Evaluation of past,present and future tools for radar-based flash-flood prediction in the USA

Abstract

The societal impacts of flash floods are more significant than any other weather-related hazard. They are often manifested in the form of damage to infrastructure, flooding of roadways and bridges, creating deadly hazards to motorists and inundation of crops and pasture. Some of these hazards can be anticipated and thus mitigated given effective warning systems. This study describes the tools proposed over recent decades in the USA to predict flash flooding and evaluates them using a common observational data set. Design recommendations for flash-flood forecasting systems are provided, taking into account today's availability of high-resolution rainfall data at scales commensurate with flash flooding, their archives, spatial data sets to describe physiographic properties, and ever-increasing computational resources.

Editor D. Koutsoyiannis; Guest editor R.J. Moore

Citation Gourley, J.J., Flamig, Z.L., Hong, Y., and Howard, K.W., 2014. Evaluation of past, present and future tools for radar-based flash-flood prediction in the USA. Hydrological Sciences Journal, 59 (7), 1377–1389. http://dx.doi.org/10.1080/02626667.2014.919391     

Effects of urbanization variables on model parameters for watershed divisions

Abstract

A semi-distributed model with a parallel connection was applied to examine the effects of urbanization variables. Data were obtained from watershed divisions that were characterized by various degrees of urbanization. The mean rainfall was calculated using the kriging method. The model inputs were obtained by subtracting mean rainfall from Ф-index values, based on the spatially uniform loss assumption. Regression analysis was applied to determine the relationship between the parameters of 64 calibrations and urbanization variables among the divisions. The results showed that overland parameters produced more consistent change in response to imperviousness than to population. Conversely, the channel parameter was unaffected by changes in urbanization. The verification results of 46 cases showed that power linkage was a potential option for linking division parameters with the corresponding imperviousness based on four evaluation criteria. The changes in imperviousness on overland parameters show the hydrological effects of division urbanizations.

Editor D. Koutsoyiannis; Associate editor T. Wagener

Citation Chen, R., Chuang, W.-N., and Cheng, S., 2014. Effects of urbanization variables on model parameters for watershed divisions. Hydrological Sciences Journal, 59 (6), 1167–1183. http://dx.doi.org/10.1080/02626667.2014.910305     

Sensitivity analysis of the hydrological response of the Gaping River basin to radar-raingauge quantitative precipitation estimates

Abstract

The generation of reliable quantitative precipitation estimations (QPEs) through use of raingauge and radar data is an important issue. This study investigates the impacts of radar QPEs with different densities of raingauge networks on rainfall–runoff processes through a semi-distributed parallel-type linear reservoir rainfall–runoff model. The spatial variation structures of the radar QPE, raingauge QPE and radar-gauge residuals are examined to review the current raingauge network, and a compact raingauge network is identified via the kriging method. An analysis of the large-scale spatial characteristics for use with a hydrological model is applied to investigate the impacts of a raingauge network coupled with radar QPEs on the modelled rainfall–runoff processes. Since the precision in locating the storm centre generally represents how well the large-scale variability is reproduced; the results show not only the contribution of kriging to identify a compact network coupled with radar QPE, but also that spatial characteristics of rainfalls do affect the hydrographs.

Editor Z.W. Kundzewicz; Guest editor R.J. Moore

Citation Pan, T.-Y., Li, M.-Y., Lin, Y.-J., Chang, T.-J., Lai, J.-S., and Tan, Y.-C., 2014. Sensitivity analysis of the hydrological response of the Gaping River basin to radar-raingauge quantitative precipitation estimates. Hydrological Sciences Journal, 59 (7), 1335–1352. http://dx.doi.org/10.1080/02626667.2014.923969     

Evaluating flash-flood warnings at ungauged locations using post-event surveys: a case study with the AIGA warning system

Abstract

This article presents a comparison between real-time discharges calculated by a flash-flood warning system and post-event flood peak estimates. The studied event occurred on 15 and 16 June 2010 at the Argens catchment located in the south of France. Real-time flood warnings were provided by the AIGA (Adaptation d’Information Géographique pour l’Alerte en Crue) warning system, which is based on a simple distributed hydrological model run at a 1-km² resolution using radar rainfall information. The timing of the warnings (updated every 15 min) was compared to the observed flood impacts. Furthermore, “consolidated” flood peaks estimated by an intensive post-event survey were used to evaluate the AIGA-estimated peak discharges. The results indicated that the AIGA warnings clearly identified the most affected areas. However, the effective lead-time of the event detection was short, especially for fast-response catchments, because the current method does not take into account any rainfall forecast. The flood peak analysis showed a relatively good correspondence between AIGA- and field-estimated peak values, although some differences were due to the rainfall underestimation by the radar and rainfall–runoff model limitations.

