Perched Water Research Articles

Preferential flow, diffuse flow, and perching in an interbedded fractured-rock unsaturated zone

Layers of strong geologic contrast within the unsaturated zone can control recharge and contaminant transport to underlying aquifers. Slow diffuse flow in certain geologic layers, and rapid preferential flow in others, complicates the prediction of vertical and lateral fluxes. A simple model is presented, designed to use limited geological site information to predict these critical subsurface processes in response to a sustained infiltration source. The model is developed and tested using site-specific information from the Idaho National Laboratory in the Eastern Snake River Plain (ESRP), USA, where there are natural and anthropogenic sources of high-volume infiltration from floods, spills, leaks, wastewater disposal, retention ponds, and hydrologic field experiments. The thick unsaturated zone overlying the ESRP aquifer is a good example of a sharply stratified unsaturated zone. Sedimentary interbeds are interspersed between massive and fractured basalt units. The combination of surficial sediments, basalts, and interbeds determines the water fluxes through the variably saturated subsurface. Interbeds are generally less conductive, sometimes causing perched water to collect above them. The model successfully predicts the volume and extent of perching and approximates vertical travel times during events that generate high fluxes from the land surface. These developments are applicable to sites having a thick, geologically complex unsaturated zone of substantial thickness in which preferential and diffuse flow, and perching of percolated water, are important to contaminant transport or aquifer recharge.

Soil properties and subsoil constraints of urban and peri-urban agriculture within Mahikeng city in the North West Province (South Africa)

PurposeUrban and peri-urban agriculture is becoming increasingly important as a source of income and food for the urban population in South Africa. While most studies on urban agriculture have focused their attention on surface soils, there is dearth of information regarding subsoil properties. This study examined properties of subsoil horizons that may impede root growth and productivity of crops under urban agriculture.Materials and methodsThe properties of topsoil (0–20 cm) and subsoil horizons (20–40 cm) of four profiles from plots within the city of Mahikeng (25° 48′ S and 25° 38′ E) were examined to determine the nature of subsoil constraints that can limit root growth and crop productivity. The plots were selected in an area extending through four residential suburbs of the city, and two plots with a long history of cultivation were purposely selected from each suburb to represent the main cropping systems and soil types. Soil physical (penetrometer resistance, bulk density, hydraulic conductivity), chemical (pH, exchangeable Ca, Mg, K, Na, phosphorus and boron) and biological (root growth, organic carbon, microbial biomass, enzyme activity) properties were measured in the profiles.Results and discussionEven though there was a large variability between profiles, the results revealed high bulk density (mean 2.06 Mg m−3) at the top of the subsoil for all the profiles. The corresponding mean penetrometer resistance was 1.89 MPa implying high mechanical resistance to root growth in this layer. The hydraulic conductivities at saturation were below 12 mm h−1 suggesting low drainage which may result in perched water table and waterlogging leading to depleted oxygen in the root zone. The pH in all the profiles was slightly acid to moderate alkaline (6.1–8.3, in water), and low levels of plant available boron (B) were found in the subsoil layers. Most of the profiles had extreme values of physical properties that would constrain root growth. All the subsoil layers had significantly (p < 0.05) lower root growth, organic carbon, microbial biomass and enzyme activity.ConclusionsIt was concluded that subsoil constraints to root growth appear to be widespread in profiles of soils used for urban and peri-urban agriculture in the city of Mahikeng. Given that studying and ameliorating subsoil constraints is difficult, time-consuming and expensive, it is recommended that periodic deep ploughing and inclusion of plants with roots which are tolerant or resistant to these conditions be considered as part of routine soil management practice in plots used for urban agriculture.

