Simulated Interactions of Soil Moisture, Drought Stress, and Regional Climate in the Amazon Basin

1. Simulated Interactions of Soil Moisture, Drought Stress, and Regional Climate in the Amazon Basin Scott Denning1, Jun Liu1, Ian Baker1, Maria Assunção F. Silva Dias2, Pedro L. Silva Dias2, John Kleist1 1Colorado State University 2University of São Paulo

2. Tropical Drought Stress • Variations in photosynthesis and respiration in tropical forests are strongly influenced by soil moisture • Variations in evapotranspiration due to ecosystem drought stress may interact with regional precipitation and climate • Ecosystem response to seasonal and interannual drought can help us to test the ability of models to represent physiological stress in tropical forests

3. IGPB News 2001 Niño period

4. Mean seasonal NEE and precipitation ( SD of interannual variation) in the Flona Tapajós, Para Model Output from: TEM (Tian et al, 1998, 2000) & IBIS (Botta & Foley, 2002) models (8 years, colored lines) Data from Tapajos Forest eddy flux sites (black line is moving average monthly NEE  SD, points are average weekly NEE) What’s missing in the models? Vertical resolution of respiration-relevant moisture? Diagnosis of moisture levels of litter and CWD? Moisture dependence of surface soil respiration? Slide and analysis provided by Scott Saleska, Harvard University

5. Point simulation driven by ECMWF reanalysis Photosynthesis is almost constant through year Respiration decreases during dry season due to dry surface soil layer Simulated net release of CO2 during wet season, net uptake during dry season, in agreement with flux tower data SiB2 Simulation: Tapajos 1993

6. 11º S: much longer and more pronounced dry season Both photosynthesis and respiration are impacted by drought stress NEE experiences complicated seasonal cycle Is this realistic? SiB2 Simulation: Reserva Jaru 1993 Month

7. Less Rainfall Low Soil Moisture + Hot, dry PBL Severe drought stress Lots of Sensible Heat No transpiration Precipitation and Ecosystem Stress: A Positive Feedback Loop

8. Change in Global Climate in HadCM3LC (expt:abuig) Interactive CO2 and Dynamic Vegetation 2090s - 1990s Lat: 15oS - 0oN Lon: 70oW - 50oW Results from the Haddley Center - England

9. Haddley Center Climate-driven Amazon dieback 1850 2000 2100

10. Change in Amazon Climate and Hydrologyin HadCM3LC (expt:abuig) Lat: 15oS - 0oN Lon: 70oW - 50oW

11. 3-year Coupled Simulation: SiB2-CSU/GCM

12. 3-year Coupled Simulation: SiB2-CSU/GCM • Exaggerated drought stress feedback in coupled model • Photosynthesis collapses, respiration increases • Simulated forest is dying! Elapsed months

13. Soil water potential calculated from soil texture and volumetric water content Exponential dependence Stress is exponential with water potential This makes a step function w.r.t. soil water content! Does this make sense at grid scale? We tried a new parameterization that is less steep in the middle (based on the idea that soil heterogeneity is important) First Step: Explore the Impact of Parameterization of Drought Stress new Control RSTFAC=1/(1+exp(-SLOPE*(www2-wHalfStress)))

14. Revised parameterization produces moderate reduction in drought stress Allows soils to dry further, so stress still develops 3 year coupled simulation still produces severe sustained drought Result: Kinder, Gentler Droughts Reserva Jaru, 1993

15. Experiment increased root zone to 10 m thickness, underlain by 5 m deep recharge zone Drought stress continues all year in this scenario, even after 10-year spinup from saturated initial condition Seasonal cycle of simulated NEE reverses sign in this experiment! Next: Effect of Rooting Depth

16. ~1 m runoff ~1 m runoff ~ 2.5m precipitation ~ 2.5m precipitation Impact of model vertical resolution in the water balance in the soil: layer1 SiB2 2cm 2 root zone 3 SiB3 4 drier upper layers 5 ~33% of saturation<wilting point ~45% Porosity 6 7 8 10m 9 more water in lower layers 10 15m 15m

17. SiB3 Soil Structure Layers Roots • 10 layers with exponentially increasing thickness • Transpiration limited by total plant available water • Transpiration distributed through root profile, weighted by water potential and root density

18. Effect of 10-layer Soil Reserva Jaru 1993 • Level of saturation shallows during wet season, deepens during dry season • Roots can access deeper layers during dry season • Only minor stress develops at end of dry season

19. Conclusions • Seasonal water stress is a major determinant of seasonal carbon exchange in tropical forests in the Amazon region • Seasonal cycles of equatorial forest physiology are well captured by SiB2 • Seasonal drought stress at higher latitudes is exaggerated by the model • Overestimation of drought stress may lead to severe climate simulation errors in coupled models • Correct coupled climate simulation requires resolution of vertical gradients of soil moisture, and probably also of soil respiration

20. Ongoing Work • Currently running coupled CSU/Coupled model; • RAMS experiments with SIB2/3: • Improving numerical efficiency and parallelism of SiB3 • Impact on the downscaling of the seasonal forecasting • Impact on regional forecasting (up to 3-5 days)

21. Impact on downscaling of the seasonal forecasting Old New How realistic are these patterns?