Publications of the program graduates

Many students of the program had published papers in different research fields of hydrology. 



Levenson Y, Emmanuel S. Quantifying micron-scale grain detachment during weathering experiments on limestone. Geochimica et Cosmochimica Acta [Internet]. 2016;173 :86–96. Publisher's VersionAbstract

Weathering in carbonate rocks is often assumed to be governed by chemical dissolution. Nevertheless, chemical processes can be coupled to mechanical mechanisms, with small grains undergoing partial dissolution along grain boundaries, followed by detachment from the rock surface. Crucially, this process can even extend down to the micron-scale. Although chemo-mechanical detachment could be critical for the understanding of carbonate weathering at the global scale, the role it plays has not been directly quantified. To calculate the contribution of grain detachment to surface retreat rates, and to determine the impact of the flow regime, we carried out a series of flow-through weathering experiments on micritic limestone. Using atomic force microscopy, we obtained high resolution in situ data of surface topography for reacting rock surfaces. In all the experiments, both grain detachment and chemical dissolution were observed. Under the laminar flow conditions we explored, we found no clear correlation between flow rate and the size of detached grains, or between the flow rate and the frequency of grain detachment events. Importantly, our results establish that grain detachment contributes significantly to the overall surface retreat, on average accelerating mass loss by 38{%}. In addition to speeding up weathering, this micron-scale mechanism could also influence the evolution of porosity in aquifers and hydrocarbon reservoirs, and provide a natural flux of colloids that could transport heavy metals or radionuclides in groundwater.

Oz I, Eyal S, Yoseph Y, Ittai G, Elad L, Haim G. Salt dissolution and sinkhole formation: Results of laboratory experiments. 2016 :1–17.
Levanon E, Shalev E, Yechieli Y, Gvirtzman H. Fluctuations of fresh-saline water interface and of water table induced by sea tides in unconfined aquifers. Advances in Water Resources [Internet]. 2016;96 :34–42. Publisher's VersionAbstract

This study examines effects of tides on fluctuations of the fresh-saline water interface and the groundwater level in unconfined coastal aquifers using a two-dimensional numerical model. The time-lags of the simulated hydraulic heads and salinities fluctuations compared to sea level fluctuations are analyzed using cross-correlation analysis. The results show that both the fresh-saline water interface and the groundwater level are affected harmonically by sea tide fluctuations. However, significantly different time-lags are obtained between the hydraulic head in the deeper and upper parts of the aquifer, and between head and salinity in the fresh-saline water interface. The hydraulic head in the deeper part of the aquifer responses much faster to sea level fluctuations than in the upper part. Surprisingly, a similar difference is detected between the time-lag of the hydraulic head in the fresh-saline water interface and the time-lag of the salinity at the same location. Furthermore, the time-lag of the salinity in the fresh-saline water interface is similar to the time-lag of the water table. We suggest a comprehensive mechanism for tidal influence on the coastal groundwater system, in which two main processes act simultaneously. First, sea tide causes a pressure head wave which propagates into the saturated zone of the aquifer, governed by the diffusivity of the aquifer (Ks/Ss). Second, this pressure head wave is attenuated at the water table due to the unsaturated flow within the capillary fringe which occurs during groundwater level oscillations. Because the tidal forcing acts on the sea-floor boundary and the attenuation of the groundwater level due to capillary effect acts on the groundwater table, two dimensional distributions of time-lag and hydraulic head amplitude are created. The capillary effect in the unsaturated zone plays a key role not only in the water table fluctuations as shown previously, but also on the salinity fluctuations in the fresh-saline water interface. Unsaturated flow dynamics controls the actual movement of the entire fresh water body, which results in simultaneous fluctuations of the groundwater level and the fresh-saline water interface.

Levenson Y, Schiller M, Kreisserman Y, Emmanuel S. Calcite dissolution rates in texturally diverse calcareous rocks. Geological Society, London, Special Publications [Internet]. 2015;406 :81–94. Publisher's VersionAbstract

The injection of reactive fluids into carbonate reservoirs during enhanced recovery operations can induce important changes in rock permeability. However, reliably predicting these changes requires accurate knowledge of calcite reaction rates. While numerous studies have examined calcium carbonate dissolution rates, most have focused on pure calcite crystals rather than actual rocks. In this study, two types of flow-through experiments were carried out to determine the calcite dissolution rates in texturally diverse calcareous rocks: the first type of experiment had a duration of 3 days, while the second type ran for 3 months. Our experiments show that rocks with differing textures and roughness (samples included coarse-grained oolitic limestone, fine-grained Solnhofen limestone, marble and calcite spar) do in fact dissolve by different mechanisms. However, despite these differences, bulk reaction rates were found to be remarkably similar (with a relative standard deviation of {\textless}20{%}) and consistent with previously reported reaction rates for calcite. Thus, our results suggest that textural differences between rock types are unlikely to have an important impact on overall reaction rates in fractured carbonate reservoirs.