Natural and anthropogenic controls on the frequency of preferential flow occurrence in a wastewater spray irrigation field

To quantify the frequency of preferential flow (PF) occurrence and its controls, and to examine the interactions of soils and irrigation on water flow dynamics, soil moisture was monitored at six depths every two minutes at four sites in a wastewater spray irrigation field from 2009 to 2011. Two irrigated sites – one cropped and another forested – were compared with two corresponding non-irrigated sites. Activation of preferential flow pathways was determined from the sequences of soil moisture responses at various depths for a total of 633 water input events, including 82 irrigation events. The results showed that the overall averaged frequencies of PF at the non-irrigated sites were 24.9% at the cropped site and 24.7% at the forested site. By contrast, the averaged frequencies at the irrigated sites were 47.1% at the cropped site and 45.0% at the forested site. The temporal stability of PF frequency was evaluated, showing that the overall frequency of PF stabilized after 70 events at non-irrigated sites, while the irrigated sites took as many as 105 events (24 of which were irrigations). Associations between 20 possible controls related to water input, moisture response, and season were quantified using the Student’s t-test between each parameter and PF detection. The most influential factors were water inputs, especially the total input and peak input intensity. Antecedent soil moisture and response slope (maximum 2-min increase in moisture content during the response) were also significant in some but not all cases. Soil horizonation influenced soil water storage and dynamics by restricting flow and causing perched water tables. Irrigated sites experienced saturated conditions for as much as 38% of the monitoring time at some depths, compared to a maximum of 3% at non-irrigated sites. Preferential flow was not only more likely to occur during irrigation events, but also during natural events at the irrigated sites, suggesting that the irrigated soils have physically adapted to accommodate large volumes of water, after decades of spray irrigation. The results of this study have implications for understanding hydrology and contaminant fate in anthropogenically-altered landscapes as well as implementing sustainable management practices.

Evaluation and localization of an artificial drainage network by 3D time-lapse electrical resistivity tomography.

In France, 10% of total arable land is equipped with subsurface drainage systems, to control winter and spring waterlogging due to a temporary perched water table. Most of these systems were installed in the1980s and have aged since then and may now need maintenance. Sometimes, the location of the systems is known, but the standard situation in France is that the original as-built master sketches are no longer available. Performance assessment of drainage systems and curative actions are complicated since drain location is unknown. In this article, the authors test the application of a non-destructive drain detection method which consists in water injection at the outfall of the drainage network combined with time-lapse electrical resistivity tomography (ERT) monitoring. To assess the performance of this methodology, which consists in measuring electrical resistivity from electrodes placed at the nodes of a 1.2-m regular mesh, the authors interpreted the signal using a two-step approach. The first step is based on 3D ERT numerical modelling during a scenario of surface infiltration processes (forward modelling followed by geophysical inversion); this step optimizes the ERT method for locating the infiltration at depths below 1m. The second step is the validation of the results obtained by numerical modelling with an experimental data set, using water injection into the drainage network combined with time-lapse ERT monitoring on an experimental field site. The results showed the relevance of time-lapse ERT monitoring on a small agricultural plot for locating the drainage network. The numerical results also showed several limitations of the combined methodology: (i) it is necessary to use an electrode spacing unit less than 1.20m, which does not facilitate investigation on large agriculture plots, (ii) measurements must be taken when resistivity contrast is the strongest between the infiltration area and the soil and (iii) the volume of water needed for injection can limit the extension of the method.

No tillage and liming reduce greenhouse gas emissions from poorly drained agricultural soils in Mediterranean regions

No tillage (NT) has been associated to increased N2O emission from poorly drained agricultural soils. This is the case for soils with a low permeable Bt horizon, which generates a perched water layer after water addition (via rainfall or irrigation) over a long period of time. Moreover, these soils often have problems of acidity and require liming application to sustain crop productivity; changes in soil pH have large implications for the production and consumption of soil greenhouse gas (GHG) emissions. Here, we assessed in a split-plot design the individual and interactive effects of tillage practices (conventional tillage (CT) vs. NT) and liming (Ca-amendment vs. not-amendment) on N2O and CH4 emissions from poorly drained acidic soils, over a field experiment with a rainfed triticale crop. Soil mineral N concentrations, pH, temperature, moisture, water soluble organic carbon, GHG fluxes and denitrification capacity were measured during the experiment. Tillage increased N2O emissions by 68% compared to NT and generally led to higher CH4 emissions; both effects were due to the higher soil moisture content under CT plots. Under CT, liming reduced N2O emissions by 61% whereas no effect was observed under NT. Under both CT and NT, CH4 oxidation was enhanced after liming application due to decreased Al3+ toxicity. Based on our results, NT should be promoted as a means to improve soil physical properties and concurrently reduce N2O and CH4 emissions. Raising the soil pH via liming has positive effects on crop yield; here we show that it may also serve to mitigate CH4 emissions and, under CT, abate N2O emissions.

