Publication Date:
2017-02-16
Description:
The interaction of land with the atmosphere is sensitive to soil moisture (W). Evapotranspiration (ET) reacts to soil moisture in a non-linear way, f (W), as soils dry from saturation to wilt point. This nonlinear behavior and the fact that soil moisture varies on scales as small as 1-10 meters in nature, while numerical General Circulation models (GCMs) have gridcell sizes on the order of 1 to 100's of kilometers, makes the calculation of gridcell-average ET problematic. It is impractical to simulate the land in GCMs on the small scales seen in nature, so techniques have been developed to represent sub-grid scale heterogeneity, including: 1) statistical-dynamical representations of grid sub-elements of varying wetness, 2) relaxation of f (W), 3) moderating f (W) with approximations of catchment hydrology, 4) ‘tiling' the landscape into vegetation types, and 5) hyper-resolution. Here we present an alternative method for representing subgrid variability in W, one proven in a conceptual framework where landscape-scale W is represented as a series of 'bins' of increasing wetness from dry to saturated. The gridcell-level f (W) is defined by the integral of the fractional area of the wetness bins and the value of f (W) associated with each. This approach accounts for the spatiotemporal dynamics of W. We implemented this approach in the SiB3 land-surface parameterization and then evaluated its performance against a control, which assumes a horizontally uniform field of W. We demonstrate that the Bins method, with a physical basis, attenuates unrealistic jumps in model state and ET seen in the control runs. This article is protected by copyright. All rights reserved.
Electronic ISSN:
1942-2466
Topics:
Geography
,
Geosciences
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