Editor Z.W. Kundzewicz; Guest editor R.J. Moore

Citation Javelle, P., Demargne, J., Defrance, D., Pansu, J. and Arnaud, P., 2014. Evaluating flash-flood warnings at ungauged locations using post-event surveys: a case study with the AIGA warning system. Hydrological Sciences Journal, 59 (7), 1390–1402. http://dx.doi.org/10.1080/02626667.2014.923970     

Catchment-scale storm velocity: quantification,scale dependence and effect on flood response

Abstract

The concept of “catchment-scale storm velocity” quantifies the rate of storm motion up and down the basin accounting for the interaction between the rainfall space–time variability and the structure of the drainage network. It provides an assessment of the impact of storm motion on flood shape. We evaluate the catchment-scale storm velocity for the 29 August 2003 extreme storm that occurred on the 700 km²-wide Fella River basin in the eastern Italian Alps. The storm was characterized by the high rate of motion of convective cells across the basin. Analysis is carried out for a set of basins that range in area from 8 to 623 km² to: (a) determine velocity magnitudes for different sub-basins; (b) examine the relationship of velocity with basin scale and (c) assess the impact of storm motion on simulated flood response. Two spatially distributed hydrological models of varying degree of complexity in the representation of the runoff generation processes are used to evaluate the effects of the storm velocity on flood modelling and investigate model dependencies of the results. It is shown that catchment-scale storm velocity has a non-linear dependence on basin scale and generally exhibits rather moderate values, in spite of the strong kinematic characteristics of individual storm elements. Consistently with these observations and for both models, hydrological simulations show that storm motion has an almost negligible effect on the flood response modelling.

Editor Z.W. Kundzewicz; Guest editor R.J. Moore

Citation Nikolopoulos, E.I., Borga, M., Zoccatelli, D., and Anagnostou, E.N., 2014. Catchment-scale storm velocity: quantification, scale dependence and effect on flood response. Hydrological Sciences Journal, 59 (7), 1363–1376. http://dx.doi.org/10.1080/02626667.2014.923889     

Isotopic tracing for conceptual models of groundwater hydrodynamics in multilayer aquifer systems of central and southern Tunisia

Abstract

New and previously published data sets including stable and radiogenic isotope measurements (¹⁸O, ²H, ³H, ¹³C and ¹⁴C) were used to investigate, conceptualize and compare groundwater hydrodynamics within three major multilayer aquifer systems located in central and southern Tunisia. It has been demonstrated that the investigated aquifer systems contain modern and palaeoclimatic waters. Modern groundwaters, which refer to contemporaneous and post-nuclear recharge waters, are characterized by enriched stable isotope contents, high carbon-14 activities and high to moderate tritium concentration. While palaeoclimatic groundwaters, which refer to Late Pleistocene and Early Holocene recharge waters, are distinguished by their depleted stable isotope contents, low carbon-14 activities and insignificant tritium concentrations. Established conceptual models have elucidated the groundwater hydrodynamics within the studied aquifer systems. They show that groundwater mixing occurs between end-members from the shallow and deep aquifers that migrate by downward and upward leakage towards the intermediate aquifer.

Editor D. Koutsoyiannis; Associate editor S. Faye

Citation Dassi, L. and Tarki, M., 2014. Isotopic tracing for conceptual models of groundwater hydrodynamics in multilayer aquifer systems of central and southern Tunisia. Hydrological Sciences Journal, 59 (6), 1240–1258. http://dx.doi.org/10.1080/02626667.2014.892206     

