News - May 16, 2013

Veluwe itself reason for rainy Veluwe

Altitude and woodland lead to more rain.
Substantial effect especially in winter.

Forests and mountains can influence the local climate, as everyone knows who has been in the Alps. It can be raining on one side of the mountain while you have bright sunshine on the other side. But those are mountains. What is the effect on rainfall of the Veluwe, for example, where height differences are 100 metres at most? But that matters more than you might think, as a study by meteorologist Herbert ter Maat (Alterra) and ­colleagues demonstrates.
Scientists have to rely on models to measure the influence of altitude or land use. After all, you cannot just fell all the trees on the Veluwe or level it out. Ter Maat used a weather model in which he simulated these two extreme situations and compared them to a control set based on real data for an average summer month and winter month in the Netherlands. Ter Maat: 'The difference compared with the control data says something about the contribution woodland or altitude makes to rainfall.'
And that contribution is considerable. Woodland and altitude have a particularly big effect in winter, leading to a difference in precipitation of nearly 18 percent in some places. The extra rainfall in the summer is less: up to 10 percent due to altitude and up to 6 percent due to woods. It should be said that the average effect for the Veluwe as a whole is rather less, with six millimetres of extra rain.
Climate models
Incidentally, the finding that more rain falls on the Veluwe is not new. Measurements by the Royal Netherlands Meteorological Institute give an annual rainfall of up to 75-100 millimetres greater (10-15 percent) than in the rest of the country. Ter Maat's simulations show why that is. Woodland and higher altitude lead to changes in evaporation, vertical wind speeds and turbulence. Those changes affect the formation of clouds and ultimately the rainfall.
So you can influence the weather by planting trees. Ter Maat: 'Woods affect precipitation. Woods are better at holding water than grassland. That means converting grassland into woodland in temperate zones such as here will have an effect.' This knowledge is particularly useful for improving existing climate and weather models. 'You see that you need to include land use and topography properly in your model as they affect the results.'