Impact of artificial subsurface drainage on groundwater travel times and baseflow discharge in an agricultural watershed,Iowa (USA)

Pollutant delivery through artificial subsurface drainage networks to streams is an important transport mechanism, yet the impact of drainage tiles on groundwater hydrology at the watershed scale has not been well documented. In this study, we developed a two‐dimensional, steady‐state groundwater flow model for a representative Iowa agricultural watershed to simulate the impact of tile drainage density and incision depth on groundwater travel times and proportion of baseflow contributed by tile drains. Varying tile drainage density from 0 to 0.0038 m^?1, while maintaining a constant tile incision depth at 1.2 m, resulted in the mean groundwater travel time to decrease exponentially from 40 years to 19 years and increased the tile contribution to baseflow from 0% to an upper bound of 37%. In contrast, varying tile depths from 0.3 to 2.7 m, while maintaining a constant tile drainage density of 0.0038 m^?1, caused mean travel times to decrease linearly from 22 to 18 years and increased the tile contribution to baseflow from 30% to 54% in a near‐linear manner. The decrease in the mean travel time was attributed to decrease in the saturated thickness of the aquifer with increasing drainage density and incision depth. Study results indicate that tile drainage affects fundamental watershed characteristics and should be taken into consideration when evaluating water and nitrate export from agricultural regions. Copyright © 2011 John Wiley & Sons, Ltd.     

Oxygen‐18 dynamics in precipitation and streamflow in a semi‐arid agricultural watershed,Eastern Washington,USA

Understanding flow pathways and mechanisms that generate streamflow is important to understanding agrochemical contamination in surface waters in agricultural watersheds. Two environmental tracers, δ¹⁸O and electrical conductivity (EC), were monitored in tile drainage (draining 12 ha) and stream water (draining nested catchments of 6‐5700 ha) from 2000 to 2008 in the semi‐arid agricultural Missouri Flat Creek (MFC) watershed, near Pullman Washington, USA. Tile drainage and streamflow generated in the watershed were found to have baseline δ¹⁸O value of ?14·7‰ (VSMOW) year round. Winter precipitation accounted for 67% of total annual precipitation and was found to dominate streamflow, tile drainage, and groundwater recharge. ‘Old’ and ‘new’ water partitioning in streamflow were not identifiable using δ¹⁸O, but seasonal shifts of nitrate‐corrected EC suggest that deep soil pathways primarily generated summer streamflow (mean EC 250 µS/cm) while shallow soil pathways dominated streamflow generation during winter (EC declining as low as 100 µS/cm). Using summer isotopic and EC excursions from tile drainage in larger catchment (4700‐5700 ha) stream waters, summer in‐stream evaporation fractions were estimated to be from 20% to 40%, with the greatest evaporation occurring from August to October. Seasonal watershed and environmental tracer dynamics in the MFC watershed appeared to be similar to those at larger watershed scales in the Palouse River basin. A 0·9‰ enrichment, in shallow groundwater drained to streams (tile drainage and soil seepage), of δ¹⁸O values from 2000 to 2008 may be evidence of altered precipitation conditions due to the Pacific Decadal Oscillation (PDO) in the Inland Northwest. Copyright © 2009 John Wiley & Sons, Ltd.     

Using environmental isotopes to investigate the groundwater recharge mechanisms and dynamics in the North-eastern Basin,Palestine

ABSTRACT

This study aims to differentiate the potential recharge areas and flow mechanisms in the North-eastern Basin, Palestine. The results differentiate the recharge into three main groups. The first is related to springs and some of the deep wells close to the Anabta Anticline, through the Upper Aquifer (Turonian) formation, with depleted δ¹⁸O and δ²H. The second is through the Upper Cenomanian formation surrounding the Rujeib Monocline in the southeast, where the lineament of the Faria Fault plays an important role, with relatively enriched δ¹³C_DIC values of about ?4‰ (VPDB). The third is the Jenin Sub-series, which shows higher δ¹³C_DIC values, with enriched δ¹⁸O and δ²H and more saline content. The deep wells from the Nablus area in the south of the basin indicate low δ¹³C_DIC values due to their proximity to freshwater infiltrating faults. The deep wells located to the northwest of the basin have δ¹³C_DIC values from ?8 to ?9‰ (VPDB), with enriched δ¹⁸O signatures, indicating slow recharge through thick soil.     

Estimation of baseflow nitrate loads by a recursive tracing source algorithm in a rainy agricultural watershed

Baseflow has become an important source of nitrate nonpoint source pollution in many intensive agricultural watersheds. Uncertainties in baseflow nutrient load separation are caused by the effects of hydrometeorological factors on both baseflow recession and baseflow nutrient load recession. These uncertainties have not been addressed well in the existing separating algorithms, which are based on simple baseflow rate–load relationships. In the present study, a recursive tracing source algorithm (RTSA) was developed based on a nonlinear reservoir algorithm and hydrometeorology-corrected baseflow nutrient load recession parameter. This approach was used to reduce the uncertainty of baseflow nitrate load estimation caused by variations in different load recessions under varying climate conditions. RTSA validation in a typical rainy agricultural watershed yielded Nash–Sutcliffe efficiency, root mean square error-observation standard deviation ratio, and R² values of 0.91, 0.30, and 0.91, respectively. The baseflow nitrate–nitrogen (N─NO₃^–) loads from 2003 to 2012 in the Changle River watershed of eastern China were estimated with the RTSA. The results indicated that baseflow nitrate export accounted for 62.0% of the mean total annual N─NO₃^– loads (18.0 kg/ha). The total baseflow N─NO₃^– export was highest in spring (3.6 kg/ha), followed by summer (3.2 kg/ha), winter (2.3 kg/ha), and autumn (2.1 kg/ha). The contribution of baseflow to total nitrate in the stream decreased in the order of winter (69.88%) >spring (66.59%) >autumn (60.36%) >summer (54.04%). The monthly baseflow N─NO₃^– loads and flow-weighted concentrations greatly increased during the research period (Mann–Kendall test, Z_s > 2.56, p < .01). Without proper countermeasures, baseflow nitrate may represent a serious long-term risk for water surfaces in the future.     

Performance of methods for estimating the time of concentration in a watershed of a tropical region

The time of concentration (T_c) is a fundamental parameter in the design of hydrological projects for watersheds. In this study a graphical methodology is described for estimating T_c in a watershed, and this is applied to 17 rainfall–runoff events from a rural watershed located near the capital city of Mato Grosso do Sul State, in the Brazilian Cerrado. The T_c values obtained through the graphical method were compared to T_c values estimated using 20 equations from various references. The equations were selected by considering those that were not developed using data for urban watersheds, and the results of the graphical method were compared to those derived by applying the equations to sub-basin data. The graphical method was reliable in determining T_c, and Ventura’s equation was found to present the best performance for a rural watershed in a tropical climate region.     

Field investigation of the groundwater contribution to baseflow in an urban stream from a Quaternary aquifer with a leaky base

Groundwater contributions to baseflow in Minnehaha Creek, a creek located in a highly developed watershed in the Minneapolis-St. Paul metropolitan area, from the watershed's Quaternary aquifer were quantified as part of an effort to manage low flow conditions in the creek. Considerable uncertainty exists with any single method used to quantify groundwater contributions to baseflow; therefore, a “weight of evidence” approach in which methods spanning multiple spatial scales was utilized. Analyses conducted at the watershed-scale (streamflow separation and stable isotope analyses) were corroborated with site-scale measurements (piezometer, seepage meter, and streambed temperature profiles) over a multi-year period to understand processes and conditions controlling connectivity between the stream, its shallow aquifer system and other flow sources. In the case of Minnehaha Creek, groundwater discharge was found to range from 6.2 to 23 mm year⁻¹, which represented only 5 to 11% of annual streamflow during the study period. From the weight of evidence, it is conjectured that regional-scale hydrogeological conditions control groundwater discharge in Minnehaha Creek. Implications of these results with regard to possible augmentation of baseflow by increasing groundwater recharge with infiltration of stormwater are discussed.     

Groundwater monitoring at the watershed scale: An evaluation of recharge and nonpoint source pollutant loading in the Clear Creek Watershed,Iowa

Determining the groundwater contribution of nonpoint source pollution at a watershed scale is a challenging issue. In this study, we utilized a top‐down approach to characterize representative groundwater response units (GRUs) based on land use and landscape position (e.g., upland, sideslope, or floodplain) in the 275‐km² Clear Creek Watershed, Iowa. Groundwater monitoring wells were then established along downslope transects in representative GRUs. This unique combination of top‐down/bottom‐up approaches allowed us to estimate groundwater pollutant loads at the watershed scale with minimal monitoring. For the 2015 study period, results indicated that more groundwater recharge occurred in the floodplain (404 mm) compared to the uplands or sideslopes (281 and 165 mm, respectively), irrespective of land use. Recharge in the floodplains consisted of 37% of the annual precipitation, whereas upland wells averaged 26% and sideslopes averaged 15% of the annual precipitation. Less recharge was found to occur beneath perennial grass compared to row crop and urbanized areas. Baseflow discharge accounted for 69% of the total NO₃‐N exported from the Clear Creek Watershed, with row crop areas contributing approximately 95% of the annual load. Orthophosphorus (OP) yields were approximately 0.72 kg/ha beneath urban and suburban areas, three times higher than those in row crop or perennial areas. Urban and suburban areas accounted for 21.4% of groundwater orthophosphorus and chloride loads in the watershed compared to only 8.5% of the land area. Overall, the groundwater load allocation model for baseflow nutrient discharge to Clear Creek can be used to target future nonpoint source load reduction strategies at the watershed scale. The use of GRUs can pinpoint better areas of concern for controlling nutrient loads.     

Relating hydrogeomorphic properties to stream buffering chemistry in the Neversink River watershed,New York State,USA

Monitoring the effects of acidic deposition on aquatic ecosystems in the Northeastern US has generally required regular measurements of stream buffering chemistry (i.e. acid‐neutralizing capacity (ANC) and calcium Ca²⁺), which can be expensive and time consuming. The goal of this paper was to develop a simple method for predicting baseflow buffering chemistry based on the hydrogeomorphic properties of ten nested watersheds in the Neversink River basin (2·0–176·0 km²), an acid‐sensitive basin in the Catskill Mountains, New York State. The tributaries and main reach watersheds have strongly contrasting mean baseflow ANC values and Ca²⁺ concentrations, despite rather homogeneous vegetation, bedrock geology, and soils. A stepwise regression was applied to relate 13 hydrogeomorphic properties to the mean baseflow ANC values and Ca²⁺ concentrations. The regression analysis showed that watersheds with lower ANC values had a higher mean ratio of ‘quickflow’ runoff to precipitation during 20 non‐snowmelt runoff events (referred to as mean runoff ratio). The mean runoff ratio could explain at least 80% of the variability in mean baseflow ANC values and Ca²⁺ concentrations among the ten watersheds. Greater mean runoff ratios also correlated with steeper slopes and greater drainage densities, thus allowing the prediction of baseflow ANC values (r² = 0·75) and Ca²⁺ concentrations (r² = 0·77) with widely available spatial data alone. These results indicate that hydrogeomorphic properties can predict a watershed's sensitivity to acid deposition in regions where the spatial sources of stream buffering chemistry from the bedrock mineralogy and soils are fairly uniform. Copyright © 2010 John Wiley & Sons, Ltd.     

Baseflow separation in a small watershed in New Brunswick,Canada, using a recursive digital filter calibrated with the conductivity mass balance method

Baseflow separation is important for obtaining critical parameters for hydrological models. As measuring the baseflow component directly is difficult, various analytical and empirical baseflow separation methods have been developed and tested. The recursive digital filter (RDF) method is commonly used for baseflow separation due to its simplicity and low data requirement. However, parameters used in the RDF method are often determined arbitrarily, resulting in high uncertainty of the estimated baseflow rate. A more accurate method is the conductivity mass balance (CMB) method, which is established based on the differences in physical processes between baseflow and surface runoff. In this research, the output of the CMB method was used to calibrate the parameters of an RDF model, and the calibrated RDF model was used to estimate monthly, seasonal and annual baseflow rate and baseflow index for the past 19 years using streamflow discharge records. The characteristics of the baseflow hydrographs were found to be consistent with the hydrological and hydrogeological conditions of the research area. Research results indicated that the accuracy of the RDF model has been greatly enhanced after being calibrated with the CMB method so that the RDF model can provide more reliable baseflow separation results for a long‐term study. Copyright © 2012 John Wiley & Sons, Ltd.     

The role of baseflow in dissolved solids delivery to streams in the Upper Colorado River Basin

Salinity has a major effect on water users in the Colorado River Basin, estimated to cause almost $300 million per year in economic damages. The Colorado River Basin Salinity Control Program implements and manages projects to reduce salinity loads, investing millions of dollars per year in irrigation upgrades, canal projects, and other mitigation strategies. To inform and improve mitigation efforts, there is a need to better understand sources of salinity to streams and how salinity has changed over time. This study explores salinity in the baseflow fraction of streamflow, assessing whether groundwater is a significant contributor of dissolved solids to streams in the Upper Colorado River Basin (UCRB). Chemical hydrograph separation was used to estimate baseflow discharge and baseflow dissolved solids loads at stream gages (n = 69) across the UCRB. On average, it is estimated that 89% of dissolved solids loads originate from the baseflow fraction of streamflow, indicating that subsurface transport processes play a dominant role in delivering dissolved solids to streams in the UCRB. A statistical trend analysis using weighted regressions on time, discharge, and season was used to evaluate changes in baseflow dissolved solids loads in streams (n = 27) from 1986 to 2011. Decreasing trends in baseflow dissolved solids loads were observed at 63% of streams. At the three most downstream sites, Green River at Green River, UT, Colorado River at Cisco, UT, and the San Juan River near Bluff, UT, baseflow dissolved solids loads decreased by a combined 823,000 metric tons (mT), which is approximately 69% of projected basin‐scale decreases in total dissolved solids loads as a result of salinity control efforts. Decreasing trends in baseflow dissolved solids loads suggest that salinity mitigation projects, landscape changes, and/or climate are reducing dissolved solids transported to streams through the subsurface. Notably, the pace and extent of decreases in baseflow dissolved solids loads declined during the most recent decade; average decreasing loads during the 2000s (28,200 mT) were only 54% of average decreasing loads in the 1990s (51,700 mT).     

Suspended sediment discharge in subsurface flow from the head hollow of a small forested watershed,northern Japan

Surface flow and suspended sediment discharge from the head hollow of the Jozankei Experimental Watershed in Hokkaido, northern Japan, were measured to clarify the implications of subsurface hydrology for soil movement. Subsurface discharges during the extremely large storms of 1993 to 1994 were measured in a V-notch weir installed at a natural spring near the bottom of the head hollow, and shallow groundwater levels were observed in the wells excavated in the hollow. Sediment samples whose particle size range from 0·001 to 0·1 mm were manually and automatically collected at 15 to 60 min intervals, by use of 1 or 21 polyethylene bottles. Maximum concentration and flux of suspended sediment during the storms preceded the peak discharge of subsurface flow by several hours. Neither the changes in concentration (mg l⁻¹) nor flux (mg s⁻¹) of suspended sediment coincided with those in subsurface discharge (l s⁻¹). Furthermore, sediment concentration was poorly correlated with the rate of change in subsurface discharge (l s⁻²) during the rising limb of the hydrograph. Suspended sediment flux during the acceleratory limb, however, was closely correlated with the rate of change in subsurface discharge. The relationship between suspended sediment flux and rate of change in subsurface discharge were in inverse proportion to initial subsurface discharge before the storm runoff and they represented rare seasonal variation. Subsurface hydraulic erosion and transport of suspended sediment resulting from changes in rate of change in subsurface discharge actively occur during the acceleratory rising limb of the hydrograph. Accordingly, subsurface hydraulic erosion during the acceleratory rising limb of the hydrograph can be physically understood by analysing suspended sediment flux associated with rate of change in subsurface discharge and initial subsurface discharge. © 1997 John Wiley & Sons, Ltd.     

Snow hydrology of a constructed watershed in the Athabasca oil sands region,Alberta, Canada

Wetlands are now being integrated into oil sands mining landscape closure design plans. These wetland ecosystems will be constructed within a regional sub‐humid climate where snowfall represents ~25% of annual precipitation. However, few studies focus on the distribution of snow and, hence, the storage of winter precipitation in reclaimed oil sands landscapes. In this study, the distribution, ablation and fate of snowmelt waters are quantified within a constructed watershed in a post‐mining oil sands environment. Basin‐averaged peak SWE was 106 mm, with no significant difference between reclaimed slopes with vegetation and those that were sparsely vegetated or bare. Snow depth was greatest and more variable near the toe of slopes and became progressively shallower towards the crest. Snow ablation started first on the vegetated slope, which also exhibited the maximum observed ablation rates. This enhanced melt was attributed to increased absorption of short‐wave radiation by vegetation stems and branches. Recharge to reclaimed slopes and a constructed aquifer during the snowmelt period was minimal, as the presence of ground frost minimized infiltration. Accordingly, substantial surface run‐off was observed from all reclaimed slopes, despite being designed to reduce run‐off and increase water storage. This could result in increased flashiness of downstream watercourses during the spring freshet that receive run‐off from post‐mining landscapes where large reclaimed slopes are prolific. Run‐off ratios for the reclaimed slopes were between 0.7 and 0.9. Thus, it is essential to consider snow dynamics when designing landscape‐scale constructed ecosystems. This research demonstrates that the snowmelt period hydrology within reclaimed landscapes is fundamentally different from that reported for natural settings and represents one of the first studies on snow dynamics in constructed watershed systems in the post‐mined oil sands landscape. Copyright © 2016 John Wiley & Sons, Ltd.     

Assessing baseflow index vulnerability to variation in dry spell length for a range of catchment and climate properties

Baseflow index (BFI) prediction in ungauged basins has largely been based on the use of catchment physiographic attributes as dominant variables. In a context where changes in climate are increasingly evident, it is also important to study how the slow component of flow is potentially affected by climate. The aim of this study was to illustrate the impact of climate variability on the baseflow process based on analysis of daily rainfall characteristics and hydrological modelling simulation exercises validated with observed data. Ten catchments were analysed that span southern to northern Europe and range from arid Mediterranean to maritime temperate climate conditions. Additionally, more than 2,000 virtual catchments were modelled that cover an extended gradient of physiographic and climate properties. The relative amounts of baseflow were summarized by the BFI. The catchment slow response delay time (Ks) was assumed to be a measure of catchment effects, and the impact of climate properties was investigated with the dry spell length (d). Well‐drained and poorly‐drained groups were identified based on Ks and d, and their response to an increase or decrease in dry spell length was analysed. Overall, for either well‐ or poorly‐drained groups, an extension in dry spell length appeared to have minor effects on the baseflow compared with a decrease in dry spell length. Under the same dry spell variation, the BFI vulnerability appeared higher for catchments characterized by large initial d values in combination with poorly‐drained systems, but attributing an equal weight to the variations in d both in the case of dry and wet initial conditions, it is in the end concluded that the BFI vulnerability appears higher for systems laying in the transition zone between well‐ and poorly‐drained systems.     

Impact of forest degradation on streamflow regime and runoff response to rainfall in the Garhwal Himalaya,Northwest India

Baseflows have declined for decades in the Lesser Himalaya but the causes are still debated. This paper compares variations in streamflow response over three years for two similar headwater catchments in northwest India with largely undisturbed (Arnigad) and highly degraded (Bansigad) oak forest. Hydrograph analysis suggested no catchment leakage, thereby allowing meaningful comparisons. The mean annual runoff coefficient for Arnigad was 54% (range 44–61%) against 62% (53–69%) at Bansigad. Despite greater total runoff Q_t (by 250 mm year^–1), baseflow at Bansigad ceased by March, but was perennial at Arnigad (making up 90% of Q_t vs. 51% at Bansigad). Arnigad storm flows, Q_s, were modest (8–11% of Q_t) and occurred mostly during monsoons (78–98%), while Q_s at Bansigad was 49% of Q_t and occurred also during post-monsoon seasons. Our results underscore the importance of maintaining soil water retention capacity after forest removal to maintain baseflow levels.

