Abstract
Abstract. This paper presents the “tran-SAS” package, which includes a set of codes to model solute transport and water residence times through a hydrological system. The model is based on a catchment-scale approach that aims at reproducing the integrated response of the system at one of its outlets. The codes are implemented in MATLAB and are meant to be easy to edit, so that users with minimal programming knowledge can adapt them to the desired application. The problem of large-scale solute transport has both theoretical and practical implications. On the one side, the ability to represent the ensemble of water flow trajectories through a heterogeneous system helps unraveling streamflow generation processes and allows us to make inferences on plant–water interactions. On the other side, transport models are a practical tool that can be used to estimate the persistence of solutes in the environment. The core of the package is based on the implementation of an age master equation (ME), which is solved using general StorAge Selection (SAS) functions. The age ME is first converted into a set of ordinary differential equations, each addressing the transport of an individual precipitation input through the catchment, and then it is discretized using an explicit numerical scheme. Results show that the implementation is efficient and allows the model to run in short times. The numerical accuracy is critically evaluated and it is shown to be satisfactory in most cases of hydrologic interest. Additionally, a higher-order implementation is provided within the package to evaluate and, if necessary, to improve the numerical accuracy of the results. The codes can be used to model streamflow age and solute concentration, but a number of additional outputs can be obtained by editing the codes to further advance the ability to understand and model catchment transport processes.
Highlights
The field of hydrologic transport focuses on how water flows through a watershed and mobilizes solutes towards the catchment outlets
The tran-StorAge Selection (SAS) package includes a basic implementation of the age master equation (Eq 1) using general SAS functions
The codes can be used to simulate the transport of solutes through a catchment and to evaluate water residence times
Summary
The field of hydrologic transport focuses on how water flows through a watershed and mobilizes solutes towards the catchment outlets. Water trajectories within a catchment are usually considered from the time water enters as precipitation to the time it leaves as discharge or evapotranspiration. As watersheds are heterogeneous and subject to time-variant atmospheric forcing, water flow paths have marked spatiotemporal variability. For this reason, a formulation of transport by travel time distributions (see Cvetkovic and Dagan, 1994; Botter et al, 2005) can be convenient as it allows the transformation of complex 3-D trajectories into a single variable: the travel time, i.e., the time elapsed from the entrance of a water particle to its exit
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