Proceedings of the International Association of Hydrological Sciences, 2021
Lake Victoria is the largest lake in Africa and its outflows strongly influence flows in the Whit... more Lake Victoria is the largest lake in Africa and its outflows strongly influence flows in the White Nile, including the availability of water for hydropower generation, irrigation and water supply. Understanding the water balance is a major challenge since the lake is large enough to influence the local climate and its catchment spans several countries. Hydrometeorological monitoring networks are also sparse in some parts of the basin. In this paper, we consider the history of water balance estimates for the lake and how the science has developed as new information and techniques have become available, including in areas such as seasonal flow forecasting and estimating the potential impacts of dam operations and climate change. These findings are placed into a wider context including the challenges arising from a changing climate and evolving ideas from international research programmes, which lead to some suggestions for future research priorities for Lake Victoria and other sub-Saharan/Rift Valley lakes. Published by Copernicus Publications on behalf of the International Association of Hydrological Sciences. K. Sene et al.: Reflections on almost a century of hydrological studies on Africa's largest lake Disclaimer. Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Special issue statement. This article is part of the special issue "Hydrology of Large River Basins of Africa". It is a result of the
Excess nutrients in surface waters, such as phosphorus (P) from agriculture, result in poor water... more Excess nutrients in surface waters, such as phosphorus (P) from agriculture, result in poor water quality, with adverse effects on ecological health and costs for remediation. However, understanding and prediction of P transfers in catchments have been limited by inadequate data and over-parameterised models with high uncertainty. We show that, with high temporal resolution data, we are able to identify simple dynamic models that capture the P load dynamics in three contrasting agricultural catchments in the UK. For a flashy catchment, a linear, second-order (two pathways) model for discharge gave high simulation efficiencies for short-term storm sequences and was useful in highlighting uncertainties in out-of-bank flows. A model with nonlinear rainfall input was appropriate for predicting seasonal or annual cumulative P loads where antecedent conditions affected the catchment response. For second-order models, the time constant for the fast pathway varied between 2 and 15 h for all three catchments and for both discharge and P, confirming that high temporal resolution data are necessary to capture the dynamic responses in small catchments (10-50 km 2). The models led to a better understanding of the dominant nutrient transfer modes, which will be helpful in determining phosphorus transfers following changes in precipitation patterns in the future. 1 Introduction The quality of both surface waters and groundwater is under increasing pressure from numerous sources, including intensive agricultural practices, water abstraction, climate change, and changes in food production and housing provisions to cope with population growth (Carpenter and Bennett, 2011). Sediment and nutrient concentrations and loads are of concern to water utility companies and to environmental regulators who are striving to meet stringent water quality standards. However, accurate estimation of loads requires accurate, high temporal resolution measurements of both discharge and nutrient concentrations (Johnes, 2007) and should include quantification of observational uncertainties (McMil-Published by Copernicus Publications on behalf of the European Geosciences Union.
The paper outlines a general approach to the digital identification, estimation and control of li... more The paper outlines a general approach to the digital identification, estimation and control of linear continuous-time systems based on the True Digital Control (TOC) design philosophy using 8 operator models. The Refined Instrumental Variable (RIV) identification and estimation procedures, which provide the modelling basis for the implementation of these 8 operator TDC concepts, are seen to be a theoretically inspired generalisation of earlier continuous-time techniques proposed in the 1960's.
Hydrology and Earth System Sciences Discussions, 2017
Excess nutrients in surface waters, such as phosphorus (P) from agriculture, result in poor water... more Excess nutrients in surface waters, such as phosphorus (P) from agriculture, result in poor water quality, with adverse effects on ecological health and costs for remediation. However, understanding and prediction of P transfers in catchments have been limited by inadequate data and over-parameterised models with high uncertainty. We show that, with high temporal resolution data, we are able to identify simple dynamic models that capture the P load dynamics in three contrasting agricultural catchments in the UK. For a flashy catchment, a linear, second-order (two pathways) model for discharge gave high simulation efficiencies for short-term storm sequences and was useful in highlighting uncertainties in out-of-bank flows. A model with non-linear rainfall input was appropriate for predicting seasonal or annual cumulative P loads where antecedent conditions affected the catchment response. For second-order models, the time constant for the fast pathway varied between 2 and 15 hours fo...
