Comparison of single and multiple flow direction algorithm for computing topographic parameters in TOPMODEL – A research report by National Institute of Hydrology

The report attempts to do a comparison of single and multiple flow direction algorithms for computing topographic parameters in TOPMODEL. Topography is an important factor that influences the physical phenomenon of rainfall-runoff. Undeniably, it plays a primary role in the gravity drainage within a basin. In the last decade, ever-developing computer techniques associated with the greater availability of terrain data in digital form has favored the production of physically based models, which have spatial variability of topography built into their structure.

The TOPMODEL developed by Beven, in 1979 is one of the many models, which uses topographic feature in simulating the flows.  The topographic features are represented in the model as topographic index, ln (a/tanβ) in which ‘a’ represents the cumulative upslope area that drains through a point, and tanβ is the local slope at the same point. The index reflects the spatial distribution of soil moisture and saturated surface within the basin.

The topographic index, ln (a/tanβ) distribution can be computed using the two different approaches, namely, Single Direction Flow Algorithm (SDF) and Multiple Direction Flow Algorithm (MDF).

In the present study, the TOPMODEL has been applied to the Malaprabha catchment. The topographic index was computed using both SDF and MDF. An attempt has been made to analyze the effect of these approaches on the simulation of flows and on the model parameters. The results show that, the SDF and MDF have slight effect on the efficiency of the model and also on the model parameters. However, choice of these approaches does not have any effect on the simulation of flows.

The study concludes that –

• The single direction flow algorithm computes greater cumulative area for the same topographic index value as compared to the multiple direction flow algorithm.
• The use of SDF and MDF algorithm results in different spatial distribution of the topographic index computed from the DEM’s
• The use of SDF and MDF algorithms results in only a marginal difference in the model efficiency and simulated flow paths. The MDF algorithm gives slightly better results.
• The use of SDF and MDF algorithms has some effect on the model parameters, especially on m (the decay parameter) and lnT_o (the transmissivity of the soil profile under saturation).

The choice of algorithm influences the simulated spatial distribution of hydrologic characteristic such as soil moisture content. Therefore, if TOPMODEL is being used to simulate the spatial pattern of hydrologic characteristics, then the MDF algorithm is preferable over SDF algorithm. If the model is used for simulation of streamflow, the choice of algorithm makes no difference.

Download the report here: