Conclusions

The main objective of the study was to evaluate the effect of rising groundwater levels on agricultural production in Flanders, and to provide a modelling tool that policy makers and researchers can easily use to predict those effects. We conducted an extensive literature review on the impact of too wet conditions on agriculture and nutrient mobilization, and on the opportunities and obstacles for wet agriculture (paludiculture) in Flanders. We applied the SWAP-WOFOST model to the entire Flemish agricultural area for five conventional crops: grass, fodder maize, potato, winter wheat and sugar beet, using public data layers. We used the agricultural area around De Zegge-Mosselgoren as an example of how the model can also be used locally to estimate the impact of groundwater management on agriculture. The model and corresponding documentation is freely available in the PEILIMPACT github repository.

Based on this study, the following main conclusions can be drawn:

Literature review

• Too shallow groundwater levels cause yield reduction since most of the arable crops are sensitive to oxygen stress, and wet conditions may lead to weed, disease and pest proliferation. It also affects agricultural practices involving the use of machinery, because wet soils have less carrying capacity.

  For example, the land can be too wet for plowing or harvesting, leading to delays in sowing or harvesting.

• Soil texture largely determines how much water can be stored in the soil and how much of it is available to plants. It also determines how roots develop and thus to what extent plants gain access to soil water.

• Too wet conditions lead to insufficient oxygen in the soil, which drastically changes its physical and electrochemical characteristics. In these new conditions, adsorbed phosphorus and organic carbon substances are more mobile, and can be diffused to surface waters or groundwater.

• Wet farming or “Paludiculture” can be an alternative to conventional agriculture in areas where rewetting projects are foreseen, with several economic and ecological benefits. The small-scale agricultural areas in Flanders can be a limiting factor for paludiculture to become profitable at industrial levels. However, it could be more suitable at a local scale. Research/pilot projects would aid in determining if paludiculture in all can offer a sufficiently robust and profitable revenue model in the Flemish conditions.

Regional analysis & Plausibility check

• Regionally, droughts cause higher yield reduction in crops than wet conditions, and among crops, potato, silage maize and sugar beet are more sensitive to water stress compared to grass and winter wheat. The high variability of weather conditions, soil and groundwater levels mostly determine the temporal and spatial yield variability. Yields are normally higher in areas with sandy loam and loamy soils than in clayey soils due to less root growth restrictions by the soil.
• Average groundwater levels less than 1 m below the soil surface generally have a negative effect on yield in wet years, but benefit in dry years. The optimal groundwater level is on average 1 m in normal and dry years, and 1.5 m in wet years. These thresholds can widely change due to variability introduced by crops, soils, groundwater dynamics and weather. It is therefore advisable to take this complexity into account and not to rely on these general guidelines in concrete cases.
• The plausibility check of the model showed that it is able to describe general multi-annual trends in average crop yield, despite many limitations in the input data and model simplifications. However, the model tends to underestimate the crop yield, except for grass. The underestimation was large for sugar beet. This can be attributed to the use of outdated crop parameters, missing site-specific field management information (e.g. irrigation), a limited yield data set for validation and the uncertain difference between harvest results from test plots compared to farmer’s fields.

Case study “De Zegge”

The model was applied to the study case De Zegge, for grass and silage maize, where the nature reserves De Zegge and Mosselgoren are situated and surrounded by agricultural lands.

• Shallow groundwater levels in the study area benefit crop production in dry years, but cause oxygen stress in crops in wet and slightly wet years. The total yield reduction caused by too dry or too wet conditions, and by indirect effects (e.g. less workability, harvest delays) is normally no higher than 30 % for the two crops in consideration, for the current climate and water management in the area.

• Currently, field management and groundwater level control in the area are optimal for the cultivation of grass and silage maize, especially in dry years. Detailed conclusions of the impact of rising groundwater levels due to rewetting strategies on agriculture in the study area could not be drawn from this study, since groundwater level scenarios from the ecohydrological study in the area were not available up to the conclusion of this project.