<p>Climate and global changes will force cities to adapt to new drastic meteorological and hydrological conditions. Within this context, urban planning has pointed to the need to restore the natural water cycle in urban cities. Restoring the natural water cycle means promoting water infiltration in urban areas to facilitate groundwater recharge and minimize runoff at the soil surface. Several techniques were developed with this goal, including those aimed at infiltrating water in specific drainage works like sustainable urban drainage systems (SUDS). However, the management of SUDS requires monitoring their capability to infiltrate water and its permanence with time. Indeed, several processes may impact the hydraulic characteristics of soils and, consequently, the infiltration capacity of bio-retention. Among others, clogging may reduce the soil's hydraulic conductivity and decrease infiltration. Conversely, plant growth and related development of root systems may promote macropore networks and increase the bulk hydraulic conductivity of soil, resulting in an increase in infiltration.</p> <p>Infiltration techniques, including single-ring water infiltration experiments, were developed to monitor the soil's hydraulic properties and investigate their evolution with time. Infiltration techniques are based on infiltration tests with rings with radii in the order of 5-10 cm. If the question of the type of condition imposed at the soil surface was already posed (e.g., the question of the value of the water pressure head to impose, the presence of a sand layer for tension infiltrometers, etc.), the question of the ring size has not been investigated in depth.</p> <p>In this study, we investigate the impact of the ring size on the results of water infiltration experiments, particularly regarding the activation of the soil macropore network and the hydraulic characterization of soils. We then performed infiltration experiments with rings of two contrasting sizes (7.5 cm versus 25 cm for the radius). Water infiltrations were carried out, involving the same total cumulative infiltration depth of 300 mm. BEST methods were then applied to derive the soil hydraulic parameters (Angulo-Jaramillo et al., 2019). The results were then compared between the large and the regular (small) rings. Differences in estimate means and standard deviations were discussed for each hydraulic parameter. Numerical modeling was also performed using HYDRUS (Radcliffe and Simunek, 2018) with synthetic soils to explain the difference in results between ring sizes with the concept of partial activation of the macropore network depending on the ring size. Our results constitute the first step toward understanding the ring effect on soil hydraulic characterization and its optimization with regard to the activation of all types of porosities.</p> <p>References</p> <p>Angulo-Jaramillo, R., Bagarello, V., Di Prima, S., Gosset, A., Iovino, M., Lassabatere, L., 2019. Beerkan Estimation of Soil Transfer parameters (BEST) across soils and scales. J. Hydrol. 576, 239&#8211;261. https://doi.org/10.1016/j.jhydrol.2019.06.007</p> <p>Radcliffe, D.E., Simunek, J., 2018. Soil physics with HYDRUS: Modeling and applications. CRC press.</p>
<p>The hydrological response of steep slopes catchments is strongly conditioned by the connectivity of subsurface preferential flows. The objective of this research is to investigate the role played by stemflow infiltration in subsurface water flow dynamics, focusing on a forested hillslope located in an Aleppo pine Mediterranean forest (Pinus halepensis, Mill.) located at Sierra Calderona, Valencia province, Spain. We combined stemflow artificial experiments with ground-penetrating radar (GPR) techniques as a non-invasive method to investigate stemflow-induced preferential flow paths activated by different trees and the related hydrological connectivity at the hillslope scale. Our observations allowed us to identify different dynamics associated with the initiation of stemflow and then lateral preferential flow, including the activation of connected preferential flow paths in soils that received stemflow water from different trees. These observations provided empirical evidence of the role of stemflow in the formation of lateral preferential flow networks. Our measurements allow estimations of flow velocities and&#160; new insight on the magnitude of stem-induced lateral preferential flow paths. The applied protocol offers a simple, repeatable and non-invasive way to conceptualize hillslope responses to rainstorms.</p>
<p>Nature-based systems are being employed to allow stormwater to infiltrate directly in the soil, which is supposed to capture pollutants. To better track the evolution of such systems performances, in particular the infiltration and filtration performances, and to be able to optimize their management, these systems need to be better known. Currently, there is a lack of knowledge and methods regarding the characterization of the macropores and matrix contributions in infiltration and filtration of urban soils, whereas the quality of groundwater and the capacities of these systems are at stake.