Salma TAFASCA (PhD student)

Supervisors: Agnès DUCHARNE and Christian VALENTIN

Impact of soil texture on simulated hydrology

in ORCHIDEE land surface model

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CESM Land Model Working Group18 June 2019, Boulder

• Soil hydraulic properties are important in hydrological and land surface models (LSMs)

• There is no global coverage of measured soil hydraulicproperties

• Pedotransfer functions derive soil hydraulic properties fromavailable soil data e.g. soil texture.

Introduction

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Soil texture: relative proportion of sand, silt and clay

What is soil texture?

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2 µm 50 µm 2000 µm

Clay Silt Sand

Soil texture: relative proportion of sand, silt and clay

What is soil texture?

12 USDA soil textures

% Sand

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2 µm 50 µm 2000 µm

Clay Silt Sand

Soil texture: relative proportion of sand, silt and clay

2 µm 50 µm 2000 µm

Clay Silt Sand

Soil texture class

Median diameter (µm)

What is soil texture?

12 USDA soil textures

% Sand

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How does soil texture impact simulated hydrology in a LSM?

Soil texture: relative proportion of sand, silt and clay

2 µm 50 µm 2000 µm

Clay Silt Sand

Soil texture class

Median diameter (µm)12 USDA soil textures

% Sand

What is soil texture?

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• State-of-the-art land surface model• Developed by the Institute Pierre Simon Laplace (Paris)• Simulates water, energy and carbon fluxes

The land surface model ORCHIDEE

• Multi-layer scheme (11 layers)

• Physically based description of soil water fluxes: Richards equation

• Soil thickness: 2m

• Free drainage at the bottom

Rsurf

D

PSoil EvapTransp.

Soil hydrology in ORCHIDEE LSM

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• Multi-layer scheme (11 layers)

• Physically based description of soil water fluxes: Richards equation

• Soil thickness: 2m

• Free drainage at the bottom

Rsurf

D

PSoil EvapTransp.

Requires K(θ) and D(θ) curves:From Van Genuchten equations

Soil hydrodynamic parameters(porosity, Ks, ts…..)

Soil texture

Soil hydrology in ORCHIDEE LSM

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• Multi-layer scheme (11 layers)

• Physically based description of soil water fluxes: Richards equation

• Soil thickness: 2m

• Free drainage at the bottom

Rsurf

D

PSoil EvapTransp.

Requires K(θ) and D(θ) curves:From Van Genuchten equations

Soil hydrodynamic parameters(porosity, Ks, ts…..)

Soil texture

Soil hydrology in ORCHIDEE LSM

K(θ) D(θ)

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Soil texture map Derived soil parameter map

Pedotransfer functions

Reynolds et al. (2000)

Deriving soil parameters in ORCHIDEE LSM

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Soil texture map Derived soil parameter map

Pedotransfer functions

Look-up table ofCarsel and Parrish (1988)

Deriving soil parameters in ORCHIDEE LSM

Reynolds et al. (2000)

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Offline ORCHIDEE simulations

0.5° resolution

Period: 1980-2010

Different soil maps and atmospheric forcings

Soil Map Atmospheric forcing Texture distrib.

Reynolds CRU-NCEP

Reynolds GSWP3

Zobler GSWP3

SoilGrids GSWP3

Uniform Loam GSWP3

Uniform Loamy Sand GSWP3

Uniform Silt GSWP3

Uniform Clay GSWP3

Numerical experiments

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Soil Map Atmospheric forcing Texture distrib.

Reynolds CRU-NCEP

Reynolds GSWP3

Zobler GSWP3

SoilGrids GSWP3

Uniform Loam GSWP3

Uniform Loamy Sand GSWP3

Uniform Silt GSWP3

Uniform Clay GSWP3

Numerical experiments

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Reynolds et al. (2000) Zobler (1987) SoilGrids (Hengl et al., 2014)

Offline ORCHIDEE simulations

0.5° resolution

Period: 1980-2010

Different soil maps and atmospheric forcings

Soil Map Atmospheric forcing Texture distrib.

