Towards regional upscaling of subirrigation as a tool for agricultural climate adaptation

Abstract

The high Pleistocene sands in the south and east of the Netherlands have been subjected to severe drought in recent years (2018/2019). As a result, agricultural pressure on freshwater resources has increased, with negative impacts on nature as a result. As bans on surface water and groundwater abstraction are looming, it has become evident that there is a need for farmers to adapts to the fickle (political) climate. In recent years, KWR has been implementing subirrigation (sub-surface irrigation) as an alternative and addition to conventional irrigation at the field scale under the project Klimaat Adaptatie in de Praktijk (KLIMAP). KWR has identified it as a promising tool for regional agricultural climate adaptation. From the field scale modelling and measurements, it was however evident that subirrigation requires substantial amounts of water during the growing season when the resource might be the most limited. Additionally, subirrigation has been shown to impact all components of the field scale water balance which might result in challenges for water management regionally and downstream. A fitting modelling tool to assess the effect of subirrigation when implemented on a regional scale in North-Limburg near the Mariapeel was however not yet in place. Using Vensim PLE system dynamics modelling (SDM) software, this research aimed to set up a new base water balance model that can operate at the scale in between the current SWAP and national hydrological models (LHM). It did so by simplifying the previous water balance model SWAP for the field scale and explored expanding it to link multiple fields to the regional surface water system. Multiple scenarios for field scale parameterisation and regional implementation of subirrigation were run. This research aimed to answer the following research questions: 1) How can the hydrological processes at the field scale, and the impact of subirrigation on these processes, be translated into a simplified qualitative field scale water balance at both field and regional scale? 2) How can the influence of subirrigation on the field- & regional scale water balance be modelled quantitatively using system dynamics modelling in Vensim? And 3) How can the developed Vensim model be applied to explore scenarios for challenges/opportunities faced during the regional upscaling of subirrigation in Limburg? The new Vensim model turned out to be capable tool, with the modelled groundwater levels closely matching those as modelled in SWAP. For the area of interest, linear extrapolation of the modelled water balance results translated into a water requirement for subirrigation of 3.3 million cubic metres of water during the growing season. This is however an overestimation, as the regional scenario modelling in this research showed the side-effect of subirrigation by increasing groundwater levels in adjacent fields. Widespread subirrigation could have detrimental effects on downstream surface water availability in times of drought and in cases of large abstractions where demand exceeds supply of surface water. Maintaining higher surface water levels might reduce subirrigation water requirements but could be difficult in times of drought. Improvements to the model still can be made. Future research should focus on improving the field scale model representation of the unsaturated zone, improving the representation of field area in the model and on improving the parameterisation of the hydraulic conductivities within the soil. The next challenge will be to link multiple fields using the linking types as explored in this research and recreate a real-life area of interest for Waterschap Limburg to more precisely grasp water distribution challenges and requirements in case of large-scale implementation of subirrigation