New equipment to accurately measure evaporation from crops and nature areas has been developed by Dutch knowledge institutes KWR, Alterra and KNMI, sensor company Eijkelkamp Soil & Water, drinking water company Vitens and the Dutch government. As small errors in estimates of evaporation represent large volumes of water, the evaporation figures obtained with this new technology are of great importance for the water sector and climate studies.
“Evaporation is usually estimated with soil-vegetation-atmosphere transfer models or techniques using satellite imagery,” Bernard Voortman, Scientific researcher–Ecohydrology, KWR Watercycle Research Institute, told The Source. “Validating estimates of evaporation remains challenging, since collecting the ground truth is usually costly and prone to numerous errors, and the mismatch in spatial resolution between estimates and measurements. There is thus a need for a system that can measure evaporation cost effectively, accurately and on a relatively large scale.”
The new device developed, called a Smart Lysimeter, measures evaporation by weighing an undisturbed soil column that is buried into the soil and vegetated at the surface. Weight increases and decreases are transferred into estimates of precipitation and evaporation, respectively.
By comparing the surface temperature of the soil column with the surroundings, the representativeness of the evaporation measurement can be assessed. The first system was successfully installed and tested in heather vegetation in the centre of the Netherlands and delivers daily measurements of evaporation with a precision of 0.1 millimetres.
“By spatially extrapolating the evaporation measurements with a thermal camera, we were able to estimate evaporation for areas of approximately 25 metres squared,” added Voortman. “Such a system could bridge the gap in spatial resolution between ground based point measurements and satellite based estimates of evaporation.”
In most regions of the world, more than half the precipitation water turns into evaporation (evaporation from the soil surface, open water and transpiration by plants). From the perspective of groundwater management, evaporation is considered a loss term in the water balance.
For instance, for one of the largest water bodies in the Netherlands, The Veluwe, an error of perhaps 20 percent in evaporation equals an amount as large as all the drinking water extractions of the area, which is equivalent to the annual tap water use of about 2 million people. Hence, accurate numbers of evaporation are required for the cost effective management of groundwater resources.
“With the advent of this measurement technique, we can reduce the uncertainty in the largest water loss term of groundwater systems: evaporation,” said Voortman. “This is essential in order to optimally organise water management and prepare for big changes in the water cycle, such as those resulting from climate change.”
