Alluvial Groundwater Research Articles

Accounting for uncertainty in complex alluvial aquifer modeling by Bayesian multi-model approach

Alluvial aquifers by nature are complex caused by varied depositional environments. Developing a reliable groundwater model to represent an alluvial aquifer is non-trivial. Also, relying on a single best calibrated model may not be sufficient because of an inadequate choice of model parameter values. To better understand groundwater dynamics and improve model prediction reliability, this study presents a Bayesian multi-model uncertainty quantification (BMMUQ) framework to account for model parameter uncertainty in complex alluvial groundwater modeling. The methodology was applied to the agriculturally intensive Mississippi River alluvial aquifer (MRAA), Northeast Louisiana. An aquifer architecture was first constructed using 7,259 well logs in the MRAA area which covers three fluvial deposits (alluvium, braided-stream terrace, and braided-stream terrace-loess). A 12-layer MODFLOW model was then developed to address the alluvial aquifer complexity and well calibrated through a genetic algorithm. This study quantified model parameter uncertainty in hydraulic conductivity and specific storage of sand facies. Bayesian model averaging (BMA) with the Expectation Maximization (EM) algorithm was adopted to derive posterior model weights and head variances of 50 alternative conceptual groundwater flow models, and thereby obtains BMA ensemble model predictions instead of only relying on the best calibrated conceptual model. Results show that an estimated around 950 million m3 of groundwater storage loss occurs in 2015 with respect to the beginning of 2004, due to high groundwater demand for irrigation in the MRAA area. Explicitly quantifying model uncertainty can produce more reliable groundwater level predictions from BMA ensemble model. The presented groundwater modeling framework improves our understanding of the MRAA and provides a valuable tool to assist agricultural water management.

How trees thrive in a dry climate: diurnal and seasonal hydrology and water relations in a riparian cottonwood grove.

In semi-arid ecoregions, trees are restricted to river valley floodplains where river water supplements the limited precipitation. To characterize the associated diurnal and seasonal dynamics in hydrology and water relations, we studied narrowleaf cottonwoods (Populus angustifolia) along a prairie river in Canada. From June through August, the shallow soil moisture was depleted but moisture remained higher above the alluvial groundwater table, which dropped to 2.3m along with river recession. Throughout the summer, with the daily rise in temperature and insolation, foliar stomatal conductance (gs) and transpiration (E) increased to midday and then fell, thus maintaining the midday leaf water potential (Ψmd) above ~-1.7MPa. This Ψmd approximated the water potential associated with 12% loss of xylem conductivity due to cavitation for branches (P12); the Ψmd and P12 varied independently across eight trees, providing differences in relative hydraulic risk. Sap flux density (Fd) was measured with thermal dissipation probes, and revealed diurnal patterns similar to foliar E. In contrast to our expectation, the daily Fd maxima were consistent through the summer despite the seasonal recession in water supply. Canopy conductances (GS), derived from Fd, sapwood area and canopy area, declined with vapor pressure deficit (D) and fell slightly in late summer along with stomatal sensitivity, which reflects the magnitude of decrease in GS with increasing D. For spatial up-scaling, satellite-derived near-infrared reflectance of vegetation revealed the woodland phenology, with leaf expansion from May through June and gradual decline in photosynthetic productivity through the summer. Thus, the phreatophytic cottonwoods: (i) sustained substantial water use and productivity through the warm and dry summer, by (ii) accessing shallow soil moisture and then deeper alluvial groundwater, and (iii) providing diurnal stomatal regulation, to (iv) avoid xylem cavitation and (v) maintain fairly constant hydraulic conductance.

Quantifying and understanding the source of recharge for alluvial systems in arid environments through the development of a seepage model

