Design of a spatially differentiated water balance modelling tool

Existing water balance assessments may lack precision due to overlooking the spatial variations in factors such as soil and topography while transferring the results from point modelling to a wider area. In cooperation with Bayer AG Crop Science Division and Hamburg University, within the DREAM (Digital Run-off Exposure Assessment and Management) project, a model is being developed which is temporally dynamic as well as spatially differentiated to provide a more nuanced and location-specific understanding of quantitative water dynamics. It is based on high resolution grid data and features a multi-layered soil water model, the goal of which is to depict volumes of water in different soil layers. It is to be employed in an agricultural context and serve as a toolbox of possible runoff reduction measures for plant protection products.

Since risk management is a highly localized undertaking, the model operates at a field- or sub-field-level with a spatial resolution of up to one meter. The temporal resolution of simulation steps is variable; from one hour to a day. The necessary input data – that being a digital terrain model, information about the vegetation as well as soil and weather data – create conditions specific to the site.

It is embedded in the open source geoinformation system (GIS) SAGA. The modular approach allows for flexible development and changes on short notice. The model includes an algorithm that determines soil water movement, incorporating the groundwater layer as the model’s lower boundary. To achieve this, an expanded bucket model for soil water movement, based on the works of Glugla, is used. Should the infiltration capacity of the soil - calculated via the Green-Ampt-Method - be surpassed, runoff occurs. In this case, the model possesses the ability to depict runoff and its flow paths through the terrain, along with the respective volumes and flow velocity based on Gauckler-Manning-Strickler.

While the current focus lies on the movement of water, the model is designed for gradual expansion and improvement, allowing for future considerations such as the tracking of solutes moving into larger depths or even into groundwater.