Modeling Nitrate Export from a Mesoscale Catchment Using StorAge
Selection Functions
Abstract
StorAge Selection (SAS) functions describe how catchments selectively
remove water of different ages in storage via discharge, thus
controlling the transit time distribution (TTD) and solute composition
of discharge. SAS-based models have been emerging as promising tools for
quantifying catchment-scale solute export, providing a coherent
framework for describing both velocity and celerity driven transport.
However, due to their application in headwaters only, the spatial
heterogeneity of catchment physiographic characteristics, land-use
management practices, and large-scale validation have not been
adequately addressed with SAS-based models. In this study, we integrated
SAS functions into the grid-based mHM-Nitrate model (mesoscale
Hydrological Model) at both grid scale (distributed model) and
catchment scale (lumped model). The proposed model provides a spatially
distributed representation of nitrogen dynamics within the soil zone and
a unified approach for representing both velocity and celerity driven
subsurface transport below the soil zone. The model was tested in a
heterogeneous mesoscale catchment. Simulated results show a strong
spatial heterogeneity in nitrogen dynamics within the soil zone,
highlighting the necessity of a spatially explicit approach for
describing near-surface nitrogen processing. The lumped model could well
capture instream nitrate concentration dynamics and the
concentration-discharge relationship at the catchment outlet. In
addition, the model could satisfactorily represent the relations between
subsurface storage, mixing scheme, solute export, and the TTDs of
discharge. The distributed model shows comparable results with the
lumped model. Overall, the results reveal the potential for large-scale
applications of SAS-based transport models, contributing to the
understanding of water quality-related issues in agricultural
landscapes.