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[1] The average length of time land-borne compounds remain within an aquatic system is one of the key parameters controlling its biochemical processes. This study explores the magnitude and sources of daily, seasonal, and interannual variability of river water residence time in the Sau Reservoir, a prototypical example of a Mediterranean water supply. Daily estimates of residence time from 1998 to 2005 were obtained from a series of tracer experiments simulated with a one-dimensional physical model, based on actual observations and synthetic scenarios. Results highlight that multiple in situ factors, both natural and managed by humans, affect the residence time in reservoirs. Simulated residence times varied on average $30% on a daily basis, as a result of natural meteorological (17%) and river inflow temperature (11%) variability. The management of withdrawal depths largely controlled the seasonal variations: The practice of withdrawing water from the shallowest outlet, which was close to the intrusion level of summer inflows, promoted shorter residence times in summers than winters. Interannual variability was primarily associated with the natural variability of inflow volume (25%), and secondarily with surface meteorology (8%). Excessive withdrawals prior to and during long dry periods, however, drastically reduced the reservoir storage capacity and withdrawal options, leading to shorter residence times in dry years. The flushing time, calculated as the ratio of storage volume and flow rates, captured the trends in annual mean residence times reasonably well but not daily and seasonal residence times.

The goal of the research was to identify the mixing features that distinguish wind forced, medium‐size fjord‐type lakes near the Arctic Circle from systems of similar geometry, but in a temperate climate. Experimental data and the results of 3D numerical hydrodynamic simulations were analyzed for Lake Lagarfljót (27 km long; 2 km wide; 110 m maximum depth; 65°N) during the 5‐month ice‐free period in 2009. The results showed that probably the most distinctive feature of arctic lakes is their low water column stability, one order of magnitude lower than that of mid‐latitude lakes, even in mid‐summer. The second characteristic is the high rate of energy imparted by wind in the Arctic, estimated as one order of magnitude larger than in a temperate climate. Frequent wind events with Lake and Wedderburn numbers below 1 occurred during the ice‐free period, leading to a strong shear at the base of the surface mixed‐layer and the upwelling of deep metalimnetic layers. As a result, nearly continuous density stratification developed in the shallowest 70 m of the water column, and longitudinal temperature gradients may therefore be very significant. On average, 15% of the wind forcing drove large‐scale internal motions. This energy was rapidly dissipated, partly as a result of stronger bottom velocities, nonlinear surges, and hydraulic jumps that repeatedly formed in the lake. The initial V1H1 setup rapidly evolved into spatially complex oscillatory modes, which, given the high latitude, are affected by the Earth's rotation, even in narrow basins.

Short-duration precipitation bursts can cause substantial property damage and pose operational risks for wastewater managers. The objective of this study was to assess the present and possible future flood hazard in the combined sewer system in Reykjavik city centre. The catchment is characterised by two hills separated by a plain. A large portion of the pipes in the aging network are smaller than the current minimum diameter of 250 mm. Runoff and sewer flows were modelled using the MIKE URBAN software package incorporating both historical precipitation and synthetic storms derived from annual maximum rainfall data. Results suggest that 3% of public network manholes were vulnerable to flooding during an 11-year long rainfall sequence. A Chicago Design Storm (CDS) incorporating a 10-minute rainfall burst with a 5-year return period predicted twice as many flooded manholes at similar locations. A 20% increase in CDS intensity increased the number of flooded manholes and surface flood volume by 70% and 80%, respectively. The flood volume tripled if rainfall increase were combined with urban re-development, leading to a 20% increase in the runoff coefficient. Results highlight the need for reducing network vulnerabilities, which include decreased pipe diameters and low or drastically varying pipe grades.

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