Impact of climatic change on river discharge in the driest region of Poland

Abstract

Discharge in the Warta River in Poland has been analysed based on long series of measurements at the Gorzów Wielkopolski gauge station (covering the whole catchment area) and at Poznań (middle and upper catchment area), and the Note? River is characterized by the gauge station at Nowe Drezdenko. The annual mean discharge of the Warta River for the period 1981–2010 was equal to the average value for the last 163 years (209 m³ s^-1), and there was no significant change in comparison with the ratio of runoff in the summer and winter half-years. In the driest region of Poland, the climate has been described on the basis of precipitation and air temperature. The annual mean precipitation for 1981–2010 (544 mm) in the Warta River catchment area was the same as that for the period 1848–2010. The precipitation has been increasing in spring and winter, and decreasing in summer. There is a positive and very significant correlation (r = 0.705) between the annual discharge and annual precipitation totals. The annual mean air temperature has risen by 0.6°C between the periods 1848–1980 and 1981–2010.

Editor D. Koutsoyiannis

Citation Ilnicki, P., Farat, R., Górecki, K., and Lewandowski, P., 2014. Impact of climatic change on river discharge in the driest region of Poland. Hydrological Sciences Journal, 59 (6), 1117–1134. http://dx.doi.org/10.1080/02626667.2013.831979     

Long-term diagnostics of precipitation estimates and the development of radar hardware monitoring within a radar product data quality management system

Abstract

Quality is key to ensuring that the potential offered by weather radar networks is realized. To achieve optimum quality, a comprehensive radar data quality management system, designed to monitor the end-to-end radar data processing chain and evaluate product quality, is being developed at the UK Met Office. Three contrasting elements of this system are described: monitoring of key radar hardware performance indicators; generation of long-term integrations of radar products; and monitoring of radar reflectivity factor using synthesized observations from numerical weather prediction model fields. Examples of each component are presented and ways in which the different types of monitoring information have been used to both identify issues with the radar product data quality and help formulate solutions are given.

Editor Z.W. Kundzewicz; Guest editor R.J. Moore

Citation Harrison, D., Georgiou, S., Gaussiat, N., and Curtis, A., 2013. Long-term diagnostics of precipitation estimates and the development of radar hardware monitoring within a radar product data quality management system. Hydrological Sciences Journal, 59 (7), 1327–1342. http://dx.doi.org/10.1080/02626667.2013.841316     

Four-dimensional reflectivity data comparison between two ground-based radars: methodology and statistical analysis

Abstract

A methodology is proposed to compare radar reflectivity data obtained from two partially overlapping ground-based radars in order to explain relative differences in radar-rainfall products and establish sound merging procedures for multi-radar observing networks. To identify radar calibration differences, radar reflectivity is compared for well-matched radar sampling volumes viewing common meteorological targets. Temporal separation and three-dimensional matching of two different sampling volumes were considered based on the original polar coordinates of radar observation. Since the proposed method assumes radar beam propagation under standard atmospheric conditions, anomalous propagation cases were eliminated from the analysis. The reflectivity comparison results show systematic differences over time, but the variability of these differences is surprisingly large due to the sensitive nature of the radar reflectivity measurement.

Editor D. Koutsoyiannis/Z.W. Kundzewicz; Guest editor R.J. Moore

Citation Seo, B.-C., Krajewski, W.F., and Smith, J.A., 2013. Four-dimensional reflectivity data comparison between two ground-based radars: methodology and statistical analysis. Hydrological Sciences Journal, 59 (7), 1312–1326. http://dx.doi.org/10.1080/02626667.2013.839872     

Groundwater-level forecasting under climate change scenarios using an artificial neural network trained with particle swarm optimization

Abstract

Artificial neural networks (ANNs) have recently been used to predict the hydraulic head in well locations. In the present work, the particle swarm optimization (PSO) algorithm was used to train a feed-forward multi-layer ANN for the simulation of hydraulic head change at an observation well in the region of Agia, Chania, Greece. Three variants of the PSO algorithm were considered, the classic one with inertia weight improvement, PSO with time varying acceleration coefficients (PSO-TVAC) and global best PSO (GLBest-PSO). The best performance was achieved by GLBest-PSO when implemented using field data from the region of interest, providing improved training results compared to the back-propagation training algorithm. The trained ANN was subsequently used for mid-term prediction of the hydraulic head, as well as for the study of three climate change scenarios. Data time series were created using a stochastic weather generator, and the scenarios were examined for the period 2010–2020.

