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Ecohydrology_Lecture3_F11

Ecohydrology_Lecture3_F11 - Ecosystem Water Balances Water...

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Ecosystem Water Balances
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Water Balance Inputs (cross-boundary flows) Rainfall Stochastic in interval, intensity and duration Runin/Groundwater? Outputs Evapo-transpiration Surface runoff Infiltration Key internal stores/processes Soil moisture Interception Stomatal regulation Sap-flow rates
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Water Balance P = ET + R + D + ΔS P – precipitation ET – evapotranspiration Contains interception (I), surface evaporation (E) and plant transpiration (T) R – runoff D – recharge to groundwater ΔS – change in internal storage (soil water) Quantities on the RHS are functions of each other ET, R and D are a function of ΔS, and vice versa
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Soil-Plant-Atmosphere Continuum ET through a chain of resistances in series Boundary layer (canopy architecture) Leaf resistance (stomatal dynamics) Xylem resistances (sapwood area, conductivity) Root resistances (water entry and movement) Soil (matrix resistance) Note that individual plasticity and changes in composition (i.e., species level variability) affect all of these at different time scales, and thus create important feedbacks between the ecosystem and it’s resistance properties
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Figuratively Process is driven by a vapor pressure deficit between the soil and atmosphere AND net radiation Soil evaporation is a minor (~5%) portion of total ecosystem water use MOST water passes through plant stomata to the atmosphere even in wet areas with low canopy cover (max. evap. ~ 14%) Soil Moisture Atmospheric Demand Boundary layer Leaf control Stem control Root control Soil resistance
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Boundary, Leaf, Stem, Soil Conductance Primary Production Soil Moisture Intercepted Water R a i n f a l l Infiltration Runoff Vapor Pressure Deficit + + + + + - - - - - - + +
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Key Regulatory Processes Interception I = S + a*t Interception (I) is canopy storage plus rain event evaporation rate * time Annual I in forests > crops and grasses because of seasonal effects Zhang et al. (1999)
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Key Regulatory Process - ET Penman-Monteith Equation Ω is a decoupling coefficient (relative importance of energy vis-à-vis aerodynamic terms (0-1) Forests is usually small and higher in grasslands; vegetation affects this s is the slope of saturation vapor pressure curve, γ is the psychrometric constant, ε is s/γ, Rn is net radiation, G is ground heat flux, ρ is the density of air, Cp is the specific heat capacity of air, Dm is the vapor pressure deficit, rs is the surface resistance and ra is the aerodynamic resistance ENERGY AERODYNAMIC
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Vapor Deficit Distance between actual conditions and saturation line Greater distances = larger evaporative potential Slope of line (s) is an important term for ET prediction equations Usually measured in kPa/°C
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ET and Surface Resistance Distinct effects of vegetation on canopy resistance Forests more sensitive to changes in rs Acquire water from deeper in the soil profile, so that effect can be compensated ET (indexed to potential) from a dry canopy as a function of surface resistance (rs) at constant aerodynamic resistance
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