Shallow Groundwater Flow Research Articles

Mountain Permafrost Hydrology—A Practical Review Following Studies from the Andes

Climate change is expected to reduce water security in arid mountain regions around the world. Vulnerable water supplies in semi-arid zones, such as the Dry Andes, are projected to be further stressed through changes in air temperature, precipitation patterns, sublimation, and evapotranspiration. Together with glacier recession this will negatively impact water availability. While glacier hydrology has been the focus of scientific research for a long time, relatively little is known about the hydrology of mountain permafrost. In contrast to glaciers, where ice is at the surface and directly affected by atmospheric conditions, the behaviour of permafrost and ground ice is more complex, as other factors, such as variable surficial sediments, vegetation cover, or shallow groundwater flow, influence heat transfer and time scales over which changes occur. The effects of permafrost on water flow paths have been studied in lowland areas, with limited research in the mountains. An understanding of how permafrost degradation and associated melt of ground ice (where present) contribute to streamflow in mountain regions is still lacking. Mountain permafrost, particularly rock glaciers, is often conceptualized as a (frozen) water reservoir; however, rates of permafrost ground ice melt and the contribution to water budgets are rarely considered. Additionally, ground ice and permafrost are not directly visible at the surface; hence, uncertainties related to their three-dimensional extent are orders of magnitude higher than those for glaciers. Ground ice volume within permafrost must always be approximated, further complicating estimations of its response to climate change. This review summarizes current understanding of mountain permafrost hydrology, discusses challenges and limitations, and provides suggestions for areas of future research, using the Dry Andes as a basis.

Rainfall, runoff and shallow groundwater response in a mixed‐use, agro‐forested watershed of the northeast, USA

AbstractStream and shallow groundwater responses to rainfall are characterized by high spatial variability, but hydrologic response variability across small, agro‐forested sub‐catchments remains poorly understood. Conceivably, improved understanding in this regard will result in agricultural practices that more effectively limit nutrient runoff, erosion, and pollutant transport. Terrestrial hydrologic response approaches can provide valuable information on stream‐aquifer connectivity in these mixed‐use watersheds. A study was implemented, including eight stream and co‐located shallow groundwater monitoring sites, in a small sub‐catchment of the Chesapeake Bay watershed in the Northeast, USA to advance this ongoing need. During the study period, 100 precipitation‐receiving days (i.e., 24‐hour periods, midnight to midnight) were observed. On average, the groundwater table responded more to precipitation than stream stage (level change of 0.03 vs. 0.01 m and rainfall‐normalized level change estimate of 3.81 vs. 3.37). Median stream stage responses, groundwater table responses, and response ratios were significantly different between sub‐catchments (n = 8; p < 0.001). Study area average precipitation thresholds for runoff and shallow groundwater flow were 2.8 and 0.6 cm, respectively. Individual sub‐catchment thresholds ranged from 0.5 to 2.8 cm for runoff and 0.2 to 1.3 cm for shallow groundwater flow. Normalized response lag times between the stream and shallow groundwater ranged from −0.50 to 3.90 s·cm−1, indicating that stormflow in one stream section was regulated by groundwater flow during the period of study. The observed differences in hydrologic responses to precipitation advance future modelling efforts by providing examples of how terrestrial groundwater response methods can be used to investigate sub‐catchment spatial variability in stream‐aquifer gradients with co‐located shallow groundwater and stream stage data. Additionally, results demonstrate asynchronous stream and shallow groundwater responses on precipitation‐receiving days, which may hold important implications for modelling hydrologic and biogeochemical fate and transport processes in small, agro‐forested catchments.

Effect of DEM-smoothing and -aggregation on topographically-based flow directions and catchment boundaries

It is generally assumed that, in humid climates, the groundwater table is a subdued copy of the surface topography. However, the general groundwater table is unlikely to be affected by the microtopography as seen in high-resolution Digital Elevation Models (DEMs). So far, there has been little guidance on the best resolution DEM to use to determine the shape of the water table or the direction of shallow groundwater flow in headwater catchments. We, therefore, looked at the effects of DEM-smoothing and -aggregation on the calculated flow directions and derived catchment boundaries, and identified areas and landscape features for which the calculated flow directions are particularly sensitive to DEM smoothing or aggregation. For > 40 % of the area of the Krycklan study catchment, the calculated flow directions depend strongly on the degree of smoothing or aggregation of the DEM. The four main landscape features for which DEM smoothing or aggregation strongly affected the calculated flow directions were: local slopes in the opposite direction of the general slope, flat areas, ridges, and incised streambanks. To determine the effects of the changing flow directions on the derived catchment boundaries for the smoothed and aggregated DEMs, we calculated the drainage area for 40 locations, representing the outlets of catchments of varying sizes. The shape and size of the catchments of first-order streams were most affected by the processing of the DEM. These streams were often almost completely smoothed out during the DEM preprocessing steps. These shifts in catchment boundaries and drainage area would have a large effect on the water balance. This study thus highlights the need to carefully consider the effects of DEM smoothing or -aggregation on the calculated flow directions and drainage areas.

