Schneebeli, Marc, Wolf, Sebastian, Kunert, Norbert, Eugster, Werner and Matzler, Christian
Abstract
First paragraph: Tropical rain forests play a dominant role in the earth's water balance. An important fraction of the hydrological cycle amounts to rain water interception and its subsequent re-evaporation. According to Lawrence et al. (2007), transpiration is the dominant process of evapotranspiration with a contribution of 58%, followed by interception evaporation (33%) and soil evaporation (9%). Rain interception is rather difficult to measure accurately. The most frequently used technique is the measurement of throughfall by setting up several rain-gages below the canopy and one or more above the canopy. The difference of the collected water is assumed to be withheld by the canopy. There are some major drawbacks to this method: Due to the complex structure of a forest canopy, the spatial variability of the throughfall is very large, hence numerous rain gages are needed in order to achieve representative and accurate sampling (Kimmins, 1973). Direct evaporation of intercepted rain water is another effect that cannot be tackled with the throughfall method, wherefore reliable measurements can only be conducted during night time. A promising technique is the attenuation measurement of a 10 GHz signal over a horizontal path through a Douglas fir stand as reported in Bouten et al. (1991). They observed a distinct linear relation of the attenuated signal to intercepted rain. The authors then employed this attenuation method in several upcoming studies as a monitoring tool for water interception in forest canopies ([Bouten et al., 1996] and [Vrugt et al., 2003]). Czikowsky and Fitzjarrald (2009) reported on a new interception estimation technique that estimates the excess evaporation following rain events with eddy covariance flux measurements. Recently, a global assessment of canopy interception from satellite data was published in Miralles et al. (2010). The authors used an analytical interception model (Valente et al., 1997) and fed it with satellite data of precipitation, lightning frequency and canopy fraction. It was found that the interception loss is sensitive to the rainfall volume, rain intensity and the forest cover. Some of these findings will be confirmed in the article at hand. Compared to intercepted rain, very little is known about the importance of dew formation and dew evaporation, most likely because no established measurement technique is available so far to measure the dew amount in a forest canopy. It is although known that dew not only promotes diseases of plant crops and is therefore an important parameter in agriculture (for tropical conditions see e.g. Holliday, 1980), according to Kabela et al. (2009), dew "also may contaminate remotely sensed measurements of important ecosystem variables such as soil moisture, land surface temperature, and vegetation biomass". Dew duration is relatively easy to measure by using electrical devices that change their resistance or capacity when becoming wet ([Kidron et al., 1965] and [Noffsinger, 1965]). However, methods for measuring dew amounts (e.g. weighing dew gages, weighing lysimeters) are limited and solely used for measurements close to the ground (Agam & Berliner, 2006).
