Using bromide data tracer and HYDRUS-1D to estimate groundwater recharge and evapotranspiration under film-mulched drip irrigation in an arid Inland Basin,Northwest China

Accurate estimation of groundwater recharge (GR) and evapotranspiration (ET) are essential for sustainable management of groundwater resources, especially in arid and semi-arid regions. In the Manas River Basin (MRB), water shortage is the main factor restricting sustainable development of irrigated agriculture, which relies heavily on groundwater. Film-mulched drip irrigation significantly changes the pattern and dominant processes of water flow in the unsaturated zone, which increases the difficulty of GR and ET estimation. To better estimate GR and ET under film-mulched drip irrigation in the MRB, bromide tracer tests and soil lithologic investigation were conducted at 12 representative sites. A one-dimensional variably saturated flow model (HYDRUS-1D) was calibrated at each site using soil evaporation data inferred from the bromide tracer tests. The results showed that average annual soil evaporation in uncultivated lands calculated from bromide trace tests was 25.55 mm. The annual GR ranged from 5.5 to 37.0 mm under film-mulched drip irrigation. The annual ET ranged from 507.0 to 747.1 mm, with soil evaporation between 35.7 and 117.0 mm and transpiration between 460.9 and 642.3 mm. Soil evaporation represented 7% to 16% of the total ET and more than 70% of precipitation and irrigation water was used by cotton plants. Spatial variations of soil lithology, water table depth and initial soil water content led to the spatial differences of GR and ET in the MRB. Our study indicated that bromide tracer tests are useful for inferring ET in the arid and semi-arid oases. The combination of bromide tracer tests and HYDRUS-1D enhances reliability for estimation of GR and ET under film-mulched drip irrigation in the MRB and shows promise for other similar arid inland basins around the world.     

Performance of the White method for estimating groundwater evapotranspiration under conditions of deep and fluctuating groundwater

The White method is a simple but the most frequently applied approach to estimate groundwater evapotranspiration (ET_g) from groundwater level diurnal signals. Because of a lack of direct measurements of ET_g, it is difficult to evaluate the performance of the White method, particularly in field environments with variable groundwater fluctuations. A 2‐year field observation in a hyper‐arid riparian tamarisk (Tamarix spp.) stand with deep groundwater depth in the lower Tarim River basin of China was conducted to measure the surface evapotranspiration (ET_s) and groundwater table. The performance of the White method and the influences of the variable groundwater fluctuations on the determinations of the specific yield (S_y) and recharge rate of groundwater (r) in the White method were investigated. The results showed that the readily available S_y determined by Loheide's method was feasible but must be finely determined based on the soil textures in the layers in which the groundwater level fluctuated. A constant S_y value for a defined porous medium could be assumed regardless of the discharge or recharge processes of groundwater. The time span of 0000 h to 0600 h for r estimation for the White method worked best than other time spans. A 2‐day moving average of r values further enhance ET_g estimation. Slight effects of environmental or anthropogenic disturbances on the diurnal fluctuations of groundwater level did not influence the ET_g estimations by the White method. Our results provide valuable references to the application of the White method for estimating daily ET_g in desert riparian forests with deep groundwater depth. Copyright © 2015 John Wiley & Sons, Ltd.     

Hydrological processes of a rainforest headwater swamp from natural chemical tracing in Nsimi watershed,Cameroon

The hydrological role of a headwater swamp in a tropical rainforest is studied using chloride mass balance (CMB) and end‐member mixing analysis. There are three main contributions to streamflow: (1) the hillside bedrock aquifer, (2) overland flow from the swamp during storm events and (3) groundwater flow from the swamp aquifer. Before rainfall events of the wet season, the pre‐event water comprises a mix of 80% of bedrock aquifer and 20% of swamp aquifer. During storms, the relative contribution of overland flow increases according to the rainfall intensity and the initial saturation rate of the pre‐event water reservoirs. The yearly contribution of overland flow from the swamp to the stream is about 31%. The relationship between the swamp and the stream fluctuates with space and time. Generally, the swamp is drained by the stream; however, at the end of long dry seasons, after the first rains, indirect recharge occurs from the stream to the swamp with a hydraulic gradient inversion in the swamp aquifer. The net contribution of the swamp aquifer to the stream is only 4%, which is much lower than the hillside aquifer contribution of about 65%. Recharge on the swamp being very low, these results suggest that, except for a few storms at the end of the dry season, the Nsimi swamp does not contribute to flood attenuation. Evapotranspiration is higher on the hillside than in the swamp. Nevertheless, depletion of water stored within the swamp is dominated by evaporation rather than by its contribution to streamflow. The export of solutes through swamp groundwater flow below the weir is low (<7%). Nevertheless, the swamp is the most active area of weathering in the watershed. Copyright © 2011 John Wiley & Sons, Ltd.     

