Jos Samuel

Estimation of Continuous Streamflow in Ontario Ungauged Basins: Comparison of Regionalization Methods

Regionalization, a process of transferring hydrological information [i.e., parameters of a conceptual rainfall-runoff model, namely, the McMaster University-Hydrologiska Byrans Vattenbalansavdelning (MAC-HBV)] from gauged to ungauged basins, was applied to estimate continuous flows in ungauged basins across Ontario, Canada. To identify appropriate regionalization methods, different regionalization methods were applied, including the spatial proximity [i.e., kriging, inverse distance weighted (IDW), and mean parameters], physical similarity, and regression-based approaches. Furthermore, an approach coupling the spatial-proximity (IDW) method and the physical similarity approach is proposed. The analysis results show that the coupled regionalization approach as well as the IDW and kriging produce better model performances than the remaining three. Further investigations show that the coupled-regionalization approach provides slightly better performances than the other two spatial proximity methods. In addit...

Influence of Indian Ocean sea surface temperature variability on southwest Western Australian winter rainfall

[1] Southwest Western Australia (SWWA) has experienced a significant drop in winter rainfall during the past three decades, the cause of which is still subject to considerable speculation and debate. This prolonged drought has heavily impacted on water resources in the state, the predicted continuation of which is of major concern to the people of Western Australia. In this paper, the possible influence of sea surface temperature (SST) variability occurring over parts of the Indian Ocean on SWWA rainfall variability is explored. The results of our statistical analyses show that winter rainfall (May to October) in SWWA is lower in years when warm SSTs dominate the southern and tropical western Indian Ocean, compared to years when cool anomalies are present in the same region. In addition, a step change that can be detected in the SST anomalies in the southern and tropical western Indian Ocean occurring around 1970 happens to ncoincide with a similar sharp reduction in SWWA rainfall. It is suggested that the possible association between the Indian Ocean SST anomalies and observed variability of SWWA rainfall may be utilized, at a minimum, as possible diagnostic tools in the evaluation of global climate model outputs, in this way constraining the uncertainties in their predictions.

CRDEMO: Combined regionalization and dual entropy‐multiobjective optimization for hydrometric network design

[1]xa0Establishing an adequate hydrometric network to provide accurate and reliable continuous flow information for various users remains a major challenge. This includes the design of new networks or the evaluation of existing networks. This study proposes a combined regionalization and dual entropy-multiobjective optimization (CRDEMO) method for determining minimum network that meets the World Meteorology Organization (WMO) standards, which is considered herein an optimal minimum network. A regionalization approach is used to estimate flows in potential locations for new additional stations, and a dual entropy-multiobjective optimization approach is used to identify optimal trade-offs between the maximum possible information content and the minimum shared information among the stations. The method was examined in two Canadian River basins, namely, the St. John River and St. Lawrence River basins. The St. John River basin has a higher network density compared to the St. Lawrence River basin. Results of the analysis indicate that the St. Lawrence River basin requires a high number of new additional stations to meet the WMO minimum network density. It was also determined that existing stations do not have a significant influence in determining the locations of new stations in the St. Lawrence River basin. Conversely, however, in the St. John River basin, existing stations have significant influence on the determination of locations of new stations due to the higher number of existing stations. Overall, the CRDEMO technique is shown to be robust for designing optimal minimum hydrometric networks and can be a useful tool for evaluating current and proposed networks.

A comparative modeling analysis of multiscale temporal variability of rainfall in Australia

[1]xa0The effects of long-term natural climate variability and human-induced climate change on rainfall variability have become the focus of much concern and recent research efforts. In this paper, we present the results of a comparative analysis of observed multiscale temporal variability of rainfall in the Perth, Newcastle, and Darwin regions of Australia. This empirical and stochastic modeling analysis explores multiscale rainfall variability, i.e., ranging from short to long term, including within-storm patterns, and intra-annual, interannual, and interdecadal variabilities, using data taken from each of these regions. The analyses investigated how storm durations, interstorm periods, and average storm rainfall intensities differ for different climate states and demonstrated significant differences in this regard between the three selected regions. In Perth, the average storm intensity is stronger during La Nina years than during El Nino years, whereas in Newcastle and Darwin storm duration is longer during La Nina years. Increase of either storm duration or average storm intensity is the cause of higher average annual rainfall during La Nina years as compared to El Nino years. On the other hand, within-storm variability does not differ significantly between different ENSO states in all three locations. In the case of long-term rainfall variability, the statistical analyses indicated that in Newcastle the long-term rainfall pattern reflects the variability of the Interdecadal Pacific Oscillation (IPO) index, whereas in Perth and Darwin the long-term variability exhibits a step change in average annual rainfall (up in Darwin and down in Perth) which occurred around 1970. The step changes in Perth and Darwin and the switch in IPO states in Newcastle manifested differently in the three study regions in terms of changes in the annual number of rainy days or the average daily rainfall intensity or both. On the basis of these empirical data analyses, a stochastic rainfall time series model was developed that incorporates the entire range of multiscale variabilities observed in each region, including within-storm, intra-annual, interannual, and interdecadal variability. Such ability to characterize, model, and synthetically generate realistic time series of rainfall intensities is essential for addressing many hydrological problems, including estimation of flood and drought frequencies, pesticide risk assessment, and landslide frequencies.

