Principles of SuperflexPy

Hydrological models are widely used in environmental science and engineering for process understanding and prediction.

Models can differ depending on how the processes are represented (conceptual vs. physical based models), and how the physical domain is discretized (from simple lumped configurations to detailed fully-distributed models).

At the catchment scale, conceptual models are the most widely used class of models, due to their ability to capture hydrological dynamics in a parsimonious and computationally fast way.

Conceptual models

Conceptual models describe hydrological dynamics directly at the scale of interest. For example, in catchment-scale applications, they are based on relationships between catchment storage and outflow. Such models are usually relatively simple and cheap to run; their simplicity allows extensive explorations of many different process representations, uncertainty quantification using Monte Carlo methods, and so forth.

Many conceptual models have been proposed over the last 40 years. These models have in common that they are composed by general elements such as reservoirs, lag functions, and connections. That said, existing models do differ from each other in a multitude of major and minor aspects, which complicates model comparison and selection.

Model differences may appear on several levels:

conceptualization: different models may represent a different set of hydrological processes;
mathematical model: the same process (e.g. a flux) may be represented by different equations;
numerical model: the same equation may be solved using different numerical techniques.

Several flexible modeling frameworks have been proposed in the last decade to facilitate the implementation and comparison of the diverse set of hydrological models.

Flexible modelling frameworks

A flexible modeling framework can be seen as a language for building conceptual hydrological models, which allows to build a (potentially complex) model from simpler low-level components.

The main objective of a flexible modeling framework is to facilitate the process of model building and comparison, giving modelers the possibility to adjust the model structure to help achieve their application objectives.

Although several flexible modeling frameworks have been proposed in the last decade, there are still some notable challenges. For example:

implementation constraints can limit the originally envisaged flexibility of the framework;
the choice of numerical model can be fixed;
the spatial organization can be limited to lumped configurations;
the ease of use can be limited by a complex software design.

These challenges can impact on usability, practicality and performance, and ultimately limit the types of modeling problems that can be tackled. The SuperflexPy framework is designed to address many of these challenges, providing a framework suitable for a wide range of research and operational applications.

Spatial organization

Hydrologists are increasingly interested in modeling large catchments where spatial heterogeneity becomes important. The following categories of spatial model organization can be distinguished:

lumped configuration, where the entire physical domain is considered uniform;
semi-distributed configuration, where the physical domain is subdivided into (usually coarse) areal fractions that are assumed to have the same hydrological response and operate in parallel (usually without connectivity between them);
fully-distributed configuration, where the physical domain is subdivided into a (usually fine) grid. This configuration typically includes flux exchanges between neighboring grid cells.

The lumped approach yields the simplest models, with a low number of parameters and often sufficiently good predictions. However, the obvious limitation is that if the catchment properties vary substantially in space, the lumped model will not capture these variations. Nor can a lumped model produce spatially distributed streamflow predictions.

The fully-distributed approach typically yields models with a large number of parameters and high computational demands, usually related to the resolution of the grid that is used.

The semi-distributed approach is intermediate between the other two approaches in terms of spatial complexity and number of parameters. A typical example is the discretisation of the catchment into Hydrological Response Units (HRUs), defined as catchment areas assumed to behave in a hydrologically “similar” way. The definition of HRUs represents a modelling choice and depends on the process understanding available in the catchment of interest.

SuperflexPy

SuperflexPy is a new flexible framework for building hydrological models. It is designed to accommodate models with a wide range of structural complexity, and to support spatial configurations ranging from lumped to distributed. The design of SuperflexPy is informed by the extensive experience of its authors and their colleagues in developing and applying conceptual hydrological models.

In order to balance flexibility and ease of use, SuperflexPy is organized in four different levels, which correspond to different degrees of spatial complexity:

elements;
units;
nodes;
network.

The first level is represented by “elements”, which comprise reservoirs, lag functions, and connections. Elements can represent entire models or individual model components, and are intended to represent specific processes within the hydrological cycle (e.g. soil dynamics).

The second level is represented by “units”, which connect together multiple elements. This level can be used to build lumped models or to represent HRUs within a spatially distributed model.

The third level is represented by “nodes”, where each node contains several units that operate in parallel. Nodes can be used to distinguish the behavior of distinct units within a catchment, e.g., when building a (semi)-distributed model where the units are used to represent HRUs (defined according to soil, vegetation, topography, etc).

The fourth level is represented by the “network”, which connects multiple nodes and routes the fluxes from upstream to downstream nodes. This level enables the representation of large watersheds and river networks that comprise several subcatchments with substantial flow routing delays. A SuperflexPy model configuration can contain only a single network.

Technical details on these components are provided in the Organization of SuperflexPy page.