The Volkerak-Zoommeer is a former estuary located the Rhine Meuse Delta in the southwestern part of the Netherlands. After the completion of the Volkerakdam, Philipsdam and Oesterdam as part of the Delta Works, this estuary was cut of from the Oosterschelde and a fresh water lake was formed in a sea water environment. This new lake became the Volkerak-Zoommeer: a lake that acts as a fresh water supply for the regional water systems of the water boards Hollandse Delta, Scheldestromen and Brabantse Delta as well as providing a tidal free shipping connection between Antwerp and the Rhine. The operational water manager of the lake is Rijkswaterstaat. They manage the water level and water quality by letting fresh water in from the Hollandsch Diep at the Volkeraksluizen and release it at Bath into the Westerschelde. This mechanism enables Rijkswaterstaat to flush the system and adhere to the target levels, as well as managing a maximum chloride concentration of 450 mg Cl/L during the growing season. In 2018, the Netherlands experienced one of the most severe droughts since the beginning of weather measurements started. Low discharges at the Rhine resulted in a decreasing water supply and more salinization in the Rhine Meuse Delta. When the discharge of the Rhine at Lobith reach below 800 m3/s, it is no longer allowed to let in fresh water through the Volkeraksluizen. This could lead to problems in the fresh water supply to the regional system of the water users of the Volkerak-Zoommeer. The question came up whether creating a fresh water buffer by temporarily heightening of the water level could be used to overcome a period with no fresh water supply from the Hollandsch Diep. In autumn 2020, the dynamics of the system were researched during a practical trial. The water level was heightened up to +0.15 m NAP. Next, the inlet was closed and it was measured how long it would take until the water level reached -0.10m NAP with normal flushing operations at Bath. During a second trial, the outlet at Bath was closed too. Based on the insights of these trials, a prototype for a Decision Support System was created. This system is able to give insights in the development of the water level at the Volkerak-Zoommeer according to operational water management decisions. This Decision Support System was used to evaluate the impact of delta and climate scenarios in 2050 and 2085 on the water level of the Volkerak-Zoommeer. The maximum possible duration for a period without a fresh water supply was researched per scenario within the target levels of -0.10 m NAP and + 0.15m NAP. Different flushing regimes at Bath were evaluated. For normal flushing operations, this duration is 5 till 7 days. For flushing every second low tide, this period could be lengthened to 8 to 13 days. When the outlet at Bath is closed, the maximum duration is 20 till 60 days. From the trials, it was found that the chloride concentration will gradually increase in this period. Unfortunately, sufficient insights in the dynamics of the chloride concentration lack at the moment to sufficiently incorporate the chloride concentration in the prototype. From historical data, the maximum duration of a period with discharges lower than 800 m3/s measured at Lobith was 16 days during the growing season and 82 days for all year data. Within the current target levels, this duration cannot be reached. It was therefore researched what initial water level theoretically could be achieved based on historical data. This turned out to be +0.50 m NAP. Here, the Volkerak-Zoommeer was considered as a closed system, neglecting the open connections with the Dintel and the Vliet. With an initial water level of +0.50 m NAP, the duration without fresh water supply could be significantly lengthened. With normal flushing operations, the period can take between 12 and 19 days. For flushing operations every second low tide, this period takes 20 till 34 days. When the outlet at Bath is closed, a period up to 120 days could be bridged.

Like other cities in Kathmandu Valley, Bhaktapur faces rapid urbanisation and population growth. Unsafe, new settlements are partly located at the floodplains and the government lags behind in implementing proper land-use policy to control unrestrained settlement. The rivers are not only constrained by uncontrolled settlements, but also by insufficient width and freeboard of bridges, and waste blockages causes problems. Combined with more extreme rain events during the monsoon due to climate change, flooding has become a reoccurring problem in Bhaktapur. To gain better understanding of the river and the corresponding flood risk, historical data is essential. Unfortunately, historical databases of water levels are non-existent for this river. Only starting from monsoon 2019, water levels and discharge have been measured on a regular basis. To reconstruct the missing historical data for a return level analysis, this research introduces the Classical Model for Structured Expert Judgment (SEJ) in combination with citizen science (CS). The objective of this research was to use Structured Expert Judgment in a flood risk analysis for the city of Bhaktapur. As a result of using SEJ, we were able to obtain sufficient water level data and estimate the return levels of extreme water levels of Hanumante river by fitting a Generalized Extreme Value distribution (GEV). This eventually led to a reverse Weibull fit, which in this case does not seem accurate. This research discusses in detail the advantages and issues of using Structured Expert Judgement in situations like this and also discusses the reliability of the results.

The Kathmandu Valley in Nepal is facing the combined effects of population growth, rapid urbanization, economic development, and climate change. This results in serious water management challenges: growing freshwater demands, declining water tables, drying of streams, and deteriorating water quality. Insufficient surface water supplies have led to increased reliance on groundwater, especially during the dry winter and pre-monsoon seasons (November - May). Despite groundwater’s importance, it is sparsely measured, poorly understood, and insufficiently managed. As it is difficult and costly to measure all groundwater extractions in the Valley, a water balance approach is an alternative method to estimate total net groundwater pumping. Therefore, the aim of this research was to develop and evaluate potential methods for quantifying total pre-monsoon baseflow supplies by extrapolating baseflow measurements of a subsample of watersheds to unmeasured watersheds. Estimated baseflow was used, together with other water balance fluxes and changes in storage, to evaluate net groundwater pumping in the Valley. Three different methods were used: (1) Spatial Analysis, (2) Regression Model, and (3) Black Box (machine learning). All methods relied on streamflow data from 2017 to 2019, collected by citizen scientists from S4W-Nepal. Based on the three methods we presented, we cautiously conclude that it is possible to determine the pre-monsoon baseflow contributions from a sub-sample of head water catchments. Total baseflow estimates for the Valley using Spatial Analysis, Regression Model, Black Box were 2.32, 2.30, 2.65 m3/s respectively. These values show orders of magnitude that correspond with expected values. By using the average baseflow values of all three methods, we were able to close the water balance and make an assumption for the net groundwater pumping in the Valley. Based on a population of 3.5 million, a net groundwater extraction of 96 L/person/day during pre-monsoon was found. This striking outcome emphasizes the need for more discharge and groundwater extraction measurements, to decrease the uncertainties and to refine the methods.