Project Topics
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MODEL COMPONENTS:
The following sections will describe the various submodels; their purpose and the reasoning behind the equations and assumptions used in their development. For clarity and conciseness, while explaining individual processes in the submodel descriptions, the ‘W’ and ‘B’ indicators at the end of each parameter code name will be omitted from the parameter names. However, to differentiate which variables have two pools for the FWS system, a parenthesized ‘W/B’ will be placed after the initial parameter description. If the process occurs in only one of the FWS pools they will be distinguished with a parenthesized W or B. The described processes will occur in the SSF system, unless otherwise stated.
Figure 7 shows the relationships between the main code and the various submodels for SET-WET. As Figure 7 shows, the main program calls every submodel a various number of times. The number of times and whether a sub-model is called are dependent on the length of the simulation and which cycles are modeled.
Figures 8 through 22 represent the relationships between modeled processes affecting the respective cycles in SET-WET. The figures represent the various stocks, parameters and flows that are accounted for by SET-WET during a simulation run. Appendix D explains the symbols used in the respective figures.
Wetland Main Program:
The wetland main program is the bookkeeper and accountant of the model. It determines when and if each submodel should be called, transfers data between time and season periods, opens input and output files, and writes to the files. There are no inputs to the main program, but it is the code that links all of the submodel components together.
Base Submodel:
The base submodel describes the basic design of the wetland system and determines which nutrient cycles will be modeled. As stated before, the input values are not in a fixed format; however, it is suggested that the values are entered in the same manner of the Fortran code, or similar to the manner that it is displayed in Appendix B.
Hydrology Submodel:
The hydrology submodel is active for every simulation. The model represents two pools of water for the FWS simulations and one for the SSF wetland simulations (Figures 8 and 9). The model assumes that the FWS wetland always has water lying upon the surface and is never completely exhausted of free lying surface water. It further assumes that the water flow for a SSF wetland is always beneath surface level. For FWS simulations, the soil and peat area is assumed to be saturated at all times. A valid assumption for many constructed wetlands would be that they are lined, thus infiltration/percolation is minimal; however, there is a percolation and infiltration option that allows a daily set amount of water to be released or enter the system. Water input could be derived from four sources; 1) watershed catchment runoff, 2) direct precipitation input, 3) percolation additions, and 4) point source additions. Although initially designed for constructed wetlands, the model may analyze any natural wetlands that follow these conditions.
where dV/dt is the change in surface storage (m3/day); Qc is the runoff from the catchment (m3/day); Qp are the water flow from possible point source additions (m3/day); Qout is the outflow rate (m3/day); ∆ BMV is the change in living biomass volume in the surface water (m3/day);
SDV is the change in standing dead volume in surface water (m3/day); P is the daily precipitation rate (m/day); PI is the percolation/infiltration rate (m/day); ET is the evapotranspiration rate (m/day); and A is the wetland surface area (m2), determined from the input wetland length and width.
As stated earlier, there are two options available for determining runoff from the watershed; the daily option and the SCS method. The daily option requires input of Qc on a daily basis, where the amount is either known or determined with an existing NPS model. If the SCS method is used, runoff will be determined from the daily precipitation and the SCS curve number.
The outflow from the system is a function of the water volume, and the type of outflow device associated with the wetland, while ET is a function of daily temperature and local climate. There are six different options that can be chosen to represent the wetland outlet, five for FWS wetlands and one for SSF wetlands.
where Q is the discharge (m3/s); L is the weir length (m); and H is the head on the weir (m). The head on the weir is the difference between the wetland water height (HI) and the bottom level of the outflow weir (HOUT, m). The Francis equation allows both suppressed and contracted weir outflows to be determined.
If the water height in the system is lower than HOUT, then it is assumed that outflow from the system is zero. If the water height in the system is higher than the HOVER (water height completely above the weir) mark, outflow is the addition of the overflow through the weir and the water that spills over the weir height.
where CONTC is the contraction coefficient for the pipe; AREAPIPE is the area of opening for the outflow pipe (m2); HI is the water level in the wetland relative to a zero datum (m); and HOUT is the height (m) of the outflow opening relative to a zero datum.
Outlet option 4 represents pumped discharge that removes a constant amount of water from the system per day. The input values are TOPPUMP and FLOWOUT and represent respectively the height necessary in the wetland for water to flow from the system (m, with respect to zero datum) and the amount removed per day when outflow is continuos through the day (m3). Flow is continuos when the wetland water level is above the TOPPUMP location.
For each of these outlet options, the water velocity in the wetland system is determined after the outflow in the system is determined. Water velocity is calculated by dividing the outflow through the system by the wetland cross-sectional area (Equation 8).
For option 5, water velocity is determined first and is then used to calculate outflow. Water velocity is determined using the rate law equation (Equation 11). In the rate law equation when flow is in the turbulent range, α (slope component) is approximately 0.5, and if flow is in the laminar range, α =1.0. The β (depth component) value is usually in the range of 2≤β≤ 4, but is dependent on microtopography as well as the stem-density depth distribution of the wetland (Hammer and Kadlec, 1986). The calculated velocity in the wetland system is multiplied by the wetland cross-sectional area to determine outflow.
1.Classification
a. Natural Wetlands
b. Constructed Wetlands
2. Constructed Wetland Design
3. Nitrogen Cycle in Wetlands
a. Nitrogen Transformation Processes .
i. Mineralization (ammonification)
ii. Nitrification
iii. Denitrification
iv. Nitrogen Fixation
v. Assimilation: Plant and Bacterial Uptake
b. Other Nitrogen Fluxes
i. Atmospheric Nitrogen Inputs
ii. Ammonia Volatilization
iii. Adsorption
iv. Burial of Organic Nitrogen
v. Biomass Decomposition
4. Phosphorous Cycle in Wetlands
a. Importance of Sediment – Sorption/Desorption
b. Precipitation
c. Biomass: Growth, Death, Decomposition, Uptake and Storage
5. Bacteria in Wetlands
6. Vegetative/Carbon Cycle in Wetlands
7. Modeling Wetland Processes
a. General Modeling Practices
b. Modeling of Specific Wetland Processes
i. Hydrology
Overall Water Budget
Surface Water Flow
Evapotranspiration
Groundwater Flow
ii. Nitrogen
iii. Phosphorous
iv. Sediment
v. Vegetation
c. Selected Wetland Models
D. LITERATURE REVIEW SUMMARY
III: MODEL DEVELOPMENT
A. MODEL OVERVIEW
1. FWS vs. SSF Modeling
B. MODEL COMPONENTS:
8. Dissolved oxygen submodel:
9. Bacteria submodel:
a. Autotrophic Dynamics
b. Heterotrophic bacteria
10. Sediment submodel:
11. Phosphorous submodel:
12. Deltaht submodel:
13. SET-WET Flow Chart
C. MODEL DEVELOPMENT SUMMARY
IV. MODEL EVALUATION
A. MODEL CALIBRATION AND VALIDATION
B. STATISTICAL ANALYSIS:
C. SENSITIVITY ANALYSIS:
D. MODELING APPLICATION
E. MODEL EVALUATION SUMMARY
V. SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS
VI. CITED WORK
VII. APPENDICES
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SET-WET: A WETLAND SIMULATION MODEL TO OPTIMIZE NPS POLLUTION CONTROL
