Picturing and modeling catchments by representative hillslopes

Abstract

Abstract. This study explores the suitability of a single hillslope as a parsimonious representation of a catchment in a physically based model. We test this hypothesis by picturing two distinctly different catchments in perceptual models and translating these pictures into parametric setups of 2-D physically based hillslope models. The model parametrizations are based on a comprehensive field data set, expert knowledge and process-based reasoning. Evaluation against streamflow data highlights that both models predicted the annual pattern of streamflow generation as well as the hydrographs acceptably. However, a look beyond performance measures revealed deficiencies in streamflow simulations during the summer season and during individual rainfall-runoff events as well as a mismatch between observed and simulated soil water dynamics. Some of these shortcomings can be related to our perception of the systems and to the chosen hydrological model, while others point to limitations of the representative hillslope concept itself. Nevertheless, our results confirm that representative hillslope models are a suitable tool to assess the importance of different data sources as well as to challenge our perception of the dominant hydrological processes we want to represent therein. Consequently, these models are a promising step forward in the search for the optimal representation of catchments in physically based models.

HESS Opinions: The complementary merits of competing modelling philosophies in hydrology

“…For example, instead of averaging van Genuchten parameters, the ensemble of actual observed water retention curves at different locations in the system could be used to estimate upper and lower bounds of effective pedotransfer functions. As recently illustrated by Loritz et al (2017), integrating some of the heterogeneity in such a way, these functions may be more representative of larger areas. This in turn allows one to reduce the spatial resolution of the model domain and the associated problems.…”

On the dynamic nature of hydrological similarity

“…Fenicia et al (2016) and Jackisch (2015) showed that topography alone might be a poor guide 120 for subdividing a 256 km 2 catchment into different functional units, and questioned the explanatory power of the topography in this respect. Our own work, Loritz et al (2017), has shown that an "effective" representation of two different catchments by a single representative hillslope was able to provide successful simulations of their inter-annual runoff responses and annual storage dynamics. Together, these findings suggest that an informationally "compressed" representation of the topographic map may be able 125 to preserve the relevant information regarding geopotential differences that drive runoff generation.…”

“…Besides the importance of lateral flow along the bedrock, several irrigation and dye staining experiments have highlighted the role of vertical structures for infiltration and subsequently for subsurface runoff formation (Jackisch et al, 2016). For a more detailed description 205 please see Loritz et al (2017), Wrede et al (2015) and Jackisch (2015).…”

“…[17] With respect to soils, bedrock topography and vegetation, the 105 hillslope models were identically parameterized using a parameter set, macropore distribution and subsurface stratification tested and derived by Loritz et al (2017) [18] Boundary conditions were set to an atmospheric boundary at the top, no flow boundary conditions at the upslope, and a gravitational flow boundary condition at the lower boundary. At the hill foot of the hillslope we selected a seepage interface for the upper 0.4 m, where outflow only occurs under saturated and no flow under unsaturated conditions.…”

“…For the lower 1.6 m of the downslope boundary we selected a 245 no flow boundary to mimic a saturated zone close to the river. All of the hillslopes are covered entirely by forest and the evapotranspiration routine is parameterized similarly to the one described in detail in Loritz et al (2017). Figure 1c shows an example of a typical CATFLOW hillslope grid and soil setup divided into soil, rapid flow paths and bedrock.…”