the Tarim River basin, China

The Tarim River basin, China, is a typical arid inland river basin in China. Management of water resources and agricultural development rely heavily on the understanding of droughts. In this study, an integrated drought index is proposed, based on the Standardized Precipitation Evapotranspiration Index and the Standardized Runoff Index, and then the changing properties of drought regimes have been analyzed using a Markov Chain model. Results indicate that: (1) the Kaidu and Aksu Rivers are dominated by prompt transition between hydrological and meteorological droughts. Long-lasting droughts heavily impact agricultural development in the Kaidu and Yarkand River basins; (2) the Kaidu and Aksu River basins are influenced mainly by hydro-meteorological droughts and hydrological drought is dominant in the Yarkand River basin; (3) different drought conditions are the results of different sources of river runoff. Increasing precipitation alleviate droughts in the Tarim River basin. However, a drying tendency can still be found in the Kaidu River basin; (4) higher probability is detected for the transition from both meteorological and hydrological wet to both meteorological and hydrological dry, implying that the Tarim River basin is sensitive to both meteorological and hydrological droughts. Results of this study are of practical value for the regional management of water resources, planning of agricultural irrigation, and measures for mitigation of hydrological and meteorological droughts.