Initial Stages in the Development of a Coupled Hillslope Hydrology- Floodplain Inundation Model

Two-dimensional hydrodynamic floodplain inundation models were originally developed for civil engineering applications and have been developed to a high level of sophistication. These two-dimensional (depth averaged) schemes are capable of a high degree of spatial representation and recent developments such as their application to longer reach lengths have enabled their application in other fields including hydrology, geomorphology and ecology. These models provide a powerful tool for investigations in these fields and may be considered as a platform for further developments which incorporate specific processes occurring within the floodplain environment. It is for the case of hydrological applications that a problem has been identified in that these models do not currently consider catchmeat hydrology; whilst the models provide a good representation of floodplain processes in a hydraulic context, catchment hydrology is essentially treated as a black box. The only input to the system is the upstream input hydrograph (occasionally rainfall over the floodplain surface or tributary inflows are included) and output only occurs at the downstream boundary. The floodplain is assumed to be impermeable and any input from the hillslopes bordering the reach is ignored. This study diiusges the initial stages of an investigation carried out to examine the significance., of contributions to the floodplain from the hillslopes bordering the reach. In order to do thii, the zero flux boundary condition at the hillslope-floodplain interface is relaxed. A two-dimensional floodplain inundation model, RMA-2, is set up for a 14 km reach of the River Cuim in Devon. A distributed hillslope hydrology model, VSAS3 is set up for a section of the hillslopes bordering the reach. The topography of the hillslope section is used as a geometric template, although a semitheoretical approach is adopted whereby it is assumed that the hillslope hydrological characteristics are homogenolpp and VSAS3 is parameterised using values reported in the literature from field observations. VSAS3 is coupled to RMA-2 using a simple external coupling mechanism where the hillslope discharge predicted by VSAS3 is applied to elements along the edge of the RMA-2 finite element mesh. Three storm events, with different return periods are simulated\ud using this coupled scheme. It has been shown that hillslope infiows can have a significant effect on the predictions made by RMA-2. In addition, the timing of the hillslope inflow peak relative to the arrival of the floodwave from upstream is of great importance

Towards defining a scallop dominant discharge for vadose conduits: some preliminary results

A well-established inverse relationship exists between mean scallop length and flow vel~city fo~ a given population of scallops. Previous authors have suggested that one or more 'sc~llop dominant diScharges' can be identified at which erosion by dissolution proceeds at the ?Teatest rate, Since scallop. populations usually indicate a single flow velocity whereas discharge and velOCity are unsteady throu~ tIme. For vadose conduits, a scallop dominant discharge is difficult to de~ne because of the unconstrained crosssection; this causes problems in determining the discharge at whIch scallops are forme?, althou~ recent developments in instrumentation allow greater flexibility in monitoring flow~ on a continUOUS b~ls. Here the relationships between monitored flow velocity and depth are compare? ~Ith the scallop velOCIty fo: an active vadose streamway in Poulnagollum, Co. Clare, Ireland. From these InItIal results, a complex relatIOnship is seen to exist between the velocity and depth of flow as discharge changes. Thr~sholds o~c~ over discrete depth ranges where there is little or no change in velocity; these are observed dunng both nsmg and falling stage. It is suggested that these thresholds may be related to changes in hydraulic radius, and hence flow resistance at different depths of flow. The scallop-derived velocity is related ~o the record~ flow data, with reference to the various controls on erosion, most notably the degree to whIch the flow IS undersaturated with CaC03, and ongoing research is outlined

Initial Stages in the Development of a Coupled Hillslope Hydrology- Floodplain Inundation Model

