John Ewen

NITROGEN TRANSFORMATION COMPONENT FOR SHETRAN CATCHMENT NITRATE TRANSPORT MODELLING

Abstract The capability to simulate nitrogen transformations has been added to the SHETRAN physically based, spatially distributed river catchment modelling system so that it can be used in 3D simulations of coupled flow and nitrate transport. In SHETRAN, the subsurface is a variably saturated heterogeneous region, comprising perched, unconfined, confined and unsaturated systems, and at the surface there is vegetation and water flow overland and in stream networks. Nitrate transport is modelled in SHETRAN using advection–dispersion equations with terms added for adsorption and a two-region (dynamic region and dead-space) representation. The nitrogen transformations taking place in and below the root zone are modelled using NITS (Nitrate Integrated Transformation component for SHETRAN), which was designed to be comprehensive, self-consistent and fully compatible with SHETRAN. NITS has pools for both carbon and nitrogen in manure, litter and humus, and further pools for ammonium and nitrate, and involves the simultaneous solution of seven ordinary differential equations plus several auxiliary equations. NITS and its integration in SHETRAN are described here, as are a series of successful verification simulations for the responses of the carbon and nitrogen pools when straw and manure are added to the soil, and a successful field validation for nitrate generation and leaching in a fertilised barley plot. The NITS equations strictly apply at a point, and are used in SHETRAN with distributed parameters (i.e. each finite difference cell in SHETRAN has its own set of transformation variables and parameters). The intention is that in addition to being used in simulations of nitrate pollution and the effectiveness of proposed remedial measures and changes in agricultural practice, SHETRAN will be used in studies of the ‘upscaling’ of the equations and parameters for nitrate transport using the ‘UP’ approach of Ewen [Ewen, J., 1997. Hydrol. Earth System Sci. 1, 125–136]. In Birkinshaw and Ewen [Birkinshaw, S.J., Ewen, J., 2000. Modelling nitrate transport in the Slapton Wood catchment using SHETRAN. J. Hydrol. 230, 18–33.] SHETRAN is used to simulate the generation of nitrate following the application of fertiliser in the Slapton Wood catchment, Devon, UK, and the subsequent leaching, lateral subsurface transport and discharge of nitrate to the ground surface and its transport in the Slapton Wood stream.

Validation of catchment models for predicting land-use and climate change impacts. 1. Method

Abstract Computer simulation models are increasingly being proposed as tools capable of giving water resource managers accurate predictions of the impact of changes in land-use and climate. Previous validation testing of catchment models is reviewed, and it is concluded that the methods used do not clearly test a models fitness for such a purpose. A new generally applicable method is proposed. This involves the direct testing of fitness for purpose, uses established scientific techniques, and may be implemented within a quality assured programme of work. The new method is applied in Part 2 of this study (Parkin et al., J. Hydrol., 175:595–613, 1996).

Validation of catchment models for predicting land-use and climate change impacts. 2. Case study for a Mediterranean catchment

Validation methods commonly used to test catchment models are not capable of demonstrating a models fitness for making predictions for catchments where the catchment response is not known (including hypothetical catchments, and future conditions of existing catchments which are subject to land-use or climate change). This paper describes the first use of a new method of validation (Ewen and Parkin, 1996. J. Hydrol., 175: 583–594) designed to address these types of application; the method involves making ‘blind’ predictions of selected hydrological responses which are considered important for a particular application. SHETRAN (a physically based, distributed catchment modelling system) is tested on a small Mediterranean catchment. The test involves quantification of the uncertainty in four predicted features of the catchment response (continuous hydrograph, peak discharge rates, monthly runoff, and total runoff), and comparison of observations with the predicted ranges for these features. The results of this test are considered encouraging.

Modelling nitrate transport in the Slapton Wood catchment using SHETRAN

Abstract In Birkinshaw and Ewen [Nitrogen transformation component for SHETRAN catchment nitrate transport modelling system, J. Hydrol. 230, 1–17] a detailed nitrogen transformation model was incorporated in the SHETRAN physically based spatially distributed river catchment modelling system. This gives SHETRAN the capability to simulate nitrate generation and leaching and the subsequent subsurface transport through combinations of confined, unconfined and unsaturated systems to seepage points and streams, and transport through stream networks. SHETRAN is applied here to explaining the complicated pattern of nitrate discharge seen at the stream outlet from the Slapton Wood catchment, Devon, UK. The nitrate concentrations simulated by SHETRAN are physically realistic and in agreement with measurements made at the catchment, and since SHETRAN was not calibrated against any of the nitrate measurements made at the catchment this represents a strong validation test of the nitrogen component of SHETRAN. The catchment is clay-loam soil underlain by slate, and the main features of the nitrate concentration at the outlet are low concentrations associated with both low and high flows and high concentrations associated with medium flows. A plume of nitrate develops above and around the loam-slate boundary following fertiliser applications in the spring and early summer, and the high outlet concentration occurs when lateral flow develops at the loam-slate boundary during the winter and acts as the main source of flow to the stream. At times when the stream flow is low it is fed from the slate below the plume, and when it is high it is fed from surface flow. SHETRAN simulates the above behaviour and there is close agreement between the simulated and measured outlet concentrations. The SHETRAN distributed results for nitrate concentrations and leaching load also agree well with existing measurements. It is concluded that SHETRAN should prove to be a powerful, practical and useful tool for studying nitrate pollution problems.