Diagnostic analysis of water balance variability: A comparative modeling study of catchments in Perth, Newcastle, and Darwin, Australia

[1]xa0A comparative study is performed to explore interactions between climate variability and landscape factors that control water balance variability in three diverse regions of Australia: Perth (temperate with distinct dry summers); Newcastle (temperate with no distinct dry season); and Darwin (tropical region affected by monsoons). This comparative analysis is carried out through adoption of a common conceptual model. The similarity and differences between the three catchments are explored through evaluation of signatures of streamflow and soil moisture variability, and systematic sensitivity analysis with respect to parameters representing various landscape characteristics. The results of the analysis show that the biggest contributor to the differences between the catchments is the distribution of soil depth and the soils drainage characteristics. The second factor is climate, as exemplified by the (annual) climatic dryness index and the intra-annual (seasonal) variability of both rainfall and potential evaporation, and associated rainfall intensity patterns, and their interactions with the soil properties (i.e., soil depth and the soils drainage characteristics). In Perth and Darwin, climate seasonality is responsible for a seasonal switching on/off of subsurface stormflow at the start/end of the wet season, respectively. In Newcastle, where soil moisture contents hover near the field capacity value throughout the year, subsurface stormflow occurs frequently throughout the year, with event-based switching on/off in response to individual storms of moderate magnitude and temporal clustering of small storms. In addition, in rare circumstance, surface runoff is triggered in response to extreme storm events and temporal clustering of moderate to large storm events.

Effects of multiscale rainfall variability on flood frequency: Comparative multisite analysis of dominant runoff processes

[1]xa0We present results of a comparative modeling analysis of the effects of multiscale rainfall variability (within-event, between-event, seasonal, interannual, and interdecadal) on estimated flood frequency curves for three catchments located in Perth, Newcastle, and Darwin, Australia. The analysis is performed using the derived distribution approach by combining long-term rainfall time series generated by a stochastic rainfall model with a continuous rainfall-runoff flood model that is able to generate runoff variability over a multiplicity of timescales. Similarities and differences of the flood frequency curves (FFCs) in these rather diverse catchments are then interpreted on the basis of differences in the dominant runoff generation processes. In Newcastle, annual maximum flood peaks are caused by saturation excess overland flow over the entire range of annual exceedance probabilities (AEPs) or return periods. On the other hand, in Darwin, the shape of the FFC is determined strongly by seasonal climatic variability, which, in combination with deep soils, leads to a switch of dominant runoff mechanisms contributing to annual maximum flood peaks, from subsurface stormflow at high AEPs (low return periods) to saturation excess overland flow at low AEPs (high return periods). This leads to FFCs exhibiting a consistent break in slope in the Darwin catchment but not so in Newcastle. On the other hand, the FFCs in Perth are affected by both seasonality and long-term climate variability and produce a variety of shapes depending on the relative strengths of these climatic controls. Because of the fact that in Perth and Darwin the shapes of the flood frequency curves depend on a possible switch of the dominant runoff generation mechanisms with increasing return period, uncertainty in hydrological model parameters relating to landscape properties contributes significantly to the uncertainty in the flood frequency curves. This uncertainty is much less pronounced in Newcastle because of the absence of such a switch of runoff generation mechanisms.

Evaluation of Canadian National Hydrometric Network density based on WMO 2008 standards

Since its establishment in the 1890s, the Canadian National Hydrometric Network (CNHN) has become the main source of water information for various users across the country. The CNHN provides the essential information and data on the status of one of the most precious natural resources of Canada: its surface water resources. However, the CNHN has never been evaluated based on international standards such as the World Meteorological Organization (WMO) guidelines for hydrometric network density. This preliminary analysis aims to fill that gap and provide background information on CNHN density. In this study, the CNHN is evaluated using WMO (2008) guidelines as a benchmark. Spatial analysis techniques are applied to determine the areas that meet the WMO (2008) minimum network density standards and those that do not meet the minimum standards. It was found that only about 12% of the Canadian terrestrial area is covered by hydrometric networks that meet the WMO (2008) minimum standards, while 49% of the terrestrial area is poorly gauged and about 39% is ungauged. The poorly gauged and ungauged areas are largely located in the mountainous and northern regions. Only a few spots in the southern parts of the country have a streamflow network density that meets the WMO (2008) minimum recommendation. An estimated total of about 5041 new stations are needed to bring the CNHN up to WMO (2008) minimum standard level. The study results clearly confirm the necessity to continue operating all existing streamflow stations and the need to add new streamflow stations. However, further analysis is required to optimize the CNHN at the regional scale by accounting for other factors such as population density, specific needs and economic constraints.