Geological Effects on the Scaling Relationships of Groundwater Contributions in Forested Watersheds

AbstractThe scaling relationships of groundwater contributions have been examined in previous studies, but whether these findings are site‐specific or applicable to other watersheds remains unclear. Here, we illustrate how scaling relationships differ between watersheds with different geology. We investigated the spatial distributions of the specific discharge (discharge per unit drainage area) and chemical compositions of stream and spring water during baseflow periods in three Cretaceous to Eocene sedimentary watersheds and three Neogene volcanic watersheds in the Ishikari River basin, Japan. End‐member mixing analysis was used to separate stream water into shallow and deep groundwater. The specific discharge of stream water only significantly increased with drainage area for two of the volcanic study sites. Conversely, the ratio of deep groundwater to stream water only significantly increased with drainage area for two of the sedimentary study sites. The specific discharge of deep groundwater increased with area in both volcanic and sedimentary sites, but the specific discharge of shallow groundwater only increased with area for the volcanic sites. These findings indicate that the shallow groundwater flow played a crucial role in making geology‐dependent differences in scaling relationships. The geological effects on shallow groundwater flow were likely characterized by the spatial distribution of highly permeable bedrock layer. This geological control provides a useful framework for generalizing knowledge about the scaling relationships of groundwater contributions.

Groundwater Flow Model In The Center Of West Progo Dome, Kaligesing, Purworejo, Central Java And Its Surrounding Area, Based On Hydrochemical And Isotopic Characteristics

Groundwater studies were carried out in the center of the West Progo Dome, at Kaligesing, Purworejo District, Central Java, and its surrounding area, with an emphasis on hydrochemical problems. As a water-scarce area, groundwater studies are urgently needed in this area. This research is intended as a hydrogeological study with the aim of knowing the conceptual groundwater flow model in the study area. The method used is a field hydrogeological survey as well as hydrochemical and natural isotope analysis supported by chemical and groundwater isotope data. Less clear hydrochemical evolution indicates that the process of groundwater flow is dominant in the local flow system. Groundwater facies is dominated by bicarbonate type, neutral pH, relatively low total dissolved solid (TDS), and electric conductivity (EC), and influenced by season or rainfall. The dominant hydrochemical processes in the groundwater system are leaching, ion exchange, sulfate reduction, and dilution. Groundwater facies is determined by the rock minerals marked by differences in hardness and TDS. Whereas, stable isotope contents of groundwater vary from light to heavy. Springs with light isotopes show the circulation of deep groundwater flow or from a relatively high recharge zone, either locally or from other places around it. Isotopic enrichment in all seasons can occur due to evaporation or mixing with surface water that has undergone previous evapotranspiration, indicated by increasing of heavy isotopes or δD-excess (d) of groundwater. There are two types of groundwater flow patterns, namely shallow and deep groundwater flow patterns. Shallow groundwater is characterized by heavy isotopes, shifted with relatively small d. Deep groundwater circulation pattern is characterized by a consistent, light δD value and appreciable d.

Unsaturated Flow Processes and the Onset of Seasonal Deformation in Slow‐Moving Landslides

