Source Of Baseflow Research Articles

Saturated hydraulic conductivity (Ksat) and topographic controls on baseflow contribution in high-altitude aquifers with complex geology

Mountains in general are important sources of water resources and correspondingly, the Upper Indus Basin (UIB) in the Himalayas provides water for hundreds of millions of users. During low-flow periods, streamflow is fed by a combination of groundwater (baseflow), snow/glacier melt and recent precipitation. However, the proportion of stream baseflow generated by different sources and the factors that control these contributions remain poorly understood. In this paper, we focus on the Western Himalayan Liddar watershed (1243 km2) in the UIB and use a multi-method approach to characterize aquifers and baseflow generation in three nested catchments which represent varying geology and topography. Our multi-method approach includes geophysical surveys, slug tests, stable isotope compositions, young water fraction (Fyw) estimation and a Bayesian end member mixing model. Specifically, we hypothesize that Ksat of the subsurface plays a critical role in the contribution of groundwater to baseflow. The watershed is subdivided into three nested catchments (Limestone, hardrock and alluvium). Our results indicate that the upstream, steep, high-permeability limestone catchment has the relatively largest groundwater contribution to streamflow (63–78 %) and a low Fyw (38 %) than the other two sites. The mid-elevation hard rock catchment has a lower permeability on average but is highly variable due to fracture networks and geologic contacts, allowing a substantial groundwater contribution to streamflow (38–66 %) and a similar Fyw (36 %) to the limestone catchment. The downstream, low-slope catchment underlain by alluvium has a highly variable hydraulic conductivity and a substantially lower groundwater contribution to streamflow (26–45 %) and a higher Fyw (55 %). We find that both hydraulic conductivity and topography exert control on the magnitude of baseflow contribution to the stream. However, Ksat acts as a key control in groundwater contribution to baseflow as observed in headwaters and downstream region of the watershed. While unconsolidated deposits are often thought of as important high-porosity mountain aquifers, our results point to the importance of fractured and karstified bedrock in baseflow generation in high mountains. As climate change alters the snow- and ice-melt regimes of UIB rivers, our study underscores the critical importance of improving our understanding of baseflow sources.

Interpretable baseflow segmentation and prediction based on numerical experiments and deep learning

Baseflow is a crucial water source in the inland river basins of high-cold mountainous region, playing a significant role in maintaining runoff stability. It is challenging to select the most suitable baseflow separation method in data-scarce high-cold mountainous region and to evaluate effects of climate factors and underlying surface changes on baseflow variability and seasonal distribution characteristics. Here we attempt to address how meteorological factors and underlying surface changes affect baseflow using the Grey Wolf Optimizer Digital Filter Method (GWO-DFM) for rapid baseflow separation and the Long Short-Term Memory (LSTM) neural network model for baseflow prediction, clarifying interpretability of the LSTM model in baseflow forecasting. The proposed method was successfully implemented using a 63-year time series (1958–2020) of flow data from the Tai Lan River (TLR) basin in the high-cold mountainous region, along with 21 years of ERA5-land meteorological data and MODIS data (2000–2020). The results indicate that: (1) GWO-DFM can rapidly identify the optimal filtering parameters. It employs the arithmetic average of three methods, namely Chapman, Chapman-Maxwell and Eckhardt filter, as the best baseflow separation approach for the TLR basin. Additionally, the baseflow significantly increases after the second mutation of the baseflow rate. (2) Baseflow sources are mainly influenced by precipitation infiltration, glacier frozen soil layers, and seasonal ponding. (3) Solar radiation, temperature, precipitation, and NDVI are the primary factors influencing baseflow changes, with Nash-Sutcliffe efficiency coefficients exceeding 0.78 in both the LSTM model training and prediction periods. (4) Changes in baseflow are most influenced by solar radiation, temperature, and NDVI. This study systematically analyzes the changes in baseflow and response mechanisms in high-cold mountainous region, contributing to the management of water resources in mountainous basins under changing environmental conditions.

