•A modified EMD–SVM model improved by removing the IMF1 is proposed.•The grid research method is employed to acquire the optimal parameter of SVM.•The EMD–SVM model has a better performance than the ...basic SVM model.•The M-EMDSVM model outperforms the SVM and EMD–SVM models.
It is of great significance for operation, planning and dispatching of hydropower station to predict monthly streamflow accurately. Therefore, the main goal of this study is to investigate the accuracy of a modified EMD–SVM model for monthly streamflow forecasting in the Wei River Basin, which has made an improvement by removing the high frequency (IMF1) based on the conventional EMD–SVM model. The EMD–SVM model is obtained by combining empirical mode decomposition and support vector machine. To acquire the optimal c and g values of SVM, the grid research method was employed. Three quantitative standard statistical performance evaluation measures, root mean squared error (RMSE), mean absolute error (MAE) and mean absolute percentage error (MAPE) were employed to evaluate the performances of the ANN, SVM, EMD–SVM and M-EMDSVM models. The comparison of results reveals that the M-EMDSVM approach has provided a superior alternative to ANN, SVM and EMD–SVM models for forecasting monthly streamflow at Huaxian hydrological station, and its pass rate of prediction reaches up to 82.6% in Huaxian station. To further illustrate the stability and representativeness of the modified EMD–SVM model, the Lintong and Xianyang stations were used to verify the model. The results show that the modified EMD–SVM model has a good stability and great representativeness as well as a high prediction precision.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
•The propagation time from meteorological to hydrological drought was examined.•The propagation time shows obvious seasonal characteristics.•ENSO and AO are strongly correlated with the propagation ...time on long timescales.•The parameter w values of the Fu’s equation exhibit positive linkages with the propagation time.
It is important to investigate the propagation from meteorological to hydrological drought and its potential influence factors, which helps to reveal drought propagation process, thereby being helpful for drought mitigation. In this study, Standardized Precipitation Index (SPI) and Standardized Streamflow Index (SSI) were adopted to characterize meteorological and hydrological droughts, respectively. The propagation time from meteorological to hydrological drought was investigated. The cross wavelet analysis was utilized to examine the correlations between hydrological and meteorological droughts in the Wei River Basin (WRB), a typical arid and semi-arid region in China. Moreover, the potential influence factors on the propagation were explored from the perspectives of large-scale atmospheric circulation anomaly and underlying surface characteristics. Results indicated: (1) the propagation time from meteorological to hydrological drought has noticeably seasonal characteristics, that in spring and summer is short, whilst that in autumn and winter is long; (2) hydrological and meteorological droughts are primarily characterized by statistically positive linkages on both long and short time scales; (3) El Niño Southern Oscillation (ENSO) and Arctic Oscillation (AO) are strongly correlated with actual evaporation, thus strongly impacting the propagation time from meteorological to hydrological drought. Additionally, the propagation time has roughly positive associations with the parameter w of the Fu’s equation from the Budyko framework.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
•An integrated drought indicator based on variable fuzzy theory was proposed.•This integrated indicator is sensitive to capture drought onset and persistence.•Detected change points were closely ...associated with human activities and ENSO events.
