A hybrid‐3D hillslope hydrological model for use in Earth system models

Abstract

Hillslope‐scale rainfall‐runoff processes leading to a fast catchment response are not explicitly included in land surface models (LSMs) for use in earth system models (ESMs) due to computational constraints. This study presents a hybrid‐3D hillslope hydrological model (h3D) that couples a 1‐D vertical soil column model with a lateral pseudo‐2D saturated zone and overland flow model for use in ESMs. By representing vertical and lateral responses separately at different spatial resolutions, h3D is computationally efficient. The h3D model was first tested for three different hillslope planforms (uniform, convergent and divergent). We then compared h3D (with single and multiple soil columns) with a complex physically based 3‐D model and a simple 1‐D soil moisture model coupled with an unconfined aquifer (as typically used in LSMs). It is found that simulations obtained by the simple 1‐D model vary considerably from the complex 3‐D model and are not able to represent hillslope‐scale variations in the lateral flow response. In contrast, the single soil column h3D model shows a much better performance and saves computational time by 2‐3 orders of magnitude compared with the complex 3‐D model. When multiple vertical soil columns are implemented, the resulting hydrological responses (soil moisture, water table depth, and base flow along the hillslope) from h3D are nearly identical to those predicted by the complex 3‐D model, but still saves computational time. As such, the computational efficiency of the h3D model provides a valuable and promising approach to incorporating hillslope‐scale hydrological processes into continental and global‐scale ESMs.

Influence of landscape heterogeneity on water available to tropical forests in an Amazonian catchment and implications for modeling drought response

“…Recent efforts to explore a new subgrid structure in ALM to represent subgrid topographic variations [ Tesfa and Leung , ] provide a useful modeling framework for developing parameterizations of lateral processes driven by surface topography. For example, the subgrid framework of ALM may be combined with the hybrid 3‐D hillslope hydrological model [ Hazenberg et al , ] where lateral responses are aggregated to the topographic land units of ALM to provide a more computationally efficient method than 3‐D distributed models for simulating both vertical and lateral hydrologic processes in Earth system models. Developing approaches to represent lateral processes, in addition to incorporating plant hydraulics and preferential flow discussed above, are key efforts needed to improve the ability of Earth system models in simulating tropical forest response to drought and the future of the land carbon sink under climate change.…”

The evolution of process-based hydrologic models: historical challenges and the collective quest for physical realism

“…For example, in the push for hillslope-resolving models across large geographical domains, one approach is to use the concept of representative hillslopes (Troch et al, 2003;Hazenberg et al, 2015;Ajami et al, 2016). The representative hillslope has a length dimension much smaller than the length scale of the model element, and the hillslope is discretized into columns along an axis perpendicular to the stream to explicitly resolve lateral flow processes.…”

“…As noted earlier, recent applications of hydrologic similarity methods have shown that it is possible to reduce run times by 2 to 3 orders of magnitude, without any loss in information content (Newman et al, 2014;Chaney et al, 2016a). Also, hydrologic similarity concepts can be effectively applied using multiscale methods to resolve the dominant spatial gradients that drive flow; for example, using representative hillslopes to explicitly resolve lateral flow processes (Troch et al, 2003;Berne et al, 2005;Hazenberg et al, 2015;Ajami et al, 2016). In exploring these solutions, we recognize that there is not necessarily a tradeoff between physical realism and computational efficiency -the linkage between spatial complexity and process complexity may be rather weak, as models that are run using a large number of spatial elements may still miss dominant processes (e.g., Hartmann et al, 2017).…”

Scaling, similarity, and the fourth paradigm for hydrology

“…An alternative, but related, concept is the representative hillslope (RH; Berne et al, 2005;Hazenberg et al, 2015). The REA-REW approach is conceptually similar to Reynolds averaging, and relies on the fundamental assumption that the physics are known on the smallest scale considered (e.g., Miller and Miller, 1956).…”