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.

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.

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.