The urban heat island effect was first described 200 years ago, but the development of ways to mitigate heat in urban areas reaches much further into the past. Uchimizu is a 17th century Japanese tradition, in which water is sprinkled around houses to cool the ground surface and air by evaporation. Unfortunately, the number of published studies that have quantified the cooling effects of uchimizu are limited and only use surface temperature or air temperature at a single height as a measure of the cooling effect. In this research, a dense three-dimensional Distributed Temperature Sensing (DTS) setup was used to measure air temperature with high spatial and temporal resolution within one cubic meter of air above an urban surface. Six experiments were performed to systematically study the effects of (1) the amount of applied water; (2) the initial surface temperature; and (3) shading on the cooling effect of uchimizu. The measurements showed a decrease in air temperature of up to 1.5 ◦C at a height of 2 m, and up to 6 ◦C for near-ground temperature. The strongest cooling was measured in the shade experiment. For water applied in quantities of 1 mm and 2 mm, there was no clear difference in cooling effect, but after application of a large amount of water (>5 mm), the strong near-ground cooling effect was approximately twice as high as when only 1 mm of water was applied. The dense measurement grid used in this research also enabled us to detect the rising turbulent eddies created by the heated surface.@en

The three-dimensional (3-D) modelling of water systems involving double-diffusive processes is challenging due to the large computation times required to solve the flow and transport of constituents. In 3-D systems that approach axisymmetry around a central location, computation times can be reduced by applying a 2-D axisymmetric model set-up. This article applies the Reynolds-averaged Navier–Stokes equations described in cylindrical coordinates and integrates them to guarantee mass and momentum conservation. The discretized equations are presented in a way that a Cartesian finite-volume model can be easily extended to the developed framework, which is demonstrated by the implementation into a non-hydrostatic free-surface flow model. This model employs temperature- and salinity-dependent densities, molecular diffusivities, and kinematic viscosity. One quantitative case study, based on an analytical solution derived for the radial expansion of a dense water layer, and two qualitative case studies demonstrate a good behaviour of the model for seepage inflows with contrasting salinities and temperatures. Four case studies with respect to double-diffusive processes in a stratified water body demonstrate that turbulent flows are not yet correctly modelled near the interfaces and that an advanced turbulence model is required.@en

An axisymmetric hydrodynamic modelling approach was developed and implemented for a salt and heat transport model. This data set presents the source code and several model results for test cases involving double-diffusion and density-driven flows. The axisymmetric model is built by extending the 2DV solution procedure of the SWASH hydrodynamic model, which is briefly introduced in this document. The added terms are derived in an accompanying article (Hilgersom et al., 2017; under review).@en

