Sensitivity Test of Parameters Influencing Flood Hydrograph Routing with a Diffusion-Wave Distributed using Distributed Hydrological Model, Wet Spa, in Ziarat Watershed

Flood routing is a procedure to calculate flood stage and water depth along a river or to estimate flood hydrograph at river downstream or at reservoir outlets using the upstream hydrography . In river basins, excess rainfall is routed to the basin outlet using flow routing techniques to generate flow hydrograph.
A GIS-based distributed hydrological model, Wet Spa, has been under development suitable for flood prediction and watershed management on a catchment scale. The model predicts outflow hydrographs at the basin outlet or at any converging point in the watershed, and it does so in a user-specified time step. The model is physically based, spatially distributed and time-continuous, and simulates hydrological processes of precipitation, snowmelt, interception, depression, surface runoff, infiltration, evapotranspiration, percolation, interflow, groundwater flow, etc. continuously both in time and space, for which the water and energy balance are maintained on each raster cell. Surface runoff is produced using a modified coefficient method based on the cellular characteristics of slope, land use, and soil type, and allowed to vary with soil moisture, rainfall intensity and storm duration. Interflow is computed based on the Darcy’s law and the kinematic approximation as a function of the effective hydraulic conductivity and the hydraulic gradient, while groundwater flow is estimated with a linear reservoir method on a small subcatchment scale as a function of groundwater storage and a recession coefficient. Special emphasis is given to the overland flow and channel flow routing using the method of linear diffusive wave approximation, which is capable to predict flow discharge at any converging point downstream by a unit response function. The model accounts for spatially distributed hydrological and geophysical characteristics of the catchment.
Determination of the river flow hydrograph is a main target in hydrology. Hydrological modeling deals with calculation of watershed hydrograph using hydro-meteorological information and terrain data, and processes of transforming rainfall into a flood hydrograph and the translation of hydrographs throughout a watershed. Flow routing subjects hydrography transformation and translation throughout a river basin. The Wet Spa model used in this study is a simple grid-based distributed runoff and water balance simulation model that runs on an hourly time step. It predicts hourly overland flow occurring at any point in a watershed, hydrography at the outlet, and provides spatially distributed hydrologic characteristics in the basin, in which all hydrologic processes are simulated within a GIS framework (Bahremand, 2007). The Wet Spa model was originally developed by Wang et al. (1997) and adapted for flood prediction by De Smedt et al. (2000) and Liu et al. (2003).
The outlet is accomplished using the first passage time response function based on the mean and variance of the flow time distribution, which is derived from the advection–dispersion transport equation. The flow velocity is location dependent and calculated in each cell by the Manning equation based on the local slope, roughness coefficient and hydraulic radius. The hydraulic radius is determined according to the geophysical properties of the catchment and the flood frequency. The total direct runoff at the basin outlet is obtained by superimposing all contributions from every grid cell.
The routing of overland flow and channel flow is implemented by the method of the diffusive wave approximation. This method has been used in some recent GIS-based flood models (Fortin et al., 2001; Olivera and Maidment, 1999). Liu et al 2003 has presented the flow routing method of the WetSpa model in detail. A two-parameter response function, based on the average flow time and the standard deviation of the flow time, is proposed in this study. The flow time and its variance are determined by the local slope, surface roughness and the hydraulic radius for each grid cell. The flow path response function at the outlet of the catchment or any other downstream convergence point is calculated by convoluting the responses of all cells located within the drainage area in the form of the probability density function (PDF) of the first passage time distribution. This routing response serves as an instantaneous unit hydrograph and the total discharge is obtained by a convolution integral of the flow response from all generated spatially distributed runoff.
Starting from the continuity equation and the St.Venant momentum equation, assuming the one-dimensional unsteady flow, and neglecting the inertial terms and the lateral inflow to the flow element, the flow process can be modeled by the diffusive wave equation (Cunge et al., 1980)
In this study, flood routing is done as the main part of flow simulation of the distributed hydrological Wet Spa model in the Ziarat watershed. In order to execute the model, hourly hydrometeorological data for a period of four years (2007-2010) including rainfall, evapotranspiration, temperature, and discharge are used as inputs. Additionally, three main maps of the digital elevation model, soil map (texture), and land use are also applied and converted to digital formats. The result of the simulation shows a good agreement between the simulated hydrography and the observed one. The routing of overland flow and channel flow is implemented by the method of the diffusive wave approximation.
Results and Disscasion: The Wet Spa model has been applied in several studies, e.g. the Barebeek catchment in Belgium, the Alzette river basin in Luxembourg, the Hornad watershed in Slovakia, In this study, flood routing is done as the main part of flow simulation of the distributed hydrological WetSpa model in the Ziarat watershed. In order to execute the model, hourly hydrometeorological data for a period of four years (2007-2010) including rainfall, evapotranspiration, temperature, and discharge are used as inputs. Additionally, three main maps of the digital elevation model, soil map (texture), and landuse are also applied and converted to digital formats. The result of the simulation shows a good agreement between the simulated hydrography and the observed one. The routing of overland flow and channel flow is implemented by the method of the diffusive wave approximation. A sensitivity test shows that the parameter of flood frequency and the channel roughness coefficient have a large influence on the outflow hydrography and the calculated watershed unit hydrograph, while the threshold of minimum slope and the threshold of drainage area in delineating channel networks have a marginal effect.