Oz I, Shalev E, Yechieli Y, Gvirtzman H. Saltwater circulation patterns within the freshwater-saltwater interface in coastal aquifers: Laboratory experiments and numerical modeling. Journal of Hydrology [Internet]. 2015;530 :734–741. Publisher's VersionAbstract

Groundwater flow patterns within the freshwater-saltwater interface in coastal aquifers include rotation in the flow direction of saltwater that originates from the sea and circulates in the aquifer. Using two types of tracer experiments we analyze the configuration of the rotating flow-lines. The experimental results are numerically reconstructed and quantitatively compared to the salinity distribution along the interface. The results show that the rotation in the direction of the saltwater flow-lines begins at the lowermost part of the interface (i.e. contour C/Cmax=99{%}), and completes within its lower tenth (contour C/Cmax=94{%}). At the upper part of the interface, after the rotation is completed, the flow is dictated by the freshwater flow seaward. Based on these results, the well-known chemical freshwater-saltwater interface is divided into two different parts, defined by their physical properties: (1) the lower part is the "Flow Rotation Region", defined by convective circulating flow-lines; and (2) the upper part is the "Dispersive Region", defined by dispersive dilution. Sensitivity analysis shows that the physical configuration of the interface depends on the transversal dispersivities. At higher dispersivities the rotation width increases, but completes within the lower third of the interface, at most. The sensitivity analysis also shows that the rotation begins at the lowermost part of the interface for dispersivities. Therefore, since no flow occurs below a line of 99{%}, the saline water that flows seaward is always diluted with respect to its original salinity. These flow patterns might affect coastal processes such as submarine groundwater discharge (SGD) and chemicals transport through the aquifer.

Peleg N, Shamir E, Georgakakos KP, Morin E. A framework for assessing hydrological regime sensitivity to climate change in a convective rainfall environment: a case study of two medium-sized eastern Mediterranean catchments, Israel. Hydrology and Earth System Sciences [Internet]. 2015;19 :567–581. Publisher's VersionAbstract

A modeling framework is formulated and applied to assess the sensitivity of the hydrological regime of two catchments in a convective rainfall environment with respect to projected climate change. The study uses likely rainfall scenarios with high spatiotemporal resolution that are de- pendent on projected changes in the driving regional meteorological synoptic systems. The framework was applied to a case study in two medium-sized Mediterranean catchments in Israel, affected by convective rainfall, by combining the HiReS-WG rainfall generator and the SAC-SMA hydrolog- ical model. The projected climate change impact on the hy- drological regime was examined for the RCP4.5 and RCP8.5 emission scenarios, comparing the historical (beginning of the 21st century) and future (mid-21st-century) periods from three general circulation model simulations available from CMIP5. Focusing on changes in the occurrence frequency of regional synoptic systems and their impact on rainfall and streamflow patterns, we find that the mean annual rain- fall over the catchments is projected to be reduced by 15% (outer range 2–23%) and 18%(7–25%) for the RCP4.5 sand RCP8.5 emission scenarios, respectively. The mean annual streamflowvolumes are projected to be reduced by 45%(10– 60%) and 47% (16–66 %). The average events’ streamflow volumes for a given event rainfall depth are projected to be lower by a factor of 1.4–2.1. Moreover, the streamflow sea- son in these ephemeral streams is rojected to be shorter by 22%and 26–28%for the RCP4.5 and RCP8.5, respectively. The amplification in reduction of streamflow volumes relative to rainfall amounts is related to the projected reduction in soil moisture, as a result of fewer rainfall events and longer dry spells between rainfall events during the wet sea- son. The dominant factors for the projected reduction in rainfall amount were the reduction in occurrence of wet synop- tic systems and the shortening of the wet synoptic systems durations. Changes in the occurrence frequency of the two dominant types of the regional wet synoptic systems (active Red Sea trough and Mediterranean low) were found to have a minor impact on the total rainfall.

