Experiences from recent droughts show that elevated sandy areas in the southern and eastern Netherlands are vulnerable to drought (see e.g. Buitink et al., 2020). Transporting water to these areas is difficult due to their elevation and distance to river systems. In addition, the use of surface water for functions such as drinking water supply, agriculture, navigation, (aquatic) ecology and water quality increases the pressure on water availability. To create a more resilient and sustainable water system, more water can be retained in backwater areas to reduce the impact of meteorological droughts on the hydrological system.
The current surface water system design in these areas is based on high flow standards without sufficiently taking low flow conditions into account, which often leads to excessive runoff. Reducing the cross-sectional area and drainage depth of the drainage network reduces groundwater runoff and increases infiltration. This makes it possible to store more water in backwater areas and increase drought resilience.
This methodology is applied earlier by the Dutch Waternood project (see Voorbeeldenboek Waternood) were the width-depth ratio of the cross-sectional profile is used to increase or reduce groundwater to surface water exchange. A higher width-depth ratio increases the exchange capacity, where a lower width-depth ratio decreases this exchange capacity. The bed level of the surface water system can also be used to manage runoff. Groundwater runoff only occurs when the groundwater level rises above bed level. By elevating the bed level, the groundwater levels remain higher, which in turn results in larger water availability.
In order to explore the potential for drought mitigation, the Waternood methodology is applied in a case study for an agricultural area in the province of Overijssel in the Netherlands. It was found that groundwater levels increased only a few centimetres when elevating the bed level of the surface water system by 25%. The largest increase was found during average winters and the smallest increase during dry summers. The small effect on groundwater level increase can be caused by the largely decoupled groundwater-surface water system during summer and the resulting low discharge of the surface water system. Due to the deep groundwater table (mostly 3m below surface), the bed level of the surface water system is rarely below the groundwater level which limits drainage. Therefore, another scenario was explored where the surface water system was totally removed to ensure all precipitation would infiltrate and could only discharge towards the river system through groundwater flow. This did significantly increase groundwater levels. Again, the largest effect was visible during winter and for average years. The smallest effects were found during summers of dry years. Crop yield estimations therefore largely decreased, because the small decrease in drought stress was countered by the larger increase in oxygen stress.
Overall, the effects of changing the cross-sectional profile on groundwater level increase and crop yield were limited. However, in other areas the effect may be larger due to stronger coupling between the groundwater and surface water system. The difference in groundwater level can be more visible on smaller timescales during the start of the growing season. In addition, raising the groundwater level can have consequences for locally depressed areas which may become unsuitable for (certain types of) agriculture. Elevating the bed level of drainage channels can be combined with adjustable weirs to optimally manage water retention and allow regulated discharge. Overall, these measures can provide a small but significant contribution to mitigation of hydrological drought in freely-draining parts of The Netherlands.
The results presented here are based on MSc internship research by Robert Lubben. The link to the complete internship report can be found below.
Further reading
Buitink, J.; A. M. Swank; M. van der Ploeg; N. E. Smith; H.-J. F. Benninga; F. van der Bolt; C. D. U. Carranza; G. Koren; R. van der Velde & A. J. Teuling (2020). Anatomy of the 2018 agricultural drought in The Netherlands using in situ soil moisture and satellite vegetation indices. Hydrology and Earth System Sciences, 24, 6021–6031, doi:10.5194/hess-24-6021-2020.