The Impact of Hydrological Parameters on Modelling Slope Safety Factor Towards Shallow Landslides: A Case Study from Oltrepò Pavese

Hydrological monitoring of slope susceptible to shallow landslides allows for identifying the triggering conditions of shallow failures and implementing slope stability analysis at site-specific scale. In this work, a case study of a long-term hydrological monitoring in a slope susceptible to shallow landslides of Oltrepo Pavese (Northern Italy) is presented. The triggering mechanism develops in wet seasons (winter and spring) due to the uprise of a perched water table at about 1 m from ground surface, in consequence of the most intense rainfalls (about > 60 mm in 48 h). Unstable conditions (safety factor < 1.0) are correctly modeled on the basis of both water content and pore water pressure, with a better prediction considering hysteresis effects. Safety factor on the basis of water content can correctly assess the triggering conditions for unsaturated and completely saturated soils. It is possible estimating shallow landslides triggering caused by positive pore water pressures only considering this parameter.

Hydrogeological Conditions Changes of Tomsk, Russia

The hydro-geological conditions of Tomsk are determined by both natural factors and the impact of the urban infrastructure. Important impact on subsurface water flows involves the complex hydraulic relationship of several geological layers and the ancient and modern relief. Increasing groundwater abstraction has generally led to lowered piezometric heads in the deeper aquifer horizons, while in the uppermost horizons, rises in the water table and formation of new perched water tables are experienced due to leaking pipes and impedance of groundwater flow by deep foundations. In this paper special attention is paid to the Quaternary aquifer complex. Barrage effects of pile foundations and the intensive development of perched water distributed on flat surfaces of the watersheds and high terraces, complicated conditions for the construction and operation of facilities, leading in some cases to emergency situations.

Preliminary hydrogeophysical investigation at the 10th of Ramadan City, Egypt, by 1D and 2D inversion of VES data

ABSTRACTElectrical resistivity survey in the form of vertical electrical sounding was conducted at the 10th of Ramadan City, Egypt, to analyse subsurface lithology distribution and the inter‐relation between the possible infiltrated wastewater from oxidation ponds and the surface seepage emerged at the city centre. Special interest was paid for tracing the low‐resistivity clayey layer, which can act as a barrier against surface water percolation and as a low permeable zone to form local perched water system. Vertical electrical sounding data by Schlumberger electrode configuration were taken at 17 stations along a line extended from the city centre to the oxidation ponds used for wastewater treatment. The data were inverted by using 1D and 2D inversion schemes to construct geoelectrical cross section along the surveyed line. The geoelectrical layers were interpreted principally in terms of lithology and water content by correlation with borehole information. From the obtained results, four layers were identified in the subsurface medium. The upper three layers are located above the regional water table, whereas the fourth one was interpreted to be fully water saturated. The second layer from the surface is of great importance as it is the low‐resistivity clayey layer. This layer is discontinuous along the survey line. Downward infiltration of wastewater from the oxidation ponds is possible especially at the pond No. 2 as the clayey layer was interpreted not to exist at the site. The infiltrated wastewater from the oxidation ponds was interpreted not to be the cause of the surface seepage emerged at the city centre because the clayey layer dips toward opposite direction. The probable sources of the seepage were considered the local surface infiltration from excess irrigation water in the nearby cultivated land and/or leakage from the underground water utilities in the city centre.

