Improvement of surface run‐off in the hydrological model ParFlow by a scale‐consistent river parameterization

Abstract

We propose an improvement of the overland‐flow parameterization in a distributed hydrological model, which uses a constant horizontal grid resolution and employs the kinematic wave approximation for both hillslope and river channel flow. The standard parameterization lacks any channel flow characteristics for rivers, which results in reduced river flow velocities for streams narrower than the horizontal grid resolution. Moreover, the surface areas, through which these wider model rivers may exchange water with the subsurface, are larger than the real river channels potentially leading to unrealistic vertical flows. We propose an approximation of the subscale channel flow by scaling Manning's roughness in the kinematic wave formulation via a relationship between river width and grid cell size, following a simplified version of the Barré de Saint‐Venant equations (Manning–Strickler equations). The too large exchange areas between model rivers and the subsurface are compensated by a grid resolution‐dependent scaling of the infiltration/exfiltration rate across river beds. We test both scaling approaches in the integrated hydrological model ParFlow. An empirical relation is used for estimating the true river width from the mean annual discharge. Our simulations show that the scaling of the roughness coefficient and the hydraulic conductivity effectively corrects overland flow velocities calculated on the coarse grid leading to a better representation of flood waves in the river channels.

Evaluation of modeled summertime convective storms using polarimetric radar observations

“…Overall, the model generally does not produce sufficient base flow compared to the GRDC observations (Figure 3), which we mainly attribute to the still coarse grid resolution compared to actual river widths (Schalge et al, 2019). The model does, however, produce reasonable daily mean discharge compared to GRDC data for rivers with catchment areas covered by the storm's precipitation.…”

Presentation and discussion of the high-resolution atmosphere–land-surface–subsurface simulation dataset of the simulated Neckar catchment for the period 2007–2015

“…Our current choice of Manning's coefficient in ParFlow (5.52 × 10 −4 h m −1/3 ) results in realistic average discharge throughout the year, albeit at too-low flow velocities. In order to compensate for this inconsistency, Manning's coefficient could be scaled such that the overland flow velocity in river cells equals the river flow velocity, as proposed by Schalge et al (2019), which improves the phasing between simulated and observed discharge and the discharge peak. Similarly, the hydraulic conductivity of the model's top layer for river cells could be scaled in order to reduce the loss of too much surface water to the subsurface caused by the too-wide river cells.…”

Required sampling density of ground-based soil moisture and brightness temperature observations for calibration and validation of L-band satellite observations based on a virtual reality

“…dkrz.de/WDCC/ui/cerasearch/q?query=neckar&page=0&rows=15 (last access: 11 April 2020). We use for this study available simulation results generated by the research unit FOR2131 (Schalge et al, 2016(Schalge et al, , 2018(Schalge et al, , 2019 over an area containing the Neckar catchment in southwestern Germany in its center (Fig. 1).…”

“…The platform, specially designed for high-performance computing environments (Gasper et al, 2014), has been extensively evaluated against observations (Sulis et al, 2015;Shrestha et al, 2018a) as well as similar regional terrestrial system models (Sulis et al, 2015). The effect of spatial resolution on simulated soil moisture and the resulting exchange fluxes between land and atmosphere has been studied with TerrSysMP by Shrestha et al (2014Shrestha et al ( , 2018b.…”