It is of great importance to construct an integrated drought indicator, which is of great importance to drought risk assessment and decision-making. Given the fuzzy nature of drought, the variable fuzzy set theory was applied to develop an Integrated Drought Index (IDI) combining meteorological, hydrological, and agricultural factors across the Yellow River basin in North China. The runoff and soil moisture were derived by driving the calibrated Variable Infiltration Capacity (VIC) model with observed atmospheric forcing. Furthermore, the law of mutual change of quality and quantity was adopted to identify qualitative change points of annual IDI series in the Yellow River basin. The results indicate that: (1) the Integrated Drought Index (IDI) has a better performance compared with Standardized Precipitation Index (SPI) and Standardized streamflow Index (SSFI), and it is more sensitive and effective to capture drought onset and persistence, largely owing to its combination with the information of different drought-related variables; (2) spatially, the middle reaches has a higher drought risk than the rest portions of the Yellow River basin; seasonally, drought risk in spring and winter is larger than other seasons; overall, the IDI of the basin is dominated by an insignificantly downward trend; (3) some qualitative change points of drought were identified in the Yellow River basin, and those are primarily induced by ENSO events and the construction of dams and reservoirs. This study proposed an alternative drought indicator coupled with multivariate drought-related variables by objectively determining their weights based on the entropy weight method, which has a great value in characterizing drought.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
•Propagation dynamics from meteorological to groundwater drought are fully explored.•The propagation in spring and summer is faster than autumn and winter.•The propagation significantly accelerates ...under the context of global warming.•The increasing soil moisture facilitates the response of groundwater to precipitation in surplus period.•The increasing evapotranspiration rate and heat wave accelerate the drought propagation in deficit period.
The propagation of meteorological drought in a complete water cycle is not limited to hydrological and agricultural droughts, but also involves groundwater drought. Moreover, the intensification of water cycle under the background of global warming may also affect the time of drought propagation. Therefore, studying the dynamic propagation and possible influence factors from meteorological to groundwater drought is helpful to monitor and assess the risk of groundwater drought. Here we use terrestrial water storage anomalies observations from the Gravity Recovery and Climate Experiment satellites and simulated soil moisture and runoff variations from the Global Land Data Assimilation System to show that the groundwater storage anomalies in the Pearl River Basin (PRB). The standardized precipitation index and drought severity index were used to characterize meteorological and groundwater drought, respectively. Results indicated that: (1) the propagation time of meteorological to groundwater drought in the PRB during 2002–2015 was 8 months, and that in spring and summer was shorter than that in autumn and winter; (2) the time of drought propagation has a significant deceasing trend (p < 0.01), indicating that the water cycle in the PRB was accelerating; (3) increasing soil moisture accelerates the response of groundwater to precipitation in the surplus period due to the stored-full runoff mechanism, whilst intensifying evapotranspiration rate and heat wave facilitate the drought propagation in the deficit period; (4) compared with Arctic Oscillation and El-Niño Southern Oscillation, Pacific Decadal Oscillation is the main driving force to accelerate drought propagation in the PRB.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
This study has investigated the spatio-temporal changes of droughts from 1961 to 2005 in the Wei River Basin. The Standardized Precipitation Index (SPI) was employed to describe the droughts. The ...trends of SPI value at all the meteorological stations were calculated by using the modified Mann-Kendall (MMK) trend test method, indicating that the western basin has a significantly wet trend, whilst the eastern basin including the Guanzhong Plain has a trend towards drought . Since the historical droughts records were too short to fully investigate drought properties in this basin, a practical nonparametric method was proposed to calculate the joint probability distribution of drought properties, which overcomes the shortcomings of the univariate and parametric frequency analysis. The frequency analysis of drought in the Wei River Basin indicates that the Guanzhong Plain and the surrounding areas of Huanxian meteorological station have a high drought risks, whilst the western and northern basin except the surrounding areas of Huanxian station has a relatively low drought risk.
•The copula-based joint probabilities of the combination of dry and wet conditions was investigated.•The Bayesian copula selection method was used to choose the best copula function.•The cross ...wavelet analysis was utilized to reveal the linkages between dry and wet conditions and sunspot activities.