Timing and characteristics of Late Pleistocene and Holocene wetter periods in the Eastern Desert and Sinai of Egypt, based on 14C dating and stable isotope analysis of spring tufa deposits

There is very little dated evidence on wet periods in the Eastern Desert and Sinai Peninsula of Egypt during the Late Pleistocene and Holocene. To obtain such information, we have studied the petrography, isotope geochemistry and AMS radiocarbon ages of mostly relict tufas deposited by springs draining perched ground water bodies in metamorphic and volcanic rocks. The tufas unconformably overly Precambrian basic igneous rocks (basalt, diabase and gabbro). As the ages of tufa carbonate are frequently older than the true ages of the deposits because of the incorporation of old, 14C-dead carbon, we have dated both the carbonate matrix and insoluble organic material of the tufas. These ages show that the tufas were largely formed during two broad time periods, the most recent from 12,058 to 6678 cal yr BP (African Humid Period), and the other from ∼31,200–22,500 cal yr BP, with preferential growth during the coldest times of this period namely during Heinrich Events 2 and 3 (H2 and H3) and the Last Glacial Maximum (LGM). The time span between 19,000–9000 cal yr BP, including the YD and H1, appears to have been relatively more arid than the earlier LGM or H2 periods or the later Holocene. The Late Pleistocene tufas are depleted in 18O relative to the Holocene tufas and were deposited at a lower temperature (∼14.0°–20.8 °C vs. 18.4°–23.4 °C). We believe that the Holocene tufas in the Sinai were formed by rainfall from the Mediterranean and those in the southern part of the Eastern Desert by African monsoon rainfall derived from the Red Sea-Gulf of Aden and Indian Ocean. In contrast, the moisture that fed the Late Pleistocene tufas, which are depleted in 18O relative to Holocene deposits, and progressively depleted from north to south, was probably brought by the Westerlies from the Atlantic-Mediterranean Sea when the Westerly circulation was pushed southwards during the coldest periods of the Late Pleistocene. Periods of tufa deposition correlate with major documented paleoclimatic events in North Africa during the late Pleistocene and Holocene; such as the Nile floods, high sea level and the formation of sapropels in the Mediterranean.

Permeability test and slope stability analysis of municipal solid waste in Jiangcungou Landfill, Shaanxi, China

ABSTRACTWith the rapid increase of city waste, landfills have become a major method to deals with municipal solid waste. Thus, the safety of landfills has become a valuable research topic. In this paper, Jiangcungou Landfill, located in Shaanxi, China, was investigated and its slope stability was analyzed. Laboratory tests were used to obtain permeability coefficients of municipal solid waste. Based on the results, the distribution of leachate and stability in the landfill was computed and analyzed. These results showed: the range of permeability coefficient was from 1.0 × 10–7 cm sec–1 to 6.0 × 10–3 cm sec–1 on basis of laboratory test and some parameters of similar landfills. Owing to the existence of intermediate cover layers in the landfill, the perched water level appeared in the landfill with heavy rain. Moreover, the waste was filled with leachate in the top layer, and the range of leachate level was from 2 m to 5 m in depth under the waste surface in other layers. The closer it gets to the surface of landfill, the higher the perched water level of leachate. It is indicated that the minimum safety factors were 1.516 and 0.958 for winter and summer, respectively. Additionally, the slope failure may occur in summer.Implications: The research of seepage and stability in landfills may provide a less costly way to reduce accidents. Landslides often occur in the Jiangcungou Landfill because of the high leachate level. Some measures should be implemented to reduce the leachate level. This paper investigated seepage and slope stability of landfills by numerical methods. These results may provide the basis for increasing stability of landfills.

Flood response for the watersheds of the Fernow Experimental Forest in the central Appalachians

AbstractWe examine flood response of high‐gradient, forested central Appalachian watersheds through analyses of rainfall, streamflow, and piezometer observations from the Fernow Experimental Forest near Parsons, West Virginia. Analyses focus on hydrologic processes that control the “upper tail” of flood distributions. The largest flood peaks in the Fernow are an order of magnitude smaller than record floods in the central Appalachian region (for basins of comparable drainage area). We examine flood distributions in the Fernow using extreme value distributions (Generalized Extreme Value and Generalized Pareto distributions) and compare them to other watersheds in the central Appalachians. To examine the role of antecedent soil moisture on flood response, we installed a network of 415 crest‐stage piezometers on two headwater watersheds (0.30 and 0.14 km2) of the Fernow. Observations show pronounced heterogeneity of subsurface saturation even within the unchannelized swales of headwater watersheds. Shallow perched water tables over large portions of a watershed occur infrequently in forested central Appalachian basins, but may play an important role in extreme flood response. Fernow watersheds include “treated” and control watersheds with stream gaging records extending back to 1951. We examine nonstationarites in flood frequency in the Fernow and show that forest management practices have had relatively minor impacts on flood frequency.