The provenance and persistence of the perennial Río Loa in the Atacama Desert: links between crustal processes and surface hydrology

The Río Loa is a perennial river that crosses the Atacama Desert. A basin-wide survey enables for the first time, the hydrologic regime origin, persistence and processes to be identified. Perennial baseflow in the Ríos Loa and Salado largely originates from intra-arc aquifers which are poorly known. However, the data indicate that despite flood flow being largely confined to the summer (DJF) season, sufficient storage exists in these aquifers to maintain year-round stream flow from high-altitude, across the Atacama Desert to the coast. The intra-arc aquifers of the Western Cordillera receive recharge from time-variable precipitation infiltration and time-invariant lithospheric inputs. Lithospheric inputs potentially include slab/mantle dehydration, upper crustal melt devolatilization and/or thermally induced upflow of deeply penetrating meteoric water through buried evaporites or carbonates. Downstream, aquifers in the Pre-Andean basins variably interact with surface water, depending on location, river stage and time of year, but they do not supply significant additional sources of baseflow. Hydrochemical processes include those related to volcanic activity, soil carbonate generation, silicate weathering, CO2 degassing and calcite precipitation. Solutes undergo concentration by evaporation, gypsum dissolution, and are further affected by localized NO3, and SO4 inputs and mixing with saline waters. Stable isotopes reveal subcatchment specific precipitation and evaporation, whilst carbon and tritium isotopes are used to analyze recharge sources and processes in the intra-arc aquifers and downstream mixing.

Integrating major ion geochemistry, stable isotopes (18O, 2H) and radioactive isotopes (222Rn, 14C, 36Cl, 3H) to understand the interaction between catchment waters and an intermittent river

Determining the locations and sources of baseflow and the transit times of water is important for understanding catchment behaviour and functioning. Major ion geochemistry, stable isotopes (18O and 2H), and radioisotopes (222Rn, 3H, 14C, and 36Cl) were used to investigate the sources and transit times of water in the upper catchment of the intermittent Avoca River in southeast Australia. 222Rn activities and Cl concentrations implied the presence of baseflow inputs and the distribution was mainly controlled by local topography. Fluctuation of Cl concentrations implied that low-salinity near-river water was an important component of baseflow. The 3H activities of laterally disconnected pool waters during the summer months were 1.64 to 5.11 TU. The higher of these values exceed those of average annual rainfall (2.8–3.2 TU), probably due to the input of later winter to spring rainfall. The stream water had 3H activities ranging from 2.21 to 2.40 TU in July and 2.39 to 2.77 TU in August, which yield mean transit times of 4.0 to 7.0 years and 1.4 to 4.8 years respectively. These 3H activities were significantly higher than those of regional groundwater (3H activities <0.1 TU), implying that the river is largely sustained by young near-river stores at all flow conditions. Regional groundwater had 14C activities of 34.0 to 98.1 pMC, which yield mean residence times of up to 12,900 years. R36Cl values of regional groundwater (50.9–61.9 × 10−15) were higher than those of modern rainfall, probably reflecting the R36Cl values of recharge. Similar R36Cl values of the pool and stream water (33.3–58.7 × 10−15) implied that some Cl is derived from the regional groundwater influx. As with other intermittent streams in southeast Australia, the upper Avoca River was mainly sustained by relatively small water stores, and it will be vulnerable to short-term changes in climate and land use.

Using multiple isotopic and geochemical tracers to disentangle the sources of baseflow and salinity in the headwaters of a large agricultural watershed

Subsurface drainage modifications are a common practice in agricultural watersheds, especially in poorly drained soils of the American Midwest, yet our understanding of their impact on baseflow generation processes is incomplete. By extension, these same practices can make baseflow vulnerable to salinization. To address these knowledge gaps, a combined geochemical and multiple isotope approach was used to identify sources of baseflow and salinity in the Wabash River, draining Indiana, Ohio, and Illinois. Anomalously high salinity and high concentrations of F−, SO42−, Na+, Sr2+, and K+ were measured in baseflow along the headwaters reach of the Wabash River from Fort Recovery, OH to Huntington, IN. Three river sites in the headwaters reach have low 36Cl/Cl ranging from 27.5 (x 10−15) to 33.4 (x 10−15) that do not fall within the 36Cl/Cl range for monthly precipitation. Their 36Cl/Cl are consistent with 36Cl dilution most likely due to the presence of old Cl− found in potash (KCl) fertilizers. However, the 87Sr/86Sr ratios of the headwaters reach range from 0.708530 to 0.708904 (+/- 0.000025) and are similar to Silurian bedrock samples (0.708292 +/- 0.000047) indicating a geologic source for some solutes. The δ18O of the headwaters reach are isotopically light and similar to cold-season recharge, whereas the δ18O of the downstream river samples are similar to summer precipitation. The Cl−, NO3–, Cl−/Br−, and 36Cl/Cl data point to an anthropogenic source of salinity, namely KCl and road salt, whereas the F−, Sr2+, SO42−, and 87Sr/86Sr data point to a geologic source of solutes. We infer that shallow groundwater that is Cl− and NO3– rich mixes in the river with regional groundwater that is Cl− and NO3– poor. The contribution of regional groundwater to baseflow in the headwaters reach is not insignificant (>58 percent) and these interactions should be considered in future hydrologic models of the Upper Wabash River. Without the aid of multiple isotopic and geochemical tracers, baseflow generation processes and sources of salinity in the headwaters reach would remain masked by the elevated solute concentrations associated with the shallow aquifers. Further research is needed to better understand the impact of KCl fertilizers on the overall salinity budgets of agricultural watersheds.

