Fog deposition to a <I>Tillandsia</I> carpet in the Atacama Desert

Abstract

Abstract. In the Atacama Desert, one of the driest places on earth, fog deposition plays an important role for the water balance and for the survival of vulnerable ecosystems. The eddy covariance method, previously applied for the quantification of fog deposition to forests in various parts of the world, was used for the first time to measure deposition of fog water to a desert. In this exploratory study we estimate the amount of water available for the ecosystem by deposition and determine the relevant processes driving fog deposition. This is especially important for the species Tillandsia landbecki living in coastal Atacama at the limit of plant existence with fog and dew being the only sources of water. Between 31 July and 19 August 2008 approximately 2.5 L m −2 of water were made available through deposition. Wholeyear deposition was estimated as 25 L m −2 . Turbulent upward fluxes occurred several times during the evenings and are explained by the formation of radiation fog. In connection with that, underestimates of the deposition are assumed. More detailed studies covering various seasons and all parameters and fluxes contributing to the local energy balance are suggested. This will help to further develop understanding about the processes of (i) deposition of water to the desert, and (ii) intensification of advection fog through additional formation of radiation fog.

Evaluating the capabilities and uncertainties of droplet measurements for the fog droplet spectrometer (FM-100)

“…Hence, the PVM-100 can be considered an appropriate reference to validate our corrections for the FM-100 measurements. The sampling angle of the FM-100 was large during most of the time, such that only 4 % of the cloudy data were within the sampling angle criteria (below 30 • ) used by Westbeld et al (2009). This is remarkable, as the inlet faced the expected mean wind direction during the first part of the campaign.…”

“…It has also been used as a single instrument for microphysical studies of fog (Gonser et al, 2011;Liu et al, 2011) and compared to other devices Schmid et al, 2010;Frumau et al, 2011). Most of the presented work used the channel configuration defined by the manufacturer in order to translate the voltage to a droplet size; while Niu et al (2010) used the 20 channel configuration, which is the one that is used by the manufacturer to calibrate the instrument, some of the authors (Burkard et al, 2002;Eugster et al, 2006;Beiderwieden, 2007;Beiderwieden et al, 2008;Westbeld et al, 2009;Frumau et al, 2011) used the 40 channel configuration in order to obtain a better resolved size distribution. A different approach was taken by Gonser et al (2011) -which is one of the most recent publications -who defined their own 23 channel sizes and widths by using Mie curves prior to sampling.…”

“…Nevertheless, these procedures do not assure isokinetic sampling conditions. Westbeld et al (2009) andLiu et al (2011) also installed the FM-100 in a fixed position for the entire measurement campaign. They established a quality criterion, by only accepting data as good data if the horizontal wind direction does not differ by a certain degree from the actual inlet orientation.…”

“…The FM-100 has been used in several ground based studies so far especially as part of an eddy covariance system to quantify fog water deposition fluxes in tropical mountain cloud forests (e.g. Eugster et al, 2006;Holwerda et al, 2006;Beiderwieden, 2007;Beiderwieden et al, 2008;Schmid et al, 2010), in temperate ecosystems (Burkard et al, 2002;Thalmann, 2002;Burkard, 2003), and deposition fluxes in rather arid areas (Westbeld et al, 2009). It has also been used as a single instrument for microphysical studies of fog (Gonser et al, 2011;Liu et al, 2011) and compared to other devices Schmid et al, 2010;Frumau et al, 2011).…”

“…They established a quality criterion, by only accepting data as good data if the horizontal wind direction does not differ by a certain degree from the actual inlet orientation. Westbeld et al (2009) used ± 30 • of the hourly mean wind direction and Liu et al (2011) used ± 7 • for this criterion. However, a clear justification why they chose these angles was not given.…”