Holocene soils/paleosols in the okstindan mountains, nordland: stratigraphy and extractable fe and al

A chronosequence of surface and buried soils in moraines and outwash, ranging in age from iddle to ate olocene, i.e., Little Ice Age, and forming under alpine and subalpine vegetation in the kstindan ountains of central‐north orway (66o ) (ksskolten) was studied to determine if topographic/stratigraphic setting, weathering characteristics, soil/paleosol properties, / analysis, and extractable and could be used to elucidate information on paleoenvironment and age. The geochemical data previously published and geological mapping by the orwegian eological urvey indicate a uniform parent material in these profiles. Slight geochemical anomalies of higher , h and r in organic‐rich horizons are related to vegetation and/or higher water content at various times, confirmed by extractable evidence. The data interpretation also shows that extractable , principally o, is useful in identifying previous perched water tables, possibly generated by permafrost. The activity ratio of o/d (oxalate to dithionite) and the ratio d/t (dithionite to total ) appear useful in distinguishing older from younger soils on the basis of the conversion of ferrihydrite to goethite+hematite. Pyrophosphate extracts, proved useful in determining movement of organically bound , and showed a pronounced difference between the alpine and subalpine environments. Oxalate‐extractable and dithionite‐extractable proved of little value in relative age determination, presumably because sodium dithionite does not extract all crystalline .

Preferential flow mechanisms identified from staining experiments in forested hillslopes

AbstractField staining experiments in five different plots at two sites in Japan (Okaya in Nagano Prefecture and Konohara in Mie Prefecture) were undertaken to improve understanding of subsurface stormflow runoff within organic layers of natural forested hillslopes. This type of shallow lateral subsurface flow, specifically referred to as biomat flow, was observed only at the Okaya site based on staining experiments conducted under controlled water application rates. When the same irrigation rate (50–100 mm h−1) was applied to the Konohara site, overland flow without a significant shallow subsurface component was the dominant flow mechanism. Even in gently sloping (15–20°) forest soils at the Okaya site, biomat flow was responsible for lateral dye transport over much longer distances than subsurface flow in the matrix of mineral soil layers. Based on analysis of staining pattern images observed in the Okaya site, we conclude that (i) the organic biomat layer could be divided into two sub‐layers of different structure, (ii) biomat flow transported the dye tracer longer distances than subsurface flow in the matrix and (iii) the biomat layer topography affected biomat flow by generating preferential flowpaths and subsequent percolation into the deeper soil. Based on field experimental results and pore‐scale consideration of water infiltration into pores of soils with varying wettability properties, we hypothesized and developed a conceptual model for two key biomat flow mechanisms. The first mechanism considers lateral subsurface flow because of a permeability contrast between the much more porous and hence permeable biomat layer and the underlying mineral soil. The second mechanism involves a hydrophobic soil layer between the biomat and the underlying mineral soil. Flow across the hydrophobic layer is believed to occur when a threshold pore pressure is exceeded, i.e. as a result of a perched water table within the biomat layer. Lower pore pressures are needed to initiate flow when preferential flow paths exist, which are less hydrophobic than the surrounding organic layers. Numerical models of catchment hydrology should include lateral biomat flow when such layers are present in hillslope soils, in addition to typical subsurface flow within the soil matrix. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Biogeomorphic influence of soil depth to bedrock, volcanic ash soils, and surface tephra on silversword distribution, Haleakalā Crater (Maui, Hawai'i)

