Ecohydrology - School of Forest Resources & Conservation
Ecohydrology Fall 2015 Core Questions Primary production is controlled by time-varying soil moisture Stochastic rainfall inputs Soil physical properties control storage and overflow Moisture-dependent ET loss
What patterns emerge from a simple model of this rainfall-soil moisture-vegetation interactions? Rainfall delivery patterns Emergent patterns of soil moisture distributions Model Formulation n is soil porosity (constant, dimensionless) Zr is the rooting depth (cm) s is soil moisture (dimensionless) R(t) is stochastic rainfall (marked Poisson process with
mean depth in cm and recurrence interval in day-1) ET[s(t)] is evaporation as a function of s (cm d-1) LQ[s(t), t] is excess rainfall loss via runoff and infiltration below the root zone (cm d-1) Stochastic Rainfall Two parameters describe time variation in rainfall Mean depth Mean of an exponential distribution
Frequency Number of rainstorms per day (days between rain events) Soil Moisture Dynamics Moisture thresholds at:
Wilting point (sw) below which ET = 0 Field capacity or (sl) where ET = Etmax ETmax is a fixed quantity, ET is a linear function of s Spatial averaging makes this tenable Max available water available wo = (sl sw)*n*Zr Define two dimensionless quantities: = wo / [soil storage per mean rainfall depth] Di = ETmax / * [dryness index]
Solving for pdfs of s Propagate stochastic rainfall through the filter of soil water storage and use to get a pdf of soil moisture (effective moisture x = (s - sw)/(sl sw) Involves gamma and truncated gamma distributions parameterized with the governing parameters (, , , ) Reproducing Macroscale
Behavior Captures characteristic behavior of the semiempirical Budyko curve (dots) Increasing The value of that captures Budykos curve is 5.5. At = 1.5 cm (and sw = 0.2, sl = 0.85 and n = 0.4) this corresponds to a value of Zr of 35 cm which is approx. the global mean
rooting depth Fraction of rainfall lost to ET Dryness Index Patterns of Vegetation Stress
From pdf of soil moisture classify water stress based on the mode of x (x* is water stress threshold) = wo / / = * wo / ETmax Predicts Effects of Experimental Change in Rainfall Pattern (not amount)
Take Home Message Simple analytical model can capture the key elements of the rainfall-vegetation-soil moisture system Stress responses can arise from changes in rainfall pattern, not just amount Interactions between soil storage and rainfall create geographic variation in water stress
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