Two-dimensional hydrodynamic floodplain inundation models were originally developed for civil engineering applications and have been developed to a high level of sophistication. These two-dimensional (depth averaged) schemes are capable of a high degree of spatial representation and recent developments such as their application to longer reach lengths have enabled their application in other fields including hydrology, geomorphology and ecology. These models provide a powerful tool for investigations in these fields and may be considered as a platform for further developments which incorporate specific processes occurring within the floodplain environment. It is for the case of hydrological applications that a problem has been identified in that these models do not currently consider catchmeat hydrology; whilst the models provide a good representation of floodplain processes in a hydraulic context, catchment hydrology is essentially treated as a black box. The only input to the system is the upstream input hydrograph (occasionally rainfall over the floodplain surface or tributary inflows are included) and output only occurs at the downstream boundary. The floodplain is assumed to be impermeable and any input from the hillslopes bordering the reach is ignored. This study diiusges the initial stages of an investigation carried out to examine the significance., of contributions to the floodplain from the hillslopes bordering the reach. In order to do thii, the zero flux boundary condition at the hillslope-floodplain interface is relaxed. A two-dimensional floodplain inundation model, RMA-2, is set up for a 14 km reach of the River Cuim in Devon. A distributed hillslope hydrology model, VSAS3 is set up for a section of the hillslopes bordering the reach. The topography of the hillslope section is used as a geometric template, although a semitheoretical approach is adopted whereby it is assumed that the hillslope hydrological characteristics are homogenolpp and VSAS3 is parameterised using values reported in the literature from field observations. VSAS3 is coupled to RMA-2 using a simple external coupling mechanism where the hillslope discharge predicted by VSAS3 is applied to elements along the edge of the RMA-2 finite element mesh. Three storm events, with different return periods are simulated\ud using this coupled scheme. It has been shown that hillslope infiows can have a significant effect on the predictions made by RMA-2. In addition, the timing of the hillslope inflow peak relative to the arrival of the floodwave from upstream is of great importance

Climate Change and Water Resources

Abstract included in text

Climate Change Impact on Catchment Hydrology & Water Resources for Selected Catchments in Ireland

This paper analyses the likely impacts of changes in climate for nine hydrologically diverse catchments throughout Ireland. When assessing the impacts of climate change on water resources there is a cascade of uncertainty that begins with the establishment of future pathways of development and ends with impact assessment (Wilby, 2005). In order to represent uncertainty in future simulations, statistically downscaled output from three Global Climate Models (GCMs), forced using two emission scenarios is used to force a lumped, conceptual rainfall-runoff model for three future time periods; the 2020s, the 2050s and 2080s. Changes in catchment storage, streamflow and extreme events are assessed through comparison with the GCM modelled control period 1961-1990. Future simulations suggest that reductions in soil moisture storage throughout the summer and autumn months are likely for each catchment. The extent of decreases are largely dependent on the storage potential of individual catchments; the lower the capacity of catchments to store water, the greater the sensitivity to climate change. Reductions in groundwater storage during the recharge period will increase the risk of severe drought, as the failure of winter or spring precipitation may result in prolonged drought periods where the groundwater system is unable to recover. Greatest reductions in streamflow are likely for the autumn months in the majority of catchments, while greatest increases are suggested for the month of February. The magnitude and frequency of flood events are shown to increase, with the greatest increases associated with floods of a higher return period. Uncertainty in future simulations derived from HYSIM parameter uncertainty is found to be more important than uncertainty due to emission scenario

Climate Change and Water Resources

Abstract included in text

Climate Change and Water Resources in Ireland: Initial investigations using downscaled GCMS and hydrological modelling techniques.

Predictions are made of changes in effective runoff at a high spatial resolution for the island of Ireland under different climate change scenarios. Although previous studies have examined the response of selected Irish catchments to future climate scenarios this is the first time that the whole area of the island has been considered. This paper discusses the initial investigation that has been carried out and outlines research currently in progress. Polynomial regression techniques are used to derive a baseline climatology for Ireland. Downscaled precipitation and evaporation data from the United Kingdom Climate Program (UKCIP)for the Medium-low 2080 scenario is used together with the baseline data for the initial investigation. The precipitation and potential evaporation (PE) data are converted to a suitable form for input to HYSIM. Two sets of simulations are carried out for 825 JO x JO km grid squares covering the land area of the island of Ireland for the baseline period and the 2080 scenario. The hydrological parameters for each of these squares are considered to be identical for this initial investigation. The results of this investigation are presented. Future work is considered, focusing on the parameterisation of HYSIM for individual grid cells. The initial investigation demonstrates that the representation of storage is of particular importance. Parameter values are derived for each square using data from the Soil Survey of Ireland, the CORINE land use database and information on major aquifers provided by the Geological Survey of Ireland. The flexible data requirements of HYSIM allow some representation of the diverse hydrological conditions found within Ireland. For example, approximately 40% of Ireland is underlain by limestones, many of which are karstified. These karst aquifers are an important water resource and can respond very rapidly to precipitation inputs. There are also extensive areas of lakes and wetlands. The land area is divided into broad hydrological zones to provide some representation of this variability