This research examines the spatial distribution of Haleakalā silverswords (Argyroxiphium sandwicense DC subsp. macrocephalum) over basaltic 'a'ā lava flows at Silversword Loop, Haleakalā Crater (Maui, Hawai'i). The overall study goal is to situate an extensive field data set within an historical geomorphic framework to propose a conceptual, biogeomorphic model of landscape development at Silversword Loop. Landscape features were investigated along two belt transects (111–120m long) crossing lava-flow deposits at ~2175m; the lava flows, >4.5km long, originated ~2700 YBP at Pu’u O Māui, a large cinder cone. Transects consist of contiguous 3 × 3m (9m2) quadrats; plot data include topographic profiles, soil depth to bedrock, size of surface tephra – determined by photosieving – and plant density; 23 soil samples were analyzed within 10 profiles. Weathered lava outcrops, tephra blocks (≥10cm), and cobbles (5–10cm) cover lava-flow ridges; the gently sloping flanks of lava-flow deposits merge into intervenient concave troughs, filled with volcanic ash and small tephra. Soils along ridges are thin (≤17cm), whereas profiles on troughs are deeper (≤110cm). Soils are sandy volcanic Inceptisols (Lithic, Typic, and Aridic Lithic Haplustepts) averaging 21.6% gravel, 96.8% sand, and 3.2% fines (silt+clay); mean organic matter is just 0.92%. These soils provide low water retention (23.2% at field capacity) and rapid infiltration. Deeper trough soils are largely devoid of plants. In contrast, silverswords are virtually restricted to shallow soils on lava-flow crests, where they densely cluster about outcrops and blocks: 90% of plants grow on soils ≤40cm deep, and 71.3% occupy plots with ≤40% ash cover; 84.4% of rosettes are rooted on outcrops or near stones; only 4.4% grow on soil; and 11.2% germinated on plant litter.Patterns of lava-flow weathering and soil depth to bedrock control silversword distribution at Silversword Loop. Shallow basalt bedrock and outcrops restrict soil drainage, providing a perched water table that silversword roots can exploit. Surface blocks also provide safe, stable microhabitats, thus rosettes grow preferentially near stones. Geomorphological, pedological, and ecological data are integrated into a biogeomorphic sequence of soil development and landscape evolution at Silversword Loop for the past three millennia, first through the eruptions and ash deposits of Pu'u O Māui, followed (~970–940 BP) by pyroclastic activity in Halāli'i and Ka lu'u o Ka 'O'o cinder cones.

Hydrological factors affecting rainfall-induced shallow landslides: From the field monitoring to a simplified slope stability analysis

Rainfall-induced shallow landslides can seriously affect cultivations and infrastructures and cause human losses. A continuous monitoring of unsaturated soil hydrological properties is needed to understand the effects of pore water pressure and water content on shallow landslides triggering and slope safety factor. In this work, the impact of water content, pore water pressure and hydrological hysteresis on safety factor reconstruction is analyzed by applying two different models (Lu and Godt's and SLIP models) to a monitored slope located in Oltrepò Pavese (Northern Italy). A shallow landslide event in the studied slope during the monitoring allowed for identifying the triggering mechanism and modeling the safety factor changes. The conditions of instability mechanism develop in periods with frequent rainfalls: the uprising of a perched water table in the soil profile gets nil or positive pore water pressure, which, linked with the nil effective soil cohesion of some soil horizons, allows for the shallow landslide triggering. The safety factor trends correctly predicted unstable conditions (safety factor≤1.0) in correspondence of the shallow landslide triggering time on the basis of both water content and pore water pressure. A better prediction gets when the safety factor is modeled considering the hysteresis effects. Modeling the safety factor on the basis of water content can make a good assessment of shallow failures triggering conditions only for failures in unsaturated soils and for completely saturated soils. Instead, considering pore water pressure, it is possible estimating safety factor changes linked with the development of positive pore water pressures, which can be linked in some cases to shallow landslide development.