AbstractPredicting rainfall‐induced landslide motion is challenging because shallow groundwater flow is extremely sensitive to the preexisting moisture content in the ground. Here, we use groundwater hydrology theory and numerical modeling combined with five years of field monitoring to illustrate how unsaturated groundwater flow processes modulate the seasonal pore water pressure rise and therefore the onset of motion for slow‐moving landslides. The onset of landslide motion at Oak Ridge earthflow in California’s Diablo Range occurs after an abrupt water table rise to near the landslide surface 52–129 days after seasonal rainfall commences. Model results and theory suggest that this abrupt rise occurs from the advection of a nearly saturated wetting front, which marks the leading edge of the integrated downward flux of seasonal rainfall, to the water table. Prior to this abrupt rise, we observe little measured pore water pressure response within the landslide due to rainfall. However, once the wetting front reaches the water table, we observe nearly instantaneous pore water pressure transmission within the landslide body that is accompanied by landslide acceleration. We cast the timescale to reach a critical pore water pressure threshold using a simple mass balance model that considers variable moisture storage with depth and explains the onset of seasonal landslide motion with a rainfall intensity‐duration threshold. Our model shows that the seasonal response time of slow‐moving landslides is controlled by the dry season vadose zone depth rather than the total landslide thickness.

Influence of groundwater and topography on stream drying in semi‐arid headwater streams

AbstractNon‐perennial streams comprise over half of the global stream network and impact downstream water quality. Although aridity is a primary driver of stream drying globally, surface flow permanence varies spatially and temporally within many headwater streams, suggesting that these complex drying patterns may be driven by topographic and subsurface factors. Indeed, these factors affect shallow groundwater flows in perennial systems, but there has been only limited characterisation of shallow groundwater residence times and groundwater contributions to intermittent streams. Here, we asked how groundwater residence times, shallow groundwater contributions to streamflow, and topography interact to control stream drying in headwater streams. We evaluated this overarching question in eight semi‐arid headwater catchments based on surface flow observations during the low‐flow period, coupled with tracer‐based groundwater residence times. For one headwater catchment, we analysed stream drying during the seasonal flow recession and rewetting period using a sensor network that was interspersed between groundwater monitoring locations, and linked drying patterns to groundwater inputs and topography. We found a poor relationship between groundwater residence times and flowing network extent (R2 < 0.24). Although groundwater residence times indicated that old groundwater was present in all headwater streams, surface drying also occurred in each of them, suggesting old, deep flowpaths are insufficient to sustain surface flows. Indeed, the timing of stream drying at any given point typically coincided with a decrease in the contribution from near‐surface sources and an increased relative contribution of groundwater to streamflow at that location, whereas the spatial pattern of drying within the stream network typically correlated with locations where groundwater inputs were most seasonally variable. Topographic metrics only explained ~30% of the variability in seasonal flow permanence, and surprisingly, we found no correlation with seasonal drying and down‐valley subsurface storage area. Because we found complex spatial patterns, future studies should pair dense spatial observations of subsurface properties, such as hydraulic conductivity and transmissivity, to observations of seasonal flow permanence.

Borehole Heat Exchangers-Addressing the Application Gap with Groundwater Science.

Hydrogeologists and mechanical engineers approach the design of geoexchange systems, and the associated borehole heat exchanger (BHE) fields, in different ways, each focusing on their knowledge areas. While these differences have created a strong research base, with well-published innovations and designs that collectively allow for sustainable systems, industry has not embraced these recent advancements. Despite abundant research demonstrating how complex shallow groundwater flow and temperature conditions can influence BHE design and operation, the low-temperature geothermal industry remains largely fixed on simple analytical codes and assumed uniform ground conditions. Geoexchange system inefficiencies become masked via reduced heat pump performance and increased electricity consumption. Similarly, interactions between BHE fields and infrastructure in urban areas are slow to manifest and are often unrealized due to a lack of field temperature data. While regulations that include hydrogeological factors have been developed in some jurisdictions, they are largely voluntary or rudimentary and can be unapplied in industry. Addressing this application gap may be unreasonable as designing and installing thermally efficient geoexchange systems can put them out of the cost envelope of competing heating and cooling systems. Perhaps for hydrogeologists, the way forward lies in the use of BHE's to facilitate contaminated sites remediation, an area we are familiar with, and one that allows for innovative technologies to reduce cost envelopes. Following that path, hydrogeologists can help improve system efficiencies while fully considering the dynamic nature of advective and thermal transport by groundwater.

