distributed ecohydrological modeling: the potential for water resources management larry band, unc...
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Distributed ecohydrological modeling: the potential for water resources
management
Larry Band, UNC
Christina Tague, SDSU
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Characteristics of watersheds regulating biogeochemical cycling and export
• In situ cycling of carbon and nutrients
• Ecosystem Processes: deposition/fixation, assimilation, uptake, decomposition, mineralization, nitrification, immobilization, denitrification, and all that….
• Transport within hydrologic flowpaths
• Hydrologic Processes: overland flow, shallow throughflow, groundwater flow,….
•Distribution of net source/sink strength along flowpaths
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Plot based ecosystem models
• water, carbon, nutrient flux computed in 1-d
• long time step (one day to one month)
• long time domain (decades to centuries) incorporating long term feedbacks to ecosystem state
• typically run without consideration of spatial heterogeneity
• no incorporation of spatial dependency along flowpaths
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Dynamic BGC
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Nitrogen saturation hypothesis:• Definition: Terrestrial ecosystems have a maximum rate of nitrogen uptake determined by the net ecosystem productivity and other (plot) N sinks, input exceeding this rate may leach to ...
• Streamwater chemistry diagnostics:
• ecosystem saturation stage diagnosed by streamwater chemistry, particularly in growing season
• Based on plot paradigm: assumes leaching below rooting zone is contributed directly to streams
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Watershed model:
• e.g. HSPF - works well for discharge based on land use
• also typically fix nutrient loading from land surface classes
• lumped models cannot address downslope divergence (variable nutrient source/sink strength along flowpaths)
• adjust for BMPs with reduction factor
• no feedback to ecosystem processes controlling source quality
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Oregon Ridge
5 0 5 10 Kilometers
N
Baltimore Ecosystem Study: Urban LTER
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Infrastructure impacts on flowpaths
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Characteristics of watersheds regulating biogeochemical cycling and export:
anthropogenic alteration
• direct addition or abstraction of material
• irrigation, fertilization, spills
• wastewater treatment and disposal
• alteration of hydrologic flowpaths
• impermeable surfaces
• street drainage
• storm and sanitary sewers
• vegetation management
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RIPARIAN ZONES
• Critical interface between terrestrial and aquatic components of a watershed.
• Demonstrated ability to prevent pollutant movement from upland land uses into streams.
• Most work on groundwater nitrate, in agricultural watersheds.
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Denitrification
NO3- NO2
- NO N2O N2
- Anaerobic
- Heterotrophic (requires organic C)
• Expect high rates in wetland soils.
• Key component of the water quality maintenance function of riparian zones.
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Model and data flow structure
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RHESSys Object Hierarchy
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Basin/hillslope/patch hierarchy
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Major carbon flux processes: patch level
• Photosynthesis: Farquhar algorithm combining conductance and enzymatic limitations
• Stomatal physiology: Conductance uses Jarvis method f(temp, LWP, PAR, VPD)
• Respiration: Organ (foliage, root, stem) specific rates (massC/massC) modified by temperature Q10
• Allocation: Root, stem, foliar allocation of net photosynthate determined by Waring/Landsberg function on water, nutrient stress, and carbon supply
• Litter decomposition: litter/soil moisture, T, quality determines rates from different soil pools
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Daily carbon flux (g.m-2day-1): Upland plot, Pond Branch
-6
-4
-2
0
2
4
6
8
1 31 61 91 121 151 181 211 241 271 301 331 361
Series1
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Pondbranch Nitrate Concentration
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
14-O
ct-9
8
8-N
ov-9
8
3-D
ec-9
8
28-D
ec-
98
22-J
an-9
9
16-F
eb-9
9
13-M
ar-9
9
7-A
pr-
99
2-M
ay-9
9
27-M
ay-9
9
21-J
un-9
9
16-J
ul-99
10-A
ug-
99
4-S
ep-9
9
Date
Nit
rate
Co
nc (
mg
/L)
Observed Nitrate Conc. Simulated Nitrate Conc.
Nitrate export from Pond Branch: Note large increase in concentrations during summer - partially derived from riparian zones
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Denitrification rates during summer and winter for hillslope 1
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NO3 export concentrations as f(hydrology)
• riparian patches typically near saturation, anaerobic, active denitrifying zones
• significant reduction of [NO3] from upslope
• low decomposition, accumulates carbon, nutrient rich material
• during dry-downs riparian patches
• may disconnect from uplands, WFPS drops
• become aerobic, strongly nitrify
• flip from sink to source of NO3
• note: growing season NO3 export not related to catchment nitrogen saturation