Executive summary

Executive Summary¶

Problem Statement¶

Climate change is causing longer periods of drought, alternating with heavy rainfall. This has a significant impact on agriculture and its negative effects have already been observed in the agriculture sector in W-Europe during the dry summers of 2017, 2018, 2019, 2020, and 2022, and this will most probably continue in the future. The Flemish coalition agreement 2019-2024 focuses on proactive measures to cope with the effects of climate change. It places a strong emphasis on increasing our resilience to drought through the creation of additional wet nature or restoration/remediation of drained wetlands to promote infiltration and water storage. This means that farmers and policy-makers do not only need to adapt to an increased occurrence of droughts, but probably also to the impacts of excessive soil water (too wet conditions) in agricultural areas close to restored wetlands. However, little information is available to estimate the impact of high groundwater tables on agriculture due to the implementation of these adaptation measures.

In this study, we developed a modeling framework to estimate the impact of groundwater levels on the yield of conventional crops in Flanders. The joint model SWAP-WOFOST, behind the Dutch initiative WaterVision Agriculture, was used to simulate the crop yield and yield reduction due to drought (too dry) and oxygen stress (too wet), for five main crops in Flanders: grass, silage maize, potato, winter wheat, and sugar beet using historical data. This model also allows us to include the effect of restrictions in normal agricultural practices due to too-wet or too-cold conditions, called indirect effects. Too-wet conditions in the root zone begin when crops start experiencing oxygen stress, that is when oxygen availability is lower than the oxygen demand of plant roots.

Freely available (online or on-demand) datasets and maps for the entire Flanders region were used, obtained from Flemish institutions or previous projects. We compiled and used a database with experimental yield observations in Flanders to evaluate the performance of the model under Flemish conditions. We also wrote three literature review chapters on the effects of groundwater on agricultural practices, the effect of shallow water tables and rewetting on nutrient mobility, and the potential of paludiculture in Flanders. Finally, the model was applied to the agricultural area around De Zegge-Mosselgoren, near Geel.

Results Model Simulations with the PEILIMPACT modeling framework¶

Regional¶

At the regional level, the yield variability is highly influenced by the regional weather variability, soil heterogeneity, and water tables. Droughts affect silage maize, potato, and sugar beet yields more than wet conditions. Areas with sandy loam and loamy soils typically have higher yields than clayey soils, since they are more favorable for root growth than other soils. Shallow groundwater levels negatively affect yield in wet years, but crops can benefit from them in dry years. Just as the yield decreases with deeper water tables, it also decreases when water tables become too shallow. Deeper water tables result in higher yields in wet years, since more precipitation compensates for the low groundwater contribution to crop root water uptake. The extent of this effect depends on the soil texture and the crop rooting pattern.

Plausibility check¶

The results of the plausibility check of the model demonstrated that the current model is able to describe the general multi-annual trends in average crop yield, despite many limitations in the input data and model simplifications. Absolute values are sometimes underestimated, especially in sugar beet, where an improved yield database and/or targeted field experiments to calibrate and validate the model are needed to get more accurate results.

Case- study¶

In the case study De Zegge-Mosselgoren, shallow groundwater levels benefit crop production in dry years, but cause oxygen stress in wet years. The total yield reduction caused by too-dry or too-wet conditions, and by indirect effects is typically lower than 30 % for grass and silage maize. Under the current situation, field management and specifically groundwater level control in the area are close to optimal for agricultural activities in dry years, but already cause restrictions in wet years. In general, oxygen stress is the main cause of yield reduction in this area. Detailed conclusions of the impact of rising groundwater levels due to rewetting strategies on agriculture cannot yet be given, since groundwater scenarios were not available during the project duration.

Main points literature review¶

Cultivation factors¶

During droughts, shallow groundwater levels benefit crops by replenishing soil moisture through capillary rise. However, negative effects on crop production may arise because most of the arable crops are sensitive to oxygen stress, and wet conditions may lead to weed, disease and pest proliferation. Too shallow groundwater levels also affect the agricultural practices involving machinery, because wet soils have less carrying capacity. Soil texture plays a role because soil water retention characteristics regulate water flow through the root zone.

Nutrient mobilisation¶

Higher groundwater levels 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. This will depend on the phosphorous availability in the soil. Leaching of soluble nitrogen is typically lower and mostly lost as gas, with less of it available in the soil.

Wet farming and Paludiculture¶

Paludiculture can be used as an alternative to conventional agriculture in areas where rewetting projects are required. These crops can guarantee the production of biomass for various industrial purposes and can also form a transition between cultivated land and wet nature, and also provide water purification and water buffering.

Knowledge of cultivation practices and adapted machinery, along with market opportunities are crucial to encourage farmers to make a transition towards these crops. In Flanders, paludiculture is not well known and more research/pilot projects are needed to determine which paludicrops are more suitable for the Flemish conditions.

The small-scale agricultural areas in Flanders can be a limiting factor for paludiculture to become profitable at industrial levels, processing and use at local scale can be more suitable.

Conclusions and Recommendations¶

Groundwater levels have an important indirect effect on crop yield, causing oxygen stress when they are too shallow. However, the “optimal” water levels change drastically with variability introduced by crops, soils, groundwater dynamics, and weather. These thresholds can be assessed in a context-specific way using the model framework developed in this study. However, there is room for improvement. Further work by the research community on this framework would enable the model to give more realistic and robust results. Improvements include updating crop and soil parameters and using groundwater level dynamics when relevant information becomes available and gathering additional yield data from farmer fields or targeted field experiments, for model calibration and validation. In the study case De Zegge, the impact of specific future rewetting scenarios in the nature reserves on the agricultural activities should be assessed once these scenarios are available. The required model framework is available on Github: https://github.com/ILVO-PEILIMPACT/model_users_growing_season.