Throughfall Data

We decided to measure throughfall (the amount of rainfall that actually reaches the ground surface) at two separate locations on Cerro Pelado using a trough collection system, denoted as "TF1" and "TF2" on the Gamboa Site Map. Three sections of 4" (101.6 mm) PVC measuring 6.0 m in length were cut in half to intercept any incident rainfall. Each section is inclined in such a way that all collected water drains into a central collection bin. Water from the collection bin then flows through a 1.5" (38.1 mm) PVC pipe with a 90 degree elbow installed at the end to discharge water directly into a large tipping bucket gage. The purpose of the central collection bin is to eliminate splashing and potential water loss that would likely occur if the pipes were allowed to drain directly into the tipping bucket gage. Each tipping bucket rain gage contains adjustable posts that where used in the laboratory to calibrate the volume of water contained in each tip. From lab tests TF1 requires 1.0 L and TF2 requires 150 mL per tip. Different tipping bucket sizes were selected before hand because we were unsure if rainfall would inundate the 150 mL sized bucket. A magnet and reed switch was installed inside each tipping bucket such that each tip would register as a momentary contact when connected to a HOBO data logger. When the volume required per tip is divided by the combined area of PVC pipe the equivalent depth of rainfall results.

Fig 1 Pictures of both trough collection systems used to measure throughfall at two seperate locations on Cerro Pelado.

In order to gain an appreciation of throughfall values in tropical environments I have tabulated a selected list of publications.

Table 1 Reported values of rainfall, interception, throughfall, and stemflow for selected studies in tropical catchments.
+Rainfall is generally reported as yearly values, depending upon the source it is reported as either a historical (*) or study duration average or gross total (**).
#Assumed based on literature and observations, not an actual measured value.