Predicting energy and water cycle response to changing climate and land cover conditions in the cultivated Sahel

Transcription

1 Predicting energy and water cycle response to changing climate and land cover conditions in the cultivated Sahel C. VELLUET, J. DEMARTY, B. CAPPELAERE, I. BRAUD, N. BOULAIN, G. CHARVET, J.-P. CHAZARIN, M. OÏ, N. BENARROSH, I. MAINASSARA, M. IBRAHIM, H. B.-A. ISSOUFOU, H. YAHOU, M. BOUCHER, D. RAMIER

2 Cultivated Sahel Mali Niamey area Niger Nigeria Sahelian belt Climate: Sahel tropical semi-arid Rainfall: 4-6 mm/yr PET: 2 3 mm/yr Mean temperature: ~3 C Ecosystem: Cropland (millet) Shrubland (fallow savannah) Soils: sandy soils prone to surface crusting Hydrosystem: endorheic 1/11

3 Motivations Mali Nigeria Niger Sahelian belt 1% Natural bush Shrubland Niamey area Cropland Leduc et al., 21 ; Cappelaere et al., 29 1/11

4 Objective Fallow savannah 1% Natural bush Shrubland Cropland Millet field Evaluate the differences in the local processes on a fallow and a millet field with a process-based LSM 2/11

5 Tools: Data Niger N Niamey area 1 km Wankama catchment 3/11

6 Tools: Data Niger N Niamey area 1 km Wankama catchment AMMA-CATCH Niger observatory: Meteorology: rainfall, wind speed, air moisture, Phenology: vegetation height, LAI Soil and surface properties: granulometry, density, albedo Water and energy fluxes and storages: net radiation, sensible heat flux, soil heat flux, evapotranspiration, soil moisture and temperature profiles Fallow savannah ~5 ha Millet field ~5 ha 3/11

7 Tools: Simple Soil Plant Atmosphere model SiSPAT Braud et al., 1995 Vertical (1D) Atmosphere Vegetation Soil R N G H Ev Tr I P R θ Water and energy fluxes and storages: Runoff (R) Drainage (D) Water storage ( θ) Evapotranspiration (ET) Net Radiation (R N ) Sensible heat flux (H) Soil heat flux (G) Soil temperature (T) Soil moisture (θ) D 4/11

8 Tools: Simple Soil Plant Atmosphere model SiSPAT Braud et al., 1995 Vertical (1D) Atmosphere Vegetation Soil R N G H Ev Tr I P R θ Water and energy fluxes and storages: Runoff (R) Drainage (D) Water storage ( θ) Evapotranspiration (ET) Net Radiation (R N ) Sensible heat flux (H) Soil heat flux (G) Soil temperature (T) Soil moisture (θ) T,θ D 4/11

9 Modelling SiSPAT Braud et al., 1995 Vertical (1D) Atmosphere Vegetation Soil R N G T,θ H Ev Tr H1: surface crust H2 1 cm H3 H4 H5 5 cm 1 cm 15 cm 2 cm 25 cm I P R θ Water and energy fluxes and storages: Runoff (R) Drainage (D) Water storage ( θ) Evapotranspiration (ET) Net Radiation (R N ) Sensible heat flux (H) Soil heat flux (G) Soil temperature (T) Soil moisture (θ) Depth (cm) D Obs (θ, T) 5/11

10 Modelling Initial conditions T i et θ i Water and energy fluxes and storages: Runoff (R) Drainage (D) Water storage ( θ) Evapotranspiration (ET) Net Radiation (R N ) Sensible heat flux (H) Soil heat flux (G) Soil temperature (T) Soil moisture (θ) Parameters: Soil and vegetation properties 5/11

11 Modelling Initial conditions T i et θ i Meteorological forcing Water and energy fluxes and storages: Runoff (R) Drainage (D) Water storage ( θ) Evapotranspiration (ET) Net Radiation (R N ) Sensible heat flux (H) Soil heat flux (G) Soil temperature (T) Soil moisture (θ) Parameters: Soil and vegetation properties Calibration: 2 years (25-26) Validation: 3 years (27-29) 5/11

12 Modelling Initial conditions T i et θ i Meteorological forcing Phenology forcing LAI shrub Fallow grass Water and energy fluxes and storages: Runoff (R) Drainage (D) Water storage ( θ) Evapotranspiration (ET) Net Radiation (R N ) Sensible heat flux (H) Soil heat flux (G) Soil temperature (T) Soil moisture (θ) Millet millet shrub Parameters: Soil and vegetation properties Calibration: 2 years (25-26) Validation: 3 years (27-29) Day of year 5/11