Impacts of Prolonged Drought on Salt Accumulation in the Root Zone Due to Recycled Water Irrigation

Continuous use of recycled water (treated sewage effluent) over a long period of time may lead to the accumulation of salt in the root zone soil. This is due to the relatively higher levels of salt content in the recycled water compared to surface water. In this study, a laboratory column study was carried out to validate the HYDRUS 1D model under no rain condition. During the validation, the relative error and the % bias between observed and simulated soil water electrical conductivity (ECSW) were found to be low and varied in a range of 5–10 and 5–6 %, respectively. The validated model was then used to predict long-term (5 years) salt accumulation under drought conditions. The analysis of model predicted salt values showed a cyclical pattern of salt accumulation in the root zone, and this related to the variation in rainfall and evapotranspiration. The mean root zone ECSW in the 5th year was found to be within the highest salinity tolerance threshold for pasture (11.2 dS/m); however, the maximum root zone ECSW was found to be about 63 % more than the threshold. Irrespective of seasons, in 5 years time, ECSW at the depth of 1.0 m increased from 3.0 to 7.0 dS/m, which may pose a salinity risk to the groundwater table if there is a perched water table at a depth <1 m below the field surface. One of the management options to minimise long-term salt accumulation was also examined. By reducing the salt in recycled water by 50 %, it was possible to keep the ECsw within the recommended threshold values. Overall, the methodology developed in this study can be used to identify appropriate management options for sustainable recycled water irrigation.

Ice collapse over trapped water bodies on Enceladus and Europa

AbstractIcy satellite surfaces are riddled with geological indicators that can inform what lies beneath. Here we investigate effects of trapped or perched subsurface liquid water on the surface above. We use two models to investigate these effects: first, an analytical model to determine the flexural response of an ice shell to the existence of trapped water, and second, a discrete element model that simulates collapse of ice through granular interactions to determine possible topography that would result from this stress being placed on the ice. We find that for Europa's likely trapped water pockets below chaos terrains and a localized sea below Enceladus' South Polar Terrain, these models can reproduce observed geology of both features. Thus, we postulate that it is likely that Europa's chaos terrains and Enceladus' South Polar Terrain formed due to a similar mechanism: ice collapse above trapped water.

Les ouvrages hydrauliques souterrains de « Mazeran » et du « Garissou » (Béziers, 34) : des qanàts romains en Gaule Narbonnaise

In 2009, the excavation of a Roman shaft in the northeast of the town of Béziers (34) resulted in an underground tunnel dug in the Miocene molasses. The whole reception a perched water table and harvested percolation fine droplets contained in the sandstone aquifer. In 2011, two new Roman shafts were excavated a few hundred meters further north. Again, the archaeological work shows a similar pattern of operation. Already observed in the last century in some archaeological publications Biterroises this operating technique of groundwater wells gallery knows many similarities with the structures identified in Roman pole and larger scale throughout the Mediterranean. These millennia-hydraulic systems are grouped under the generic term «qanat » .

Long-term agroecosystem research in the central Mississippi river basin: introduction, establishment, and overview.

Many challenges currently facing agriculture require long-term data on landscape-scale hydrologic responses to weather, such as from the Goodwater Creek Experimental Watershed (GCEW), located in northeastern Missouri, USA. This watershed is prone to surface runoff despite shallow slopes, as a result of a significant smectitic clay layer 30 to 50 cm deep that restricts downward flow of water and gives rise to a periodic perched water table. This paper is the first in a series that documents the database developed from GCEW. The objectives of this paper are to (i) establish the context of long-term data and the federal infrastructure that provides it, (ii) describe the GCEW/ Central Mississippi River Basin (CMRB) establishment and the geophysical and anthropogenic context, (iii) summarize in brief the collected research results published using data from within GCEW, (iv) describe the series of papers this work introduces, and (v) identify knowledge gaps and research needs. The rationale for the collection derives from converging trends in data from long-term research, integration of multiple disciplines, and increasing public awareness of increasingly larger problems. The outcome of those trends includes being selected as the CMRB site in the USDA-ARS Long-Term Agro-Ecosystem Research (LTAR) network. Research needs include quantifying watershed scale fluxes of N, P, K, sediment, and energy, accounting for fluxes involving forest, livestock, and anthropogenic sources, scaling from near-term point-scale results to increasingly long and broad scales, and considering whole-system interactions. This special section informs the scientific community about this database and provides support for its future use in research to solve natural resource problems important to US agricultural, environmental, and science policy.

Producing Accurate and Complete Profile Descriptions

Producing Accurate and Complete Profile Descriptions