This thesis explores a new measuring approach to quantify the seepage flux from boils. Boils are preferential groundwater seeps and are a consequence of the groundwater flow that works its way through the soil matrix by creating vents of higher conductive material. In the Netherlands, boils often occur in deep polders (reclaimed lakes situated 4–7 m below sea level), transporting water directly from the deep aquifer. Because this saline aquifer is connected to the sea, the pressure difference between the sea water level and the polder water level is the main driver of the upward seepage flux. At the surface, boils seep out through canal beds and sometimes on land. This thesis focusses on boils that directly discharge into polder drainage canals. Although boils are usually highly saline compared to the fresh surface water, this research also includes an example of a relatively fresh boil. The seeping groundwater has a fairly constant temperature throughout the year. Because the surface water temperature fluctuates over the year and over the day, temperature is an ideal tracer to measure the groundwater - surface water interaction. Previous studies applied temperature and salinity samples taken at different depths in the soil to quantify the boil seepage flux. Because the boil vents are usually not strictly vertical and can be disturbed when probing the soil, this research aims to measure the boil seepage flux from a surface water perspective. The intended measurement approach samples the surface water at a very high resolution in three dimensions, compares the temperature profiles with those in a free-surface transport model, and infers the boil flux as the bottom boundary flux of the model. Over the past decades, fibre-optic distributed temperature sensing (DTS) has developed toward an effective means to obtain spatially distributed temperature samples. When releasing a laser signal through an optical fibre, the returning signal carries temperature information in its wavelengths. Current DTS machines allow measuring temperature down to every 25 cm along a fibre-optic cable. To obtain even higher resolutions, researchers often wrap cables to a coil. However, cable bends and the construction supporting the coil affect the measurement accuracy. Chapter 3 investigates how cable bends influence the temperature measurements in coil-wrapped DTS set-ups in order to account for this in the design of a highresolution DTS set-up for this research. It is concluded that, with a decreasing bending radius, the cable bends increasingly affect the temperature measurements in multiple ways. The non-linearity in the bend-induced decay of the laser signal complicates compensation for these effects and requires a very careful temperature calibration approach. To avoid continuously bent cables, the design of the three-dimensional (3-D) high-resolution DTS set-up applied a weaving pattern instead of coils (Chapter 4). This way, cables are only bent at each turnaround, intermitted by straight stretches of 1 m. By selecting the desired vertical spacing of the woven ’layers’, one can customize the vertical resolution of the set-up. To infer the seepage flux from the stream bed, the design of the set-up required very high resolutions near the bottom boundary. The set-up proved to measure very detailed temperature profiles in a water body, and even uncovered unexpected seeps in a laboratory set-up to simulate boil seepage. In the field, the measured temperatures near the stream bed displayed an accumulation of sediment around the boil during the measurement periods. Most interestingly for the current application, the detailed temperature profiles were able to capture double-diffusive phenomena. Double-diffusion occurs when two adjacent water layers have different temperatures and salinities, and the density gradients for the temperature and salinity are opposed. For example, when cold (denser) and fresh (lighter) water overtops a warm and saline water layer, a system of convective layers develops with a very sharp temperature and salinity interface between the convective layers (i.e., double-diffusive convection). A more curious phenomenon occurs when the warm and saline layer is on top. In this case, a finger-like pattern develops at the sharp interface between the layers (i.e., salt-fingering). These systems are different from normal diffusive interfaces, which tend to fade over time. Therefore, water bodies with salt and temperature gradients demand a careful modelling of the flow processes. To accurately model the boil-covering water body with large density gradients, a mass and momentum conservative free-surface model was selected. The model was extended with a transport module and modules accounting for temperature and salinity dependent densities, viscosities, and specific heats. Moreover, the model was extended with the option to include atmospheric heat exchange in the calculations. The performance of the model was tested on a solar pond (Chapter 5). Such ponds are double-diffusive convective water bodies with very strong density gradients, which store solar energy as heat in their bottom hypersaline layer. The model well captured the flow of warm water along the sloping edge of the solar pond and demonstrated the onset of small seiches in the pond due to the density gradients. The onset of convective layers was also captured, although their extents were not in complete agreement with measurement data. In general, the results confirmed the model capability to simulate double-diffusive convection. Due to the boil’s circular shape and the availability of 3-D temperature profiles, a 3-D modelling grid would be preferable for the boil seepage simulations. The dense grid needed for the transport simulation, however, yields too large computation times. Therefore, Chapter 6 investigated the potential of a quasi 3-D axisymmetric set-up for these simulations. To this end, the 2-DV model code was extended with few additional terms which hardly increased the computation time and kept the solution procedure mass and momentum conservative. Qualitative case studies demonstrated the model capability to simulate salt-fingers and double-diffusive convection. An analytical benchmark was set up for the axisymmetric expansion of an unconditionally stable layer from a central cold and saline seepage inflow. For the case of laminar flow conditions, the model results were in agreement with the analytical solution. Turbulent convection dispersed heat and salt significantly quicker. The unexpected seeps in the laboratory set-up for boil seepage simulations complicated the comparison of these measurements with model output, because the exact flow paths were unknown and could not be modelled. Chapter 7 shows a comparison of the measurements with model results for the intended seepage flow. Although double-diffusive convective and unconditionally stable layers develop in both the model and the measurement results, the growth rates, and specifically the locations where the layers grow at a faster rate, are different. Moreover, the unexpected seeps seem to have a higher flow velocity, leading to a larger mixing of heat at the interface between the layers. It is concluded that the model can not be validated based on the laboratory data and additional measurements are recommended. Although the horizontal stream flow across the boil should be negligible when applying an axisymmetric modelling approach, knowledge of the stream discharge is still relevant. For this reason, this thesis starts with exploring the possibilities to modernize and potentially automate the rising bubble technique for discharge measurement (Chapter 2). The study shows that the complicated dual camera set-up and position calculations for the air bubbles in previous publications can be avoided with modern image processing algorithms. Reflecting sun light sometimes impedes the visibility of the air bubbles on the water surface. We displayed an example of how a statistical tool still uncovers the signatures of air bubbles in digital images that would normally hardly be visible. Such tools could also be applied in pattern recognition algorithms that automatically find the air bubbles on the water surface. Although further research is necessary, the results seem to support the hypothesis that the rising bubble technique can be applied as an automatic discharge measurement technique. We conclude that the boil seepage inversion from double-diffusive models is currently still very challenging (Chapter 8). The locations and extents of double-diffusive convection cells and salt-fingers are dependent on sub-grid processes. Moreover, these phenomena are very sensitive to local density gradients which will never be modelled ’perfectly’. The importance of model boundary conditions when the layer of seepage water is still thin could also affect the inversion of the seepage flux at the bottom boundary. For all these issues, the local temperature deviations can highly influence the inversion step, yielding a high noise in the outcome. Nevertheless, we see potential in a less complicated inversion of the growth of an unconditionally stable layer above a cold and saline boil after the water body is fully mixed. This approach still requires high-resolution temperature measurements. Further research to this method is recommended.@en