&#160;</p> <p>To tackle these limits, a large infiltrometer of 50 cm in diameter with two water-supply reservoirs of approximately 40 L each, was developed to characterize both hydrodynamic and nanotracers transfer parameters. Cumulative water infiltration was carried out at a constant hydraulic pressure head of 10 cm. Superparamagnetic iron oxide nanoparticles (SPIONs), which mimic both colloidal pollutants and bacteria flow behaviors in soils, were designed to be detectable by ground-penetrating radar (GPR). Fifty volumes of SPIONs solution (i.e., 50 x 5 mL at 3.35g/L) were injected into the ring and the GPR was passed along different survey lines around the ring several times during the infiltration experiment. GPR data was treated with ReflexW (Sandmeier Scientific Software, Karlsruhe, Germany) and Rockware (RockWare, Inc, 2015) to define a 3D block diagram of the infiltration bulb. The probability of presence of the nanoparticles was obtained from comparing the radargrams, before and after nanoparticle injection, by using two methods (Allroggen and Tronicke, 2015; Di Prima et al., 2020) on a R software (https://www.R-project.org/).</p> <p>The large infiltrometer device, compared with a smaller one (Di Prima et al., 2015), is proved effective for estimating water and transfer parameters. The dispersion of SPIONs gave an idea of the relative importance of the transfer through the soil macropores as compared to the soil matrix. The probability of SPIONs presence gave information on the filtration function of soils. The whole device application will be illustrated and discussed with regard to its use for the assessment of the infiltration and filtration functions of bio-infiltration systems.&#160;</p> <p>&#160;</p> <p>Allroggen, N., Tronicke, J., 2015. Attribute-based analysis of time-lapse ground-penetrating radar data. Geophysics 81, H1&#8211;H8. https://doi.org/10.1190/geo2015-0171.1</p> <p>Di Prima, S., Lassabatere, L., Bagarello, V., Iovino, M., Angulo-Jaramillo, R., 2015. Testing a new automated single ring infiltrometer for Beerkan infiltration experiments. Geoderma 262, 20&#8211;34. https://doi.org/10.1016/j.geoderma.2015.08.006</p> <p>Di Prima, S., Winiarski, T., Angulo-Jaramillo, R., Stewart, R.D., Castellini, M., Abou Najm, M.R., Ventrella, D., Pirastru, M., Giadrossich, F., Capello, G., Biddoccu, M., Lassabatere, L., 2020. Detecting infiltrated water and preferential flow pathways through time-lapse ground-penetrating radar surveys. Sci. Total Environ. 726, 138511. https://doi.org/10.1016/j.scitotenv.2020.138511</p>
<p>In the Sahel region, agroforestry is a land-use system widely adopted as a more sustainable agricultural production system. In this type of system, woody perennials that are grown in association with agricultural crops and pastures, constitute spatially disconnected zones where microclimate and soil&#8217;s infiltrability, physical, chemical, and biological conditions are assumed locally improved. Particularly the stemflow concentrates a part of the intercepted rainfall from the canopies to the stems. Hence stemflow can induce preferential infiltration around the stem base and promote groundwater recharge.</p> <p>In the West African Sahel, <em>Faidherbia albida</em> (Delile) A.Chev. is commonly adopted as multi-purpose woody perennial in agroforestry systems. It is a deciduous tree with an inverse phenology as it loses the leaves during the rainy season. Although, the absence of leaves during the rainy season is expected to decrease the interception and to consequently decrease stemflow, evidence of stemflow at the base of <em>F. albida</em> trees were reported in the literature when the stems were partially covered with green leaves (Chinen, 2007).</p> <p>In this study, we carried out timelapse ground penetrating radar (GPR) surveys in conjunction with a simulated stemflow event to investigate stemflow-induced infiltration by an <em>F. albida</em> tree trunk and root system. We established a survey grid (2.1 m &#215; 2.1 m) around an <em>F. albida</em>, consisting of twelve horizontal and ten vertical parallel survey lines with 0.3 m intervals between them. Two stemflow pulses, each of 20 L, were poured on the tree trunk using a PVC pipe with a 1-mm-diameter hole every 50 mm. The pipe was connected to a plastic funnel and positioned around the tree trunk at 0.4 m from the soil surface. One grid GPR survey was carried out before the stemflow simulation experiment. A total of 40 L of water was used during the experiment. A second survey was carried out after the injection of the first 20 L, while the last survey was carried out after the second stemflow pulse. We collected a total of 66 (3 GPR surveys &#215; 22 survey lines) radargrams using a GSSI (Geophysical Survey System Inc., Salem, NH) SIR 3000 system with a 900-MHz antenna. We therefore obtained for each survey line a pre-wetting and two post-wetting radargrams. Next, we created other forty-four matrixes based on absolute differences between pre- and post-wetting amplitude values. Higher differenced values occurred because of amplitude changes and time shifts related to wave propagation.</p> <p>The analysis of the differentiated radargrams provided evidence of deep infiltration along the tap roots. The wetted zone extended mainly in-depth providing evidence of the potential role played by the <em>F. albida</em> trees in groundwater recharge processes due to their deep rooting, preferably reaching the groundwater table. Put all together, this study shows a first signal of the importance of accounting for stemflow infiltration in the water balance of agroforestry systems with <em>F. albida</em> trees.</p> <p><strong>References</strong></p> <p>Chinen, T., 2007. An observation of surface runoff and erosion caused by acacia albida stemflow in dry savanna, in the south-western republic of Niger 10.</p>