Subsurface hydrology of tile‐drained headwater catchments: Compatibility of concepts and hydrochemistry

AbstractConcepts and terms used in previous multidisciplinary studies of tile‐drained aquitard‐dominated catchments (TDADC) are inconsistent and confusing. We provide a well‐defined, comprehensive conceptual model of the subsurface hydrology of TDADC by selecting seven mutually compatible and consistent concepts. These concepts are: (1) groundwater as the main source of baseflow in headwater streams, (2) dominance of ‘pre‐event’ water in stormflow, (3) importance of both macropores and matrix, (4) changes in flowpaths with rate of stream discharge, (5) dominance of shallow, lateral subsurface flow, (6) interactive nature of subsurface water, (7) transpiration of groundwater. This conceptual model was successfully ‘field‐tested’ by examining data collected in a TDADC in a rural area of southern Ontario, Canada. The data consist mainly of chemical and isotopes tracers in water samples (headwater streams, groundwater, precipitation, tile water, soil‐surface water), supplemented by water levels and meteorological data.

Groundwater inflow in rivers as a controlling factor to surface water nitrate concentrations and impact of groundwater age distribution on response times for remediation strategies

Diffuse groundwater contamination by intense fertilizer use is a widespread problem in most of the agricultural regions in West-Europe and in many other countries worldwide. As a result elevated nitrate concentrations in groundwater are found, and as groundwater is the source of baseflow into rivers and streams, also surface waters show increased nitrate levels. Flanders in the north of Belgium is no exception and despite restrictions on the fertilizer use and remediation practices initiated in the mid-1990s, and still ongoing, in many stream monitoring points, peak concentrations are measured above the 50 mg/l criterium. In many monitoring stations there is apparently a long delay between the start of the restrictions and effect on water quality. Often this delay is related to high concentrations in the groundwater inflow and long residence and travel times of groundwater feeding the streams. To better understand this behavior and estimate response times a tool was developed that incorporates the main water flows in a typical catchment and considers the effects of varying groundwater ages. This tool is conceived as a parsimonious lumped parameter model, simple and easy enough to be used by many people but still capturing the main mechanisms end processes. In this paper the tool is explained and it is applied on two testcases in Flanders to show its performance.

Including Regional Knowledge Improves Baseflow Signature Predictions in Large Sample Hydrology

AbstractA catchment's hydrological response is controlled by climatic forcing and by the landscape through which water moves. Yet when we compare large samples of catchments, we often find climate to be the only good predictor of the hydrological response and a lot of variability is left unexplained. This contradicts extensive evidence from field and regional studies which shows the importance of catchment form (e.g., geology) on catchment hydrological processes, particularly on baseflow processes. We hypothesize that this is due to limitations in (a) the catchment attributes we use to inform our analyses and (b) the hydrological signatures we use to describe the hydrological response. To test these hypotheses, we use a large sample of catchment data across the contiguous United States. By reviewing literature from several U.S. regions, we show that region‐specific knowledge is underutilized in large sample studies. To organize the findings from these regions, we propose and apply a framework based on standardized perceptual models. Informed by these perceptual models, we use both available and newly calculated catchment attributes to show that baseflow signature predictions can be improved regionally. Multiple baseflow signatures are needed to better distinguish between different baseflow sources, such as the subsurface, surface water bodies, and snow. We conclude with pointing at potential future directions and argue that we should aim at a more systematic and hydrologically motivated selection of catchment attributes and hydrological signatures.