EDITOR D. Koutsoyiannis; ASSOCIATE EDITOR D. Gerten     

Isotope hydrology of a tropical coffee agroforestry watershed: Seasonal and event‐based analyses

Stable isotope variations are extremely useful for flow partitioning within the hydrologic cycle but remain poorly understood throughout the tropics, particularly in watersheds with rapidly infiltrating soils, such as Andisols in Central America. This study examines the fluctuations of stable isotope ratios (δ¹⁸O and δ²H) in the hydrologic components of a tropical coffee agroforestry watershed (~1 km²) with Andisol soils in Costa Rica. Samples were collected in precipitation, groundwater, springs, and stream water over 2 years. The local meteoric water line for the study site was δ²H = 8.5 δ¹⁸O + 18.02 (r² = 0.97, n = 198). The isotope ratios in precipitation exhibited an enriched trend during the dry season and a notable depletion at the beginning of the wet season. The δ¹⁸O compositions in groundwater (average = ?6.4‰, σ = 0.7) and stream water (average = ?6.7‰, σ = 0.6) were relatively stable over time, and both components exhibited more enriched values in 2013, which was the drier year. No strong correlation was observed between the isotope ratios and the precipitation amount at the event or daily time‐step, but a correlation was observed on a monthly scale. Stream water and base flow hydrograph separations based on isotope end‐member estimations showed that pre‐event water originating from base flow was prevalent. However, isotope data indicate that event water originating from springs appears to have been the primary driver of initial rises in stream flow and peak flows. These results indicate that isotope sampling improves the understanding of water balance components, even in a tropical humid location, where significant variations in rainfall challenge current modelling efforts. Further research using fine‐scale hydrometric and isotopic data would enhance understanding the processes driving spring flow generation in watersheds.     

Surface water and groundwater interactions in an extensively mined watershed,upper Schuylkill River,Pennsylvania, USA

Streams crossing underground coal mines may lose flow, whereas abandoned mine drainage (AMD) restores flow downstream. During 2005–2012, discharge from the Pine Knot Mine Tunnel, the largest AMD source in the upper Schuylkill River Basin, had near‐neutral pH and elevated concentrations of iron, manganese and sulphate. Discharge from the tunnel responded rapidly to recharge but exhibited a prolonged recession compared with nearby streams, consistent with rapid infiltration of surface water and slow release of groundwater from the mine complex. Dissolved iron was attenuated downstream by oxidation and precipitation, whereas dissolved CO₂ degassed and pH increased. During high flow conditions, the AMD and downstream waters exhibited decreased pH, iron and sulphate with increased acidity that were modelled by mixing net‐alkaline AMD with recharge or run‐off having low ionic strength and low pH. Attenuation of dissolved iron within the river was least effective during high flow conditions because of decreased transport time coupled with inhibitory effects of low pH on oxidation kinetics. A numerical model of groundwater flow was calibrated by using groundwater levels in the Pine Knot Mine and discharge data for the Pine Knot Mine Tunnel and West Branch Schuylkill River during a snowmelt event in January 2012. Although the calibrated model indicated substantial recharge to the mine complex took place away from streams, simulation of rapid changes in mine pool level and tunnel discharge during a high flow event in May 2012 required a source of direct recharge to the Pine Knot Mine. Such recharge produced small changes in mine pool level and rapid changes in tunnel flow rate because of extensive unsaturated storage capacity and high transmissivity within the mine complex. Thus, elimination of stream leakage could have a small effect on the annual discharge from the tunnel, but a large effect on peak discharge and associated water quality downstream. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.     

The role of surface and sub-surface runoff processes in controlling cation export from a wetland watershed