Software, Data and Modelling News: Graphical user interface for rapid set-up of SHETRAN physically-based river catchment model

The SHETRAN physically-based distributed rainfall-runoff modelling system gives detailed simulations in time and space of water flow and sediment and solute transport in river catchments. It is therefore a powerful tool for studying hydrological and environmental impacts associated with land-use and climate change. A Graphical User Interface (GUI) has been developed that allows a catchment data set to be set up quickly using a minimum of information. The GUI has an algorithm for the automatic generation of river channel networks from a DEM and has access to libraries of soil and vegetation parameters.

Susceptibility to drying of unsaturated soil near warm impermeable surfaces

Abstract Drained soil near warm impermeable surfaces heats up and may dry. It is proposed here, based on a theoretical study, that a soil property—critical temperature difference-is a measure of the susceptibility of soil to drying; near a surface of any shape or size, drying will take place if a temperature difference greater than the critical temperature difference is maintained between any two points in the soil. The design of an experiment to measure critical temperature difference is discussed.

Implementing a Grid/Cloud eScience Infrastructure for Hydrological Sciences

The objective of this chapter is to describe building an eScience infrastructure suitable for use with environmental sciences and especially with hydrological science applications. The infrastructure allows a wide range of hydrological problems to be investigated and is particularly suitable for either computationally intensive or multiple scenario applications. To accomplish this objective, this research discovered the shortcomings of current grid infrastructures for hydrological science and developed missing components to fill this gap. In particular, there were three primary areas which needed work: first, integrating data and computing grids; second, visualization of geographic information from grid outputs; and third, implementing hydrological simulations based on this infrastructure. This chapter focuses on the first area, which is focusing on grid infrastructure system integration and development. A grid infrastructure, which consists of a computing and a data grid, has been built. In addition, the computing grid has been extended to utilize the Amazon EC2 cloud computing resources. Users can conduct a complete simulation job life cycle from job submission, and data management to metadata management based on the tools available in the infrastructure.

Geo-visualization Fortran library

Geobrowser tools offer easy access to geographical and map images over which geospatial data can be overlaid, a process that provides a powerful new visualization resource for scientists. Many of these tools make use of the well-documented KML/XML data formats, and the challenge for the scientist is to generate KML files from their simulation and analysis programs. Since many of these programs are written in the Fortran language, which does not have native tools to support XML files, we have developed a new library - WKML - that enables KML files to be produced directly and automatically. This paper describes the WKML library, gives a number of different examples to illustrate the breadth of its functionality, and describes in more detail an example of its use for hydrology.

A physically based approach to modelling radionuclide transport in the biosphere

Calculations of radiological risk are required to assess the safety of any potential future UK deep underground repository for intermediate-level and certain low-level solid radioactive wastes. In support of such calculations, contaminant movement and dilution in the terrestrial biosphere is investigated using the physically based modelling system SHETRAN. Two case studies are presented involving modelling of contaminants representing long-lived poorly sorbed radionuclides in the near-surface aquifers and surface waters of hypothetical catchments. The contaminants arise from diffuse sources at the base of the modelled aquifers. The catchments are characterised in terms of detailed spatial data for topography, the river network, soils and vegetation. Simulations are run for temperate and boreal climates representing possible future conditions at a repository site. Results are presented in terms of the concentration of contaminants in the aquifer, in soils and in surface waters; these are used to support the simpler models used in risk calculations.

Modelling long-term contaminant migration in a catchment at fine spatial and temporal scales using the UP system

A method has been developed to simulate the long-term migration of radionuclides in the near-surface of a river catchment, following their release from a deep underground repository for radioactive waste. Previous (30-year) simulations, conducted using the SHETRAN physically based modelling system, showed that long-term (many decades) simulations are required to allow the system to reach steady state. Physically based, distributed models, such as SHETRAN, tend to be too computationally expensive for this task. Traditional lumped catchment-scale models, on the other hand, do not give sufficiently detailed spatially distributed results. An intermediate approach to modelling has therefore been developed which allows flow and transport processes to be simulated with the spatial resolution normally associated with distributed models, whilst being computationally efficient. The approach involves constructing a lumped model in which the catchment is represented by a number of conceptual water storage compartments. The flow rates to and from these compartments are prescribed by functions that summarize the results from physically based distributed models run for a range of characteristic flow regimes. The physically based models used were, SHETRAN for the subsurface compartments, a particle tracking model for overland flow and an analytical model for channel routing. One important advantage of the method used in constructing the lumped model is that it makes down scaling possible, in the sense that finescale information on the distributed hydrological regime, as simulated by the physically based distributed models, can be inferred from the variables in the lumped model that describe the hydrology at the catchment scale. A 250-year flow simulation has been run and the down scaling process used to infer a 250-year time-series of three-dimensional velocity fields-for the subsurface of the catchment. This series was then used to drive a particle tracking simulation of contaminant migration. The concentration and spatial distribution of contaminants simulated by this model for the first 30 years were in close agreement with SHETRAN results. The remaining 220 years highlighted the fact that some of the most important transport pathways to the surface carry contaminants only very slowly so both the magnitude and spatial distribution of concentration in surface soils are not apparent over the shorter SHETRAN simulations.