November 30, 2010

Large-Scale Throughfall Measurement System Installed at Agua Salud

by Frank Base

Following the long-term approach of the Agua Salud Project, researchers recently installed three automatic-logging trough systems to measure throughfall in a 5-year-old secondary succession forest at Agua Salud. The purpose of the permanent trough system is to document the temporal changes in throughfall as the secondary forest grows and develops.

Juan Carlos BriseƱo, technical assistant, finishing one of the new trough systems. Photo by Frank Base.

The trough systems consist of 32 three-meter open tubes (troughs) connected like feathers on the wings of a bird. One wing spreads over an area of 9 m x 3 m. The system in total has an opening of about 12 m². At the center of the construction, a logging tipping bucket catches all drained water and has a capacity of 3 L per tip. Within one hour, 2160 L can be measured. This capacity equals a rain intensity of 195 mm/h, which is sufficient for the strongest rain events in the Agua Salud area based on data from 2008-2010.

Before installing the system, Agua Salud researchers had been using 100 funnels to measure throughfall manually since 2008. The new automatic system now provides continuous measurement of throughfall over the course of the year, enabling us to characterize variations over time and correlate them with rainfall intensity.

Measuring throughfall with trough systems is not new, however. Cuartas et al. (2007) used a trough system covering 1.8 m² with 6-m troughs and a tipping bucket capacity of 125 ml per tip in central Amazonia, Brazil. And McJannet et al. (2007) used 6-m long troughs in a star formation covering an area of 2.4 m² - 3.6 m² in an Australian tropical rainforest. Their tipping buckets had a capacity of 1.8 L - 2.1 L per tip.

Scale is the main difference between the systems used in Brazil and Australia and the one at Agua Salud. We have built, as far as we know, one of the biggest trough systems in the world. Such a large measuring system is necessary to reduce the effect of throughfall on spatial variability caused by extreme structural differences in the canopy of secondary forests. By sampling such a large area, we hope to smooth out the variation caused by canopy structure.

Cuartas, LA, J Tomasella, AD Nobre, MG Hodnett, MJ Waterloo, and JC Munera. 2007. Interception water-partitioning dynamics for a pristine rainforest in Central Amazonia: Marked differencesbetween normal and dry years. Agricultural and Forest Meteorology 145: 69-83.

McJannet, D, J Wallace, and P Reddell. 2007. Precipitation interception in Australian tropical rainforests: I. Measurement of stemflow, throughfall and cloud Interception. Hydrological Processes 21: 1692–1702.