13 Modelling Initial conditions T i et θ i Meteorological forcing Phenology forcing LAI shrub Fallow grass Water and energy fluxes and storages: Runoff (R) Drainage (D) Water storage ( θ) Evapotranspiration (ET) Net Radiation (R N ) Sensible heat flux (H) Soil heat flux (G) Soil temperature (T) Soil moisture (θ) Millet Grazing millet Parameters: Soil and vegetation properties shrub Calibration: 2 years (25-26) Validation: 3 years (27-29) Day of year 5/11

14 Calibration: Hydrodynamic properties θ = θ r + 1+ θ sat h h θ g n r 1 2/ n Retention capacity: van Genuchten (198) Hydraulic conductivity: Brooks & Corey (1964) ( θ) h g n θ sat K sat β m - m 3 /m 3 m/s - K = K sat θ θ sat β 6/11

15 Calibration: Hydrodynamic properties θ = θ r + 1+ Fallow savannah H1: -1 cm -1,2 h g n θ sat K sat β m - m 3 /m 3 m/s - H2: 1-2 cm -,5 2, H3: 2-7 cm, , H4: 7-12 cm -, ,6 H5: 12-4 cm Millet field θ sat h H1: -1 cm ,5,29 6, H2: 1-2 cm 2,7 2,5.1-6 H3: 2-7 cm h θ g n r 1 2/ n Retention capacity: van Genuchten (198) Hydraulic conductivity: Brooks & Corey (1964) H4: 7-12 cm -,3, ,6 5, H5: 12-4 cm K ( θ) = 5, ,5 K sat θ θ sat β 6/11

16 Calibration: Hydrodynamic properties θ = θ r + 1+ θ sat h h θ Fallow savannah g n r 1 2/ n Retention capacity: van Genuchten (198) H1: -1 cm -1,2 Hydraulic conductivity: Brooks & Corey (1964) K ( θ) h g n θ sat K sat β m - m 3 /m 3 m/s - H2: 1-2 cm -,5 2, H3: 2-7 cm,3 = , H4: 7-12 cm -, ,6 H5: 12-4 cm Millet field H1: -1 cm ,5,29 6, H2: 1-2 cm 2,7 2,5.1-6 H3: 2-7 cm H4: 7-12 cm -,3, ,6 5, H5: 12-4 cm , ,5 K sat θ θ sat β Parameters: Physically realistic Similar for both sites 6/11

17 Calibration: Hydrodynamic properties θ = θ r + 1+ θ sat h h θ Fallow savannah g n r 1 2/ n Retention capacity: van Genuchten (198) H1: -1 cm -1,2 Hydraulic conductivity: Brooks & Corey (1964) K ( θ) h g n θ sat K sat β m - m 3 /m 3 m/s - H2: 1-2 cm -,5 2, H3: 2-7 cm,3 = , H4: 7-12 cm -, ,6 H5: 12-4 cm Millet field H1: -1 cm -,5 H2: 1-2 cm,29 2,5.1-6 H3: 2-7 cm 2,7 5, , ,5 H4: 7-12 cm -,3, ,6 5, H5: 12-4 cm K sat θ θ sat β Parameters: Physically realistic Similar for both sites, except at surface Stronger surface crust on fallow 6/11

18 Calibration-Validation: Soil moisture θ soil (m 3.m -3 ) Fallow cm Millet cm 15 cm 25 cm Model Obs Obs uncertainty Day of year Model Performance: Satisfactory for both sites, wet & dry years Similar for calibration & validation Better for fallow site Highlights: Fallow: little infiltration below 1m Millet: infiltration through the whole profile 7/11

19 Calibration-Validation: Energy fluxes Model (W/m²) Fallow R N Millet G H ET Observations (W/m²) 8/11

20 Calibration-Validation: Energy fluxes Model (W/m²) Fallow Observations (W/m²) Millet R N RMSE: 12 2 Bias: 6 1 RMSE: Bias: -5 5 G RMSE: Bias: -7-5 RMSE: Bias: -5 5 H RMSE: Bias: -9-2 RMSE: 24 3 Bias: -7 1 ET RMSE: Bias: -5 3 RMSE: 25 3 Bias: Model Performance: Satisfactory for both sites, wet & dry years Similar for calibration & validation Very good for R N Good for G Better H & ET for millet site 8/11