In rural settings, groundwater is often the most utilised freshwater resource and can be subject to a number of factors that impact its availability and distribution. River seepage plays an important role in recharging alluvial groundwater systems, especially for arid environments where ephemeral rivers dominate. In this study, a river seepage component was developed for a spatially distributed rainfall/runoff model, the J2000 model. To represent river seepage, physical channel properties such as channel width, slope, length as well as a lumped riverbed hydraulic conductivity under ‘dry’ and ‘wet’ conditions were used. To facilitate and account for enhanced evapotranspiration in the alluvial channel, seepage firstly satisfied the potential evaporation demand and thereafter contributed to simulated recharge. The results show that by accounting for seepage, observed low flows were well simulated (Log E2: 0.74). Although, the input precipitation data and quality of the measured precipitation was associated with a high uncertainty and further impacted by a change in station density between the headwater and valley region (3 times higher in the headwaters). As a result, peak flows were less well represented (E2: 0.39) and a second downstream calibration was not possible. On average, river/reach seepage was estimated to be 8 mm d−1, where 150–300 mm yr−1 of seepage was simulated during wet years, exceeding simulated percolation on a reach level and a significant source over the catchment area. Given streamflow only comprised 1.3% of the total water balance, the efficiencies attained by this approach were good considering the extremely harsh climatic conditions. This approach is likely transferable to other arid environments and rivers with intermittent flow, where river seepage may be overlooked as an important contributor to hydrological functioning.

Comparing four machine learning model performances in forecasting the alluvial aquifer level in a semi-arid region

Groundwater level fluctuation is a nonlinear and non-stationary system as it depends on several factors in the time and space scales. Conceptual models require several physical parameters whose estimation is delicate in poorly monitored areas. However, data-based models may be valuable for modelling and forecasting groundwater level over short and long terms. To that end, four machine learning models, namely: Support Vector Regression, k- Nearest Neighbour (k-NN), Random Forest (RF), and Artificial Neural Network (ANN), are trained, validated, and compared for predicting groundwater level (GWL) at seven piezometers on alluvial groundwater of Tanobart aquifer in Morocco. The results revealed that the ANN models succeeded properly in simulating GWL at five piezometers out of the total seven piezometers considered in this study (NSE = 0.69 to 0.8); the RF was satisfactory at five piezometers (NSE = 0.41 to 0.72) and SVR at three piezometers (NSE = 0.57 to 0.81); the k-NN was the poorest model among all the investigated models (NSE = −1.05 to −0.15). The uncertainty analysis showed that the selected models are accurate overall; the SVR model showed the best forecasting accuracy with the smallest 95% interval prediction error (−0.25 m and 0.11 m) at one piezometer. This study provides new insight to forecast the GWL under a semi-arid context such Tanobart aquifer in Khemesset province, Morocco.

The use of regional and alluvial groundwater by riparian trees in the wet‐dry tropics of northern Australia

AbstractRiparian trees play a critical role in the ecological function of rivers, yet are threatened by anthropogenic change to the hydrological cycle. Identifying the sources of water used by riparian trees can inform sustainable water policy. We used isotopic analysis complemented by measurements of plant water relations to assess water sources for riparian trees at two sites with contrasting hydrogeological processes; one with an alluvial aquifer overlaying an aquitard, and one where fault‐induced preferential pathways in the aquitard allowed the flow of deeper, older groundwater from a regional aquifer to the alluvium. At both sites, plant water potential, stomatal conductance, and plant water isotope composition in the xylem sap of riparian trees were collected from two landscape positions, the riverbank and floodplain. We used a Bayesian mixing model (MixSIAR) to assess differences in the proportion of water sources for sites and landscape positions. We found that xylem water isotope values differed between the two sites in line with their hydrogeological characteristics, with trees at the regional aquifer site using water sourced from the regional groundwater and trees at the site with only an alluvial aquifer present using a mixture of water sources, with no dominant source identified. Higher plant predawn water potential values at the regional site indicated greater water availability and support the inference that plants were using more groundwater at the regional site compared to the alluvial site. Trees closer to the river had higher isotope values, indicative of surficial water sources i.e. shallow soil water and river water. Our findings show that the water sources used by riparian trees reflect local hydrogeology and resource availability. Water managers should identify and protect plant water sources to ensure maintenance of riparian trees.

Hydrodynamic groundwater modeling and hydrochemical conceptualization of the mining area of Moulares Redeyef (southwestern of Tunisia): New local insights

The Moulares Redeyef Mining basin (MMR-B) is located on Southwestern of Tunisia. The problem of groundwater mineralization in this basin has long been attributed to the significant decline in groundwater reserves. In order to quantify the aquifers reserves and identify processes and factors governing the groundwater quality, a detailed multidisciplinary study was proposed in this study. The approach used various GIS databases, including hydrodynamic characteristics, geochemistry, geology and climatic parameters. A Modflow hydrodynamic model was developed over 67-year modeling period (1953–2020) and many simulations were performed. Water samples were collected from 39 boreholes from the deep aquifer and 13 wells from the superficial aquifer in Tabeddit area. The hydrogeological model shows a flow convergence zone around the Tabeddit area. The reserve of the aquifer system was simulated to 10 Mm3/year for 2020, while it was 26 Mm3/year in 2000. Yet, 61% of exploitable reserves was consumed in 20 years. The quality of groundwater showed that MMR-b is characterized by 4 main trends: Ca–Mg–HCO3, Ca–Mg–SO4, Na–Cl–NO3 and Na–Mg–HCO3. For the heavy metals, the 13 alluvial groundwater samples contain high concentration of Cadmium that exceed the Tunisian Standards. Finally, a Risk Index of Aquifer is elaborated based on the Modflow model and the geochemical quality. The index gives that the PQ layer is under very high risk and the M aquifer is in a moderate situation. Accordingly, it is recommended that the mining deciders revise their exploitation strategy urgently in order to preserve water resources for future use.

Multiple processes contribute to methane emission in a riparian cottonwood forest ecosystem.

Methane emission from trees may partially or completely offset the methane sink in upland soils, the only process that has been regularly included in methane budgets for forest ecosystems. Our objective was to analyze multiple biogeochemical processes that influence the production, oxidation and transport of methane in a riparian cottonwood ecosystem and its adjacent river. We combined chamber flux measurements on tree stems, forest soil and the river surface with eddy covariance measurements of methane net ecosystem exchange. In addition, we tested whether methanogens were present in cottonwood stems, shallow soil layers and alluvial groundwater. Average midday peak in net methane emission measured by eddy covariance was c. 12nmolm-2 s-1 . The average uptake of methane by soils (0.87nmolm-2 s-1 ) was largely offset by tree stem methane emission (0.75nmolm-2 s-1 ). There was evidence of methanogens in tree stems but not in shallow soil. Growing season (May-September) cumulative net methane emission (17.4mmolCH4 m-2 ) included methane produced in cottonwood stems and methane input to the nocturnal boundary layer from the forest and the adjacent river. The multiple processes contributing to methane emission illustrated the linked nature of these adjacent terrestrial and aquatic ecosystems.

Machine Learning Models for Predicting the Ammonium Concentration in Alluvial Groundwaters

Considering the great importance of groundwater quality for water supply, in the last decade, significant scientific attention has been devoted to nitrate reduction transformation pathways and nitrogen conservation in groundwaters in the form of ammonium. To evaluate and assess the ability of machine learning models to predict the ammonium concentration, four machine learning models were applied: a three-layer neural network (NN), a deep neural network (DNN), and two variants of support vector regression (SVR) models: with linear and with Gaussian radial basis function kernel. A dataset of 322 samples with 13 predictor variables representing selected parameters responsible for oxidative/reductive nitrogen transformations in shallow alluvial groundwater was acquired from measurements in 55 monitoring wells during a 6-year monitoring period (2011–2016) in Serbia. Applied principal component analysis and cluster analysis gave an insight into conditionality and relations between the selected parameters, distinguishing four main factors, which explained 70.97% of total variance, and classifying examined objects by similarity. Extracted factors correlated the concentration patterns, implying the main nitrogen transformations in examined groundwater. The machine learning models were successfully applied for predicting the ammonium concentration with high determination coefficients (R2) in tests: 0.84 for DNN and 0.64 for NN, while the SVR did not prove to be adequate with the best R2 of 0.24.

Use of mixing models to explain groundwater quality time and space variation in a narrowed fluctuating alluvial aquifer

Hydrogeochemical studies can provide insights to pursue a flexible and responsive groundwater resources management.Origin of water, mixing and reactive processes are three key parameters that allow to understand hydrological systems and resulting groundwater quality.However, those parameters may evolve strongly and quickly over time and space, especially for hydrological system impacted by pumping activities and seasonal variations.Within this framework, the current study aims to pursue a survey of the hydrological parameters in a small dynamic alluvial aquifer fed by 3 contrasted end-members and used for water supply.The variations of the end-members and the alluvial groundwaters quality (Ca2+, Mg2+, K+, Na+, Cl−, CO32−, NO3−, δ2H and δ18O) will be used as a record of those parameters. 26 sampling campaigns were carried out fortnightly from October 2017 to October 2018 and data processed with two distinct mixing models: GLUE-EMMA model, which define likelihood of each mixing component, and PHREEQC inverse modeling, which account for the reactive processes that occur as water chemistry evolves. For each studied groundwater, appropriate time variant end members signatures as well as analytical uncertainties were accounted and models constrained to fit within 5.0% for the major ions and 0.5‰ for water stable isotopes.With suitable models for all alluvial groundwater samples, both GLUE-EMMA and PHREEQC highlight the mixing process between two end-members as the main parameter of the alluvial groundwater quality. Nonetheless, Mg2+ and K+ reactive fluxes are needed to fit with the required constraints.Hence, the alluvial groundwater quality variations are either associated with the typical seasonal variations of the end-member signature and the change of the mixing processes.For both models, mixing process exhibits slow and small as well as strong and sudden changes. The first can be related with the relative capacity of the end members to feed the aquifer according to their individual high and low water periods. The second to the effects of alternative pumping regime between sampling campaign.The capacity of both models to evaluate and monitor main hydrological processes, over time and space, confirms the interest to combine them as a flexible multi process investigation suited for dynamic hydrological system.

Estimating groundwater-river connectivity factor for quantifying changes in irrigation return flows in the Murray–Darling Basin

ABSTRACT Concerns have been raised that the use of infrastructure and water efficiency projects to recover water entitlements for the environment in the Murray–Darling Basin (MDB) could be undermined by a reduction in return flows to rivers. Water use efficiency changes prior to 2009 had already reduced surface return flows. A dimensionless and normalised variable, connectivity factor (CF), has been used to analyse the cumulative impact on river flow from actions that alter groundwater recharge or extraction. CF is objectively estimated from existing modelling outputs for three large alluvial groundwater systems in south-eastern MDB relevant to a major water use efficiency program. Lower values of CF (0.05–0.45) imply lower reductions of return flows, less impacts on recovered water entitlements, with impacts more likely to be managed adaptively within current water management plans. The study should help allay concerns by explaining why previous estimates had been overly high.

Sensitivity of hydrologic and geologic parameters on recharge processes in a highly heterogeneous, semi-confined aquifer system

Abstract. An increasing reliance on groundwater resources has been observed worldwide during the past 50–70 years and has led to unsustainable groundwater abstraction in many regions, especially in semi-arid and arid alluvial groundwater basins. Managed aquifer recharge (MAR) has been promoted to replenish overdrafted groundwater basins and augment surface water supply. However, MAR feasibility in alluvial groundwater basins is complicated by complex geologic architecture that typically includes laterally continuous, fine-texture confining units that can impede both recharge rates and regional propagation of increases in the hydraulic head. A greater feasibility of MAR hinges on identifying locations where rapid, high-volume recharge that provides regional increases in pressure head are possible, but relatively little research has evaluated the factors that control MAR feasibility in alluvial groundwater basins. Here, we combine a transition probability Markov chain geostatistical model of the subsurface geologic heterogeneity of the eastern side of the northern Central Valley, California, with the three-dimensional, variably saturated water flow code ParFlow to explore the variability of MAR feasibility in this region. We use a combination of computationally efficient local- and global-sensitivity analyses to evaluate the relative importance of factors that contribute to MAR feasibility. A novel proxy parameter approach was used to describe the configuration and proportions of subsurface hydrofacies and the water table depth for sensitivity analyses, and results suggest that recharge potential is relatively more sensitive to the variability of this proxy parameter than to the variability of individual hydrofacies hydraulic properties. Results demonstrate that large variability of MAR feasibility is typical for alluvial aquifer systems and that outsized recharge rates are possible in select locations where interconnected, coarse-texture hydrofacies occur.

Symmetrical Rank-Three Vectorized Loading Scores Quasi-Newton for Identification of Hydrogeological Parameters and Spatiotemporal Recharges

In a multi-layered groundwater model, achieving accurate spatiotemporal identification and solving the ill-posed problem is the vital topic for model calibration. This study proposes a symmetry rank three vectorized loading scores (SR3 VLS) quasi-Newton algorithm by modifying the Levenberg–Marquardt algorithm and importing a rank three structure from Broyden–Fletcher–Goldfarb–Shanno algorithm for identification of hydrogeological parameters and spatiotemporal recharge simultaneously. To accelerate directional convergence and approach a global optimum, this study uses a vectorized limited switchable step size in the transmissive groundwater inverse problem. The Hessian approximation rank three uses high and low-rank factor loading scores analyzed from simulated storage fluctuation between adjacent iterations for calculation and matrix correction. Two numerical experiments were designed to validate the proposing algorithm, showing the SR3 VLS quasi-Newton reduced the error percentages of the identified parameters by 1.63% and 9.65% compared to the Jacobian quasi-Newton. The proposing method is applied to the Chou-Shui River alluvial fan groundwater system in Taiwan. Results show that the simulated storage error decreased rapidly in six iterations, and has good head convergence as small as 0.11% with a root-mean-square-error (RMSE) of 0.134 m, indicating that the proposing algorithm reduces the computational cost to converge to the true solution.

Access of Ground Water Quality with Associated Physico-Chemical Parameters of Cuttack District, Odisha, India

The aim of present investigation was to analyze the variations in the physio-chemical properties of the ground water of Cuttack district Odisha. In the present study 98 samples were collected and analyzed to assess the quality of ground water. The pH, electrical conductivity (EC), total hardness and total alkalinity of the collected 98 samples were in the range of 4.6-7.3, 36-4550 μS/cm, 40-200 mgl-1, 20-680 mgl-1 respectively. Similarly, the other important water quality parameters such as; chloride, nitrate sulphate and phosphate concentration were varies between BDL-327, 1.8-86.25, BDL-194 and BDL to 3.2 mgl-1 respectively. The pH of the alluvial groundwater is controlled by the HCO3. The fluoride concentration was varies from BDL to 2.38 mgl-1. Apart from few samples, 90.81% fluoride contaminated samples comes under the category of quality group A (< 1 mgl-1flouride). Similarly, out of total samples collected only in three samples the uranium concentration estimated to be more than 5ppb. Among the water quality parameters there exist a positive correlation between pH and fluoride with a correlation coefficient of 0.641. From the correlation analysis it is found that, higher concentration of fluorid correlated with higher pH. Similarly the correlation coefficient between calcium and chloride is very high i.e. 0.500, which strongly supported the existence of calcium in the study area is predominantly in the form of CaCl2. Most of the ground water samples meet the requirements of the WHO drinking water standards with respect to salinity, main constituents and potentially toxic trace elements such as uranium

Influence of oxic and anoxic groundwater conditions on occurrence of selected agrochemicals

Abstract The occurrence and mitigation of selected pesticides and nitrates in surface and groundwater samples from two alluvions in the Republic of Serbia are investigated in this study. The influence of aquifer conditions on the degradation processes of pesticides and nitrates is considered as a potential removal mechanism of compound residues in alluvial groundwater. Nitrate concentration was analyzed in 144 groundwater samples in the Kovin-Dubovac drainage system and 63 samples at the Ključ groundwater source. The occurrence of 15 pesticides was monitored in groundwater in a total of 34 samples in the Kovin-Dubovac area and 14 samples at the Ključ groundwater source. Concentrations of selected pesticides and nitrates were monitored in eight samples from the Danube in the Kovin-Dubovac area and 15 samples from Velika Morava at the Ključ groundwater source. Both selected locations are agricultural areas. Results show that concentrations of nitrates (NO3−) are much higher in oxic groundwater conditions compared with concentrations in typical anoxic conditions. The opposite is the case for the concentration of pesticides that occurred in surface and groundwater samples. These results are very important for a better understanding of the self-purification potential of alluvial aquifers and assessment of aquifer condition influence, especially oxic or anoxic conditions, on the occurrence and mitigation of selected agrochemical residues in groundwaters.

Enhanced Flocculation Using Drinking Water Treatment Plant Sedimentation Residual Solids

Inefficient removal of total organic carbon (TOC) leads to the formation of carcinogenic disinfection by-products (DBPs) when a disinfectant is added. This study is performed in an effort to develop a simple, non-invasive, and cost-effective technology that will effectively lower organic precursors by having water utilities reuse their treatment residual solids. Jar tests are used to simulate drinking water treatment processes with coagulants—aluminum sulfate (alum), poly-aluminum chloride (PACl), and ferric chloride and their residual solids. Ten coagulant-to-residual (C/R) ratios are tested with water from the Missouri River at Coopers Landing in Columbia, MO versus alluvial ground waters. This treatment results in heavier floc formation and leads to improved sedimentation of organics and additional removal of aluminum and iron. An average of 21%, 28%, and 33% additional TOC removal can be achieved with C/R ratios <1 with alum, PACl, and ferric chloride, respectively.