Editor Z.W. Kundzewicz; Associate editor L. See

Citation Tapoglou, E., Trichakis, I.C., Dokou, Z., Nikolos, I.K., and Karatzas, G.P., 2014. Groundwater-level forecasting under climate change scenarios using an artificial neural network trained with particle swarm optimization. Hydrological Sciences Journal, 59(6), 1225–1239. http://dx.doi.org/10.1080/02626667.2013.838005     

Dependence of radar quantitative precipitation estimation error on the rain intensity in the Cévennes region,France

Abstract

Radar quantitative precipitation estimates (QPEs) were assessed using reference values established by means of a geostatistical approach. The reference values were estimated from raingauge data using the block kriging technique, and the reference meshes were selected on the basis of the kriging estimation variance. Agreement between radar QPEs and reference rain amounts was shown to increase slightly with the space–time scales. The statistical distributions of the errors were modelled conditionally with respect to several factors using the GAMLSS approach. The conditional bias of the errors presents a complex structure that depends on the space–time scales and the considered geographical sub-domains, while the standard deviation of the errors has a more homogeneous behaviour. The estimation standard deviation of the reference rainfall and the standard deviation of the errors between radar and reference rainfall were found to have the same magnitude, indicating the limitations of the available network in terms of providing accurate reference values for the spatial scales considered (5–100 km²).

Editor D. Koutsoyiannis; Guest editor R.J. Moore

Citation Delrieu, G., Bonnifait, L., Kirstetter, P.-E., and Boudevillain, B., 2013. Dependence of radar quantitative precipitation estimation error on the rain intensity in the Cévennes region, France. Hydrological Sciences Journal, 59 (7), 1300–1311. http://dx.doi.org/10.1080/02626667.2013.827337     

Hydrological impact and potential flooding of convective rain cells in a semi-arid environment

Abstract

t Spatio-temporal storm properties have a large impact on catchment hydrological response. The sensitivity of simulated flash floods to convective rain-cell characteristics is examined for an extreme storm event over a 94 km² semi-arid catchment in southern Israel. High space–time resolution weather radar data were used to derive and model convective rain cells that then served as input into a hydrological model. Based on alterations of location, direction and speed of a major rain cell, identified as the flooding cell for this case, the impacts on catchment rainfall and generated flood were examined. Global sensitivity analysis was applied to identify the most important factors affecting the flash flood peak discharge at the catchment outlet. We found that the flood peak discharge could be increased three-fold by relatively small changes in rain-cell characteristics. We assessed that the maximum flash flood magnitude that this single rain cell can produce is 175 m³/s, and, taking into account the rest of the rain cells, the flash flood peak discharge can reach 260 m³/s.

Editor Z.W. Kundzewicz; Guest editor R.J. Moore

Citation Morin, E. and Yakir, H., 2013. Hydrological impact and potential flooding of convective rain cells in a semi-arid environment. Hydrological Sciences Journal, 59 (7), 1275–1284. http://dx.doi.org/10.1080/02626667.2013.841315     

Development of an operational rainfall data quality-control scheme based on radar-raingauge co-kriging analysis

Abstract

Automatic raingauge data often serve as an important input to hydrological and weather warning operations. They are not only fundamental in quantitative rainfall analysis, but also act as the ground truth in warning operation and forecast validation. Quality control is required before the data can be used quantitatively due to systematic and random errors. Extremely large random errors and unreasonably small or false zero values can hamper effective monitoring of heavy rain. Yet both are difficult to detect in real-time by objective means. In an attempt to address these problems, a rainfall data quality-control scheme based on radar-raingauge co-kriging analysis was developed. The important threshold values required in the data quality control of 60-min raingauge rainfall were determined from a detailed analysis of the distributions of rainfall residuals defined as the arithmetic difference and the logarithm of the ratio between a raingauge measurement and its co-kriging estimate. The scheme has been developed and is in real-time use in Hong Kong, a coastal city of about 1100 km² area with more than 150 raingauges installed. Geographically, it is located in the subtropics and dominated by heavy convective rainfall in the summer. As a basis of the quality-control scheme, the co-kriging rainfall analysis was shown through a verification exercise to be superior to those obtained by the Barnes analysis and ordinary kriging of raingauge data. The performance of the quality-control algorithm was assessed using selected cases and controlled tests, and was found to be satisfactory, with a high error detection rate for the two targeted types of error. Limitations and operational issues identified during a real-time trial of the quality-control scheme are also discussed.