This paper is concerned with adaptive, off-line signal processing and forecasting for nonstationa... more This paper is concerned with adaptive, off-line signal processing and forecasting for nonstationary signals described by the unobserved component model yt=Tt+St+f(ut)+Nt +et, for et~N{O, σ2}, where yt is the observed time series, Tt is a trend or low-frequency component, St is a periodic component possibly exhibiting temporal changes in both amplitude and phase, f(ut) captures the influence of a vector
An important aspect of flood risk management is the issuing of timely flood alerts. The spatial, ... more An important aspect of flood risk management is the issuing of timely flood alerts. The spatial, as well as temporal, scale of these warnings is important. In many situations efficient risk management may be aided by the provision of local flood predictions at a high spatial resolution. Examples of such situations include issuing warnings for small groups of outlying houses or key infrastructure locations such as power sub-stations. In this paper a methodology for providing automated, detailed and location specific warnings which are computed 'on-site' is presented.
A systematic, semi-automatic method for imaging the cards from the widely used Campbell-Stokes su... more A systematic, semi-automatic method for imaging the cards from the widely used Campbell-Stokes sunshine recorder is described. We show how the application of inexpensive commercial equipment and practices can simply and robustly build an archive of high-quality card images and manipulate them into a form suitable for easy further analysis. Rectified and registered digital images are produced, with the card's midday marker in the middle of the longest side, and with a temporal scaling of 150 pixels per hour. The method improves on previous, mostly manual, analyses by simplifying and automating steps into a process capable of handling thousands of cards in a practical timescale. A prototype method of extraction of data from this archive is then tested by comparison with records from a co-located pyrheliometer at a resolution of the order of minutes. The comparison demonstrates that the Campbell-Stokes recorder archive contains a time series of downwelling solar-irradiance-related data with similar characteristics to that of benchmark pyrheliometer data from the Baseline Surface Radiation Network. A universal transfer function for card burn to direct downwelling shortwave radiation is still some way off and is the subject of ongoing research. Until such time as a universal transfer function is available, specific functions for extracting data in particular circumstances offer a useful way forward. The new image-capture method offers a practical way to exploit the worldwide sets of long-term Campbell-Stokes recorder data to create a time series of solar irradiance and atmospheric aerosol loading metrics reaching back over 100 yr from the present day.
Phosphorus losses from land to water will be impacted by climate change and land management for f... more Phosphorus losses from land to water will be impacted by climate change and land management for food production, with detrimental impacts on aquatic ecosystems. Here we use a unique combination of methods to evaluate the impact of projected climate change on future phosphorus transfers, and to assess what scale of agricultural change would be needed to mitigate these transfers. We combine novel high-frequency phosphorus flux data from three representative catchments across the UK, a new high-spatial resolution climate model, uncertainty estimates from an ensemble of future climate simulations, two phosphorus transfer models of contrasting complexity and a simplified representation of the potential intensification of agriculture based on expert elicitation from land managers. We show that the effect of climate change on average winter phosphorus loads (predicted increase up to 30% by 2050s) will be limited only by large-scale agricultural changes (e.g., 20–80% reduction in phosphorus...
A unified approach to Multiple and single State Dependent Parameter modelling, termed Extended St... more A unified approach to Multiple and single State Dependent Parameter modelling, termed Extended State Dependent Parameters (ESDP) modelling, of nonlinear dynamic systems described by timevarying dynamic models applied to ARX or transfer-function model structures. Crucially, the approach proposes an effective model structure identification method using a novel Information Criterion (IC) taking into account model complexity in terms of the number of states involved. In ESDP, model structure involves not only the model orders, but also selection of the states driving the parameters, which effectively prevents the use of most current IC. This leads to a powerful methodology for investigating nonlinear systems building on the Data-Based Mechanistic (DBM) philosophy of Young and expanding the applications of the existing DBM methods. The methodologies presented are tested and demonstrated on both simulated data and on high frequency hydrological observations, showing how structure identification leads to discovery of dynamic relationships between system variables.
