Quality and trends of groundwater-dependent ecosystems, 2016

Heath and marsh ecosystems most affected by water table drawdown

In many areas the water table has been lowered for agricultural and residential land uses or by drinking water abstraction, which can lead to falling groundwater levels in natural areas as well. Lowered groundwater levels are one of the aspects of the drying out of wet natural habitats. Some plant species are dependent on a high groundwater level, especially in the spring, such as species of wet heath, wet grassland and moist forest and carr ecosystems.
The national trends in average spring groundwater level are stable in semi-natural grassland and open dunes. The trends in heath, marsh and forest ecosystems are slightly downwards. Marsh ecosystems are particularly affected by falling groundwater levels, causing the drying out of soils. This reduces the availability of soil moisture to plants and has other effects, such as changes to their physical, hydrochemical and microbiological properties. A possible explanation for the reduction in moisture availability, together with a reduction in nutrient availability in marshland, is the process of natural ageing, in which the accumulation of organic matter lowers the groundwater level and as a result the influence of rainwater is increased. The marsh becomes slightly drier, nutrient poorer and more acidic. Sites with moderate or bad groundwater levels are found throughout the country and in all ecosystem types. The average spring groundwater level is too low in many marshes, wet heaths and raised bogs.

Results of policy to reduce drying-out

The Netherlands has international commitments to the goals of the Convention on Biological Diversity, the Birds and Habitats Directives (Natura 2000) and the EU Biodiversity Strategy. In the Nature Pact (Natuurpact) the national and provincial governments have agreed to raise ecological quality through the creation of the National Ecological Network and through more intensive restoration and management measures to improve water and environmental conditions.
Since 1990, various measures have been introduced to prevent further water table drawdown and the drying out of areas, and to restore hydrological conditions. However, previous surveys of water table drawdown and its effects by the Association of Provinces (IPO) and the Institute for Inland Water Management and Wastewater Treatment (IPO/RIZA, 2005) as well as progress reports for the Rural Areas Investment Budget (ILG) have shown that progress has been slow and the measures taken have not been sufficient for the task at hand. One of the reasons for this is that land holdings had to be purchased before the water levels could be raised. In addition, securing support for the measures has proved to be problematic, because the consequences for land uses are considerable (such as agricultural damage resulting from higher groundwater levels and wet soils).
Under the Nitrogen Reduction Programme (PAS) the provinces and the national government are currently examining how restoration measures in existing natural areas can improve environmental quality. Many of these measures are also aimed at combating water table drawdown, and prevent the drying-out of areas. This is because soil acidity and nutrient availability depend not only on the current deposition of eutrophying substances, but are also influenced by the lowering of water tables and the quality of surface water and groundwater.

Presence of plants indicative of prevailing environmental conditions

Direct measurements of soil and water (groundwater levels) can be used to determine the extent to which changes in environmental conditions occur and whether or not current conditions are suitable for the sustainable conservation of ecosystems. However, the availability of measurement data is limited. Environmental conditions can also be estimated indirectly from the presence of certain plant species and changes in species composition, because plants have specific environmental requirements. The vegetation composition found in an area is therefore indicative of the prevailing environmental conditions at site level.
In the 'monitoring and evaluation methodology' (WMBN), the provinces and conservation management organisations have set out how environmental conditions can be determined. In anticipation of the first set of monitoring data from the provinces, these and the two indicators below illustrate current environmental conditions.

• Environmental quality of terrestrial ecosystems: nitrogen deposition 1994–2022
• Ecosystem quality and trends in soil pH, 2018