Introduction to the TOPMODEL

1. Introduction to the TOPMODEL Venkatesh Merwade

2. Brief History • Development Initiated by Prof. Mike Kirkby, School of Geography, University of Leeds in 1974. • First version programmed by Keith Beven in Fortran IV (Who is currently the head of group working on the model at Lancaster University) • Since 1994 many versions of TOPMODEL have been developed at Leeds, Lancaster and some other places. • Not intended to be a traditional model package but is more a collection of concepts that can be used where appropriate. • The present version will be best suited to catchments with shallow soils and moderate topography which do not suffer from excessively dry periods.

3. Description ……. “the problem is what to route and how to route”. (Cordova and Rodriguez-Iturbe, 1983) • Two components • Water Balance at the soil surface (run off prediction ) • Routing (transfer of runoff to the basin outlet)

4. Description Cont’d. • The water balance at the soil level is the component which characterizes the model and constitutes the most important part. • The transfer component is divided into two phases, the first representing transfer along the slopes towards the drainage network and the second representing transfer along the drainage network to the basin outlet. • In the TOPMODEL, by its very nature (distributed!), both the surface component and the flow in the soil are made available directly along the drainage network at each time interval.

5. Parameters • Five parameters • M : the parameter of the exponential transmissivity function or recession curve (m). • Ln (To) : the natural logarithm of the effective transmissivity of the soil when just saturated. A homogenous soil throughout the catchment is assumed (m2/h). • SRmax: the soil profile storage available for transpiration, i.e. an available water capacity (m). • SRint: the initial storage deficit in the root zone (m). • ChVel : routing velocity for scaling the distance over area – also called network width function. Linear routing is assumed (m/h).

6. Vegetation interception Capacity: represented by a reservoir with a capacity of SRmax. The water is extracted from the reservoir at potential evapotranspiration rate; the net precipitation in excess of the capacity SRmax reaches the soil and forms the input for the subsequent model components. Physical Processes

7. Physical Processes Cont’d. • Surface runoff from saturation excess • In the TOPMODEL the saturated hydraulic conductivity of the soil follows a negative exponential law vs. depth. • It is also assumed that the water table is parallel to the soil surface so that downslope flow beneath water table at a depth zi is given for any point i by • Series of calculations gives the value of zi. The value of zi is determined only by the parameter f and the topographic index.

8. Physical Processes Cont’d. • Surface runoff from infiltration excess • In the TOPMODEL, the infiltration excess computation is based on the Philip equation. • Calculation of flow in the saturated zone and the sequence of calculation in the TOPMODEL • The value of permits the estimation of the saturated basin fraction. The value of is updated at every time step by using the equation • which is basically a continuity equation.

9. Initial Conditions • Initial Conditions • The continuity equation is initialized by assuming that the simulation begins after a long dry period; in other words, the unsaturated zone is held to be totally dry and the flow observed at the basin outlet is deemed to have been generated only by the subsurface flow contribution:

10. Project Files • Input Data File • The input data file provides the rainfall, potential evapotranspiration and observed discharge data required for a particular application of TOPMODEL. • Topographic Index Map Data File • This file should be the map of the topographic index values from which the distribution input as part of the catchment data files can be derived. • Catchment Data File • It provides the distributions for the catchment or for each subcatchment

11. Output Options • The Hydrograph Prediction Option • This option allows the model to be run and hydrographs displayed. Parameter values can be changed on screen. After each run four indices of goodness of fit are given for evaluation, viz., The Nash and Sutcliffe Efficiency Criterion, SSE, SLE and SAE. • Sensitivity Analysis Option • The allows to observe the sensitivity of the objective functions to changes of one or more of the parameters to be explored. • The Monte Carlo Analysis Option • In this option a large number of runs of the model can be made using uniform random samples of the parameters chosen for inclusion in the analysis.