Automated wetting of a fiber-optic cable for forest evaporation partitioning

Measuring evaporation through Bowen ratios requires measuring a wet and a dry air temperature, something that is challenging to reliably accomplish in outdoor field conditions. In the context of forest evaporation, the desire to estimate Bowen ratios as a function of height (e.g., to partition evaporation above and below the canopy) adds another dimension of complexity to this measurement challenge.

As part of the Ruijsdael Observatory in the Loobos, Netherlands, we aim to continuously measure evaporation throughout a forest profile, using a dry and a wetted fiber-optic table along a 40 m tower to measure temperature profiles using Distributed Temperature Sensing (DTS). Previous studies have used continuous pumping with relatively large flow rates to ensure wetness, but this is not feasible for long term installations because of large water volume requirements.

In this contribution a smart and open-source solution for keeping a wet temperature wet and a dry temperature dry over a 40 m profile is presented. Two peristaltic pumps are regulated using two microcontrollers that modulate the pumping rate along different environmental conditions. For instance, no pumping could be needed at nighttime since there is negligible evaporation and pumping is stopped at low temperatures to prevent frost damage. A capacitive method is presented to attempt to quantify wetness, tank levels are monitored, and solutions for recycling water to limit the water volume requirements are introduced. Microcontrollers are connected to WiFi to enable convenient monitoring from the office.

With this contribution we hope to contribute to generalized solutions to measure evaporation or, in general, to inspire on methods about how to keep hydrological sensors wet or dry.