Oz I, Shalev E, Yechieli Y, Gavrieli I, Gvirtzman H. Flow dynamics and salt transport in a coastal aquifer driven by a stratified saltwater body: Lab experiment and numerical modeling. Journal of Hydrology. 2014;511 :665–674.Abstract

This paper examines the transient development and the steady-state configuration of groundwater within a coastal aquifer adjacent to a stratified saltwater body. Such systems consist of three different water types: the regional fresh groundwater, and low and high salinity brines forming the upper and lower water layers of the stratified water body, respectively. The dynamics, location and the geometry of the interfaces and the density-driven circulation flows that develop in the aquifer are examined using laboratory experiments and numerical modeling at the same scale. The results show that the transient intrusion of the different water bodies into the aquifer takes place at different rates, and that the locations of the interfaces between them change with time, before reaching steady-state. Under steady-state conditions both the model and the experiments show the existence of three interfaces between the three water types. The numerical model, which is calibrated against the salinity distribution and groundwater discharge rate in the laboratory experiments, allows the quantification of the flow rates and flow patterns within the aquifer. These flow patterns, which cannot be derived from laboratory experiments, show the transient development of three circulation cells which are confined between the three interfaces. These results confirm the hypothesis that has been previously suggested based solely on a steady-state numerical modeling defined by a conceptual understanding. Parametric analysis shows that the creation of three circulation cells and three interfaces is limited to certain conditions and defines the ranges for the creation of this unique system. ?? 2014 Elsevier B.V.

Emmanuel S, Levenson Y. Limestone weathering rates accelerated by micron-scale grain detachment. Geology. 2014;42 :751–754.Abstract

The weathering of carbonate rocks plays a critical role in the evolution of landscapes, the erosion of buildings and monuments, and the global-scale shifting of carbon from the atmosphere to the ocean. Chemical dissolution is often assumed to govern the rates of weathering of carbonate rocks, although some studies have suggested that mechanical erosion could also play an important role. Quantifying the rates of the different processes has proved challenging, in part due to the high degree of variability encountered across different scales in both field and laboratory conditions. To constrain the rates and mechanisms controlling long-term limestone weathering, we analyze a lidar scan of the Western Wall, a Roman-period edifice located in Jerusalem. We find that extreme erosion rates in fine-grained micritic limestone blocks are as much as two orders of magnitude higher than the average rates estimated for coarse-grained limestone blocks at the same site. Atomic force microscope imaging of dissolving micritic limestone suggests that these elevated reaction rates are likely to be the result of rapid dissolution along micron-scale grain boundaries, followed by mechanical detachment of tiny particles from the surface. Our analysis indicates that such grain detachment could be the dominant erosional mode for fine-grained carbonate rocks in many regions on Earth.

Peleg N, Bartov M, Morin E. CMIP5-predicted climate shifts over the East Mediterranean: implications for the transition region between Mediterranean and semi-arid climates. International Journal of Climatology. 2014;5 :n/a–n/a.Abstract

The effect of climate change on the Eastern Mediterranean (EM) region, a region that reflects a transition between Mediterranean and semi-arid climates,was examined. This transition region is affected by global changes such as the expansion of the Hadley cell, which leads to a poleward shift of the subtropical dry zone. The Hadley cell expansion forces the migration of jet streams and storm tracks poleward from their standard course, potentially increasing regional desertification. This article focuses on the northern coastline of Israel along the EM region where most wet synoptic systems (i.e. systems that may lead to precipitation) are generated. The current climate was compared to the predicted mid-21st century climate based on Intergovernmental Panel on Climate Change (IPCC) Representative Concentration Pathway (RCP) RCP4.5 and RCP8.5 scenarios using four Coupled Model Intercomparison Project Phase 5 (CMIP5) models. A warming of 1.1–2.6 ∘C was predicted for this region. The models predicted that rain in the region will become less frequent, with a reduction of 1.2–3.4% in 6-h intervals classified as wet synoptic systems and a 10–22% reduction in wet events. They further predicted that the maximum wet event duration in the mid-21st century would become shorter relative to the current climate, implying that extremely long wet systems will become less frequent. Three of the models predicted shrinking of the wet season length by up to 15%. All models predicted an increasing occurrence frequency of Active Red Sea Troughs (ARSTs) for the RCP8.5 scenario by up to 11% by the mid-21st century. For the RCP4.5 scenario, a similar increase of up to 6%was predicted by two of the models.

