Forests are known to have a negative effect on water yield (the total annual streamflow). However, little is known about the effect of forest on hydrological droughts. Considering the relatively high evapotranspiration of forest, hydrological droughts might be expected to increase when forest establishes.
The effect of forest on hydrological drought is difficult to detect in larger basins, because most basins have mixed land cover. However it can be investigated with data of large scale lysimeter experiments. A thirty year record of daily drainage data was collected from the lysimeters in the dunes at Castricum (The Netherlands), starting in the 1940s. These are four large concrete basins of 25 x 25 m and 2.5 m deep which were filled with sand. One of the lysimeters was left bare, while the others were vegetated with dune shrubs, deciduous forest and coniferous forest respectively. Drainage was measured at the outlet at the bottom of the basin.
To identify the drought periods a varying threshold method based on the 20th quantile of the lysimeter with bare sand was used, since this was the common reference at the start of the experiment. This means that 20% of the time the drainage of this lysimeter is below the threshold value in this time of the year. Consequently, hydrological drought characteristics were calculated and compared between the different lysimeters.
Results show very clear effects of forest on hydrological drought. In the forested lysimeters, more days per year were identified as drought. Also the average drought duration was longer: 69 days in deciduous forest and 128 days in coniferous forest, compared to only 17 days for bare soil. Finally, the annual water deficit was 7 times higher under deciduous forest than under bare soil. Under coniferous forest this was even 11 times higher than under bare soil. These results are showing the big influence of forest on hydrological drought and the substantial difference between coniferous forest and deciduous forest in this respect. Also the effects were bigger when the forest was older than in the first period after planting.
These results show that land use changes can have big effects on the occurrence and intensity of hydrological droughts. Loon et al. (2016) argue that more attention should be given to the role of human action in drought development. The important effect of land use type on hydrological drought supports their statement that drought cannot be seen as a phenomenon caused purely by natural processes, but that human actions should also be taken into account.
Analogous to the way drought and society influence each other, these results also stress the bilateral dependency between drought and vegetation. While forests can be seriously affected by drought, also the opposite process is going on.
The results presented here are based on BSc thesis research by Anne Hogenboom. The link to the complete thesis can be found below.
Further reading
Van Loon, A. F.; T. Gleeson; J. Clark; A. I. J. M. van Dijk; K. Stahl; J. Hannaford; G. Di Baldassarre; A. J. Teuling; L. N. Tallaksen; R. Uijlenhoet; D. M. Hannah; J. Sheeld; M. Svoboda; B. Verbeiren; T.Wagener; S. Rangecroft; N.Wanders & H. A. J. van Lanen (2016), Drought in the Anthropocene. Nature Geoscience, 9, 89–91, doi:10.1038/ngeo2646.