Geologic Controls on Source Water Drive Baseflow Generation and Carbon Geochemistry: Evidence of Nonstationary Baseflow Sources Across Multiple Subwatersheds

AbstractThe contributions and composition of baseflow sources across an extended recession period were quantified for six subwatersheds of varying size in a structurally complex watershed in coastal California using endmember mixing analysis and related to catchment characteristics (e.g., topography, geology, land use, and soil characteristics). Both shallow subsurface and deep groundwater reservoirs were important contributors for streamflow during low flow periods, and the composition of baseflow sources across subwatersheds was directly related to geologic indices. A binary classification of underlying bedrock permeability (e.g., low vs. high) best explained the changes in shallow subsurface water and deeper groundwater inputs through the seasonal recession. Dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and specific UV absorbance at 254 nm (SUVA254) were used to provide additional insight into endmember characteristics and their contributions to baseflow. Stream water DIC concentrations were broadly controlled by mixing of groundwater and shallow subsurface water endmembers with relatively constant DIC concentrations, while stream water DOC concentrations reflected both spatial and temporal changes in shallow subsurface water DOC. Results from this study show (1) the importance of considering baseflow as a dynamic mixture of water from multiple sources, (2) the effect of geology on source composition at the subwatershed scale during low flow conditions, and (3) the impact of shifting baseflow sources on stream water dissolved carbon concentrations and the utility of using dissolved carbon concentrations to obtain additional insight into temporal variability in baseflow sources.

Water balance analysis of Talau-Loes Watershed, a cross border watershed of Indonesia and East Timor

Abstract. Riwu-Kaho M, Mella WII, Mau YS, Riwu-Kaho NPLB, Nur MSM. 2020. Water balance analysis of Talau-Loes Watershed, a cross border watershed of Indonesia and East Timor. Trop Drylands 21: 17-24. The Talau-Loes watershed is a cross-border watershed in Indonesia and Timor Leste (East Timor) which has traditionally been a natural resource for living, cultivation and conservation areas for communities in several districts of Belu Regency, Indonesia, and several areas in the Timor Leste territory. Water shortages in the land system are thought to play a significant role in the low productivity of agricultural crops. This study aimed to predict water balance in the Talau watershed, and to estimate river discharge in Malibaka Sub-watershed of Talau-Loes watershed. The study results showed that the amount of annual precipitation of 1679 mm was smaller than the total amount of water lost, amounting to 1914.79 mm per year, originating from evapotranspiration (ET) of 1650.91 mm and runoff water of 263.85 mm. Thus, theoretically, there would be an annual deficit of 235.79 mm in the Talau watershed. However, at the peak of the groundwater deficit, it turned out that the Malibaka River in the Malibaka sub-watershed and the Talau watershed had a discharge of 18 mm3/sec. This gives a clue about the ability of the groundwater level to gradually release its water reservoir as a source of base flow. Soil and water conservation actions that combine integrated systems such as agroforestry systems with mechanical conservation actions are proposed as solutions that need to be studied and developed in the future.

Cannabis and residential groundwater pumping impacts on streamflow and ecosystems in Northern California

Cannabis is an emerging agricultural frontier, but due to its quasi-legal status its environmental impacts are poorly understood. Where cannabis is irrigated by groundwater, pumping can lead to streamflow depletion in surrounding streams which may impair other water users or aquatic ecosystems. Here, we investigate the impacts of groundwater pumping for cannabis irrigation at the scale of the watershed, the individual well, and the stream segment, and contextualize by comparing with residential groundwater use. Combining mapped cannabis cultivation and residential structure locations with grower reports of irrigation water sources, we develop distributed estimates of groundwater pumping and associated streamflow depletion caused by cannabis and residential users within the Navarro River Watershed in Northern California (USA). An estimated 73% of cannabis cultivation sites and 92% of residential structures in the watershed rely on groundwater, and groundwater abstraction leads to streamflow depletion during late summer when groundwater is a critical source of baseflow to ecologically important streams. However, streamflow depletion caused by cannabis cultivation is dwarfed by the impacts of residential use, which causes >5 times as much streamflow depletion and is concentrated close to ecologically important stream segments. Focusing on cannabis, a small number of wells (<25%) cause a disproportionate amount of depletion (>50%), and significant predictors for impacts of a well are the annual pumping rate, the distance to the closest stream, and the transmissivity between the well and the stream. Streamflow depletion increases nonlinearly when pumping occurs within 1.2 km of streams, and most cannabis and residential groundwater use is within this critical distance. Given the rapid increase in cannabis cultivation, these results indicate that potential streamflow depletion from groundwater irrigation of cannabis is a current and future concern, and will be superimposed on top of significant depletion already occurring due to residential use in the region studied.