Combining Uranium, Boron, and Strontium Isotope Ratios (234U/238U, δ11B, 87Sr/86Sr) to Trace and Quantify Salinity Contributions to Rio Grande River in Southwestern United States

In semi-arid to arid regions, both anthropogenic sources (urban and agriculture) and deeper Critical Zone (groundwater with long flow paths and water residence times) may play an important role in controlling chemical exports to rivers. Here, we combined two anthropogenic isotope tracers: uranium isotope ratios (234U/238U) and boron isotope ratios (δ11B), with the 87Sr/86Sr ratios to identify and quantify multiple solute (salinity) sources in the Rio Grande river in southern New Mexico and western Texas. The Rio Grande river is a major source of freshwater for irrigation and municipal uses in southwestern United States. There has been a large disagreement about the dominant salinity sources to the Rio Grande and particularly significant sources are of anthropogenic (agriculture practices and shallow groundwater flows, groundwater pumping, and urban developments) and/or geological (natural groundwater upwelling) origins. Between 2014 and 2016, we collected monthly river samples at 15 locations along a 200-km stretch of the Rio Grande river from Elephant Butte Reservoir, New Mexico to El Paso, Texas, as well as water samples from agricultural canals and drains, urban effluents and drains, and groundwater wells. Our study shows that due to the presence of localized and multiple salinity inputs, total dissolved solids (TDS) and isotope ratios of U, B, and Sr in the Rio Grande river show high spatial and temporal variability. Several agricultural, urban, and geological sources of salinity in the Rio Grande watershed have characteristic and distinguishable U, Sr, and B isotope signatures. However, due to the common issue of overlapping signatures as identified by previous tracer studies (such as δ18O, δD, δ34S), no single isotope tracer of U, Sr, or B isotopes was powerful enough to distinguish multiple salinity sources. Here, combining the multiple U, Sr, and B isotope and elemental signatures, we applied a multi-tracer mass balance approach to quantify the relative contributions of water mass from the identified various salinity end members along the 200-km stretch of the Rio Grande during different river flow seasons. Our results show that during irrigation (high river flow) seasons, the Rio Grande had uniform chemical and isotopic compositions, similar to the Elephant Butte reservoir where water is stored and well-mixed, reflecting the dominant contribution from shallow Critical Zone in headwater regions in temperate southern Colorado and northern New Mexico. In non-irrigation (low flow) seasons when the river water is stored at Elephant Butte reservoir, the Rio Grande river at many downstream locations showed heterogeneous chemical and isotopic compositions, reflecting variable inputs from upwelling of groundwater (deeper CZ), displacement of shallow groundwater, agricultural return flows, and urban effluents. Our study highlights the needs of using multi-tracer approach to investigate multiple solutes and salinity sources in rivers with complex geology and human impacts.

Permafrost Promotes Shallow Groundwater Flow and Warmer Headwater Streams

AbstractThe presence of permafrost influences the flow paths of water through Arctic landscapes and thereby has the potential to impact stream discharge and thermal regimes. Observations from 11 headwater streams in Alaska showed that July water temperatures were higher in catchments with more near‐surface permafrost. We apply a fully coupled cryohydrology model to investigate if the impact of permafrost on flow path depth could cause the same pattern in temperatures of groundwater discharging from hillslopes to streams. The model simulates surface energy and water balances, snow, and subsurface water and energy balances for two‐dimensional hillslope model cases with varying permafrost extent. We find that hillslopes with continuous permafrost have more shallow flow paths and twice as high rates of evapotranspiration, compared to hillslopes with no permafrost. For our simulated cases, 6.7% of the horizontal water flux moves through the top organic soil layers when there is continuous permafrost, while only 0.5% moves through organic layers without permafrost. The deeper flow paths in permafrost‐free simulations buffer seasonal temperature extremes, so that summer groundwater discharge temperatures are highest with continuous permafrost. Our results suggest that permafrost thawing alters groundwater flow paths and can lead to decreases in summer stream temperatures and reductions in evapotranspiration in headwater catchments. These changes are of potential importance for stream biotic components of ecosystems, however, the full impact remains unknown.

Reduction of nutrient loads from agricultural subsurface drainage water in a small, edge-of-field constructed treatment wetland

Constructed treatment wetlands are a common practice for filtering nitrogen from agricultural sub-surface (i.e. tile) drainage in the Midwest. Wetland size recommendations typically depend on watershed size, but little is known about the effectiveness of wetlands with a pooled area as small as 0.11 ha combined with a high infiltration rate and sub-surface treatment. A small, edge-of-field wetland treating tile drainage from a 10.1-ha row-crop field was constructed in 2013 adjacent to Elm Creek in Martin County, MN. The water, nitrate‑nitrogen (NO3−-N), and phosphorus (P) budgets were determined for 2013 to 2016. The wetland received an annual average of 12.6 m drainage water, 185 kg NO3−-N, and 1.3 kg total phosphorus (TP) each of the four years of this study. It removed 67% of the input NO3−, but TP removal was inconclusive. The majority of NO3− was removed in the shallow groundwater flow after it infiltrated from the surface. Approximately 3.2 kg-P ha−1 yr−1 was removed by harvesting the vegetation in the wetland. Therefore, it can be concluded that NO3− removal in the shallow groundwater and TP removal through vegetation harvest were the primary means of removing each nutrient.