A new method known as Unobserved Components-Dynamic Harmonic Regression (UC-DHR) was applied to a... more A new method known as Unobserved Components-Dynamic Harmonic Regression (UC-DHR) was applied to a 39-year record of rainfall and streamflow for three sub-catchments of the Sarukawa Experimental Watershed in southwestern Japan. Some 25% of the timber was harvested from one of the sub-catchments in May-July 1982 and the objective was to quantify the magnitude of this effect relative to the effects of climate cycles (e.g. Southern Oscillation Index). The observed effects of inter-annual climate cycles (i.e. 0.89-1.36 mm/d) were seen to be comparable (i.e. 0.70-1.17 mm/d) to the effects of harvesting 25% of the standing timber. This result underlines the importance of always quantifying the effect of climate on streamflow response when harvesting impacts are studied.
ABSTRACT The identification of periodic patterns in water cycle variables is critical to the unde... more ABSTRACT The identification of periodic patterns in water cycle variables is critical to the understanding of land-atmosphere interactions, climate change and the evaluation of General Circulation Model (GCM) output. SE Asia in particular plays a very important role on the global climate because it is a large source of energy and water fluxes into the upper atmosphere. Cycle identification is carried out following the Data Based Mechanistic (DBM) philosophy, which focuses on the use of parsimonious, rigorous models which are characterised by lack of a priori assumptions, built in uncertainty analysis and final model acceptance dependent on the physical interpretation of the results. The DBM tool used here is the Unobserved Component - Dynamic Harmonic Regression (UC-DHR) model, which is a statistical method that allows the identification of variability in time series by introducing Time Variable Parameter (TVP) estimation of harmonic components. UC-DHR is not scale dependent and was thus applied to both hourly (to investigate diurnal variation) and fortnightly datasets (for intra- and inter-annual variability). The data used in the analysis has been gathered from existing catchment datasets for three regions of tropical SE Asia, namely Northern Thailand, Central Peninsular Malaysia and Northeast Borneo. These regions were chosen because they represent the hydro-climatic gradient (seasonal to equatorial) present within the tropics and because SE Asia has the most extensive set of catchment/plot studies within the humid tropics. Results show modeling tools were able to quantify the main patterns present in the observations throughout different time scales (diurnal, intra-annual and inter-annual) and the strength of the correlation pattern between the four hydro-climatic variables. The subsequent discussion focuses on the physical processes behind those patterns (e.g. diurnal variability caused by local convection due to solar heating; impact of El Niño Southern Oscillation on inter-annual variability of rainfall and river discharge).
The technique of Diffusional Gradients in Thin-films (DGT) can be used in situ to obtain high res... more The technique of Diffusional Gradients in Thin-films (DGT) can be used in situ to obtain high resolution profiles of trace-metals in sediment pore waters. Substances sampled by DGT continuously diffuse through a 'diffusion layer' comprising a hydrogel prior to being immobilized by binding to a resin layer. DGT therefore measures a time averaged flux from the pore water to the resin. Interpretation of this flux as pore water concentration is problematic for two reasons. Firstly, the pore water concentration adjacent to the sampler may become depleted by the DGT induced flux. Secondly, if there are steep vertical chemical gradients in the pore waters, they may relax by diffusion along the gradient within the gel layer. The extent of relaxation depends on the diffusion coefficient, gradient steepness, and diffusion layer thickness. Two dimensional (2D) numerical models of DGT deployments in horizontally uniform sediments were used to investigate to what extent DGT measured profiles accurately reproduced (a) the shape of pore water concentration profiles, and (b) the magnitude of pore water concentrations. A method is developed which translates high resolution DGT measured flux profiles into reliable estimates of pore water concentrations. Linear relationships are given which estimate the minimum DGT measured peak width (as a function of diffusion layer thickness) that ensures accurate reproduction of the shape and the magnitude of peaks in pore water concentrations. Peaks in DGT profiles obtained from assemblies with diffusion layer thicknesses of 0.3 mm (0.5 mm) should be at least 1.2 mm (1.8 mm) wide for their shape to reflect accurately their true shape in the pore water, and at least 1.7 mm (2.7 mm) wide to ensure the peak concentration is accurately estimated.
