Evapotranspiration is nearly always the main source of moisture loss in water balance studies. Most of the precipitation in an area is actually evaporated by crops, trees, natural vegetations and the ground.
In this already completed study, a commercially attractive field set-up was developed for the direct measurement of the real rates of evapotranspiration. This was the objective of the broad partnership between research institutes KWR, Alterrra and KNMI, technology developer and supplier Eijkelkamp Soil & Water, drinking water company Vitens, and the waterboards (via STOWA). One of the end-users, the Province of Gelderland, was represented in the project group alongside the partners; end-user involvement ensured that the project harmonised with their requirements. The measurement of real evapotranspiration with lysimeters (soil columns with a weighing system buried at ground level) provides reliable information for estimates on groundwater recharge, the source of our groundwater and the force driving groundwater flow in the landscape. The downside of the limited amount of lysimeters produced to-date is their large dimension, and their development, installation and maintenance costs. This research targeted the development of a smaller, cheaper, and thus more practical product. In addition, the scaling-up of the measurements to cover a larger surface area was also examined.
The objective of the project was to develop a commercially attractive field set-up for the national and international markets, which would directly measure real evapotranspiration. Besides achieving the technological objectives, we also acquired insight into the evapotranspiration behaviour of grasslands and heathlands. The measurements were also used to compare the different measurement methods with each other.
Two lysimeters were tested at the CESAR observatory at Cabauw. The lysimeters provide information on the evapotranspiration on a daily basis. A second monitoring station, consisting of six lysimeters has been set up in the Hoge Veluwe park The measurements from both stations are compared with other evapotranspiration measurements and satellite images. Our measurements reveal a wide distribution and confirm the uncertainty in evapotranspiration model simulations. To achieve an accurate, national picture of real evapotranspiration, we would therefore argue for a national network of field measurements.
Models, remote sensing algorithms and on-site measurements need to be combined to achieve a calculation of real evapotranspiration which is as optimally comprehensive as possible. Precipitation in the Netherlands is measured at 325 stations; it is now high time that the largest source of moisture loss in the water balance be measured: the real evapotranspiration. This is essential in order to optimally organise water management in the country and for us to prepare for big changes in the water system, such as those resulting from climate change.