Mass Balance Calculation for Waterbodies using EEMS
A waterbody typically has multiple tributaries contributing flow and other constituents to the domain. Modeling the waterbody requires the estimation of mass balance of all the constituents that enter and exit the modeling domain. Mass balance is an essential aspect of weight-of-evidence based model calibration.
Once you develop a model using EEMS, you can use the EE interface to quickly estimate the mass balance. This blog describes the process of doing this estimation for the Caloosahatchee Estuary model as an example. A simplified version of this model for hydrodynamics is available for download from our website, and the steps shown can be performed with EEMS in the free demo mode.
The Caloosahatchee River is located on the southwest Gulf Coast of Florida, USA, and is approximately 67 miles (108 km) long. It drains rural areas on the northern edge of the Everglades, east of Fort Myers. The river serves as an important link in the Okeechobee Waterway, a manmade inland waterway system of southern Florida. The river forms a tidal estuary along most of its course and has become the subject of efforts to restore and preserve the Everglades.
The EFDC+ Caloosahatchee model domain is shown in Figure 1, and is comprised of 4,041 grid cells. The average cell size is 370 m x 330 m; the model is configured with four vertical layers. Boundaries include those from the widely used hydrologic and water quality model HSPF, as well as waste water treatment plants (WWTPs).
After the model is completely configured (as is the case with this example model), you can calculate the mass balance using the “Boundary Mass Loading” tool. It is not required to run the model and generate the model output for this calculation. You can open the model in EE and browse to the Tools menu. From this menu, select the Boundary Mass Loading option (Figure 2).
The Mass Loading form appears (Figure 3). You can set limits for the calculation with the beginning and end times. This time should be fully covered by time series of boundaries. In this case, it is for the year 2003 as measured in Julian days. All of the constituents that have been configured in the model are listed in the drop-down menu.
After clicking the Compute button, EE will calculate the mass balance and produce a report (Figure 4). This first report is for the flow boundaries. This process can then be repeated for total suspended sediment (TSS) and nutrients such as total organic carbon (TOC), total nitrogen (TN), and total phosphorus (TP). These contributions have been summarized in Table 1. You may want to normalize this calculation per unit of time (per unit of year or day).
A mass balance summary table is important for the simulation plan development. This information can be used for QA/QC and to focus the model calibration efforts. If you have any questions on generating this report, please feel free to ask in our EEMS forums.
Talk to the experts
Anurag Mishra, Ph.D, PE
Senior Hydrologic Engineer
Pham Huy Duy