1. Department of Hydrology and Water Resources
Assessing the limiting factors of CO2 exchange in the Amazon rainforest during the dry season using the Simple Biosphere Model 3
Rafael Rosolem
Department of Hydrology and Water Resources
May 6th, 2008

2. Motivation
Saleska et al (Science)
Myneni et al (PNAS)

3. Motivation
Nemani et al (Science)

4. Hypothesis How to solve this issue?
- No water stress is observed during the dry season because deep roots can still extract water from the soil;
Dry season = less cloud development; as a result more incoming solar energy to the vegetation which enhances photosynthesis.
How to solve this issue?
Vegetation phenology
Soil-water dynamics
Available water
depth
Stockli et al. (in preparation)

5. Objective
Identify the mechanisms associated with the dry season “green-up” of the Amazon rainforest using a widely known land surface parameterization scheme;
- Understand the soil-water dynamics component of the model;

6. Site location
Located at FLONA (“Floresta Nacional”) Tapajós (lat/lon S / W);
Vegetation: tropical humid forest on a broad flat plateau;
The soil is mainly clay with some patches of sandy soil

7. Simple Biosphere 3 (SiB3) – Colorado State University
New Features:
- Prognostic T, e, CO2, other tracers in “canopy air space”;
- 10-layer soil (T and w), with adjustable water extraction profile (roots);
- Snowpack of 0-5 layers;
- Mixed canopy physiology (e.g., savanna); and
- Stable isotope fractionation of CO2.
Adapted from Dr. Denning slides

8. Model configuration
Forcing data: Temp. (oC), q (g kg-1), U (m s-1), Press. (hPa), Sdown (W m-2), Ldown (W m-2), and Prec. (mm hr-1): 1 hr timestep available from Jan/2001- Dec/2003;
Highly parameterized scheme (36 parameters + 8 time-varying inputs);
Soil and veg. type dependent parameters: SiB2 look-up tables applied to UMD vegetation and FAO soil maps;
biophysical parameters and time-varying inputs: global version of the European Fourier-Adjusted and Interpolated NDVI (EFAI-NDVI): 20 year long NASA/NOAA AVHRR Pathfinder NDVI data (10-day temporal and 0.1 degree spatial resolution) at
No model calibration and validation experiments: LARGE SOURCE OF UNCERTAINTY!!!
30-yr spin-up has been applied to stabilized soil moisture and carbon quantities in the model;

9. Simulation experiments
Control
Saturation
Saturation - Control

10. Results: Monthly
sat
con

11. Results: Diurnal – Dry season
con

12. Results: Diurnal and seasonal variation of NEE

13. Results: Cumulative NEE
con
sat - con

14. Conclusions
Control experiment fails to simulate the energy partion (H and LE);
Simulation is clearly improved in the dry season when water is available at the bottom layers: energy partition is improved!!!
Saturation experiment: available energy becomes the only limiting factor in the Amazon rainforest (consistent with past studies);
Model calibration MUST be performed (e.g, diurnal pattern of gorund heat flux);
At the end of the 3 years, the saturation simulation uptakes approximately 30% more carbon (0.25 g m-2 s-1) than the control experiment;
IMPORTANT: This study is just a QUICK FIX to the issue of Amazon “green- up”; the best approach is actually to incorporate a powerful phenology prognostic model and develop a rooting dynamics formulation to account for water extraction under stress conditions;