<p>Stormwater management zones must enable water to infiltrate easily, primarily due to macropores, but little is known about the transport of pollutants through these macropores. Some coupled methods using, for example, dyes, disc, or ring infiltrometers were developed to give insight on the respective contributions of the macropores versus the matrix to the bulk infiltration. However, these methods do not visualize where and how water infiltrates. Besides, no information is given on the solute transfer in soils, whereas this issue is crucial regarding the quality of soils and groundwater. One of the goals of the national program INFILTRON (https://infiltron.org) project granted by the French national research agency is to develop superparamagnetic iron oxide nanoparticles (SPIONs) which mimic both pollutant and bacteria flow behavior in soils (Raimbault et al., 2021) and are detectable by ground-penetrating radar (GPR). Within its framework (Lassabatere et al., 2020), we aim to show how nano-tracers can help detect preferential treatment flows (lithological heterogeneity, root system) and quantify or qualify the pollutant transfer in heterogeneous soils. A specific device was designed and presented for the concomitant monitoring of water infiltration and nano-tracer injection. This specific infiltrometer involves two water supply reservoirs and a ring diameter of 50 centimeters. This device maintains a constant depth of water (10 cm) above the soil and delivers the water infiltration into the soil. Two rules posed on the reservoirs allow monitoring the water drop and computation of the cumulative infiltration. Fifty volumes of SPIONs solutions (5 mL of 3.35g/l SPIONs solution) were injected into the ring to maintain a constant concentration in the ring. GPR monitors the bulb of infiltrated water and SPIONs. GPR data is treated with ReflexW (&#169;Sandmeier geophysical research) and RockWorks (RockWare&#174;) software. Combining this specific prototype with the use of GPR for the detection of water and the SPIONs gives insight into the processes of infiltration and SPIONs transfer and localization in the soil. These data allow us to understand and model pollutant transfer into the vadose zone.</p><p>Raimbault, J., Peyneau, P.-E., Courtier-Murias, D., Bigot, T., Gil Roca, J., B&#233;chet, B., and Lassabat&#232;re, L.: Investigating the impact of exit effects on solute transport in macropored porous media, 2020, 1&#8211;20, https://doi.org/10.5194/hess-2020-494, 2020</p><p>Lassabatere, L., De Giacomoni, A.-C., Angulo-Jaramillo, R., Lipeme Kouyi, G., Martini, M., Louis, C., Peyneau, P.-E., Rodriguez-Nava, V., Cournoyer, B., Aigle, A., and others: INFILTRON package for assessing infiltration & filtration functions of urban soils, in: EGU General Assembly Conference Abstracts, 11269, 2020. https://doi.org/10.5194/egusphere-egu2020-11269</p>
<p>Infiltration basins are among the most spread techniques for managing stormwater. Infiltration basins allow the infiltration of stormwater, which prevents their piping towards treatment systems. However, stormwater contains loads of pollutants and suspended solids that accumulate at the surface of the basin and form a sedimentary layer. That sedimentary lay may clog the infiltration basin partially, thus reducing its bulk infiltration capability. Fortunately, plants and fauna colonize spontaneously this sedimentary layer, thus preventing complete clogging and restoring soils' infiltration functions. The knowledge of the effect on restoring the infiltration function requires properly characterize fauna, notably earthworms, with the aim to predict their impact on infiltration. Besides, earthworms, considered as ecosystem engineers, are known to be good candidates for integrating soil chemical pollution.</p><p>If earthworms have been intensively studied in natural and agricultural soil, very few studies have focused on the characterization of earthworms' communities in urban soils and, in particular, in infiltration basins. This study presents the description of earthworms sampled at several places over one infiltration basins. This basin receives the stormwater collected over an industrial peri-urban catchment. The infiltration basin has been functioning for more than two decades, thus, plants and fauna have colonized the surface related to water ponding at surface and water infiltration. The sampled places were selected to follow three specific water pathways at the surface. High population variability was measured with densities ranging from 0 to 300 earthworms per square meter with the presence of adults but also juveniles. But, only endogenic and epigeic functional groups were found. The characterization of abundance, age, and species over the sampled places was correlated to water content and sediment thickness, in addition to pollutant loads.</p><p>The results show that earthworms require given edaphic conditions (including thick enough sedimentary layer) to settle. We then expect most earthworms to colonize those specific places, increasing water infiltration punctually at these places. Put all together, our findings participate in the understanding of colonization of basin infiltration by organisms and their contribution to their primary function: infiltrating water.</p>
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