The Urban Heat Island (UHI) was first described 200 years ago, but ways to mitigate heat in urban areas reach much further into the past. Uchimizu is a 17th century Japanese tradition, in which water is sprinkled around houses, temples, and in gardens to cool the ground surface and the air, and to settle the dust. Nowadays, megacities such as Tokyo are aiming to revive the - by modern technology suppressed - method, and uchimizu is promoted by local authorities as a "clever way to feel cool". Unfortunately, the number of published studies that have quantified the cooling effects of uchimizu is limited, and only uses measurements of the surface temperature, or air temperature at a single height, as a measure of the cooling effect. In this research a dense 3D Distributed Temperature Sensing (DTS) setup was used to measure air temperature within once cubic meter of air above an urban surface with high spatial and temporal resolution. Six experiments were performed to systematically study the effect of (1) applied water amount, (2) initial surface temperature, and (3) shading on the cooling effect of uchimizu. We present the results and the subsequent analyses of these experiments, done during summer in Delft, The Netherlands. We show that this simple water sprinkling method has the potential to decrease extreme temperatures in impervious and paved parts of urban areas considerably. Besides mitigating the UHI, uchimizu practice is also an opportunity to increase awareness among citizens, and stimulate citizen participation in solving heat stress problems and energy saving. By providing refreshing insights on the cooling effect of uchimizu, we aim to contribute to the modern revival of this old tradition.@en

The three-dimensional (3-D) modelling of water systems involving double-diffusive processes is challenging due to the large computation times required to solve the flow and transport of constituents. In systems that approach axisymmetry around a central location, computation times can be reduced by applying a quasi 3-D axisymmetric model setup. This article applies the Navier-Stokes equations described in cylindrical coordinates, and integrates them to guarantee mass and momentum conservation. The discretized equations are presented in a way that a Cartesian finite volume model can be easily extended to this quasi 3-D framework, which is demonstrated by the implementation into a non-hydrostatic free-surface flow model. This model employs temperature and salinity dependent densities, molecular diffusivities, and kinematic viscosity. Four qualitative case studies demonstrate a good behaviour with respect to expected density and diffusivity driven flow and stratification in shallow water bodies. A fifth case study involves a new validation method that quantifies the radial expansion of a dense water layer developing from a central inflow at the bottom of a shallow water body.@en

Distributed temperature sensing has proven a useful technique for geoscientists to obtain spatially distributed temperature data. When studies require high-resolution temperature data in three spatial dimensions, current practices to enhance the spatial resolution do not suffice. For example, double-diffusive phenomena induce sharp and small-scale temperature patterns in water bodies subject to thermohaline gradients. This article presents a novel approach for a 3-D dense distributed temperature sensing setup, the design of which can be customized to the required spatial resolution in each dimension. Temperature is measured along fiber-optic cables that can be arranged as needed. In this case, we built a dense cage of very thin (1.6 mm) cables to ensure that interference with flow patterns was minimal. Application in water bodies with double-diffusion-induced sharp temperature gradients shows that the setup is well able to capture small-scale temperature patterns and even detects small unsuspected seeps and potential salt-fingers. However, the potential effect of the setup on the flow patterns requires further study.@en

