Integration of Biozone Algorithm

The biozone algorithm was integrated into the latest version of (SWAT+ version 2009 and the GIS interface version ArcSWAT+ 2009). Each unit of onsite septic systems is represented by a septic HRU. A septic HRU represents either one OWS or a hypothetical system that aggregates several OWSs that have similar HRU properties such as soil type, landuse and slope. In the latter case, septic HRUs in a subbasin are not spatially referenced and only lumped simulation is available. Septic HRUs are generated during the GIS interface processing based on the septic land use type defined by the users. Biozone and other properties of a septic system specific to a HRU is listed in septic input files (*.sep files). The septic water quality database developed based on literature lists STE loading rate and pollutants concentration in STE of the conventional system 25 kinds of advanced systems and an untreated effluent system (septwq.dat). Pollutant characteristics included in the water quality database are per capita septic effluent flow rate, Bio-chemical Oxygen Demand (BOD), Total Suspended Solid (TSS), Total Nitrogen (TN), ammonium, nitrite, nitrate, organic nitrogen, Total Phosphorus (TP), phosphate, organic P, and Fecal Coliform. A generic type for conventional system and advanced system are listed as the first and the second in the water quality database list, so the user can select these types with limited information. The hydrologic linkage to SWAT+ allows daily STE directly to be introduced to the biozone layer by increasing the soil moisture content in the subroutine percmain. The movement of soil water through percolation in and out of the biozne layer is then simulated by a SWAT+ soil water model in percmicro. The movement of nutrients through soil profile from the biozone layer is simulated by a combination of nutrient modules in SWAT+ and biozone. SWAT+ septic input parameters are presented in the updated SWAT+ Input/Output File Documentation (Neitsch et al., 2010).