21 Results: Annual water budget P Rainfall variability: P=42 58 mm/yr Evapotranspiration (ET): Fallow: 8 95% of P Millet: 85 11% of P Evaporation vs.transpiration: Ev>Tr Fallow: T = 2 8% of ET Millet: T = of ET Runoff (R): Fallow: 5 2% of P Millet: 2 15% of P Drainage (D): Fallow: no drainage Millet: -5% of rainfall GW recharge Water stock ( θ): Fallow: no interannual storage θ=mm Millet: θ= 9/11

22 Results: Annual water budget Fallow P ET Millet Rainfall variability: P=42 58 mm/yr Evapotranspiration (ET): Fallow: 8 95% of P Millet: 85 11% of P Evaporation vs.transpiration: Ev>Tr Fallow: T = 2 8% of ET Millet: T = of ET Runoff (R): Fallow: 5 2% of P Millet: 2 15% of P Drainage (D): Fallow: no drainage Millet: -5% of rainfall GW recharge Water stock ( θ): Fallow: no interannual storage θ=mm Millet: θ= 9/11

23 Results: Annual water budget Fallow P T Ev Millet Rainfall variability: P=42 58 mm/yr Evapotranspiration (ET): Fallow: 8 95% of P Millet: 85 11% of P Evaporation vs.transpiration: Ev>Tr Fallow: T = 2 45% of ET Millet: T = 4 47% of ET Runoff (R): Fallow: 5 2% of P Millet: 2 15% of P Drainage (D): Fallow: no drainage Millet: -5% of rainfall GW recharge Water stock ( θ): Fallow: no interannual storage θ=mm Millet: θ= 9/11

24 Results: Annual water budget Fallow P R T Ev Millet Rainfall variability: P=42 58 mm/yr Evapotranspiration (ET): Fallow: 8 95% of P Millet: 85 11% of P Evaporation vs.transpiration: Ev>Tr Fallow: T = 2 45% of ET Millet: T = 4 47% of ET Runoff (R): Fallow: 5 2% of P Millet: 2 15% of P Drainage (D): Fallow: no drainage Millet: -5% of rainfall GW recharge Water stock ( θ): Fallow: no interannual storage θ=mm Millet: θ= 9/11

25 Results: Annual water budget Fallow P D R T Ev Millet Rainfall variability: P=42 58 mm/yr Evapotranspiration (ET): Fallow: 8 95% of P Millet: 85 11% of P Evaporation vs.transpiration: Ev>Tr Fallow: T = 2 45% of ET Millet: T = 4 47% of ET Runoff (R): Fallow: 5 2% of P Millet: 2 15% of P Drainage (D): Fallow: no drainage Millet: -5% of P GW recharge Water stock ( θ): Fallow: no interannual storage θ=mm Millet: θ= 9/11

26 Results: Annual water budget Fallow P θ D R T Ev Millet Rainfall variability: P=42 58 mm/yr Evapotranspiration (ET): Fallow: 8 95% of P Millet: 85 11% of P Evaporation vs.transpiration: Ev>Tr Fallow: T = 2 45% of ET Millet: T = 4 47% of ET Runoff (R): Fallow: 5 2% of P Millet: 2 15% of P Drainage (D): Fallow: no drainage Millet: -5% of P GW recharge Water storage ( θ): Fallow: no interannual storage θ=mm Millet: θ = mm/yr 9/11

27 Results: 3-day moving average mm/day Fallow Evaporation Transpiration Millet Evaporation Transpiration 1/11

28 Results: 3-day moving average mm/day Fallow Evaporation Transpiration Millet Evaporation Transpiration Stronger inter-annual variability on the fallow 1/11

29 Results: 3-day moving average mm/day Fallow Evaporation Transpiration Millet Evaporation Transpiration Stronger inter-annual variability on the fallow Different seasonal distribution 1/11

30 Results: 3-day moving average mm/day Fallow Evaporation Transpiration Millet Evaporation Transpiration Stronger inter-annual variability on the fallow Different seasonal distribution Sheep and cow have more effect than the climate! 1/11

31 Conclusions Evaluate the differences in the local processes on a fallow and a millet field with a process-based model Model performance: calibration & validation, satisfactory/stable for wet & dry years, millet & fallow Process-based modeling complete, detailed water & energy budgets, for variable meteorological and vegetation conditions Comparison of the millet & fallow water balances ( Ramier et al., 29) main conclusions confirmed Inter-annual variability comparison of land cover types varies with years Perspectives Longer periods to account for the marked variability of the Sahel climate Effects of land management practices (agro forestry, grazing, )? Impact of uncertainties on inputs and parameters? 11/11

32 Thank you for your attention!