Peleg N, Morin E. Stochastic convective rain-field simulation using a high-resolution synoptically conditioned weather generator (HiReS-WG). Water Resources Research. 2014;50 :2086–2107.Abstract

A new stochastic high-resolution synoptically conditioned weather generator (HiReS-WG) appropriate for climate regimes with a substantial proportion of convective rainfall is presented. The simulated rain fields are of high spatial (0.53 0.5 km2) and temporal (5 min) resolution and can be used for most hydrological applications. The WG is composed of four modules: the synoptic generator, the motion vector generator, the convective rain cell generator, and the low-intensity rainfall generator. The HiReS-WG was applied to a study region on the northwestern Israeli coastline in the Eastern Mediterranean, for which 12 year weather radar and synoptic data were extensively analyzed to derive probability distributions of convective rain cells and other rainfall properties for different synoptic classifications; these distributions were used as input to the HiReS-WG. Simulated rainfall data for 300 years were evaluated for annual rain depth, season timing, wet-/dry-period durations, rain-intensity distributions, and spatial correlations. In general, the WG well represented the above properties compared to radar and rain-gauge observations from the studied region, with one limitation—an inability to reproduce the most extreme cases. The HiReS-WG is a good tool to study catchments’ hydrological responses to variations in rainfall, especially small-size to medium-size catchments, and it can also be linked to climate models to force the prevailing synoptic conditions.

Levenson Y, Emmanuel S. Pore-scale heterogeneous reaction rates on a dissolving limestone surface. Geochimica et Cosmochimica Acta. 2013;119 :188–197.Abstract

The rate at which limestone dissolves determines the resistance of buildings and monuments to weathering, the efficiency of carbon capture in deep geological reservoirs, and the processes by which soils, rocks, and landscapes form and evolve. However, the normalized rates of mineral dissolution measured in laboratory experiments are often found to be far greater than those measured in field settings. Here, we use atomic force microscopy (AFM) measurements to demonstrate experimentally that the rate of calcite dissolution within micron-size pores at the surface of a limestone sample is much lower than the rate of dissolution in the surrounding calcite surface. In addition, we use numerical simulations to show that this difference cannot be explained using a simple diffusion-surface reaction model. We suggest that the observed heterogeneous reaction rates could instead be due to the elevated density of reactive high curvature features on the polished surface surrounding the pore. These high curvature features can strongly affect local interfacial free energy, making such surfaces more prone to dissolution. As a result, polished surfaces should be more reactive than pore surfaces that have effectively been smoothed during prolonged contact with natural fluids. As rate experiments routinely use polished and powdered samples, the results may help to explain the widely reported discrepancy between lab and field-based dissolution rates. ?? 2013 Elsevier Ltd.

Peleg N, Ben-Asher M, Morin E. Radar subpixel-scale rainfall variability and uncertainty: Lessons learned from observations of a dense rain-gauge network. Hydrology and Earth System Sciences. 2013;17 :2195–2208.Abstract

Runoff and flash flood generation are very sen- sitive to rainfall’s spatial and temporal variability. The in- creasing use of radar and satellite data in hydrological ap- plications, due to the sparse distribution of rain gauges over most catchments worldwide, requires furthering our knowl- edge of the uncertainties of these data. In 2011, a new super- dense network of rain gauges containing 14 stations, each with two side-by-side gauges, was installed within a 4km2 study area near Kibbutz Galed in northern Israel. This net- work was established for a detailed exploration of the un- certainties and errors regarding rainfall variability within a common pixel size of data obtained from remote sensing systems for timescales of 1 min to daily. In this paper, we present the analysis of the first year’s record collected from this network and from the Shacham weather radar, located 63km from the study area. The gauge–rainfall spatial cor- relation and uncertainty were examined along with the esti- mated radar error. The nugget parameter of the inter-gauge rainfall correlations was high (0.92 on the 1 min scale) and increased as the timescale increased. The variance reduction factor (VRF), representing the uncertainty from averaging a number of rain stations per pixel, ranged from 1.6% for the 1 min timescale to 0.07% for the daily scale. It was also found that at least three rain stations are needed to adequately represent the rainfall (VRF<5%) on a typical radar pixel scale. The difference between radar and rain gauge rainfall was mainly attributed to radar estimation errors, while the gauge sampling error contributed up to 20% to the total dif- ference. The ratio of radar rainfall to gauge-areal-averaged rainfall, expressed by the error distribution scatter parameter, decreased from 5.27dB for 3 min timescale to 3.21dB for the daily scale. The analysis of the radar errors and uncertainties suggest that a temporal scale of at least 10 min should be used for hydrological applications of the radar data. Rainfall measurements collected with this dense rain gauge network will be used for further examination of small-scale rainfall’s spatial and temporal variability in the coming years.