Analysis of Spatiotemporal Variability of Corn Yields Using Empirical Orthogonal Functions

We used empirical orthogonal functions (EOF) to analyze the spatial and temporal patterns of corn (Zea mays L.) yields at three hydrologically-bounded fields with shallow subsurface preferential lateral flow pathways. One field received uniform application of manure, the second field received the uniform applications of the chemical nitrogen fertilizer, and the third field received variable rate applications of the chemical fertilizer. The preferential subsurface flow and storage pathway locations were inferred from the ground penetration radar survey. Six-year monitoring data were analyzed. Statistical distributions of EOFs across fields were approximately symmetrical. Semivariograms of the first EOF differed between fields receiving manure and chemical fertilizer. This EOF accounted for 52 to 56% of the interannual variability of yields, and its values reflected the distance to the subsurface flow and storage pathways. The second and third EOF explained 17 to 20% and 10 to 13% of the interannual variability of yields, respectively. The precision applications of the nitrogen fertilizer minimized corn yield variability associated with subsurface preferential flow patterns. Investigating spatial patterns of yield variability under shallow groundwater flow control can be beneficial for the within-field crop management resource allocation.

Extension of Bayesian chemistry-assisted hydrograph separation to reveal water quality trends (BACH2)

A Bayesian chemistry-assisted hydrograph separation (BACH) approach was previously demonstrated using 15 years of monthly total phosphorus (TP) and total nitrogen (TN) data from eight mesoscale catchments in New Zealand’s North Island. Calibration was done separately for three 5-year data periods, and in each period, concentrations of the two tracers (TP and TN) discharged from each of the three separated flow paths—fast (event-response near-surface flow), medium (seasonal shallow local groundwater flow), and slow (persistent deeper regional groundwater flow)—were assumed to be constant. This approach has now been extended to reveal non-linear trends in the tracer concentrations in each flow path, each represented using a four-parameter curve (initial and final values of a linear trend plus two harmonics). The extended method (called BACH2) identified clear TP and TN concentration trends in the medium and slow flow paths in most of the eight catchments. TP and TN concentration trends in the fast flow path were generally uncertain, however, due to the infrequency and inherent variability of concentrations sampled during high flow conditions. Concentrations closely matched previously published results from the constant-concentration BACH model calibrated to shorter data series. The BACH2 approach is a powerful tool for revealing concentration trends in the different pathways that sustain stream flow using commonly available water quality and flow data. This type of analysis has not previously been available outside of complex distributed simulation models.

Impacts of long-term climate change on the groundwater flow dynamics in a regional groundwater system: Case modeling study in Alashan, China

Groundwater flow dynamics plays a crucial role in social development and ecosystem protection. This study investigated the impacts of long-term climate change on the regional groundwater flow in Alashan, China to reveal the spatial variability of groundwater flow response to climate change and the dynamic patterns of both deep and shallow groundwater flow velocity. To this end, the hydrogeologic conditions and climate evolution history were analyzed. Based on these, four future scenarios of long-term climate change were designed, including linearly varying and cyclic changes, and numerical groundwater flow modeling was then conducted to investigate the effects of long-term climate changes on groundwater flow dynamics (including groundwater level and flow velocity). It was found that groundwater level dynamics were notably influenced by climate changes; the time scale and initial change pattern of cyclic climate influenced the amplitude and mean level of groundwater level fluctuations. In addition, our results also revealed that, for a regional groundwater system, climate change impacts on groundwater level had obvious spatial variability. Groundwater level in recharge and discharge areas were sensitive to drying and wetting climates, respectively. Thus, the spatial variability of groundwater level dynamics reflected climate change information. Moreover, groundwater flow velocity dynamics were also affected by climate changes. However, linearly varying and cyclic climates had different influences. Linearly wetting climate generally increased the groundwater flow velocity in recharge and discharge areas, but decreased that in runoff area, whereas cyclic climate induced periodical fluctuation of groundwater flow velocity. The fluctuation period of shallow groundwater flow velocity matched the full climate cycle period, whereas of the deep groundwater flow velocity was only half. This unique behavior of deep groundwater flow velocity is caused by fluctuation of the vertical velocity alternating between positive (upward flow) and negative (downward flow) values. This finding of groundwater flow velocity dynamics contributes to deeper understanding of long-term climate change effects on groundwater flow dynamics.