Using stable isotopes to quantify water sources for trees and shrubs in a riparian cottonwood ecosystem in flood and drought years

AbstractRiparian cottonwood forests in dry regions of western North America do not typically receive sufficient growing season precipitation to completely support their relatively high transpiration requirements. Water used in transpiration by riparian ecosystems must include alluvial groundwater or water stored in the potentially large reservoir of the unsaturated soil zone. We used the stable oxygen and hydrogen isotope composition of stem xylem water to evaluate water sources used by the dominant riparian cottonwood (Populus spp.) trees and shrubs (Shepherdia argentea and Symphoricarpos occidentalis) in Lethbridge, Alberta, during 3 years of contrasting environmental conditions. Cottonwoods did not exclusively take up alluvial groundwater but made extensive use of water sourced from the unsaturated soil zone. The oxygen and hydrogen isotope compositions of cottonwood stem water did not strongly overlap with those of alluvial groundwater, which were closely associated with the local meteoric water line. Instead, cottonwood stem water δ18O and δ2H values were located below the local meteoric water line, forming a line with a low slope that was indicative of water exposed to evaporative enrichment of heavy isotopes. In addition, cottonwood xylem water isotope compositions had negative values of deuterium excess (d‐excess) and line‐conditioned (deuterium) excess (lc‐excess), both of which provided evidence that water taken up by the cottonwoods had been exposed to fractionation during evaporation. The shrub species had lower values of d‐excess and lc‐excess than had the cottonwood trees due to shallower rooting depths, and the d‐excess values declined during the growing season, as shallow soil water that was taken up by the plants was exposed to increasing, cumulative evaporative enrichment. The apparent differences in functional rooting pattern between cottonwoods and the shrub species, strongly influenced the ratio of net photosynthesis to stomatal conductance (intrinsic water‐use efficiency), as shown by variation among species in the δ13C values of leaf tissue.

Mobility and sorption assessment of selected pesticides in alluvial aquifer.

The objective of the paper was to use transport model of selected pesticides (carbendazim, acetamiprid and imidacloprid) in determination of linear sorption coefficients in alluvial aquifer. For constructing transport model, results of a field experiment at the location of Kovin-Dubovac drainage system in Serbia were used in order to set hydraulic parameters (hydraulic conductivity, aquifer layer thickness of the observed area, effective porosity etc.). The field experiment consisted of a tracer test during which concentrations of non-reactive tracer (Cl-) and selected pesticides (carbendazim, acetamiprid and imidacloprid) were monitored. For better characterization of hydraulic parameters, a pumping test was conducted at the observed well and results were used in designing transport model. Simulation model was constructed with Lizza groundwater flow software and W.O.D.A. (Well Outline and Design Aid) solver. Obtained linear sorption coefficients in the sand and gravel water-bearing layer were 0.14mLg-1 for carbendazim and 0.11mLg-1 for acetamiprid and imidacloprid. Results from this study are a unique insight into mobility of observed pesticides in the alluvial groundwater in natural conditions and can be used in contamination assessment for drinking water wells.

Coupling hydrodynamic, geochemical and isotopic approaches to evaluate oxbow connection degree to the main stream and to adjunct alluvial aquifer

Wetlands as oxbows play a fundamental socio-economic and environmental role. They behave as efficient tools in regards to hydrological and ecological concerns: they contribute to control rivers discharge (flood prevention and contribution to drought flow) as well as to manage water quality (excess nutrients retention) and they constitute refuge habitats for flora and fauna. They may so become more and more determinant as nature-based solutions in a near future. However, the establishment of complete and reliable functioning models is complicated by the different connection degrees that can present these specific wetlands to the main stream and to the adjunct alluvial aquifer. Moreover, monodisciplinary approaches usually conducted can underestimate or even neglect sources of water supply. The present study offers a multidisciplinary approach coupling hydrodynamic (surface water/groundwater levels), geochemical (T, EC, pH, major ions) and isotopic (δ2H-δ18O) characterization in order to properly assess the dynamic of these specific hydrosystems and so be able to establish a complete and reliable hydrodynamic conceptual model. The coupling approach has been tested on the Auzon Oxbow, one of the fluvial annexes of the Allier River (Massif Central, France). The resulting conceptual model testifies for connection degree higher that expected between the oxbow and both the main stream and alluvial aquifer. Indeed, a non-negligible supply of the oxbow by the alluvial groundwater is observed, especially in its downstream part. As this part of the oxbow is the site of its confluence to the Allier River, groundwater supply could have been underestimated. The geochemical approach shows that Allier River also supply the oxbow through the upstream paleochannel, result of a channel migration 30 years ago. Hydrodynamic and isotopic approaches complete the understanding showing that the paleochannel pathway is only active during high flow periods (from November to June). Based on these results, the strategic utility of the proposed approach to study oxbow’s hydrosystems has been proved.

