New “Visual” Tools for Load Allocation Analyses: Using STELLA Software for Water Quality Accounting

STELLA is a visual platform for customized modeling of flow networks and tracking multiple constituents through a water system. STELLA has recently been used as a conservative water quality accounting tool to support total phosphorus (TP) and total suspended solids (TSS) TMDL allocation analyses for a large watershed with multiple reaches and discharge inputs. STELLA was chosen as the tool for this purpose due to its visual presentation of reaches, inputs, and variables, as well as its flexibility.STELLA is essentially an open worksheet in which a user builds a network of flow elements, storage elements, and variables to represent any water system. For this TMDL analysis, the mainstem and tributaries were set up in a model within STELLA, and nonpoint source flows and loads from a previously developed model were incorporated, along with point source flows and loads compiled from existing data. MS4 flows and loads were also entered. The baseline pollutant loads were reduced through a series of reduction factors until concentrations met proposed water quality objectives. As needed, STELLA has also been used to simulate first-order decay or attenuation of total phosphorus in impounded flow networks.STELLA provides a visual feature not available with other modeling platforms. The network constructed for the load allocation analyses has a similar “on-screen” visual structure to the actual river network and interdependencies between system elements are clearly represented. The visual aspect of STELLA makes for a straightforward explanation of how the model works to public stakeholders. It is easy to visualize how the entire water network is constructed conceptually, how reaches and discharges connect to one another, and how pollutants “flow” through the system and are mathematically reduced. STELLA was chosen for these analyses because the platform can be used to quickly “screen through” reduction percentages and identify effective allocations reach by reach by examining resulting in-stream concentrations. The model was set up to include both global and local reduction factors. Global reduction factors are applied universally throughout the entire watershed, while local reduction factors can be applied to each individual reach. For each model run, these reduction factors are easily edited on a control interface screen and results can be exported to a spreadsheet for further analysis and load allocation calculations.