Citation Yeung, H.Y., Man, C., Chan, S.T., and Seed, A., 2014. Development of an operational rainfall data quality-control scheme based on radar-raingauge co-kriging analysis. Hydrological Sciences Journal, 59 (7), 1285–1299. http://dx.doi.org/10.1080/02626667.2013.839873     

Exercice de scénarisation hydrologique en Afrique de l’Ouest—Bassin du Bani

Résumé

Les pays sub-sahariens basent principalement leur économie sur l’agriculture pluviale. Les projections démographiques à moyen ou long terme montrent que la pression démographique va s’accroître très fortement. A l’avenir, il faudra soutenir encore plus fortement le développement de stratégies agricoles de ces pays : cela nécessite une meilleure connaissance de leurs ressources en eau futures. Cette connaissance de la ressource en eau dans le futur passe par l’élaboration de scenarios climatiques et hydrologiques qui font intervenir différents acteurs, compétents dans des activités très variées, qu’ils soient scientifiques de divers domaines (climatologues, hydrologues, sociologues, …) mais aussi financiers, politiques, acteurs sociaux et décideurs. Ce processus d’élaboration de scenarios s’accompagne de nombreuses incertitudes, plus ou moins bien maitrisées, avec lesquelles doivent s’accommoder les gestionnaires d’ouvrages et planificateurs d’aménagements régionaux. Illustré par l’exemple du bassin du Bani, affluent du fleuve Niger, ce travail propose une réflexion sur la « scénarisation hydrologique » et la prédétermination des paramètres d’un modèle hydrologique dans un contexte global non stationnaire. Le choix de jeux de paramètres a de nombreux impacts sur les résultats de la scénarisation comme l’estimation de la ressource et la variabilité de cette ressource. Ces deux éléments sont fondamentaux pour qui doit conduire un processus de décision en réponse à une demande de stratégie d’aménagement ou à une demande de stratégie de développement. Nous proposons une méthodologie de détermination des jeux de paramètres se basant sur la construction de plusieurs jeux de paramètres issus de calages glissants, qui permet d’aller dans le sens de ce que de plus en plus de gestionnaires préconisent : ne pas essayer de prévoir le futur mais plutôt des futurs envisageables qui forceront des modèles d’impact, ce qui leur permettra de fournir des éléments de décision afin de s’adapter ou d’adapter des solutions d’aménagement aux conditions de ces futurs.

Editeur Z.W. Kundzewicz; Editeur associé C. Perrin

Citation Paturel, J.-E., 2014. Exercice de scénarisation hydrologique en Afrique de l’Ouest—Bassin du Bani. Hydrological Sciences Journal, 59 (6), 1135–1153. http://dx.doi.org/10.1080/02626667.2013.834340     

Comment on “Complete Eulerian-mean tracer equation for coarse resolution OGCMs” by M. S. Dubovikov and V. M. Canuto

Dubovikov and Canuto (Dubovikov, M.S. and Canuto, V.M., Complete Eulerian-mean tracer equation for coarse resolution OGCMs. Geophys. Astrophys. Fluid Dyn., 2006, 100, 197–214), after averaging the tracer conservation equation in density coordinates and transforming to height coordinates, concluded that present ocean models are missing important terms in their mean tracer equations. Here we point out some errors made by Dubovikov and Canuto (2006 Dubovikov, MS and Canuto, VM. 2006. Complete Eulerian-mean tracer equation for coarse resolution OGCMs. Geophys. & Astrophys. Fluid Dynam., 100: 197–214. [Taylor & Francis Online], [Web of Science ®] , [Google Scholar]) in their isopycnal averaging procedure. We draw on the temporal-residual-mean (TRM) theory and show that when the isopycnal averaging and coordinate transformation are performed correctly, the tracer equations of present ocean circulation models are recovered. We therefore conclude that present ocean circulation models are not neglecting the leading order terms identified by Dubovikov and Canuto (2006 Dubovikov, MS and Canuto, VM. 2006. Complete Eulerian-mean tracer equation for coarse resolution OGCMs. Geophys. & Astrophys. Fluid Dynam., 100: 197–214. [Taylor & Francis Online], [Web of Science ®] , [Google Scholar]).     