Controls on ecosystem water-use and water-use efficiency: Insights from a comparison between grassland and riparian forest in the northern Great Plains

Within the grassland-dominated landscape of the North American Great Plains, riparian forest ecosystems exist along river floodplains. We compared cumulative evapotranspiration (ET) and ecosystem water-use efficiency (WUE) between a cottonwood forest and a nearby native grassland ecosystem in southern Alberta, using eddy covariance measurements during May-September (growing season) of three study years. Our objective was to test predictions about mechanistic controls on ecosystem water-use, and to provide insights into the amount of alluvial groundwater and stored soil water required to support a healthy riparian forest within the Great Plains biome. Grassland ET was dependent on precipitation inputs during the growing season. Cumulative growing season ET at the cottonwood site (375–451 mm) exceeded grassland ET (111–213 mm) by 2.1- to 3.4-fold depending on study year, despite slightly higher WUE in the cottonwood ecosystem. The difference in cumulative ET between ecosystems ranged from 238 to 264 mm in different years, in a region that normally receives 258 mm of cumulative precipitation during May-September. The large ET at the cottonwood site was caused by two-fold higher LAI, and associated greater canopy conductance than was apparent at the grassland site. The additional soil water required for the higher cottonwood ET was supplied by access to alluvial groundwater, which is recharged by river water, and was also supported by a larger soil volume to store water from precipitation and river flooding inputs. These factors resulted in a relatively long interval for cottonwood photosynthetic gas exchange that was consistent among years despite widely different environmental conditions, while the grassland had shorter growing season lengths that were constrained further as precipitation and soil moisture declined among years. Our analyses contribute to understanding the water requirements of these contrasting ecosystems and will help to improve management procedures for regulating river flow rates in order to sustain healthy riparian cottonwood ecosystems.

Hydroclimatic drivers of the growth of riparian cottonwoods at the prairie margin: River flows, river regulation and the Pacific Decadal Oscillation

Cottonwoods, riparian poplars, are facultative phreatophytes and can obtain water from shallow soil moisture originating from rainfall, or from the deeper capillary fringe above the alluvial water table that is recharged by river water infiltration. The correspondence between cottonwood growth and river flows should reveal the dependency upon alluvial groundwater and subsequently, the vulnerability to reduced river flows. To explore this association, we analyzed historic growth patterns of plains cottonwoods (Populus deltoides) along the Red Deer River (RDR), which is at the northwestern limit of the North American Great Plains. We developed chronologies of yearly radial increments (RI) and basal area increments (BAI) and explored correspondences with the environmental records from the past century. In this semi-arid region, the RI or BAI were not correlated with local precipitation while negative correlation with growth season temperature (T) (r = −0.37, p < 0.01) could reflect reduced growth with hot summers. There was correlation between growth and annual river discharge (Q, and particularly log Q that approximates river stage) and this increased with two year averaging (r = 0.51, p < 0.01), reflecting carry-over in the watershed hydrology and in the ecophysiological response. There was correspondence with the Pacific Decadal Oscillation index (PDO, r = −0.45, p < 0.01), which provides multi-decade transitions that influence Rocky Mountain headwater precipitation and other weather characteristics, and river flows. The combination of Q, PDO and T provided the strongest multiple regression model, accounting for 44% of the historic growth variation (52% correspondence for 1953–2013). The RDR was dammed in 1983, enabling winter flow augmentation, but summer flows were sustained and cottonwood growth and the streamflow correspondence persisted. This indicates that it is the pattern of dam operation and not damming per se that determines the fate of established riparian cottonwoods downstream. This study revealed that these cottonwoods are phreatophytic and dependent upon alluvial groundwater that is recharged from the river. This provides a research strategy to determine whether riparian woodlands along other regulated rivers are similarly groundwater-dependent and could be vulnerable to river flow reductions from excessive water withdrawal for irrigation or other uses, or with climate change.