It is of great significance to investigate the joint probabilities of the combination of dry and wet conditions between adjacent seasons under the background of global warming. The Guanzhong Plain, a very important agricultural production area, was selected as the study area. The joint probabilities of the combination of dry and wet conditions between adjacent seasons based on the Standardized Precipitation Index (SPI) in 1961–2010 were computed by copulas functions. The Mann–Kendall trend test method and wavelet analysis were used to investigate the trend and period of dry and wet conditions, respectively, and the Bayesian copula selection method was used to choose the best copula. Furthermore, the cross wavelet analysis was utilized to reveal the correlation between dry and wet conditions and sunspot activities. Results indicate the following: (1) the Guanzhong Plain is characterized by an relatively obvious trend towards dry in non-flood season and a relatively striking tendency towards wet in flood season; (2) the dry and wet conditions in the Guanzhong Plain have a statistically significant in-phase relationship with sunspot activities in 1983–1997 with a period of 8–12 years, whilst they have a non-statistically significant anti-phase relationship with sunspot activities in 1961–1973 with a period of 9–12 years, hence, sunspot activities exert a strong influence on dry and wet conditions in the Guanzhong Plain; (3) the plain is characterized by a high-frequency continuous dry condition in the entire year and a high risk of continuous wet condition in the season of Summer–Autumn;(4) the joint return period of the combination of dry and wet conditions between adjacent seasons gradually increases from east to west in the west and middle of the Guanzhong Plain, which is possibly closely linked to different geomorphic terrains, intensity of human activities and ENSO events.
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•Impacts of vegetation restoration on groundwater drought in the Loess Plateau (LP), China were fully explored.•The GWSA-DSI is developed to characterize groundwater drought conditions.•The growth ...rate of vegetation is a dominant factor affecting groundwater drought.•The groundwater consumption rate caused by vegetation dynamics in significant area is higher than non-significant area.•The large-scale vegetation restoration in the LP exerts strong impacts on groundwater drought dynamics.
The Loess Plateau (LP) is a typical water-limited area. Since the revegetation plan started in 1999, the rapid growth of vegetation has not only significantly changed the local water cycle, but also probably affected regional groundwater drought. However, the effect of revegetation on groundwater drought remains largely unexplored. To this end, we isolated the groundwater anomalies from terrestrial water storage of the Gravity Recovery and Climate Experiment satellites and soil moisture of the Global Land Data Assimilation System, and further used the drought severity index to characterize groundwater drought. The evolution characteristics of groundwater drought in the LP were analyzed, and the effect of revegetation plan on groundwater drought were explored. Results indicated that: (1) Normalized Difference Vegetation Index (NDVI) during 2003 ~ 2015 in the LP was growing rapidly, meanwhile, groundwater storage significantly decreased (p < 0.01) and groundwater drought intensified in terms of its area and intensity; (2) compared with meteorological factors, NDVI is more strikingly correlated (p < 0.05) with groundwater drought on annual, seasonal and monthly scales, especially near the key areas of vegetation restoration; (3) the growth rate of vegetation is a dominant factor affecting groundwater drought in the LP, in which the groundwater consumption rate caused by vegetation dynamics in the significant area is higher than non-significant area. Our research results provide guidance for formulation scientific and sustainable ecological restoration policies in the LP, and also offer new ideas for the study of the relationship between vegetation and groundwater drought.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Rainfall erosivity is one of the key factors influencing soil erosion by water. Improved knowledge of rainfall erosivity is critical for prediction of soil erosion and the implementation of soil and ...water conservation plan as well as sediment management projects under climate change. In this study, the Jing River Basin (JRB), a typical eco-environmentally vulnerable region of the Loess Plateau in China was selected as a case study. Spatial-temporal changing patterns of rainfall erosivity in the JRB were first examined, followed by detailed investigations of the underlying causes through exploring the relations among annual rainfall, large-scale atmospheric circulation patterns and rainfall erosivity using the cross wavelet technique. Furthermore, implications of changing rainfall erosivity for sediment load and vegetation cover were analyzed. Results indicated that: (1) the year 1985 was a turning point in the time series of annual rainfall erosivity, demonstrating the non-stationary feature. Seasonal rainfall erosivity showed a spatial gradient with decrease from the upper to the lower stream. Rainfall erosivity was the largest in summer, and has increased significantly in the eastern basin; (2) annual rainfall erosivity showed a strong positive correlation with annual rainfall amount, implying that decrease of rainfall may have led to the reduction of rainfall erosivity in recent decades; (3) El Niño-Southern Oscillation and Pacific Decadal Oscillation were correlated with rainfall erosivity during 1982–1991, suggesting that large-scale atmospheric circulation patterns have strong influences on the changing patterns of rainfall erosivity; (4) changing rainfall erosivity had negligible impacts on the variation of vegetation cover (as indexed by the Normalized Differential Vegetation Index), but has detectable influence on sediment discharge which was further modulated by local soil and water conservation practice since the 1970s. These findings are helpful for prediction of soil erosion and adaptation strategies through local soil erosion control measures and sediment control projects.
