TOPMODEL and the role of topography and variable contributing areas in runoff production Learning objectives Be able to describe the topographic wetness index used in TOPMODEL. Be able to use TOPMODEL principles to calculate soil moisture deficit and saturated areas as a function of wetness index and use this information in the calculation of runoff.

TOPMODEL Beven, K., R. Lamb, P. Quinn, R. Romanowicz and J. Freer, (1995), "TOPMODEL," Chapter 18 in Computer Models of Watershed Hydrology, Edited by V. P. Singh, Water Resources Publications, Highlands Ranch, Colorado, p.627-668. “TOPMODEL is not a hydrological modeling package. It is rather a set of conceptual tools that can be used to reproduce the hydrological behaviour of catchments in a distributed or semi-distributed way, in particular the dynamics of surface or subsurface contributing areas.”

Hydrological processes within a catchment are complex, involving: Macropores Heterogeneity Fingering flow Local pockets of saturation The general tendency of water to flow downhill is however subject to macroscale conceptualization

Runoff generation processes Infiltration excess overland flow aka Horton overland flow P P f P qo f Partial area infiltration excess overland flow P P P qo f P Saturation excess overland flow P P qo qr qs

Map of saturated areas showing expansion during a single rainstorm Map of saturated areas showing expansion during a single rainstorm. The solid black shows the saturated area at the beginning of the rain; the lightly shaded area is saturated by the end of the storm and is the area over which the water table had risen to the ground surface. [from Dunne and Leopold, 1978] Seasonal variation in pre-storm saturated area [from Dunne and Leopold, 1978]

Topmodel – Key Ideas A b a=A/b Surface saturation and soil moisture deficit is based on topography (Slope, specific catchment area, topographic convergence) The soil profile at each point has a finite capacity to transport water laterally downslope Variable Saturated Areas (partial contributing area concept) Specific catchment area a is the upslope area per unit contour length [m2/m  m] A b a=A/b S Dw D

GIS Terrain Analysis Digital elevation model grid representation of topography

Channels, Watersheds, Flow Related Terrain Information The terrain flow information model for deriving channels, watersheds, and flow related terrain information. Watersheds are the most basic hydrologic landscape elements Raw DEM Pit Removal (Filling) Channels, Watersheds, Flow Related Terrain Information Flow Field This slide shows the general model for deriving flow field related derivative surfaces from digital elevation data. The input is a raw digital elevation model, generally elevation values on a grid. This is basic information used to derive further hydrology related spatial fields that enrich the information content of this basic data. The first step is to remove sinks, either by filling, or carving. Then a flow field is defined. This enables the calculation of flow related terrain information. My focus has been on flow related information working within this framework. I try to leave other GIS functionality, like radiation exposure, line of sight analyses and visualization to others. I have distributed my software in ways that it easily plugs in to mainstream systems, such as ArcGIS to enhance ease of use.

Slope Specific Catchment Area a/TS

Numerical Example Given Compute Ko=10 m/hr R=0.0002 m/h f=5 m-1 Qb = 0.8 m3/s A (from GIS) ne = 0.2 Compute R=0.0002 m/h l=6.9 T=2 m2/hr Raster calculator -( [ln(sca/S)] - 6.9)/5+0.46