1. Watershed Modeling with PnET-BGC
Charles Driscoll
Syracuse University

2. Modeling aims
Simulate the response of forest soils and surface waters to changes in atmospheric deposition, land disturbance and climate change
Depict the cycles of the major nutrients and elements
Reflect the acid-base and nutrient indicator properties of the soil and surface waters
Simulate ecosystem and regional response in long- and short-time scales

3. Content General concept of the PnET-BGC model PnET-BGC in detail
Model basics and structure
PnET-BGC in detail
Input
Hydrology
Vegetation
Element fluxes and soil processes

4. General concept of PnET-BGC
Water properties and hydrologic processes
Vegetation and biologic processes
Soil properties and the geological and chemical processes

5. General concept of PnET-BGC
Homogenous spatial distribution of properties over site
Ecosystem is divided in mass pools (state variables)
Pools are connected by internal processes (fluxes)
Pools are influenced by boundary conditions

6. General concept of PnET-BGC
Dynamic model (only transient calculations)
Time discretization
Initial conditions
Allow the system to come to steady-state under background deposition/climate/land cover (~1000 yrs)
Apply hindcast and forecast scenarios
Mathematical formulation:
Mass fluxes out of the pool
Mass fluxes into the pool
Mass change of pool i with time

7. PnET-BGC in detail Based on PnET-CN Developed for forest ecosystems
Carbon and nitrogen special processes
Developed for forest ecosystems
Main forest vegetation has to be specified
Different validated vegetation types
e.g. Northern Hardwood, Spruce – Fir, Pine, Red Oak – Red Maple
Mainly driven by environmental conditions
Climate, deposition, soil/bedrock
PnET = Photosynthesis and EvapoTranspiration; BGC = BioGeoChemical; CN = Carbon, Nitrogen

8. PnET-BGC in detail - Hydrology
Hydrology: Water flux into, through and out of the system and water storage in the system
Snowpack pool
Snow flux
Melt flux
Soil water pool
Inflow
Drainage
Transpiration 6 3 7
Input flux
Precipitation 1 3 1
Output flux
Evaporation
Shallow flux
Stream flux = 2 7 2 8 4 8 4 5 5 5 8 6
Climate
Vegetation
Soil water

9. PnET-BGC in detail - Vegetation
PnET-CN briefly
Balanced pools:
Foliage C and N
Fine roots C and N
Wood C and N
Bud C and N for foliage production
C for wood production
C for fine root production
Mobile C and N in plant for annual allocation
Processes:
Photosynthesis (C)
Respiration (C)
Transpiration
Driven by Photosynthesis
Limited by soil water
Plant uptake
Litter / biomass production of plant parts

10. PnET-BGC in detail - Elements
Components H C N
P Na Mg Al K Ca Cl S F Si H+
DOC, CO32-
NH4+, NO3-
PO43+
Na+
Mg2+
Al3+ K+
Ca2+
Cl-
SO42- F-
H4SiO4

11. PnET-BGC in detail - Elements1 3
Plant
Pools with element composition
Fluxes driven by
Atmospheric deposition
Water fluxes
Biomass fluxes
Pool composition
Transformation processes
Foliage
Root 2
Wood
HOM
Deadwood
Snowpack 4
Soil 6 5 1
Depostion 3
Foliage exchange 5
Weathering
Soil & soil solution interaction / nitrification 2
Plant uptake 4
Mineralization 6

12. PnET-BGC in detail - Elements
Soil
Soil pool processes
2 pools
Aqueous components in soil solution
Components bound to soil
Chemical equilibrium in soil
Open system with CO2 (gaseous)
Gibbsite dissolution/precipitation
Cation exchange with soil
Anion adsorption at soil surface
Properties: pH, base saturation, Al/Ca
Nitrification
Mobilization of bound NH4+ possible
Bound to soil
Soil solution
Component sinks/sources of soil pool
Inflow, mineralization and weathering
Drainage
Only aqueous components
Plant uptake
Mobilization of bound components possible

13. PnET-BGC in detail – Surface water
Surface water composition
Stream composition
Sum of shallow flow and drainage
Chemical equilibrium to quantify stream properties
pH, ANC, DIC, organic Al
Open system with CO2 (gaseous)
Gibbsite dissolution/precipitation
Wetland transformations
Lake composition
Stream composition inflow
In-lake processes (SO4, NO3)
Soil solution
Stream flux
Bound to soil
Lake

14. The model requires the Sulfate and Nitrate deposition as inputs
The model requires the Sulfate and Nitrate deposition as inputs. The past and current deposition was reconstructed based on historical emissions and spatial models for Adirondack region. The future deposition was constructed based on three scenarios of future emissions, including: a Base Case scenario, a moderate additional emissions controls scenario, and a more aggressive additional emissions controls scenario. This is the total sulfate and nitrate deposition for one site-Huntington Forest, which is located in the middle of Adirondack region.

15. The model performance. As I mentioned before, there are six sites also belong to the ALTM sites. Because of the limited time, I will only show model results for one ALTM site-Indian Lake. The model effectively simulated mean SO42- concentrations, and generally captured the decreasing trends. There is also decreasing trend in base cation concentrations over the measured period, and the model performed well for simulating them. The model generally captured the magnitude of ANC and pH values; however, it failed to depict short term dynamics in values. This lake showed marked increases in ANC and pH over the observational period, but the model showed relatively stable values.

16. Measurement
Basis
Use
Priority
Inputs
Max-min air temperature
oC; Monthly or finer timescale; long time series
Model input 1
Precipitation quantity
cm/mo; Monthly or finer timescale; long time series
Incident solar radiation (PAR)
Monthly or finer (mmol/m2-s) 2
Soil bulk density
(Soil mass per unit area)
Once (kg m-2)
Wet deposition
g/m2-mo; all major solutes; Monthly or finer; long time series
Dry to wet ratio
Molar ratio; all major solutes; once
Forest disturbance (logging, fire, storm)
Year; Intensity: the fraction of the watershed that is disturbed by the disturbance event;
Removal Fraction: the fraction of the forest biomass removed from the watershed during the disturbance event;
Soil loss fraction: Quantity of soil forest floor removal during disturbance event
Watershed area and latitude m2
Model calculations
Major tree species
e.g.,
Northern Hardwoods, Spruce-Fir,
Red Oak-Red Maple, Red Pine

17. Measurement
Basis
Use
Priority
Parameters
Vegetation chemistry
Vegetation stoichiometry
(Element Organic Content and Element Plant Tissue)
g/g DW; g/g N; Once 3
Soil exchangeable cations
eq/kg; Once
Soil selectivity coefficients; test model 2
Soil solution chemistry
µmol/L; Once
Soil selectivity
Adsorbed anions; or anion adsorption isotherms
Once
Anion adsorption
Foliar exchange/uptake (H, Mg, K, Ca, NH4)
Regulating net throughfall flux
Weathering
g / m2 - mo; Once
Model Input

18. Measurement
Basis
Use
Priority
Outputs
Stream discharge
mm/m2-mo; Monthly or finer; long time series
Model testing 1
Stream chemistry
µmol/L; monthly or finer; long time series
Litterfall and chemistry
g/m2-mo; g/g; once or whenever available 3
N mineralization rates
g N/m2-mo; once or whenever available
Nitrification rates
Aboveground biomass
g/m2 ; once or whenever available 2
Aboveground biomass increment
NPP / NEP
g C/m2-yr
Elements in different soil pools (Humus, litterfall, solid phase, mineralization, uptake)
g element/m2-mo

19. Software, documentation, examples
General information
Software, documentation, examples
are available at