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3. The soil water balance
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
Soil moisture
• Soil moisture is the amount of water in the upper layer of soil that interacts with the atmosphere (active layer).
• Soil moisture controls for plant establishment, growth and reproduction and soil functioning (nutrient cycling)
• Soil moisture dynamics (seasonal, interannual) provides the link among climate, soil, and vegetation
• Soil moisture is the key variable of ecohydrological models thatcouple climate-vegetation models
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Soil Water variables• Soil volume, depth Z, 1 m2
Vsol = Vair + Vwater + Vmat = Z x 1 m2
• Porosity [m3 m-3] : n = (Vair + Vwater ) / Vsol
• Soil Water Content = Soil Water Store (W) [m3 m-2 = m] W = Vwater s Vsol
0< W < Wsat
• Volumetric Soil Water content [m3 m-3] = Vwater / Vsol
Vair = 0 -> n = sat (Saturated Volumetric Water Content)
• Relative Soil Water Content [0-1] s • s = Vwater / (Vair + Vwater )• s = W / Wsat = W / (sat Z) = W / (n Z)
Vair
Vwat
Vsoil
Vmat
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Version du 14/01/2014Bioclimatologie 92
Critical values of the Soil Water Content
If soil porosity is completely filled by water, the amount of water left in the soil is the water-holding capacity (W=Wsat and s = 1) (Reserve Utile sensphysique)
If the excess water has drained away (2-3 days), the amount of water left in the soil is the field capacity (W=Wfc and sfc = Wfc/Wsat) (Capacité au Champ)
If vegetation has extracted water until it wilted, the amount of water left in the soil is the wilting level (W=Wwil and sw = Ww/Wsat) (Point de flétrissement)
The available plant soil moisture (Réserve Utile sens agronomique) (m) is : • Wfc - Wwil = Z * (fc – wil) [m]
• Exemple : fc = 0.45 m3 m-3 , wil = 0.31 m3 m-3, Z= 1m • 0.14 m = 140 mm
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Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
Master equation : Soil Water balance
Bioclimatologie 94
• Zr Depth of the root zone = active soil depth = rooting depth [m]
• Mass conservation balance
( ) ( ) ( , ) ( , ) ( , )dW P t L t E W t T W t R W tdt
W, s
Transpiration (T)
Precipitation (P)
Evaporation (E)
Leakage - Infiltration(Deep drainage, Percolation)
Ruissellement (R)
RunoffZr
Z
Canopy Interception (L)
( ) ( ) ( , ) ( , ) ( , )rdsnZ P t L t E s t T s t R s tdt
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Soil moisture and evapotanspiration3
Relative Soil Water Content (s)
Soi
l wat
er lo
sses
(e
vapo
trans
pira
tion
& le
akag
e)
Source: Porporato A. (2003) Journal of geophysical research, 108.
max
( ) ( , ) ( , )
( , ) *( , ) *( , ) 0
( , ) 0
( , ) ( )
r
h
h
fc
fc
dsnZ P t ET s t R s tdtET s t E if s sET s t s if s s sET s t if s s
R s t if s s
R s t P t if s s
Emax
non-saturated soil saturated soil
s* is the soil moisture level at which the plant begins to close stomata in response to water stress
unst
ress
edE
T
stre
ssed
ET
excessrainfalls
Accounting for stomatal control
max
( ) ( , ) ( , )
( , )
( , )
( , ) 0
( , ) 0
( , ) ( )
r
fc
h fc
h
fc
fc
dsnZ P t ET s t R s tdtET s t E if s s
ET s t s if s s s
ET s t if s s
R s t if s s
R s t P t if s s
Neglecting stomatal control
Version du 14/01/2014
Water Retention Curve
sat : water retained at = −0.1 bar = - 10 kPafc : water retained at = −0.33 bar = - 33 kPa wil = water retained at = −15 bar = - 1500 kPa
33 kPa10 kPa
1500 kPa
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Tropical Vegetation in Northern Australia
Fox (2001), modified from Hill (2005)
2 million km2
12% of the savanna biome
Northern Territories
Queensland
Western Australia
PART 1
Mitchell grasslands : C4 perennial grasslands on vertic soils
Astrebla sp. (Mitchell grass)
PART 1
Toorak (Qld)PART 2
Plant –soil moisture feedbacks
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
( ) ( ) ( , , ) ( , , ) ( , , )
( , , ) ( , , )G L
dsnZ P t L t E s p t T s p t R s p tdt
dp f p s t f p s tdt
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W, s
Transpiration (T)
Precipitation (P)
Evaporation (E)
Leakage - Infiltration(Deep drainage, Percolation)
Ruissellement (R)
RunoffZr
Z
Canopy Interception (L)
Climate diagram (diagramme ombrothermique)
Mean Annual Temperature(MAT)
Annual Precipitation(AP)
Tem
pera
ture
Précipitation
Climat tempéré (Thot>10, 0<Tcold<18)No dry season
Cold summer (Thot <22)
Temperature of the coldestmonth (Tcold)
Temperature of the hottestmonth (Thot)
One bucket soil model suitable for climate studies
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013) Source: Stephenson N.L. (1998) J. Biogeogr., 25, 855-870.
Precipitation ≈ supply
PET ≈ demand
energylimited ET
AET = PET
supplylimited
AET < PET
energylimited ET
AET = PET
Surplus
Deficit
surplus
replenishmentof soil reserve
dS >0
use ofof soil reserve
dS<0
Soil water balance and global distribution of vegetation
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
Key issues
• Impact of soil parameters and plant functional properties on plot scale water balance
• Effects of land use change and vegetation dynamics on soil water balance (Upscaling plot -> watershed)
• Coupled soil-vegetation models to understand the dynamics of drylands under global change
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
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Advanced readings
• Peel, M. C., et al. 2007. Updated world map of the Koppen-Geiger climate classification. - Hydrology and Earth System Sciences 11: 1633-1644.
• Porporato, A., et al. 2002. Ecohydrology of water-controlled ecosystems. - Advances in Water Resources 25: 1335-1348.
• Porporato, A., et al. 2004. Soil water balance and ecosystem response to climate change. - Am. Nat. 164: 625-632.
• Rodriguez-Iturbe, I., et al. 2001. Plants in water-controlled ecosystems: active role in hydrologic processes and response to water stress - I. Scope and general outline. Advances in Water Resources. 24: 695-705.
• Stephenson, N. L. 1998. Actual evapotranspiration and deficit: biologically meaningful correlates of vegetation distribution across spatial scales. - J. Biogeogr. 25: 855-870.
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
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