Catering for Uncertainty in a Conceptual Rainfall Runoff Model: Model Preparation for Climate Change Impact Assessment and the Application of GLUE using Latin Hypercube Sampling

Changes in Irish climate may pose a number of obstacles for water resource management. There is a need to approach this problem using the catchment as the basic unit of analysis. The application of a lumped conceptual rainfall-runoff model for simulating beyond a baseline calibration set is a major challenge for climate change impact assessment. This is due in no small part to the limitations associated with the use of these models, with uncertainty in model output being associated with model structure and the non-uniqueness of optimised parameter sets. In this paper, HYSIM, an “off-the-shelf” conceptual rainfall runoff model using data on a daily time-step is applied to a suite of catchments throughout Ireland in preparation for use with downscaled climate data. Uncertainties relating to process parameter calibration due to parameter interaction and equifinality are highlighted. In an attempt to improve the reliability of model output the generalised likelihood uncertainty estimation (GLUE) framework is adopted to analyse the uncertainty in model output derived from parametric sources. Traditionally this approach has been applied using Monte Carlo random sampling (MCRS). However, when using an “off-the-shelf” type model, source code may not be available and it may not be feasible to run the model for large MCRS samples without user intervention. In order to make the propagation of uncertainty through the model more efficient, input parameter sets are generated using Latin Hypercube sampling (LHS). A number of acceptable parameter sets are generated and uncertainty bounds are constructed for each time step using the 5th and 95th percentile at each temporal interval. These uncertainty bounds will be used to quantify the uncertainty in simulations carried out beyond the baseline calibration period as they include the error derived from data measurement, model structure, and parameterisation

Climate Change Scenarios and Challenges for the Water Environment

The provision of downscaled global circulation output is the first stage in assessing the implications of climate change for the water environment. Using only one global climate model Sweeney and Fealy, 2003 concluded that projected changes in climate will have potentially large effects on the water environment in Ireland, particularly on flood and drought frequencies. Increased winter runoff in western parts as a result of wetter winters and decreased summer runoff, especially in eastern Ireland as a result of substantial reductions in summer rainfall are projected. Considerable uncertainties however exist from such projections since they are based on only one GCM. These uncertainties limit the reliability of such climate scenarios for future water resource management since different GCMs tend to show different results for areas such as Ireland. This arises from inherent weaknesses they possess due to problems of scale and feedback. One way of addressing these uncertainties and providing more reliable inputs to hydrological models is to use multi- model downscaling, and this approach is presented here

Changing Precipitation Scenarios: preliminary implications for groundwater flow systems and planning.

Statistical downscaling of a suite of three global climate models for two emission scenarios are used to produce precipitation scenarios for Ireland to 2090. One of these was used to drive a rainfall-runoff model for the River Boyne. The model was calibrated over the 1961-90 base period, validated using 1991-2000 data and run for three future time periods using downscaled GCM output. Significant changes in monthly flow regimes, soil moisture storage and groundwater storage were noted, with summer flows typically reduced by 20%. Negative changes in soil moisture storage also resulted, with soil moisture deficits increasingly extending into the Autumn as the century proceeds. Such a situation is seen to potentially compromise groundwater recharge in individual years and an increasing lag in groundwater recharge was detected. By the 2080s the groundwater recharge lag has developed to the extent that spring and early summer surface flows appear to be still benefiting from winter groundwater recharge while by late autumn groundwater is seriously depleted due to drier summer conditions. Serious implications for water yield from groundwater-fed sources would thus arise in the event of a dry winter being experienced. Greater conservatism in estimating water yields from groundwater sources would seem appropriate and may require to be formally incorporated into planning procedures