Controlled drainage with subirrigation (CD-SI) could be a viable measure to i) retain, ii) recharge, and iii) discharge recharge fresh water in agricultural fields. Four field CD-SI pilots with varying geohydrological conditions in the sandy Pleistocene uplands in the Netherlands were monitored (minimum 5 years) to study the effects on groundwater level, soil moisture content and soil water potential. Measurements include time series of groundwater level, soil moisture content, ditch level and CD-SI crest level, pit levels, and water supply. Field data were used for dynamic modelling with the agro-hydrological model Soil-Water-Atmosphere-Plant (SWAP). Calibrated SWAP models were used to i) model CD-SI systems dynamically and ii) model the hydrological consequences of subirrigation.
Field measurements on four experimental plots showed that the water supply by to CD-SI systems can be high (ranging between roughly 500 mm to 1000 mm in the field sites), but CD-SI systems are able to raises the groundwater level such that soil water availability for crops increases. Furthermore, this study showed that CD-SI systems alter the hydrological fluxes significantly. Comparison of the four experimental fields also showed that a resistance to downward flow is needed to reduce downward seepage losses. Excessive downward seepage, or drainage losses towards surface water, increase the required water supply. However, unnecessary ditch drainage losses can be avoided by adapting the surface water level to the groundwater level.
The data of these field experiments are required to understand the real-world situation better, leading to better models in terms of schematization, processes modelled, and model parameter values. This study showed that field pilots varying in geohydrological conditions could be modelled acceptably well with using SWAP. Both the required water supply and the water level in the control pit of the CD-SI system were simulated dynamically, which is a key element in understanding the functioning of CD-SI systems.
The process-based model results lead to insight in the water balance components, also those components that cannot be (easily) measured in the field, and for in (extreme dry or wet) meteorological conditions that were not part of the experimental periods. Based on this research a number of recommendations are given to improve the implementation and operation of CD-SI systems.
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