Hydrotel, a Distributed Hydrological Model Compatible with Remote Sensing and Geographical Information Systems
As hydrological processes vary both in space and time as a function of meteorological inputs, land-use, topography and soil type, to mention only those, a model able to make the best use of data from remote sensing and geographic information systems (GIS) has been developed. One of the objectives in developing HYDROTEL was to be able to apply the model to as many watersheds as possible, with a minimum of calibration. Also, as the availability of data varies both in type and density from watershed to watershed, it was considered necessary to develop a model offering a choice of algorithms allowing adaptation of the model to data availability on various watersheds. Another objective was to program HYDROTEL on a micro-computer with a user-friendly interface. The complete drainage structure of a watershed is obtained with PHYSITEL, a software program designed specifically to prepare the watershed database for HYDROTEL. The area of interest is first discretized in square cells allowing the creation of a digital elevation model (DEM), with a pre-determined accuracy, from which the slope and aspect of each cell are obtained next. The aspect of each cell being known, that is the direction of flow from cell to cell, it is necessary to identify the cell considered as the outlet of the watershed to identify all cells upstream of that cell, with a recursive algorithm. When all cells constituting a watershed are identified, together with the drainage structure, it is possible to trace the river network corresponding to cells draining a number of cells greater than a specified threshold. Finally, sub-watersheds are determined, with outlets at the river junctions. Those sub-watersheds can be further sub-divided or grouped to obtain relatively homogeneous hydrological units (RHHU). A modular approach has been adopted with HYDROTEL allowing easy addition or modification of algorithms. A choice of algorithms, selected when possible for their compatibility with remotely sensed and GIS data, is generally offered for each sub-model. Moreover, it is possible, instead of choosing one of the available simulation options, to read data from disk. This permits using, for instance, rainfall data estimated from weather radars by another program. Also, one can decide to activate only specific sub-models for a run and read data from disk for the others or just ignore the others, if they are not needed for the run. The simulation runs can be done, using as a unit for the estimation of the vertical water budget, the original cells or the RHHU's. For the precipitation sub-model, solid or liquid precipitations, together with air temperatures, are interpolated to each simulation unit either by the Thiessen method or by a method leading to a weighted average of the measured amounts at the nearest three stations, taking into account, if desired, of the precipitation and temperature lapse rates. Daily variation and metamorphism of the snowpack are estimated by a modified degree-day method in which the energy budget at the snow-air interface is estimated by the degree-day approach but that within the pack by a more physical approach. Four equations are available to estimate potential evapotranspiration, those of Thornthwaite, Linacre, Penman-Monteith and Priestley-Taylor, in order to use the best equation for a given data set. The vertical water budget is simulated by the vertical algorithm of the CEQUEAU model or by a new algorithm more suited to remote sensing and GIS information. A kinematic wave approach is used to estimate downward flow from cell to cell, whereas river routing is simulated with the kinematic or diffusive wave equations. When the vertical water budget is done for a RHHU, internal routing of the available flow within the RHHU is performed through the use of a geomorphological hydrograph derived from its drainage structure. The HYDROTEL model has been applied to watersheds located in Québec, Ontario and British-Colombia in Canada and to one in Southern France, in order to test its applicability to watersheds of different types and areas in various climat es. The results obtained on those watersheds, using the available algorithms, show that the model does bave a normal reaction to precipitation and temperature impulses on all watersheds. At the same time, those results confirm the need for accurate spatial information, which is likely to be available more from remotely sensed and GIS data. A new version of HYDROTEL is now under development and it will be run on OS\2 and other environments. Finally, with its simulation options allowing monitoring of various variables during a simulation run, HYDROTEL appears to be a good tool for understanding and managing phenomena related to hydrological processes.