3. The soil water balance. Fundamentals of Ecohydrology (Philippe CHOLER CNRS France). Wuwei (09/2013)

Transcription

1 3. The soil water balance Fundamentals of Ecohydrology (Philippe CHOLER CNRS France). Wuwei (09/2013)

2 3 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 that couple climate-vegetation models

3 3 Soil Water variables Soil volume, depth Z, 1 m2 Vsol = Vair + Vwater + Vmat = Z x 1 m2 Vsoil Porosity [m3 m-3] : n = (Vair + Vwater ) / Vsol Vmat Vwat Vair 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)

4 3 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=W sat and s = 1) (Reserve Utile sens physique) If the excess water has drained away (2-3 days), the amount of water left in the soil is the field capacity (W=W fc and s fc = W fc /W sat )(Capacité au Champ) If vegetation has extracted water until it wilted, the amount of water left in the soil is the wilting level (W=W wil and s w = W w /W sat ) (Point de flétrissement) The available plant soil moisture (Réserve Utile sens agronomique) (m) is : W fc -W wil = Z * ( fc wil ) [m] Exemple: fc = 0.45 m 3 m -3, wil = 0.31 m 3 m -3, Z= 1m 0.14 m = 140 mm 92 Bioclimatologie Version du 14/01/2014

5 Fundamentals of Ecohydrology (Philippe CHOLER CNRS France). Wuwei (09/2013)

6 3 Master equation : Soil Water balance Precipitation (P) Transpiration (T) Z r Depth of the root zone = active soil depth = rooting depth [m] Canopy Interception (L) Evaporation (E) Mass conservation balance dw P() t L() t E( W,) t T( W,) t R( W,) t dt Runoff Ruissellement (R) Z Zr W, s nz ds r P() t L() t E(,) s t T (,) s t R(,) s t dt Leakage - Infiltration (Deep drainage, Percolation) Bioclimatologie 94

7 3 Soil moisture and evapotanspiration Soil water losses (evapotranspiration & leakage) non-saturated soil stressed ET unstressed ET saturated soil excess rainfalls Relative Soil Water Content (s) s* is the soil moisture level at which the plant begins to close stomata in response to water stress E max Accounting for stomatal control nz ds r P() t ET (,) s t R(,) s t dt ET (,) s t Emax if s s * ET (,) s t s if sh s s * ET (,) s t 0if s sh Rst (,) 0ifs sfc Rst (,) Ptifs () s Neglecting stomatal control nz ds r P() t ET (,) s t R(,) s t dt ET (,) s t Emax if s s fc ET (,) s t s if sh s s fc ET (,) s t 0if s sh Rst (,) 0ifs sfc Rst (,) Ptifs () s fc fc Source: Porporato A. (2003) Journal of geophysical research, 108.

8 3 Water Retention Curve 1500 kpa 33 kpa 10 kpa sat : water retained at = 0.1 bar = - 10 kpa fc : water retained at = 0.33 bar = - 33 kpa wil = water retained at = 15 bar = kpa Version du 14/01/2014

9 PART 1 Tropical Vegetation in Northern Australia Western Australia 2 million km 2 12% of the savanna biome Northern Territories Queensland Fox (2001), modified from Hill (2005)

10 PART 1 Mitchell grasslands : C4 perennial grasslands on vertic soils Astrebla sp. (Mitchell grass)

11 PART 2 Toorak (Qld)

12 3 Plant soil moisture feedbacks Precipitation (P) Transpiration (T) Canopy Interception (L) Evaporation (E) Runoff Ruissellement (R) Z Zr W, s ds nz Pt () Lt () Es (, pt,) Ts (, pt,) Rs (, pt,) dt dp fg( p,,) s t fl( p,,) s t dt Leakage - Infiltration (Deep drainage, Percolation) Fundamentals of Ecohydrology (Philippe CHOLER CNRS France). Wuwei (09/2013)

13 Climate diagram (diagramme ombrothermique) Temperature of the coldest month (T cold ) Mean Annual Temperature (MAT) Annual Precipitation (AP) Temperature of the hottest month (T hot ) Temperature Précipitation Climat tempéré (T hot >10, 0<T cold <18) No dry season Cold summer (T hot <22)

14 One bucket soil model suitable for climate studies energy limited ET AET = PET supply limited AET < PET energy limited ET AET = PET Deficit PET demand replenishment of soil reserve ds >0 surplus Precipitation supply Surplus use of of soil reserve ds<0 Fundamentals of Ecohydrology (Philippe CHOLER CNRS France). Wuwei (09/2013) Source: Stephenson N.L. (1998) J. Biogeogr., 25,

15 Soil water balance and global distribution of vegetation Fundamentals of Ecohydrology (Philippe CHOLER CNRS France). Wuwei (09/2013)

16 3 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)

17 3 Advanced readings Peel, M. C., et al Updated world map of the Koppen-Geiger climate classification. - Hydrology and Earth System Sciences 11: Porporato, A., et al Ecohydrology of water-controlled ecosystems. - Advances in Water Resources 25: Porporato, A., et al Soil water balance and ecosystem response to climate change. - Am. Nat. 164: Rodriguez-Iturbe, I., et al 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: Stephenson, N. L Actual evapotranspiration and deficit: biologically meaningful correlates of vegetation distribution across spatial scales. - J. Biogeogr. 25: Fundamentals of Ecohydrology (Philippe CHOLER CNRS France). Wuwei (09/2013)