Cost-efficient management of coastal aquifers via recharge with treated wastewater and desalination of brackish groundwater: general framework

Abstract

Semi-arid coastal zones often suffer water-stress, as water demand is high and markedly seasonal, due to agriculture and tourism. Driven by scarcity of surface water, the communities in semi-arid coastal regions turn to aquifers as prime water source; but intensive exploitation of coastal aquifers causes seawater intrusion, which degrades the quality of groundwater. The cost-efficient and sustainable development of coastal aquifers can be achieved through a holistic management scheme which combines two non-traditional water sources: (a) saltwater, to be treated to the desired quality, and (b) wastewater, to be re-claimed to augment aquifer recharge for control of seawater intrusion, and also to meet certain demands. This management scheme is based on the idea that it is cost-advantageous to: (i) desalt brackish groundwater, instead of seawater, as the former requires far less energy, and (ii) to re‐use wastewater at only the differential cost to any treatment already practiced. In this paper, we present the general framework of the proposed management scheme, and a decision aid tool (DAT) which has been developed to assist decision makers to explore the scheme's decision space. The DAT uses cost as optimization criterion to screen various management scenarios, via modelling of the dynamic natural-engineered system behaviour, and identifies those cost-efficient ones that meet the water demand and achieve aquifer protection.

Citation Koussis, A. D., Georgopoulou, E., Kotronarou, A., Lalas, D. P., Restrepo, P., Destouni, G., Prieto, C., Rodriguez, J. J., Rodriguez-Mirasol, J., Cordero, T. & Gomez-Gotor, A. (2010 Koussis, A. D., Georgopoulou, E., Kotronarou, A., Mazi, K., Restrepo, P., Destouni, G., Prieto, C., Rodriguez, J. J., Rodriguez-Mirasol, J., Cordero, T., Schwartz, J., Ioannou, C., Georgiou, A. and Zacharias, I. 2010. Cost-efficient management of coastal aquifers in water-stressed regions via recharge with treated wastewater and desalination of brackish groundwater: application to the Akrotiri basin and aquifer, Cyprus. Hydrol. Sci. J, 55(7): 1234–1245. [Taylor & Francis Online], [Web of Science ®] , [Google Scholar]) Cost-efficient management of coastal aquifers via recharge with treated wastewater and desalination of brackish groundwater: general framework. Hydrol. Sci. J. 55(7),1217–1233.     

Grid-enabled Monte Carlo analysis of the impacts of uncertain discharge rates on seawater intrusion in the Korba aquifer (Tunisia)

Abstract

The Korba aquifer, located in the north of Tunisia, suffers heavily from salinization due to seawater intrusion. In 2000, the aquifer was exploited from more than 9000 wells. The problem is that no precise information was recorded concerning the current extraction rates, their spatial distribution, or their evolution in time. In this study, a geostatistical model of the exploitation rates was constructed based on a multi-linear regression model combining incomplete direct data and exhaustive secondary information. The impacts of the uncertainty on the spatial distribution of the pumping rates on seawater intrusion were evaluated using a 3-D density-dependent groundwater model. To circumvent the large amount of computing time required to run transient models, the simulations were run in a parallel fashion on the Grid infrastructure provided by the Enabling Grid for E-Science in Europe project. Monte Carlo simulations results showed that 8.3% of the aquifer area is affected by input uncertainty.

Citation Kerrou, J., Renard, P., Lecca, G. & Tarhouni, J. (2010 Kerrou, J., Renard, P. and Tarhouni, J. 2010. Status of the Korba groundwater resources (Tunisia): observations and three-dimensional modelling of seawater intrusion. Hydrogeol. J., 18(5): 1173–1190. doi:10.1007/s10040-010-0573-5[Crossref], [Web of Science ®] , [Google Scholar]) Grid-enabled Monte Carlo analysis of the impacts of uncertain discharge rates on seawater intrusion in the Korba aquifer (Tunisia). Hydrol. Sci. J. 55(8), 1325–1336.     

