High-resolution global topographic index values for use in large-scale hydrological modelling

Abstract

Abstract. Modelling land surface water flow is of critical importance for simulating land surface fluxes, predicting runoff and water table dynamics and for many other applications of Land Surface Models. Many approaches are based on the popular hydrology model TOPMODEL (TOPography-based hydrological MODEL), and the most important parameter of this model is the well-known topographic index. Here we present new, high-resolution parameter maps of the topographic index for all ice-free land pixels calculated from hydrologically conditioned HydroSHEDS (Hydrological data and maps based on SHuttle Elevation Derivatives at multiple Scales) data using the GA2 algorithm (GRIDATB 2). At 15 arcsec resolution, these layers are 4 times finer than the resolution of the previously best-available topographic index layers, the compound topographic index of HYDRO1k (CTI). For the largest river catchments occurring on each continent we found that, in comparison with CTI our revised values were up to 20 % lower in, e.g. the Amazon. We found the highest catchment means were for the Murray-Darling and Nelson-Saskatchewan rather than for the Amazon and St. Lawrence as found from the CTI. For the majority of large catchments, however, the spread of our new GA2 index values is very similar to those of CTI, yet with more spatial variability apparent at fine scale. We believe these new index layers represent greatly improved global-scale topographic index values and hope that they will be widely used in land surface modelling applications in the future.

Modeling spatiotemporal dynamics of global wetlands: comprehensive evaluation of a new sub-grid TOPMODEL parameterization and uncertainties

“…The comparison between HydroSHEDS and GMTED also indicated that, for capturing inundated areas under the same spatial resolution, the parameter maps derived from DEM without hydrological corrections have less accuracy compared to corrected ones (Lehner and Grill, 2013). Without hydrological corrections, valleys would appear as closed depressions in the DEM, leading to an underestimation of inundated areas (Marthews et al, 2015). It could be foreseen that if DEMs in process-based models are being applied at higher resolution, this drawback could be amplified.…”

“…As a result, it might underestimate the wetland areas within each grid cell and slightly underestimate the temporal pattern of saturated areas because of improper estimates of parameter C s (Güntner et al, 2004). One limitation of HydroSHEDS is that its projection is not equal-area like HYDRO1k (Marthews et al, 2015) and will cause a potential bias in slope calculation along east-west directions at high latitudes. However, since there is no common method to calculate slope or flow direction, we believe that our calculations provide a reasonable approximation for global applications.…”

“…To avoid mismatch of CTI value inherent in computing CTI with different CTI algorithms, we generated three global CTI maps based on the three DEM products instead of relying on existing CTI products (e.g., HYDRO1k CTI, HydroSHEDS CTI product from the Centre for Ecology and Hydrology) (Marthews et al, 2015). Since studies show that multiple flow direction algorithms for calculating CTI give better accuracy compared to single-flow algorithms in flat areas (Kopecký andČížková, 2010;Pan et al, 2004), thus we selected an algorithm from R library "topmodel" (Buytaert, 2011), which applies the multiple flow routing algorithm of Quinn et al (1995) to calculate the global CTI maps.…”

“…While prognostic wetland dynamics schemes are promising to resolve these observational issues, the configuration parameters for TOPMODEL are a potential source of uncertainty in estimating wetland dynamics (Marthews et al, 2015). Among all parameters in TOPMODEL, the compound topographic index (CTI) is of critical importance for determining inundated areas in terrain-related hydrological applications (Ward and Robinson, 2000;Wilson and Gallant, 2000).…”

Improving river flow generation over Great Britain in a land surface model required for coupled land-atmosphere interactions

“…The topographic index λ has been calculated for Great Britain at 50 m 2 resolution from the CEH-IHDTM database (Morris and Flavin, 1990;1994), following the methodology in Marthews et al (2015), and then its mean and standard deviation at the 1 km 2 model grid are used as input data for the TOPMODEL approach.…”