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

Motivation Saleska et al. 2003 (Science) Myneni et al. 2007 (PNAS)

Motivation Nemani et al. 2003 (Science)

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)

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;

Site location Located at FLONA (“Floresta Nacional”) Tapajós (lat/lon 3.01 S / 54.58 W); Vegetation: tropical humid forest on a broad flat plateau; The soil is mainly clay with some patches of sandy soil

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

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 1982-2001 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 http://www.iac.ethz.ch/staff/stockli/efaindvi_page/efaindvi.html; 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;

Simulation experiments Control Saturation Saturation - Control

Results: Monthly sat con

Results: Diurnal – Dry season con

Results: Diurnal and seasonal variation of NEE

Results: Cumulative NEE con sat - con

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;