Drought Simulation

Rainfall Exclusion Experiment

Floresta Nacional do Tapajós, location of the experiment

It comes as a surprise to many that vast areas of Amazon “rainforest” get little or no rain for five or six months of every year. Deep, absorptive soils store the water that falls during the intense rainy season; during the dry months, trees send roots down into this stockpiled water to remain green and physiologically active. But lately, several factors appear to be threatening the rainfall system of the Amazon basin - deforestation and smoke alter local and regional precipitation patterns, El Niño events bring increasingly severe and prolonged droughts, and global warming could further dry the region.

In 1998, we began a rainfall exclusion experiment to see how a typical Amazon lowland forest would react to a severe, prolonged drought.

Located in the Tapajós National Forest near Santarém, in the lower Amazon basin, our experiment consists of two one-hectare parcels (2.5 acres) of moist tropical forest - a treatment parcel and a control parcel. We selected two parcels only 25 meters apart to ensure that any localized climatic factors will affect them equally.

Because species diversity is so great in this forest, we had to survey 22 hectares before finding adjacent parcels with similar species composition, soils, and terrain. Of the 535 trees in the experimental parcel alone with DBH (Diameter at Breast Height) greater than 10 cm, 166 species are represented. In both parcels combined, over 6,000 plants were eventually tagged, identified, measured, and mapped.

Three-dimensional sketch of the experimental layout. (artist: Kemel Kalif)

Setting up the experiment

The infrastructure in both parcels is so extensive it took our crew of 15 workers a year to construct. Each parcel has a 1.5 meter trench dug completely around it to isolate the soils inside and prevent any tree roots from tapping water from outside the parcel. In the case of the experimental parcel, the trench was lined with plastic and leads to a drainage ditch which was dug to channel the water falling onto the parcel to a lower area 300 meters away. Above ground, each parcel has four towers averaging 30 meters in height for canopy access and weather instruments, and beneath the canopy, elevated catwalks meander through the forest to provide access to branches for physiological measurements and observations.

Overhead view of the panels

Finally, we constructed a system of elevated gutters and stands to support 5,600 plastic-covered wooden frames to divert rainfall from most of the experimental parcel. These panels are cleaned weekly when in place, but are removed during the dry season to minimize microclimatic differences due to our localized greenhouse effect.

We installed our instruments before beginning construction wherever possible, in order to gather a full year of baseline data before implementing the drydown experiment.

Sampling scheme

The view from beneath the panels

Control parcel infrastructure: towers, soil pits, elevated walkways, sample grid points, and bordering trench

We take extensive measurements beneath, above, and within our parcels. We monitor temperature, relative humidity, rainfall, and light input above the canopy; we also have dozens of temperature/relative humidity sensors at assorted heights beneath the canopy to compare microclimatic changes brought about by our experiment.

Each parcel has five twelve-meter deep soil shafts for access to deep soil to measure soil moisture and trace gas emissions, and monitor rooting activity.

We make measurements of the forest canopy density on a monthly basis over our standard 10m sampling grid of 144 points per parcel. We continously capture litter falling from the tree canopy at these same gridpoints

Also on this sample grid, we take monthly readings of soil moisture near the surface using permanently installed Time Domain Reflectometry (TDR) probes. These measurements complement the vertical soil moisture measurements taken at 1-meter depth intervals from all five 11-meter deep soil shafts in each parcel.

More Measurements»

©Woods Hole Research Center, 2007