In general terms, existing distributed process models are designed to work at the scale of the "research catchment", where detailed data are available to meet the exacting requirements of their numerous internal processes. Such models are also intended to be applicable to single storm events, or for periods covering just one or two storms, wherein intricate finite difference modelling is still able to produce an acceptable run time. It is often the case, however, that these models are just too demanding in terms of their spatial data requirements and computational processing time - to be of widespread use as effective management tools. Indeed, in terms of land degradation associated with global warming, the required data is often not available for those regions which are most at risk. Moreover, in such regions, it is also the case that modelling would need to be implemented at the larger scale of the "management area", and to be extended over periods ranging from single storms to several decades; this being the time frame in which the distribution of important environmental variables could change.
MEDRUSH, a combined geographical information system and large scale distributed process model, has been built to meet the need for a dynamic macro-model that can overcome these spatial and temporal bottlenecks. This model, which is intended to be applicable to areas of up to 5,000 km2 and for periods of up to 100 years, is designed to provide scenarios of vegetation growth and the distribution of functional types, and to forecast water runoff, sediment yield, and the various ways in which these factors evolve in response to short term sequences of storms, seasonal/annual variations in climate, and long term trends in climate or land use.
MEDRUSH is innovative in several respects, in particular with regard to the scale of its spatial and temporal modelling, its specific application to desertification processes, and its high level of integration with GIS. Problems of scale are tackled using three levels of generalisation: sub-catchment, flow-strip, and section. Desertification factors are based on other work being carried out elsewhere within the Medalus Group. A geographical information system is the principal run-time database management and visualisation tool, working in tandem with other components, as an integrated part of the larger MEDRUSH model. At present the model is restricted to physical processes; although it is intended to incorporate human influences at a later date, via an integrated "expert system" shell.
In the development of this model, emphasis has been placed on generalisation and modelling, as opposed to widespread finite difference engineering and commensurate large scale number crunching. Detailed, purpose-built, digital elevation models are divided into sub-catchments, which are smallest in the headwater regions, and increase in size downstream. Each sub-catchment is treated as a distribution of elementary flow-strips that are defined by their profile and plan morphology; a flow-strip in this instance comprising a hillslope catena of varying width that runs from the perimeter of a sub-catchment to the outlet point of that sub-catchment. The behaviour of one flow-strip per sub-catchment is simulated in detail, to generate water and sediment yield in the short term, and changes in vegetation, surface armour, micro-topography, and soil in the longer term - up to 100 years. This part of MEDRUSH is an explicit generalisation of the MEDALUS Catena Model; but with simplified soil water, vegetation, and evapotranspiration components. Calculations are computed for a limited number of sections on each flow-strip wherein statistical functions are used to represent grain size distributions, micro-topography and inter-hour rainfall intensities. The diversity of vegetation is generalised with mechanistic functional type models, covering groups of species that are similar in structure and phenology, which are expected to show common responses to changes in climate.
The method with which data is transferred to and fro between the spatial database that resides in the GIS, and the representative flow-strips where detailed computations are carried out, is a crucial element in the modelling process. Instances on the representative flow-strip are assumed to represent sub-catchment areas with a similar "unit area" value - in this case accumulated drainage area divided by gradient. This generalisation has been shown to be an appropriate empirical basis for modelling the evolution of slope gradients over moderate periods of time, and it is reasonable to infer that other variables which are related to soil properties and erosion will change with gradient in a consistent manner, and in turn provide common environmental influences for the vegetation. The evolution of each representative flow-strip is modelled in detail, and the rate of change for each variable is applied to all points in the sub-catchment that have the same unit area value, to provide a spatial distribution of soil properties and vegetation at the end of each year. Since unit area values do not change within the time span of the simulation, it follows that the relative proportions of the different input variables will remain constant over time, an approximation that is considered valid based on the small changes that are expected to occur within the time frame envisaged.
Flow-strip behaviour is integrated up to the sub-catchment level, to provide inputs for the network routing component, on a one hour time step. Water flow in the main channel network is simulated using a cascade system, with water being routed between sub-catchment outlets, to the end of the network (basin outlet). Discharge is calculated at each input node in the network and a pre-calculated linearised transfer function is used to transfer water from one input node to the next. Sediment transport is simulated using a finite-difference method coupled to the water flow model, working with pre-calculated sediment transport capacities, and several size fractions. The appropriate transfer functions and sediment transport capacities are selected according to the current discharge level. Long term channel changes are forecast from changes in the flood frequency distribution - linked to hydraulic geometry characteristics of channel and floodplain.
MEDRUSH is at present being applied to the Agri Basin, Basilicata, Italy (1,700 km2), and to the Guadalentin Basin, Murcia, Spain (3,300 km2), so as to illustrate its use in the development of guidelines for integrated catchment management in threatened Mediterranean areas.
Bob Abrahart : bob@geog.leeds.ac.uk