To sweep or not to sweep? Investigating controversial parameters in hydrological models

In bucket-type hydrological models (also known as conceptual models), the paths of the water in a catchment are represented in a simplified way. In general, there is one way for water to enter – via precipitation – and two ways to leave – streamflow and evaporation. However, as the simple water balance equation P=Q+E this concept is based on is often not fulfilled, many bucket-type models include ‘sweep parameters’, parameters that represent an additional way for water to enter or leave the catchment. Sweep parameters come as correction factors that are used to align inputs and outputs, but also in more sophisticated ways, such as representations of groundwater inflows or outflows. The X2 parameter (Intercatchment Groundwater Flow parameter) in the GR4J model is a well-known example of a sweep parameter.Compared to a model in which the water balance is enforced, a model that includes a sweep parameter is usually more successful in simulating streamflow volumes: Too much or too little water can be compensated thanks to the sweep parameter, while otherwise the only options for compensation are via evaporation or large simulated storage volumes.Because including a sweep parameter improves model performance, sweep parameters are often seen as ‘cheat parameters’. This accusation is understandable, since sweep parameters can also compensate for incorrect input data. Still, there are many reasons why the use of sweep parameters should not be frowned upon. Many catchments are not closed systems along their topographic borders and sweep parameters are one way of representing this knowledge. In addition, we should avoid compensating for incorrect input data or additional water gains or losses via evaporation, a flux that is generally not included in  model calibration – and that could be considered cheating as well. If a mismatch in the basic water balance can be represented via a sweep parameter, this is arguably a reasonable and transparent way to do so.To investigate the effects of sweep parameters, we tested the model performance and model robustness towards variations in precipitation input data for hydrological models with and without a sweep parameter. Using a large-sample approach for more than 500 catchments in France, we could not find any evidence that model robustness is affected by the use of a sweep parameter. Furthermore, we clearly illustrate that models benefit from using a sweep parameter. Based on these results, we argue that it is justifiable to decide to sweep, but also stress that the way and effect of the sweeping should be communicated transparently and interpreted with caution.