Reynolds CRU-NCEP

Reynolds GSWP3

Zobler GSWP3

SoilGrids GSWP3

Uniform Loam GSWP3

Uniform Loamy Sand GSWP3

Uniform Silt GSWP3

Uniform Clay GSWP3

Numerical experiments

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Offline ORCHIDEE simulations

0.5° resolution

Period: 1980-2010

Different soil maps and atmospheric forcings

Soil Map Atmospheric forcing Texture distrib.

Reynolds CRU-NCEP

Reynolds GSWP3

Zobler GSWP3

SoilGrids GSWP3

Uniform Loam GSWP3

Uniform Loamy Sand GSWP3

Uniform Silt GSWP3

Uniform Clay GSWP3

Numerical experiments

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Grid point scale response of ORCHIDEE to different soil textures

Soil Map Atmospheric forcing Texture distrib.

Reynolds GSWP3

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Grid point scale response of ORCHIDEE to different soil textures

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Global scale response of ORCHIDEE to different soil maps

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Different realistic soil mapsSimilar climate forcing

Global scale response of ORCHIDEE to different soil maps

• Similar water budgets for different soil maps.

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Contrasted uniform soil mapsSimilar climate forcing

Global scale response of ORCHIDEE to different soil maps

• Similar water budgets for different soil maps.

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Global scale response of ORCHIDEE to different soil maps

• Similar water budgets for different soil maps.

• Climate forcing impacts more than soil texture.

Similar soil mapDifferent climate forcing

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Estimated total runoff(Rodell et al., 2015)

Estimated ET (Rodell et al., 2015; Jung et al., 2010; Miralles et al., 2011)

• Similar water budgets for different soil maps.

• Climate forcing impacts more than soil texture.

• Differences in water budget smaller thanuncertainty in observations.

Global scale response of ORCHIDEE to different soil maps

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• Similar spatial patterns with Reynolds map and uniform Loam map.

Evapotranspiration (mm/d)

Spatial distribution of simulated water fluxes

Reynolds map Reynolds map

Uniform Loamy map Uniform Loamy map

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• Similar spatial patterns with Reynolds map and uniform Loam map.

• Higher spatial sensitivity of surface runoff to soil maps.

Evapotranspiration (mm/d) Surface runoff (mm/d)

Spatial distribution of simulated water fluxes

Reynolds map Reynolds map

Uniform Loamy map Uniform Loamy map

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Sensitivity of evapotranspiration spatial patterns

Reynolds - Zobler

Grey areas: differences not statisticallysignificant (student t-test, α=5%)

Differences of ET (mm/d)

GSWP3 – CRU-NCEP

SoilGrids – Uniform loam Reynolds – Obs (GLEAM)

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Sensitivity of evapotranspiration spatial patterns

Reynolds - Zobler

Grey areas: differences not statisticallysignificant (student t-test, α=5%)

GSWP3 – CRU-NCEP

SoilGrids – Uniform loam Reynolds – Obs (GLEAM)

Change of soil map

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Sensitivity of evapotranspiration spatial patterns

Reynolds - Zobler

Grey areas: differences not statisticallysignificant (student t-test, α=5%)

GSWP3 – CRU-NCEP

SoilGrids – Uniform loam Reynolds – Obs (GLEAM)

Change of climate forcing

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Sensitivity of evapotranspiration spatial patterns

Reynolds - Zobler

Grey areas: differences not statisticallysignificant (student t-test, α=5%)

GSWP3 – CRU-NCEP

SoilGrids – Uniform loam Reynolds – Obs (GLEAM)

Evapotranspiration bias

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Conclusion and perspectives

• Small scale: realistic sensitivity of ORCHIDEE to soil texture

• Global scale: weak sensitivity of simulated water budget to soil texture maps

compared to other forcings (climate, vegetation …)

• Why?

• Maps of dominant soil texture at 0.5° favoring loam (over 40% overlap)

• Common input soil data (FAO/UNESCO soil map of the world)

• What next?

• Sub-grid variability of soil properties

• Other soil components: organic matter, soil structure

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