Water and cation budgets were calculated for two sub-basins within a small low relief watershed in South-Central Ontario during a period of ephemeral runoff which was initiated by spring snow melt. The hydrology of one (upland) sub-basin was strongly influenced by seasonal fluctuations in the level of regional ground water. Saturated contributing areas formed in low lying regions adjacent to the stream channel where the water table rose to the surface, and stream discharge was a mixture of ground water and saturation overland flow. In the second sub-basin a wetland provided a large and spatially less variable saturated contributing area. Clay soils underlying the wetland resulted in a shallow perched water table, poorly drained and highly organic soils, and greatly reduced inputs of regional ground water. Stream discharge was largely the result of surface runoff from the wetland and adjacent areas of saturated soil.Inter-basin variations in water export were by far greater than variations in stream chemistry. As a result, inter-basin variations in cation export strongly reflected variations in water export over the time interval in which the majority of a given ion was lost from the watershed. Spatial differences in water export were least at the onset of runoff when basin saturation was greatest and overland flow made large contributions to the discharge from both sub-basins. Potassium and hydrogen had high concentrations at this time which caused these ions to show only small spatial differences in export. With decreases in the areal extent of soil saturation, and increases in the storage capacity of the wetland, the hydrologic contrast between sub-basins increased. Greater water loss from the upland area resulted from a greater discharge of regional ground water, and a more rapid expansion of the saturated contributing areas during storm events. Calcium, magnesium, and sodium concentrations increased steadily during the first 3 weeks of runoff, so that the peak export of these cations occurred later in the runoff period at times of higher concentration, but lower and spatially more variable discharges. Consequently, spatial differences in the loss of these ions was great and favoured the upland sub-basin, since the majority of export occurred when the hydrologic contrast between sub-basins was largest.     

Groundwater evolution and mean water age inferred from hydrochemical and isotopic tracers in a tropical confined aquifer

The coastal confined aquifer in the Gulf of Urabá (Colombia) is an important water source for the banana agro‐industry as well as for urban and rural communities. However, the main processes controlling recharge and mixing in the aquifer are still poorly understood. Hydrochemical analyses and stable isotope monitoring were conducted to (a) determine groundwater recharge origin, mean groundwater age, and the main processes governing groundwater chemistry and the potential mixing of marine water and the influence of diffusive processes from the two surrounding aquitard layers. Hydrochemical data indicate that the main processes affecting the dissolved chemical composition include cation exchange, dissolution of carbonated and CO₂, and silicate weathering. δ¹⁸O and δ²H compositions combined with ¹⁴C data highlight the differences in climatic conditions between the recharge zone and the confined section of the aquifer, which is close to the Atlantic Ocean. Groundwater samples with ¹⁴C ages from recent to 28,300 years BP show a depleted isotopic trend ranging from ?6.43‰ to ?9.14‰ in δ¹⁸O and from ?43.2‰ to ?65.7‰ in δ²H. The most depleted δ¹⁸O and δ²H compositions suggest a cooler recharge climate than the current conditions (corresponding to the last glacial period of the late Pleistocene). Depleted δ¹³C values in the total dissolved inorganic carbon indicate the existence of organic material oxidation processes within the geologic formation. These results can be used or transferred to enhance groundwater modelling efforts in other confined coastal aquifers of South America where scarcity of long‐term monitoring data limits water resources planification under a changing climate.     

Thermal and isotopic evidence for surface and subsurface water contributions to baseflow in a high Arctic river

Seven longitudinal water temperature tow surveys were conducted to attempt to identify the location of surface and subsurface river water exchanges along the length of the West River at the Cape Bounty Arctic Watershed Observatory, Melville Island, Nunavut, Canada (74°55′ N, 109°35′ W). Water temperature data were collected using a calibrated thermistor with an accuracy of ±0.002 °C (resolution <0.00005 °C) along the river during July 2014 in conjunction with stable water isotope sampling to support the thermal data and to determine the extent of surface water mixing from different sources such as precipitation, snowmelt, and surface/subsurface water contributions to the river. Atmospheric conditions were found to be the main contributor to seasonal temperature variance in the river, whereas tributary inflows and residual channel snow also had important thermal effects to river temperatures. Residual channel snow was a sustained source of cold water during much of the 2014 summer season (June–August) and substantially offset downstream warming. The longitudinal temperature profiles indicate notable changes to the thermal state of the river, which are interpreted to be indicative of subsurface and surface water exchange through inputs of relatively cold or warm water. Broadly, surface inflows were found to provide warmer water relative to the West River, and contributed to downstream warming of the river, along with downstream enrichment of δD and δ¹⁸O. Subsurface inflows provided cooler water relative to the river, and contributed to downstream depletion of δD and δ¹⁸O and downstream cooling of river temperatures. These results demonstrate that localized changes in river temperature, in conjunction with isotopic tracers, can be used to track channel–slope water interactions in Arctic hydrological systems, work previously limited to alpine and temperate settings.