Shallow groundwater flow and inverted fresh/saline-water interface in a hypersaline endorheic basin (Great Basin, USA)

Pilot Valley is an 828-km2 arid-region endorheic basin in western USA. Bounding mountain ranges rise as much as 1,900 m above the nearly flat 379-km2 playa floor. Up to 3.8 m of Pleistocene Lake Bonneville mud and thin oolitic sand layers form the surface layer of the basin floor. Groundwater conditions were evaluated using data from shallow monitoring wells and borings, springs, infiltrometer measurements, slug and dilution tests, geophysical transects, and precision elevation surveys. Alluvial fan groundwater discharges at fan/playa interface springs and underflows to the shallow basin sediments along the western side of the basin; the groundwater only underflows along the eastern side. Precision surveying established a Lake Bonneville shore-line break in slope as the cause of the spring discharges. Tectonic tilting causes groundwater to flow from east to west and to the topographic low. Monthly measured and pressure transducer data established seasonal pressure responses and upward groundwater gradients. All basin groundwater is lost to evapotranspiration at the topographic low, where a thin salt pan has developed. Groundwater evolves from fresh to hypersaline near the alluvial fan/playa interface where there is an inverted salinity gradient and a groundwater pressure ridge. The pressure ridge and inverted salinity interface are due to: (1) osmotic pressure established between the oolitic sand of high hydraulic conductivity and the overlying low-hydraulic-conductivity lake mud at the fan/playa interface, and (2) the collision between fresh groundwater flow driven by a steep hydraulic head and hypersaline groundwater flow driven by a nearly flat hydraulic head.

Calibrated Simulation of the Long‐Term Average Surficial Groundwater System and Derived Spatial Distributions of its Characteristics for the Contiguous United States

AbstractWhile the physical processes governing groundwater flow are well understood, and the computational resources now exist for solving the governing equations in three dimensions over continental‐scale domains, there remains substantial uncertainty about the subsurface distribution of the properties that control groundwater flow and transport for much of the contiguous United States (CONUS). The transmissivity of the shallow subsurface is a key parameter for the simulation of water table position, shallow groundwater flow, and base‐flow discharge, but is not well‐characterized at large regional to continental scales. We used a process‐based inversion of CONUS‐extent groundwater information to generate national data sets of (a) the transmissivity of the shallow groundwater system, (b) the depth to the water table, (c) groundwater discharge as base‐flow, and (d) long‐term average water content in the unsaturated zone. CONUS‐extent coverage was developed in the form of 75 subdomain models, with the spatial distribution of long‐term average transmissivity for each subdomain model calibrated against water‐levels derived from U.S. Geological Survey (USGS) observation wells, NHDPlusV2 first‐order perennial streams, and National Wetlands Inventory (NWI) freshwater wetlands. Estimated transmissivities were lower in the western CONUS than the eastern CONUS, and across the CONUS both transmissivity and depth to water correlate with recharge, elevation, and topographic slope. These generated data sets provide spatially distributed, long‐term average estimates of subsurface properties and hydrological states that we anticipate will complement other environmental modeling efforts as explanatory variables, boundary conditions, or transport pathways.

Precipitation recharges the shallow groundwater of check dams in the loessial hilly and gully region of China

Check dams affect regional hydrological cycles and ecological environments. We conducted a field monitoring experiment in the Liudaogou Catchment on the Loess Plateau in China to determine the spatiotemporal response of shallow groundwater recharge and base flow by precipitation in check dams of this loessial hilly and gully region. The amount and seasonal distribution of precipitation directly affected the changes in shallow groundwater and base flow. The shallow groundwater was recharged by in situ vertical infiltration and lateral underground movement. Precipitation >30.0 mm d−1 recharged the shallow groundwater by piston flow when the water table in the check dam was <4.0 m. When the water table varied from the head (>4.0 m) to the middle and tail (<4.0 m) of the check dam, the influx of precipitation was by runoff in the catchment above the trench head, replenishing the groundwater vertically through the trench head and then moving laterally underground to the end of the dam. The response of the base flow and groundwater lagged the precipitation by a certain period. The lag time of the base flow was about 5–16 days, in which the response was more sensitive than for shallow groundwater. The lag time of the vertical supply for the response of the precipitation to the shallow groundwater was about 12–54 days, whereas the lag time for the lateral supply was about 72–93 days. The lag time may be associated with precipitation, temperature, solar radiation, vegetation water consumption and soil porosity. These findings will help elucidate the processes of groundwater recharge and provide new insight for managing the water balance in this loessial hilly and gully region.