•Changing patterns, causes and implications of rainfall erosivity were fully examined.•Both PDO warm phase and ENSO events have strong impacts on rainfall erosivity.•Rainfall erosivity is closely related to sediment load variation in the JRB.•No significant correlations between rainfall erosivity and NDVI were observed in the JRB.
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•A reliable nonparametric multivariate drought index was constructed.•The drought structure of the Yellow River basin (YRB) was fully investigated.•The preferred seasons of drought onset and recovery ...in the YRB were found.•ENSO events have a strong impact on droughts in the YRB.
Investigation of drought structure in terms of drought onset, termination, and their transition periods as well as drought duration helps to gain a better understanding of drought regime and to establish a reliable drought early warning system. In this study, a Nonparametric Multivariate Standardized Drought Index (NMSDI) combining the information of precipitation and streamflow was introduced to investigate the spatial and temporal characteristics of drought structure in the Yellow River basin (YRB). Furthermore, the correlations between the El Niño-Southern Oscillation (ENSO) events and NMSDI variations were explored using the cross wavelet technique. The results showed that (1) The variations of NMSDI were consistent with those of 6-month SPI (Standardized Precipitation Index) and SSFI (Standardized Streamflow Index), indicating that the proposed nonparametric multivariate drought index was reliable and effective in characterizing droughts. (2) The preferred seasons of drought onset were spring and summer, and winter was the preferred season of drought recovery in the YRB. The long-term average drought duration in the whole basin was nearly 5.8months, which was clearly longer than the average drought onset and termination transition periods. (3) Overall, the drought structure in terms of drought duration, onset and termination transition periods in the YRB remained stable, and no appreciable change trend was found. (4) ENSO events exhibited a statistically negative correlation with NMSDI variations, suggesting that they showed strong impacts on drought evolutions in the YRB. Although the YRB was selected as a case study in this paper, the approach/indicator can be applied in other regions as well.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
•The stationarity assumption of extreme precipitation events in the WRB is invalid, especially in the upstream.•Compared with annual precipitation, precipitation extremes are much more sensitive to ...changing environments.•Impacts of ENSO events on precipitation extremes in the WRB are stronger than those of PDO.
The investigation of extreme precipitation events in terms of variation characteristics, stationarity, and their underlying causes is of great significance to better understand the regional response of the precipitation variability to global climate change. In this study, the Wei River Basin (WRB), a typical eco-environmentally vulnerable region of the Loess Plateau in China was selected as the study region. A set of precipitation indices was adopted to study the changing patterns of precipitation extremes and the stationarity of extreme precipitation events. Furthermore, the correlations between the Pacific Decadal Oscillation (PDO)/El Niño-Southern Oscillation (ENSO) events and precipitation extremes were explored using the cross wavelet technique. The results indicate that: (1) extreme precipitation events in the WRB are characterized by a significant decrease of consecutive wet days (CWD) at the 95% confidence level; (2) compared with annual precipitation, daily precipitation extremes are much more sensitive to changing environments, and the assumption of stationarity of extreme precipitation in the WRB is invalid, especially in the upstream, thereby introducing large uncertainty to the design and management of water conservancy engineering; (3) both PDO and ENSO events have a strong influence on precipitation extremes in the WRB. These findings highlight the importance of examining the validity of the stationarity assumption in extreme hydrological frequency analysis, which has great implications for the prediction of extreme hydrological events.
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