<p>In large parts of Europe, the year 2018 is known as an extremely dry year. In the Netherlands this 2018 drought caused over 1 billion euros of economic damage to different sectors like agriculture, nature, industry, shipping, infrastructure and buildings. A large part of economic damage was due to extreme low groundwater levels and large soil moisture deficits. Many streams stopped flowing since groundwater levels were too low to feed the streams. The extreme low rainfall amount, in combination with above average high potential evaporation rates, caused a precipitation deficit of 300 mm in the growing season, which is normally less than 100 mm. In 2019, the year after, the spatial variability of precipitation in the Netherlands was high with only a precipitation deficit in the growing season of a few tens of millimeters in the low-lying western part of the Netherlands. However, in the higher sandy areas in the south and east part of the Netherlands, the precipitation deficit was again extreme and more than 240 mm. For the higher sandy areas this was the second dry year in a row and the question arose what the effect of two consecutive dry years on the water system was and how fast it may recover.</p><p>This question has been analyzed by applying an integrated nationwide groundwater and surface water model (De Lange et al., 2014). The model results showed that for the higher sandy areas, groundwater levels and stream discharges were even lower in the second than in the first dry year. In addition, the recovery period of the groundwater system after two extremely dry years was examined by simulating ten "normal" years with average precipitation and evaporation patterns following the two extremely dry years. The model results showed a large spatial variation in groundwater level recovery.&#160; In the first recovery year groundwater levels increased for most of the area, except for the higher-lying sandy areas lacking any surface waters (ditches and streams), like the largest Dutch forest area, the Veluwe. In these slow-responding regional recharge areas, groundwater levels are still dropping. For the central part of the Veluwe, this dropping continues until the seventh recovery year.&#160; The model results showed that two consecutive dry years have a large impact on the water system, and that full recovery of groundwater levels and stream discharges may take 2 to 4 years in most of the sandy areas, yet the recovery of the highest parts may take up to 7 to 8 years.</p><p>&#160;</p><p>De Lange, W.J., Prinsen, G.F., Hoogewoud, J.C., Veldhuizen, A.A., Verkaik, J., Oude Essink, G.H.P., Van Walsum, P.E.V., Delsman, J.R., Hunink, J.C., Massop, H.Th.L., Kroon T. (2014). An operational, multi-scale, multi-model system for consensus-based, integrated water management and policy analysis: The Netherlands Hydrological Instrument. Accepted for publication in Environmental Modelling & Software</p>
Ensemble Data Assimilation for Flood Forecasting in Operational Settings: From Noah-MP to WRF-Hydro and the National Water Model