Heavy Metal (As and Hg) contamination of shallow groundwater in the coastal areas of Pati and Rembang, Central Java, Indonesia

The current study aims to assess the contamination level of heavy metal in the shallow aquifer of groundwater in Pati and Rembang coastal areas. The analysis result indicated that arsenic and mercury were detected in the groundwater in the range 0.004 µg L−1 and 0.115-0.310 µg L−1, respectively. Referred to the standard of the heavy metals contents in the consumption water by WHO and Indonesian Water Quality Standard for Ground Water (IWQS), groundwater of coastal areas in Pati and Rembang Regions were polluted by arsenic and mercury. The result has confirmed the existence of arsenic and mercury pollution of some shallow groundwater flow in the coastal areas of Pati and Rembang.

Lateral and longitudinal distribution of riparian vegetation along an ephemeral river in Namibia using remote sensing techniques

Ephemeral rivers are key features of desert ecosystems, providing shallow groundwater and occasional surface flow, but compared to perennial systems, the structure of riparian forests in ephemeral rivers and their interactions with the hydrologic regime are poorly understood. This study examines the distribution of riparian woody species in the ephemeral Kuiseb River in central Namibia. Unmanned aerial vehicle (UAV) and satellite imagery were used to examine the structure of the riparian forest across lateral and longitudinal gradients. Woody plant abundance peaked in the terminal alluvial zone, the reach of the river where floods typically run dry. The five dominant species exhibited similar spatial patterns at latitudinal and longitudinal scales. Faidherbia albida and Salvadora persica were found more proximal to the active channel and concentrated in the terminal alluvial zone. Acacia (Vachellia) erioloba, Euclea pseudebenus, and Tamarix usneoides were more evenly dispersed laterally across the riparian zone and along the river's longitudinal profile. These spatial patterns correspond to plant physiological properties of hydrological dependence and gradients of water availability and soil composition. The tight coupling of plant distributions with water availability highlights the need for long-term monitoring of ephemeral riparian vegetation in light of potential future changes to the global hydrologic regime.

Geochemical Evolution and Groundwater Flow System in Batujajar Groundwater Basin Area, West Java, Indonesia

DOI:10.17014/ijog.7.1.87-104Batujajar and its surrounding areas, situated in the west of the Bandung Basin, are geologically composed of various Tertiary rock formations with complex fold and fault systems. Springs and drilled wells in sandstone aquifers, tuffaceous sand, and tuffaceous breccias mark the development of their aquifer systems. This study aimed to determine the characteristics of the hydrogeochemistry by analyzing major ions and stable isotopes (18O and 2H) of thirty-four hydrogeological objects. The groundwater flow pattern in this area is controlled by northwest-southeast lineament, as indicated by the emergence of springs along the transition zone between areas with high and low lineament density. In order of dominance, the groundwater facies are as follows:Ca-Mg-HCO3&gt;Ca-HCO3&gt;Ca-Na-Mg-HCO3&gt;Na-Ca-HCO3. Hydrochemical evolutions were detected from the change of cations from Ca2+ to Mg2+ and then Na+, and this is believed to be the product of cation exchange and dissolution of silicate minerals. However, evolutions toward anion changes were not apparent yet, meaning that HCO3- ions still prevail, or in other words the groundwater flow system is local. Nevertheless, the geological and hydrogeochemical analyses, along with the relative compositions of stable isotopes, revealed that the groundwater had three systems, namely shallow, intermediate, and deep groundwater flows; all of which were segmented in three subgroundwater basins (Sub-GWB). Aquifer systems with shallow to intermediate groundwater flow were found in Sub-GWB-A, Sub-GWB-B, and Sub-GWB-C, while the other ones with deep groundwater flow system were identified in Sub-GWB-B and Sub-GWB-C.Fracture system has an important role as a connector between recharge system in hilly areas and discharge system in plain areas.