Climate, Landforms, and Geology Affect Baseflow Sources in a Mountain Catchment

AbstractBaseflow is essential for stream ecosystems and human water uses, particularly in areas with Mediterranean climates. Yet the factors controlling the temporal and spatial variability of baseflow and its sources are poorly understood. Measurements of oxygen and hydrogen isotopic composition (δ18O and δ2H) were used to evaluate controls on baseflow in the stream network of a 64‐km2 catchment in western Oregon. A total of 607 water samples were collected to contrast baseflow in a year of near average precipitation (2016) to a year with low winter snowpack and subsequent summer drought conditions (2015). Spatial autocorrelation structures and relationships between surface water isotopic signatures and geologic and topographic metrics throughout the network were determined using Spatial Stream Network models. Isotope values varied widely in space and between years, indicating disparate baseflow water sources. During average flow conditions, the spatial variation in δ18O was primarily related to elevation, reflecting the influence of prior precipitation and input of water from snowmelt at higher elevation. In contrast, during drought conditions, the spatial variation in δ18O was also related to terrain slope and roughness—proxies for local water storage in deep‐seated earthflows and other Quaternary deposits. A prominent spring‐fed tributary with high unit baseflow discharge illustrated the importance of subsurface water storage in porous volcanic bedrock. As drought increases in a warming climate, baseflow in mountain catchments may become more dependent on storage in geologic and geomorphic features.

Multiyear Increase in the Stable Isotopic Composition of Stream Water From Groundwater Recharge Due to Extreme Precipitation

AbstractThe isotopic composition of surface and groundwater is impacted by many hydrologic processes. The long‐term stable water isotopic response of systems to hydrologic change is critical for interpreting isotopic information for streamflow generation, stream‐aquifer‐coupling, and recharge processes. To evaluate the response of stream‐aquifer systems to extreme precipitation events we use 743 surface and groundwater isotopes with drainage areas ranging from 0.1 to &gt;800 km2. Results show multiannual trends from high to low isotopic compositions associated with increases in the composition of shallow groundwater. The year 2011 was one of the wettest years and the months of August and September were the wettest consecutive 2‐month period in the 123‐year record. This increase in the isotopic composition has long‐term impact on the isotopic composition of surface and groundwater highlighting the importance of groundwater sources of baseflow to streams and the transient storage and release mechanisms of groundwater at the catchment scale.

Seasonal and Flood‐Induced Variations in Groundwater–Surface Water Exchange in a Northern Coldwater Fishery

AbstractGroundwater upwelling is important to coldwater fisheries survival. This study used stable isotopes to identify upwelling zones within a watershed, then combined isotope analyses with reach‐scale monitoring to measure surface water–groundwater exchange over time. Research focused on Amity Creek, Minnesota, a basin that exemplifies conditions limiting coldwater species survival along Lake Superior's North Shore where shallow bedrock limits groundwater capacity, lowering baseflows and increasing temperatures. Groundwater‐fed reaches were identified through synoptic isotope sampling, with results highlighting the importance of isolated shallow surficial aquifers (glacially derived sands and gravels) for providing cold baseflow waters. In an alluvial reach, monitoring well results show groundwater was stored in two reservoirs: one that reacts quickly to changes in stream levels, and one that remained isotopically isolated under most flow conditions, but which helps sustain summer baseflows for weeks to months. A 500‐year flood demonstrated the capacity of high‐flow events to alter surface water–groundwater connectivity. The previously isolated reservoir was exchanged or mixed during the flood pulse, while incision lowered the water table for years. The results here provide insight for streams that lack substantial groundwater inputs yet maintain coldwater species at risk in a warming climate and an approach for managers seeking to protect cold baseflow sources.

