Using Scintillometry for Assessment of Evapotranspiration in the Seasonal Tropics of Panama

Using Scintillometry for Assessment of Evapotranspiration in the Seasonal Tropics of Panamaarticle picture
Published: Thursday, September 29, 2016 By Mario Bretfeld, Smithsonian Tropical Research Institute - Accurately quantifying evapotranspiration in the seasonal tropics of Panama poses several challenges. During the rainy season, from early May to late December, frequent cloud cover hampers acquisition of remotely sensed data and results in considerable data gaps.

Increasing demand for water

This is true for the Panama Canal Watershed (PCW) in central Panama, which supplies water for the operation of the canal, domestic and industrial uses, and for generation of hydroelectricity. An increasing demand for water due to urbanization and the expansion of the Panama Canal, along with a predicted transition into a dryer climatic period, necessitate a better understanding regarding the effects of land use on energy fluxes and hydrological processes.

Development of an improved hydrological model

As part of a collaborative research supported by the National Science Foundation with the overarching goal “to assess the feedback between hydrology and humans by developing an improved hydrological model coupled with a model of socio-economic behaviour”, we have taken several approaches to measure evapotranspiration across different types of land uses in the Panama Canal Watershed.

These approaches include in-vivo photosynthesis and sap flow measurements in secondary forests of different ages and a shaded coffee plantation, an eddy flux tower in a pasture environment, and two first generation Kipp & Zonen Large Aperture Scintillometers (LAS) measuring across a mosaic of pasture and re-growing secondary forest patches.

Our research sites are located in the 15 km2 Agua Salud study area. Established in 2008, the Agua Salud project is a collaboration between the Smithsonian Tropical Research Institute, the Panama Canal Authority, Panama's Environment Authority and other partners.

Post-doctorate Mario Bretfeld and intern Laura Lyons during LAS alignment


The use of LAS Scintillometers

The main objectives of the scintillometers are: 1) to test the agreement between devices over an identical, long-distance transect spanning multiple land cover types and complex terrain in a high humidity environment, and 2) to provide an additional method and measurement scale to assess evapotranspiration, complementing sap flow and eddy flux measurements.

In February 2016, we installed two scintillometers in parallel, with a receiver and transmitter at each end of a 4217 m transect. Although signal strength was expectedly low, measurements on dry, clear days agree well between the devices (~5% difference) and a comparison of preliminary data for sensible heat flux between scintillometers and the nearby eddy flux tower further confirm the applicability of scintillometers during the dry season in the Panama Canal Watershed, with the added benefit of generally requiring less maintenance than eddy flux systems. However, expectedly, once the rains started in late April, signal strength frequently approached the noise levels of the instruments.

Next steps

Our next objective is to relocate one scintillometer and measure across a shorter transect (~ 2000 m) that traverses a 7 ha watershed currently covered by 25 year old secondary forest, which will be deforested in the next dry season, with the aim to get reliable rainy season data and to detect changes in fluxes from the forest-to-pasture conversion.


Share this page