Modelling subsurface hydrological processes in riparian zones

Abstract

Soil moisture is a crucial but unseen component of the hydrologic cycle. It provides a hydrologic link and storage between surface waters and groundwater. This unexposed water sanctuary could be the key to sustain life during ever-prevalent climatic extremes. To quantify and understand the role of soil moisture in catchment hydrology, the HaughFlow model has been developed as a new tool to simulate subsurface hydrology and water flow in floodplain soils. The model is constructed to be flexible, adaptable and logically composed in order to encourage interaction from the wider scientific community. Model transparency and parsimony are key; with assumptions clearly explained and only the dominant processes simulated to limit the number of inputs required and retain efficiency. The dominant processes for hydraulically connected floodplains comprise lateral saturated flow, driven by river-stage fluctuations, and vertical unsaturated flow, driven by precipitation and evapotranspiration. HaughFlow is applied to study sites in Australia and Germany with distinct climate conditions to exhibit the model’s range of applicability. The lateral and vertical components of the model are calibrated and evaluated in detail to determine the accuracy of model outputs. Evaluation methods (i.e. graphical comparison, standard regression, Nash-Sutcliffe efficiency and percentage bias) are used to suggest best practice for model calibration and the best outputs are then used to establish dominant trends in subsurface hydrology at each site; including climatic changes and the long-term effects of drought periods. Oxygen and hydrogen isotope tracers are also incorporated into the model, allowing the user to decouple the intermixed contributions from lateral and vertical processes in the soil. Observed precipitation and river isotope inputs can be used to predict actual isotope fractionation and mixing patterns in the soil. Using artificial isotope inputs, however, can be used to clearly delineate the capillary fringe and water plumes from individual rainfall events, within the soil. Hence, HaughFlow is a powerful numerical tool that can be used to establish hydrologic trends and their drivers in the soil, to understand the role of soil moisture in the hydrologic cycle. Ultimately these outputs can be used to investigate key questions in vegetative water sourcing and how plants adapt to changes in water availability.