Spatial delineation of riparian groundwater within alluvium deposit of mountainous region using Laplace equation

AbstractThe interaction between surface and groundwater plays a key role in a riparian ecosystem while the size of riparian groundwater has not been typically incorporated into hydrological modelling systems. An extensive geophysical survey composed of 25 individual DC electrical resistivity profiles was conducted at the Blair–Wallis site in Wyoming. The observed resistivity images show a near‐surface aquifer interpreted as the saturated alluvium deposit along the channel, rather than the geological bedrock. Based on the electrical resistivity images, it can be inferred that only the near‐surface portion of the groundwater actively interacts with the stream flow in the mountainous and hilly watershed. This study attempted the spatial extrapolation of the measured riparian aquifer depths by means of fitting functions based on the surface topography. The analysis indicated that the boundary of the riparian aquifer well corresponds to the topographical inflexion point of the hill slope profile. It was also demonstrated that the extent of alluvium deposit, where the area of riparian aquifer is indicated, can be delineated using the slope and curvature maps in the geographic information system. Then, the parabolic and biharmonic functions were tested for the groundwater depth estimation using the developed alluvium deposit map. The proposed methodology was effective if geological diffusion processes by wind and water dominated the topography. The spatial map of the active aquifer will be useful in hydrological drought analysis because it is considered to be a main source of baseflow during dry seasons.

Artificial Sweeteners Reveal Septic System Effluent in Rural Groundwater.

It has been widely documented that municipal wastewater treatment plant effluents are a major source of artificial sweeteners to surface waters. However, in rural areas, the extent to which septic systems contribute these same compounds to groundwater aquifers is largely unknown. We examined the occurrence of four commonly used artificial sweeteners in an unconfined sand aquifer that serves as a water supply for rural residents, as a receptor of domestic wastewater from septic systems, and as a source of baseflow to the Nottawasaga River, ON, Canada. Groundwater from the Lake Algonquin Sand Aquifer in the southern Nottawasaga River Watershed was collected from private domestic wells and as groundwater seeps discharging along the banks of the Nottawasaga River. Approximately 30% of samples had detectable levels of one or more artificial sweeteners, indicating the presence of water derived from septic system effluent. Using acesulfame concentrations to estimate the fraction of septic effluent in groundwater samples, ∼3.4 to 13.6% of the domestic wells had 1% or more of their well water being derived from septic system effluent. Similarly, 2.0 to 4.7% of the groundwater seeps had a septic effluent contribution of 1% or more. No relationship was found between the concentration of acesulfame and the concentration of nitrate, ammonium, or soluble reactive phosphorus in the groundwater, indicating that septic effluent is not the dominant source of nutrients in the aquifer. It is expected that the occurrence of artificial sweeteners in shallow groundwater is widespread throughout rural areas in Canada.

What is the source of baseflow in agriculturally fragmented catchments? Complex groundwater/surface‐water interactions in three tributary catchments of the Wabash River, Indiana, USA

AbstractSome conceptual models suggest that baseflow in agriculturally fragmented watersheds may contain little, if any, groundwater. This has critical implications for stream quality and ecosystem functioning. Here, we (a) identify the sources and flowpaths contributing to baseflow using 222Rn and 87Sr/86Sr and (b) quantify mean apparent ages of groundwater and baseflow using multiple isotopic tracers (CFC, SF6, 36Cl, and 3H) in 4 small (0.08 to 0.64 km2) tributary catchments to the Wabash River in Indiana, USA. 222Rn activities and 87Sr/86Sr ratios indicate that baseflow in 3 catchments is sourced primarily from groundwater; baseflow in the fourth is dominated by a source similar to agricultural run‐off. CFC‐12 data indicate that springs in 1 catchment are discharging significant proportions of water that recharged between 1974 (42 ± 2 years) and 1961 (55 ± 2 years). Those same springs have 36Cl/Cl ratios between 1,381.08 ± 29.37 (×10−15) and 1,530.64 ± 27.65 (×10−15) indicating that a substantial proportion of the discharge likely recharged between 1975 (41 years) and 1950 (66 years). Groundwater samples collected from streambed mini‐piezometers in a separate catchment have CFC‐12 concentrations indicating that a large proportion of the recharge occurred between 1948 (68 ± 2 years) and 1950 (66 ± 2 years). Repeat sampling conducted in September 2015 after above‐average summer rainfall did not show significant decreases in mean apparent age. The relatively old ages observed in 3 of the catchments can be explained by geological complexities that are likely present in all 4 catchments, but overwhelmed by flow from the shallow phreatic aquifer in the fourth catchment.

