Watershed Modeling with PnET-BGC Charles Driscoll Syracuse University

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

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

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

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

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

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

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

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

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

PnET-BGC in detail - Elements 1 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

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

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

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.

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.

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

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

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

Software, documentation, examples General information Software, documentation, examples are available at http://www.ecs.syr.edu/faculty/driscoll/personal/index.asp