Proceedings of the International Association of Hydrological Sciences, 2021
Lake Victoria is the largest lake in Africa and its outflows strongly influence flows in the Whit... more Lake Victoria is the largest lake in Africa and its outflows strongly influence flows in the White Nile, including the availability of water for hydropower generation, irrigation and water supply. Understanding the water balance is a major challenge since the lake is large enough to influence the local climate and its catchment spans several countries. Hydrometeorological monitoring networks are also sparse in some parts of the basin. In this paper, we consider the history of water balance estimates for the lake and how the science has developed as new information and techniques have become available, including in areas such as seasonal flow forecasting and estimating the potential impacts of dam operations and climate change. These findings are placed into a wider context including the challenges arising from a changing climate and evolving ideas from international research programmes, which lead to some suggestions for future research priorities for Lake Victoria and other sub-Saharan/Rift Valley lakes. Published by Copernicus Publications on behalf of the International Association of Hydrological Sciences. K. Sene et al.: Reflections on almost a century of hydrological studies on Africa's largest lake Disclaimer. Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Special issue statement. This article is part of the special issue "Hydrology of Large River Basins of Africa". It is a result of the
Excess nutrients in surface waters, such as phosphorus (P) from agriculture, result in poor water... more Excess nutrients in surface waters, such as phosphorus (P) from agriculture, result in poor water quality, with adverse effects on ecological health and costs for remediation. However, understanding and prediction of P transfers in catchments have been limited by inadequate data and over-parameterised models with high uncertainty. We show that, with high temporal resolution data, we are able to identify simple dynamic models that capture the P load dynamics in three contrasting agricultural catchments in the UK. For a flashy catchment, a linear, second-order (two pathways) model for discharge gave high simulation efficiencies for short-term storm sequences and was useful in highlighting uncertainties in out-of-bank flows. A model with nonlinear rainfall input was appropriate for predicting seasonal or annual cumulative P loads where antecedent conditions affected the catchment response. For second-order models, the time constant for the fast pathway varied between 2 and 15 h for all three catchments and for both discharge and P, confirming that high temporal resolution data are necessary to capture the dynamic responses in small catchments (10-50 km 2). The models led to a better understanding of the dominant nutrient transfer modes, which will be helpful in determining phosphorus transfers following changes in precipitation patterns in the future. 1 Introduction The quality of both surface waters and groundwater is under increasing pressure from numerous sources, including intensive agricultural practices, water abstraction, climate change, and changes in food production and housing provisions to cope with population growth (Carpenter and Bennett, 2011). Sediment and nutrient concentrations and loads are of concern to water utility companies and to environmental regulators who are striving to meet stringent water quality standards. However, accurate estimation of loads requires accurate, high temporal resolution measurements of both discharge and nutrient concentrations (Johnes, 2007) and should include quantification of observational uncertainties (McMil-Published by Copernicus Publications on behalf of the European Geosciences Union.
The paper outlines a general approach to the digital identification, estimation and control of li... more The paper outlines a general approach to the digital identification, estimation and control of linear continuous-time systems based on the True Digital Control (TOC) design philosophy using 8 operator models. The Refined Instrumental Variable (RIV) identification and estimation procedures, which provide the modelling basis for the implementation of these 8 operator TDC concepts, are seen to be a theoretically inspired generalisation of earlier continuous-time techniques proposed in the 1960's.
Hydrology and Earth System Sciences Discussions, 2017
Excess nutrients in surface waters, such as phosphorus (P) from agriculture, result in poor water... more Excess nutrients in surface waters, such as phosphorus (P) from agriculture, result in poor water quality, with adverse effects on ecological health and costs for remediation. However, understanding and prediction of P transfers in catchments have been limited by inadequate data and over-parameterised models with high uncertainty. We show that, with high temporal resolution data, we are able to identify simple dynamic models that capture the P load dynamics in three contrasting agricultural catchments in the UK. For a flashy catchment, a linear, second-order (two pathways) model for discharge gave high simulation efficiencies for short-term storm sequences and was useful in highlighting uncertainties in out-of-bank flows. A model with non-linear rainfall input was appropriate for predicting seasonal or annual cumulative P loads where antecedent conditions affected the catchment response. For second-order models, the time constant for the fast pathway varied between 2 and 15 hours fo...