Biogeochemical processes controlling aquatic quality during drying and rewetting events in a Mediterranean non-perennial river reach

Desiccation and re-flooding processes play a key role on hydrological features of non-perennial rivers. This study addresses the effects of these processes on the aquatic quality and unravels underlying biogeochemical processes of an intermittent river reach in southern Greece containing a spring-fed pool. Combined spatio-temporal sampling for physicochemical parameters, major ions and nutrients and high frequency automatic monitoring during a hydrological year (2010–2011) indicate that during the dry period, solute variation was controlled by “concentration” processes (i.e. evaporative concentration and high dissolved ion input from base flow sources). Metabolic and “concentration” processes appear intensified during desiccation and water temperature rise. Photosynthesis induced carbonate precipitation, while respiration increased with gradual desiccation, but did not cause carbonate dissolution. In certain cases, photosynthesis and respiration may have occurred simultaneously as a result of differing microhabitat metabolism within the same water body. However, during the entire desiccation cycle, autotrophic production exceeded respiration resulting in relatively high oxygen concentrations, even during the night. With increasing desiccation, a rise in nutrient assimilation occurred as well as ammonification and/or desorption of ammonium from sediments, with simultaneous loss of nitrate. During initial floods, flushing of carbonate phases was not significant. In contrast, initial flood events were characterized by the dissolution of very soluble salts, i.e. epsomite-type (MgSO4∗7H2O) and gypsum (CaSO4∗2H2O). Regarding sediment transport and nutrients, a 1000-times increase of suspended sediments was observed during re-flooding, while the nutrient quality degraded, particularly for N-species. Results of the current research may serve to better understand the links of hydrological and biogeochemical processes in non-perennial rivers and streams towards their efficient management and conservation.

Sea level and ground water table depth (WTD): A biogeochemical pacemaker for glacial-interglacial cycling

Abstract The role that changes in sea level have on potential carbon-climate feedbacks are discussed as a potential contributing mechanism for terminating glacial periods. Focus will be on coastal wetlands because these systems can be substantially altered by changing sea level and ground water table depth (WTD); in addition to being important moderators of the exchange of nutrients and energy between terrestrial and marine ecosystems. A hypothesis is outlined that describes how the release of carbon from formerly anaerobic wetland soils and sediments can influence climate when sea levels begin to decline. As ground WTD deepens and eventually recedes from the surface, coastal wetland basins may become isolated from their belowground source of water. With their primary source of base flow removed, coastal wetlands likely dried up, promoting decomposition of the carbon compounds buried in their sediments. Depending on the timing of basin isolation and the timing of decomposition, glacial sea level lows could have triggered a relatively large positive carbon feedback on climate warming, just at the time when a new interglacial period is about to begin.

Isotopic Characterization of River Waters and Water Source Identification in an Inland River, Central Asia

Understanding runoff generation and dynamics is the basis for water resource management, while water isotopic ratios are a potential tool for studying the mechanism on a large scale. In this paper, spatial variations of δ18O and δD of river water and their sources within a large region of the Tarim River were investigated. The results showed obvious spatial variations of both water isotope values along the river flow direction, and significant seasonal variation occurred within the river water isotopes. This indicated that different proportions of rain and melt water entering river water should lead to spatial variation, and for mid-stream and downstream regions, the transformation relationship between surface water and groundwater should consider less input of melt water. Furthermore, we quantitatively determine the ratio of different water sources using the stable isotope mass balance method and other stable tracer elements. Results showed the contribution of ice-snowmelt water varied from 14.97% to 40.85%, that of rain varied from 9.04% to 54.80%, and that of groundwater varied from 15.34% to 58.85%, and they also showed that baseflow is a factor connecting melt water and groundwater, which meant the Hotan River and the Yarkand River are melt water–dependent rivers, and seasonal precipitation is the main water supply source of baseflow in the Aksu River and the Kaidu River.