Minimum watershed model structure for representation of runoff processes
Creators: name: Micovic, Zoran
Publisher: name: University of British Columbia
Year: 2009

description: The aim of this study is to examine what degree of complexity in watershed model structure is required to obtain realistic representation of the runoff processes. Spatial and temporal distribution of hydrological processes are investigated, especially linear and non-linear algorithms and the sub-division of runoff time response according to the soil moisture status of the watershed. To carry out this investigation, hydrologic models, with structures of increasing complexity, were constructed and their capability to simulate runoff from a watershed was evaluated. The initial evaluation, with the point input meteorological data, was performed on a large (1150 km2) snowdominated watershed and the findings were then verified on a smaller (188 km2) rain-dominated watershed. The continuous long term runoff simulations were performed for both watersheds using daily time steps. The results indicated that, for both types of watersheds, the modelling capability increased significantly if the following elements were introduced: at least two altitude zones; fast and slow runoff components, determined by soil moisture control of watershed impermeable fraction; routing of each component through a linear reservoir system; and two land cover representations, namely treed and open areas, which influenced snowmelt process. Further increase in model complexity gave negligible improvement in Nash-Sutcliffe efficiency. The nature of the Nash-Sutcliffe coefficient of efficiency was tested with both real and synthetic data and it was shown theoretically and from model results that the efficiency would always be higher when calculated for a longer time period. The investigation of meteorological point input data focused on the influence of the location and number of stations. In terms of location, it was shown that a low elevation station was inferior to a high elevation station because it was less representative of the average watershed climate and required greater complexity in model structure to compensate for the unrepresentative climate input at low altitude. In terms of the number of meteorological stations, it was shown that using two stations, each representing an adequate portion of the watershed, improved runoff simulation. Further investigation of the role of non-linearity in model design was performed by modelling several of the largest historical storms on record using an hourly time step. It was discovered that certain parameters that appeared unimportant during the long term simulation had significant effect on the short-term extreme event model simulation. In addition, the simpler model structure that was proven satisfactory for the long term simulation was, in some cases, inferior to the slightly more complex model structure when focus was on extreme event simulation. Therefore, the results of this work indicated that, in the most cases, a reasonably simple watershed model structure would provide satisfactory representation of watershed behaviour. Such cases comprise long term runoff simulation from large and small, snow-dominated and rain-dominated watersheds as well as some extreme flood simulations. However there are occasions when this simple model structure is inadequate and extra model structure is required to model the effects of intense precipitation events. The overall conclusion is that relatively simple model structures are capable of giving good estimation of runoff from complex mountain watersheds, and there is no measurable gain from further increase of model complexity.