This paper is concerned with adaptive, off-line signal processing and forecasting for nonstationa... more This paper is concerned with adaptive, off-line signal processing and forecasting for nonstationary signals described by the unobserved component model yt=Tt+St+f(ut)+Nt +et, for et~N{O, σ2}, where yt is the observed time series, Tt is a trend or low-frequency component, St is a periodic component possibly exhibiting temporal changes in both amplitude and phase, f(ut) captures the influence of a vector
An important aspect of flood risk management is the issuing of timely flood alerts. The spatial, ... more An important aspect of flood risk management is the issuing of timely flood alerts. The spatial, as well as temporal, scale of these warnings is important. In many situations efficient risk management may be aided by the provision of local flood predictions at a high spatial resolution. Examples of such situations include issuing warnings for small groups of outlying houses or key infrastructure locations such as power sub-stations. In this paper a methodology for providing automated, detailed and location specific warnings which are computed 'on-site' is presented.
A systematic, semi-automatic method for imaging the cards from the widely used Campbell-Stokes su... more A systematic, semi-automatic method for imaging the cards from the widely used Campbell-Stokes sunshine recorder is described. We show how the application of inexpensive commercial equipment and practices can simply and robustly build an archive of high-quality card images and manipulate them into a form suitable for easy further analysis. Rectified and registered digital images are produced, with the card's midday marker in the middle of the longest side, and with a temporal scaling of 150 pixels per hour. The method improves on previous, mostly manual, analyses by simplifying and automating steps into a process capable of handling thousands of cards in a practical timescale. A prototype method of extraction of data from this archive is then tested by comparison with records from a co-located pyrheliometer at a resolution of the order of minutes. The comparison demonstrates that the Campbell-Stokes recorder archive contains a time series of downwelling solar-irradiance-related data with similar characteristics to that of benchmark pyrheliometer data from the Baseline Surface Radiation Network. A universal transfer function for card burn to direct downwelling shortwave radiation is still some way off and is the subject of ongoing research. Until such time as a universal transfer function is available, specific functions for extracting data in particular circumstances offer a useful way forward. The new image-capture method offers a practical way to exploit the worldwide sets of long-term Campbell-Stokes recorder data to create a time series of solar irradiance and atmospheric aerosol loading metrics reaching back over 100 yr from the present day.
Phosphorus losses from land to water will be impacted by climate change and land management for f... more Phosphorus losses from land to water will be impacted by climate change and land management for food production, with detrimental impacts on aquatic ecosystems. Here we use a unique combination of methods to evaluate the impact of projected climate change on future phosphorus transfers, and to assess what scale of agricultural change would be needed to mitigate these transfers. We combine novel high-frequency phosphorus flux data from three representative catchments across the UK, a new high-spatial resolution climate model, uncertainty estimates from an ensemble of future climate simulations, two phosphorus transfer models of contrasting complexity and a simplified representation of the potential intensification of agriculture based on expert elicitation from land managers. We show that the effect of climate change on average winter phosphorus loads (predicted increase up to 30% by 2050s) will be limited only by large-scale agricultural changes (e.g., 20–80% reduction in phosphorus...
A unified approach to Multiple and single State Dependent Parameter modelling, termed Extended St... more A unified approach to Multiple and single State Dependent Parameter modelling, termed Extended State Dependent Parameters (ESDP) modelling, of nonlinear dynamic systems described by timevarying dynamic models applied to ARX or transfer-function model structures. Crucially, the approach proposes an effective model structure identification method using a novel Information Criterion (IC) taking into account model complexity in terms of the number of states involved. In ESDP, model structure involves not only the model orders, but also selection of the states driving the parameters, which effectively prevents the use of most current IC. This leads to a powerful methodology for investigating nonlinear systems building on the Data-Based Mechanistic (DBM) philosophy of Young and expanding the applications of the existing DBM methods. The methodologies presented are tested and demonstrated on both simulated data and on high frequency hydrological observations, showing how structure identification leads to discovery of dynamic relationships between system variables.
