The relationship between ditch water level and peat subsidence: 50 years of measurements with levelling and subsidence plates in the Netherlands

One of the consequences of deeper drainage of peat soils is the increase in surface level subsidence. Over a period of 50 years, soil surface subsidence and subsidence at different depths in the soil profile were measured using levelling and subsidence plates at 29 sites in seven locations in the Netherlands. Over those 50 years, the surface level for these sites has dropped by an average of 0.7 cm/year. A significant relationship between the measured ditch water level and the soil surface level subsidence rate was found. The subsidence plates, which were installed at different depths, allowed us to compare the contribution of the various soil layers to the subsidence of the surface. This showed that about 2/3 of the subsidence took place in the layers, where oxygen can penetrate (<0.8 m below soil surface). In those layers, oxidation and shrinkage are the most important subsidence processes. In the layers that are always saturated, 1/3 of the ground level subsidence still occurred, which is mainly caused by consolidation and creep but with a contribution of anaerobic decomposition. Finally, the results showed that subsidence rates during the period 1970–1990 were on average higher than during the period 1990–2024. Variability over sites was large, illustrating that site-specific conditions have a large impact on soil subsidence rates. The implications of our research are that peat subsidence monitoring, in the Netherlands and elsewhere, has to (1) be long-term, (2) monitor at different depths and (3) include multiple sites.