Fibre optic distributed temperature sensing (DTS) is widely applied in Earth sciences. Many applications require a spatial resolution higher than that provided by the DTS instrument. Measurements at these higher resolutions can be achieved with a fibre optic cable helically wrapped on a cylinder. The effect of the probe construction, such as its material, shape, and diameter, on the performance has been poorly understood. In this article, we study data sets obtained from a laboratory experiment using different cable and construction diameters, and three field experiments using different construction characteristics. This study shows that the construction material, shape, diameter, and cable attachment method can have a significant influence on DTS temperature measurements. We present a qualitative and quantitative approximation of errors introduced through the choice of auxiliary construction, influence of solar radiation, coil diameter, and cable attachment method. Our results provide insight into factors that influence DTS measurements, and we present a number of solutions to minimize these errors. These practical considerations allow designers of future DTS measurement set-ups to improve their environmental temperature measurements.@en

Over the past ten years, Distributed Temperature Sensing (DTS) has been applied for monitoring many different environmental processes, from groundwater movement, to seepage into streams and canals, to soil moisture, and internal waves in lakes. DTS uses optical fibres, along which temperatures are determined by measuring Raman shifts in light that scatters back after a laser pulse has been sent into the fiber. Over the past decade, performance of DTS equipment has dramatically improved. It is now possible to determine fiber temperatures with 0.05 K accuracy, for each 25 cm along a fiber optic cable. With typical spatial resolutions of 1 m, cable lengths can run up to 5 km. Accuracy improves with integration over longer sampling intervals, but measurements over 60 s can give 0.1 K accuracy with proper in-field calibration. DTS can also be used for atmospheric properties such as air temperature, vapor pressure, and wind speed. This presentation provides a complete overview of recent advances in atmospheric DTS observations. Air temperature is the simplest, as one simply has to suspend a fiber optic cable along the profile of interest. This can be from a balloon or along poles. Care has to be taken to correct for radiative heating of the cable. Using a thin white cable minimalizes radiative effects and normally brings the measured temperature to within 1 K of actual air temperature, sufficient for studies on effects of shading in natural and urban landscapes. It is also possible to correct for radiative heating by modeling in some detail the cable’s thermal behavior or by using two cables of different diameters. Supporting structures may also have an effect on cable temperatures, which should be minimized or corrected for. Water vapor can be measured by comparing the temperatures of wet and dry cables. These wet and dry bulb temperatures allow derivation of humidity profiles, which, in turn, allows for Bowen-ratio type of calculations of latent and sensible heat fluxes. This has proven especially useful in otherwise difficult to measure profiles such as through forest canopies. Wind speed can be measured by including a conductive element in the fiber optic cable and heating the cable actively by sending a current through that element. In effect, the cable then acts as a hot wire anemometer but then over long lengths of cable and with high spatial resolutions. When carefully executed, experiments with heated cables give very detailed insight into turbulent processes in the lower boundary. It is even possible to resolve bigger individual turbulent and sub-meso-scale eddies for studying fast evolving fluid flows (orders of seconds). A comprehensive overview of atmospheric applications will be presented, together with pitfalls, common errors, and practical tips to avoid those in the field. @en

Already in the 19th century, d’Auria described a discharge measurement technique that applies floats to find the depth-integrated velocity (d’Auria, 1882). The basis of this technique was that the horizontal distance that the float travels on its way to the surface is the image of the integrated velocity profile over depth. Viol and Semenov (1964) improved this method by using air bubbles as floats, but still distances were measured manually until Sargent (1981) introduced a technique that could derive the distances from two photographs simultaneously taken from each side of the river bank. Recently, modern image processing techniques proved to further improve the applicability of the method (Hilgersom and Luxemburg, 2012). In the 2012 article, controlling and determining the rising velocity of an air bubble still appeared a major challenge for the application of this method. Ever since, laboratory experiments with different nozzle and tube sizes lead to advances in our self-made equipment enabling us to produce individual air bubbles with a more constant rising velocity. Also, we introduced an underwater camera to on-site determine the rising velocity, which is dependent on the water temperature and contamination, and therefore is site-specific. Camera measurements of the rising velocity proved successful in a laboratory and field setting, although some improvements to the setup are necessary to capture the air bubbles also at depths where little daylight penetrates.@en