A new method known as Unobserved Components-Dynamic Harmonic Regression (UC-DHR) was applied to a... more A new method known as Unobserved Components-Dynamic Harmonic Regression (UC-DHR) was applied to a 39-year record of rainfall and streamflow for three sub-catchments of the Sarukawa Experimental Watershed in southwestern Japan. Some 25% of the timber was harvested from one of the sub-catchments in May-July 1982 and the objective was to quantify the magnitude of this effect relative to the effects of climate cycles (e.g. Southern Oscillation Index). The observed effects of inter-annual climate cycles (i.e. 0.89-1.36 mm/d) were seen to be comparable (i.e. 0.70-1.17 mm/d) to the effects of harvesting 25% of the standing timber. This result underlines the importance of always quantifying the effect of climate on streamflow response when harvesting impacts are studied.
ABSTRACT The identification of periodic patterns in water cycle variables is critical to the unde... more ABSTRACT The identification of periodic patterns in water cycle variables is critical to the understanding of land-atmosphere interactions, climate change and the evaluation of General Circulation Model (GCM) output. SE Asia in particular plays a very important role on the global climate because it is a large source of energy and water fluxes into the upper atmosphere. Cycle identification is carried out following the Data Based Mechanistic (DBM) philosophy, which focuses on the use of parsimonious, rigorous models which are characterised by lack of a priori assumptions, built in uncertainty analysis and final model acceptance dependent on the physical interpretation of the results. The DBM tool used here is the Unobserved Component - Dynamic Harmonic Regression (UC-DHR) model, which is a statistical method that allows the identification of variability in time series by introducing Time Variable Parameter (TVP) estimation of harmonic components. UC-DHR is not scale dependent and was thus applied to both hourly (to investigate diurnal variation) and fortnightly datasets (for intra- and inter-annual variability). The data used in the analysis has been gathered from existing catchment datasets for three regions of tropical SE Asia, namely Northern Thailand, Central Peninsular Malaysia and Northeast Borneo. These regions were chosen because they represent the hydro-climatic gradient (seasonal to equatorial) present within the tropics and because SE Asia has the most extensive set of catchment/plot studies within the humid tropics. Results show modeling tools were able to quantify the main patterns present in the observations throughout different time scales (diurnal, intra-annual and inter-annual) and the strength of the correlation pattern between the four hydro-climatic variables. The subsequent discussion focuses on the physical processes behind those patterns (e.g. diurnal variability caused by local convection due to solar heating; impact of El Niño Southern Oscillation on inter-annual variability of rainfall and river discharge).
The technique of Diffusional Gradients in Thin-films (DGT) can be used in situ to obtain high res... more The technique of Diffusional Gradients in Thin-films (DGT) can be used in situ to obtain high resolution profiles of trace-metals in sediment pore waters. Substances sampled by DGT continuously diffuse through a 'diffusion layer' comprising a hydrogel prior to being immobilized by binding to a resin layer. DGT therefore measures a time averaged flux from the pore water to the resin. Interpretation of this flux as pore water concentration is problematic for two reasons. Firstly, the pore water concentration adjacent to the sampler may become depleted by the DGT induced flux. Secondly, if there are steep vertical chemical gradients in the pore waters, they may relax by diffusion along the gradient within the gel layer. The extent of relaxation depends on the diffusion coefficient, gradient steepness, and diffusion layer thickness. Two dimensional (2D) numerical models of DGT deployments in horizontally uniform sediments were used to investigate to what extent DGT measured profiles accurately reproduced (a) the shape of pore water concentration profiles, and (b) the magnitude of pore water concentrations. A method is developed which translates high resolution DGT measured flux profiles into reliable estimates of pore water concentrations. Linear relationships are given which estimate the minimum DGT measured peak width (as a function of diffusion layer thickness) that ensures accurate reproduction of the shape and the magnitude of peaks in pore water concentrations. Peaks in DGT profiles obtained from assemblies with diffusion layer thicknesses of 0.3 mm (0.5 mm) should be at least 1.2 mm (1.8 mm) wide for their shape to reflect accurately their true shape in the pore water, and at least 1.7 mm (2.7 mm) wide to ensure the peak concentration is accurately estimated.
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