Levanon E, Yechieli Y, Shalev E, Friedman V, Gvirtzman H. Reliable monitoring of the transition zone between fresh and saline waters in coastal aquifers. Groundwater Monitoring and Remediation. 2013;33 :101–110.Abstract

This study deals with the reliability of monitoring the transition zone between fresh and saline waters in coastal aquifers, considering the effect of tides in long-perforated boreholes. Electric conductivity (EC) fluctuations in the coastal aquifer of Israel, as measured in long-perforated borehole, were found to have the same periodicities as the sea tide, though some orders of magnitude larger than sea-level or groundwater level fluctuations. Direct measurements in the aquifer through buried EC sensors demonstrate that EC measurements within the long-perforated boreholes might be distorted due to vertical flow in the boreholes, whereas actual fluctuations of the transition zone within the aquifer are some orders of magnitude smaller. Considering these field data, we suggest that monitoring of the transition zone between fresh and saline water adjacent to the sea through long-perforated boreholes is unreliable. EC fluctuations in short-perforated boreholes (1 m perforation at the upper part of the transition zone) were somewhat larger than in the aquifer, but much smaller than those in the long-perforated borehole. The short-perforation diminishes the vertical flow and the distortion and therefore is more reliable for monitoring the transition zone in the shoreline vicinity.

Peleg N, Morin E. Convective rain cells: Radar-derived spatiotemporal characteristics and synoptic patterns over the eastern Mediterranean. Journal of Geophysical Research. 2012;117 :D15116.Abstract

This paper examines the spatiotemporal characteristics of convective rain cells over the eastern Mediterranean (northern Israel) and their relationship to synoptic patterns. Information on rain cell features was extracted from high-resolution weather radar data. The radar-gauge adjustment, validation, cell segmentation and tracking techniques are discussed at length at the beginning of the paper. Convective rain cells were clustered into three synoptic types (two winter lows—deep Cyprus lows and shallow lows—and one tropical intrusion, Active Red Sea Trough) using several NCEP/NCAR parameters, and empirical distributions were computed for their spatial and temporal features. In the study region, it was found that the Active Red Sea Trough rain cells are larger, live for less time and possess lower rain intensities than the rain cells generated by the winter lows. The Cyprus low rain cells were found to be less intense and slightly larger on average than the shallow low rain cells. It was further discovered that the preferential orientation of the rain cells is associated with the direction and velocity of the wind. The effect of distance from the coastline was also examined. An increase in the number and area of the rain cells near the coastline was observed, presumably due to the sea breeze convection. The mean rainfall intensity was found to peak near the shore and decrease with distance inland. This information is of great importance for understanding rain patterns and can be further applied in exploring the hydrological responses of the basins in this region.

Oz I, Shalev E, Gvirtzman H, Yechieli Y, Gavrieli I. Groundwater flow patterns adjacent to a long-term stratified (meromictic) lake. Water Resources Research. 2011;47.Abstract

This paper examines, for the first time, the unique situation of a groundwater system adjacent to a long-term stratified (meromictic) lake. Using conceptual and numerical models, the configuration of groundwater interfaces between the three different water bodies (regional groundwater and upper and lower lake waters) and the flow patterns were quantitatively evaluated assuming a homogenous aquifer. A complex flow system, controlled by density difference, is created near the lake, where three circulation cells are developed. These results are different from the classic circulation cell that is found adjacent to nonstratified water bodies (lakes or oceans). Sensitivity analyses reveal that the results are sensitive to changes in thickness and density of the upper water mass of the lake. The Dead Sea, under its possible future meromictic conditions, serves as an ideal example of such a system. Thus, the model's results can be used as a preliminary assessment for groundwater behavior adjacent to the lake, if and when stratification will develop.