Spatio-temporal variability of streamflow in the Yellow River: possible causes and implications

Abstract

The water shortage in the Yellow River, China, has been aggravated by rapid population growth and global climate changes. To identify the characteristics of streamflow change in the Yellow River, approximately 50 years of natural and observed streamflow data from 23 hydrological stations were examined. The Mann-Kendall and Pettitt tests were used to detect trends and abrupt change points. The results show that both the natural and the observed streamflow in the Yellow River basin present downward trends from 1956 to 2008, and the decreasing rate of observed streamflow is generally faster than that of the natural streamflow. Larger drainage areas have higher declining rates, and the declining trends are intensified downstream within the mainstream. The possibility of abrupt changes in observed streamflow is higher than in natural streamflow, and streamflow series in the mainstream are more likely to change abruptly than those in the tributaries. In the mainstream, all the significant abrupt changes appear in the middle and latter half of the 1980s, but the abrupt changes occur somewhat earlier for observed streamflow than for natural streamflow. The significant abrupt change for the observed streamflow in the tributaries is almost isochronous with the natural streamflow and occurs from the 1970s to 1990s. It is implied that the slight reduction in precipitation is not the only direct reason for the streamflow variation. Other than the effects of climate change, land-use and land-cover changes are the main reasons for the natural streamflow change. Therefore, the increasing net water diversion by humans is responsible for the observed streamflow change. It is estimated that the influence of human activity on the declining streamflow is enhanced over time.

Editor Z.W. Kundzewicz

Citation Miao, C.Y., Shi, W., Chen, X.H., and Yang, L., 2012 Miao, C.Y., Yang, L. and Chen, X.H. 2012. The vegetation cover dynamics (1982–2006) in different erosion regions of the Yellow River basin, China. Land Degradation and Development, 23(1): 62–71. [Crossref], [Web of Science ®] , [Google Scholar]. Spatio-temporal variability of streamflow in the Yellow River: possible causes and implications. Hydrological Sciences Journal, 57 (7), 1355–1367.     

Uncertainty in levee heights and its effect on the spatial pattern of flood hazard in a floodplain

Levees are not usually built to a uniform height due to the varying priority of protecting urban and agricultural lands and they are often maintained in segments. Ad hoc alteration of the heights of these segments may aggravate flood conditions. Alterations lead to complex feedback loops in velocity and depth of water that are difficult to predict. A large number of possible configurations of the levee segments renders a deterministic modelling approach ineffective. The current analysis, based on a two-dimensional hydrodynamic model involving 1000 Monte Carlo realizations of randomly varying levee heights in segments, presents a methodology of dealing with the effect of uncertainty in levee heights on the inundation pattern in a probabilistic framework. Spatially distributed model outcomes include the likelihood of inundation, range and standard deviation of flood depths and maximum speed of water. The results indicate the necessity of adopting a probabilistic approach for robust flood hazard assessment when dealing with levee segments with uncertain heights.

EDITOR M.C. Acreman; ASSOCIATE EDITOR H. Kreibich     

A fine-scale point process model of rainfall with dependent pulse depths within cells

Abstract

A cluster point process model is considered for the analysis of fine-scale rainfall time series. The model is based on three Poisson processes. The first is a Poisson process of storm origins, where each storm has a random (exponential) lifetime. The second is a Poisson process of cell origins that occur during the storm lifetime, terminating when the storm finishes. Each cell has a random lifetime that follows an exponential distribution (or terminates when the storm terminates, whichever occurs first). During cell lifetimes, a third Poisson process of instantaneous pulses occurs. The model is essentially an extension of the well-known Bartlett-Lewis rectangular pulses model, with the rectangular profiles replaced with a Poisson process of instantaneous pulse depths to ensure more realistic rainfall profiles for fine-scale series. Model equations, derived in Cowpertwait et al. (2007 Cowpertwait, P., Isham, V. and Onof, C. 2007. Point process models of rainfall: developments for fine-scale structure. Proceedings of the Royal Society of London, Series A, 463: 2569–2587. [Crossref], [Web of Science ®] , [Google Scholar]), are used to fit different sets of properties to a 60 year record of 5-min data taken from Kelburn, New Zealand. As in the previous work, two superposed processes are used to account for two main and distinct precipitation types (convective and stratiform). By treating the within-cell pulses as dependent random variables, it is found, by simulation, that improved fits to extreme values and the proportion of dry intervals are obtained.

Citation Cowpertwait, P. S. P., Xie, G., Isham, V., Onof, C. & Walsh, D. C. I. (2011) A fine-scale point process model of rainfall with dependent pulse depths within cells. Hydrol. Sci. J. 56(7), 1110–1117.