Tracking water movement through a small agricultural catchment using StorAge Selection functions and hydrologic modeling

 

Determining the processes that drive streamflow generation and catchment-scale transport of nutrients and pollutants by water is one of the challenges of modern hydrology. In the last decades, substantial knowledge has been gained from high-frequency and high-resolution measurements of tracers to track water movements within a hydrological system. For example, the stable isotopes of oxygen (d¹⁸O) and hydrogen (d²H) have been widely used to disentangle the contributions of different runoff generation mechanisms by modeling water travel and residence times. However, quantifying the effects of catchment internal factors for similar or different transit time distributions, particularly in characteristically complex and heterogeneous catchments, remains a challenge. Here we test different shapes for age selection functions (StorAge Selection) of individual distinct storage components (e.g., the root zone, groundwater) of the agricultural Hydrological Open Air Laboratory (HOAL) catchment in Petzenkirchen, Austria. The HOAL has a variety of runoff generation mechanisms, including overland flow, wetlands, and tile drains, as well as high-resolution tracer and hydrological data that allows for broad storage-discharge relationship testing. The main goal of this study is to estimate the transit time distributions associated with varying fluxes from these components (e.g., overland flow, groundwater recharge) to learn about catchment internal streamflow generation processes. Water flow is modeled with a water age balance model and are replaced by selecting the appropriate transfer functions. Testing different age selection functions for various storage components of the catchment will provide a better understanding of catchment dynamics under different environmental conditions, allowing for better calibration of catchment-scale water and nutrient transport models.