Impact of redox and transport processes in a riparian wetland on stream water quality in the Fichtelgebirge region,southern Germany

Biologically mediated redox processes in the riparian zone, like denitrification, can have substantially beneficial impacts on stream water quality. The extent of these effects, however, depends greatly on the hydrological boundary conditions. The impact of hydrological processes on a wetland's nitrogen sink capacity was investigated in a forested riparian fen which is drained by a first‐order perennial stream. Here, we analysed the frequency distributions and time‐series of pH and nitrogen, silica, organic carbon and oxygen concentrations in throughfall, soil solution, groundwater and stream water, and the groundwater levels and stream discharges from a 3‐year period. During baseflow conditions, the stream was fed by discharging shallow, anoxic groundwater and by deep, oxic groundwater. Whereas the latter delivered considerable amounts of nitrogen (～0·37 mg l^?1) to the stream, the former was almost entirely depleted of nitrogen. During stormflow, near‐surface runoff in the upper 30 cm soil layer bypassed the denitrifying zone and added significant amounts to the nitrogen load of the stream. Nitrate‐nitrogen was close to 100% of deep groundwater and stream‐water nitrogen concentration. Stream‐water baseflow concentrations of nitrate, dissolved carbon and silica were about 1·6 mg l^?1, 4 mg l^?1 and 7·5 mg l^?1 respectively, and >3 mg l^?1, >10 mg l^?1 and <4 mg l^?1 respectively during discharge peaks. In addition to that macroscale bypassing effect, there was evidence for a corresponding microscale effect: Shallow groundwater sampled by soil suction cups indicated complete denitrification and lacked any seasonal signal of solute concentration, which was in contrast to piezometer samples from the same depth. Moreover, mean solute concentration in the piezometer samples resembled more that of suction‐cup samples from shallower depth than that of the same depth. We conclude that the soil solution cups sampled to a large extent the immobile soil‐water fraction. In contrast, the mobile fraction that was sampled by the piezometers exhibited substantially shorter residence time, thus being less exposed to denitrification, but predominating discharge of that layer to the stream. Consequently, assessing the nitrogen budget based on suction‐cup data tended to overestimate the nitrogen consumption in the riparian wetland. These effects are likely to become more important with the increased frequency and intensity of rainstorms that are expected due to climate change. Copyright © 2006 John Wiley & Sons, Ltd.     

The Manawatu aquifers,North Island,New Zealand: Clarification of hydrogeology using a multidisciplinary environmental tracer approach

A recently developed approach to carbon isotope methodology (process recognition via isotope diagrams) is applied in a multidisciplinary study of precipitation‐recharged aquifers of the lowlands–plains area of the Manawatu (south‐west North Island). Urban and rural areas rely on groundwater from the upper levels of a deep basin sequence comprising marine and terrestrial sediments of Pleistocene age. Hydrochemical and isotopic (¹⁸O, ³H, ¹³C and ¹⁴C) data are merged with known details of geology and hydrogeology to reveal two separate confined aquifers within the depth range to 200 m. The shallower of these, below unconfined, locally recharged groundwater, is recharged on the foothills of the Ruahine Range to the north‐east of the study area; flow direction is NE–SW. The deeper confined aquifer is recharged on the Tararua Range to the immediate east; flow direction essentially is transverse (SE–NW) to that in the shallower aquifer. Two processes are identified as dominant contributors to concentration and isotopic composition of dissolved inorganic carbon (DIC), namely addition of CO₂ from decay of organic materials and carbonate dissolution. Limitations of carbon isotope methods in determining residence times are illustrated by the data. Although the confined groundwater is essentially tritium‐free, only a few samples showed conclusive evidence of significant ageing on the time‐scale of ¹⁴C. Copyright © 2001 John Wiley & Sons, Ltd.     

Depth‐averaged,spatially distributed flow dynamic and solute transport modelling of a large‐scaled,subtropical constructed wetland

Constructed wetlands are being utilized worldwide to effectively reduce excess nutrients in agricultural runoff and wastewater. Despite their frequency, a multi‐dimensional, physically based, spatially distributed modelling approach has rarely been applied for flow and solute transport in treatment wetlands. This article presents a two‐dimensional hydrodynamic and solute transport modelling of a large‐scaled, subtropical, free water surface constructed wetland of about 8 km² in the Everglades of Florida, USA. In this study, MIKE 21 was adopted as the basic model framework. Field monitoring of the time series hydrological and chloride data, as well as spatially distributed data such as bathymetry and vegetation distribution, provided the necessary model input and testing data. Simulated water level profiles were in good agreement with the spatio‐temporal variations of measured ones. On average, the root‐mean‐square error of model calibration on annual water level fluctuations was 0·09 m. Manning's roughness coefficients for the dense emergent and submerged aquatic vegetation areas, which were estimated as a function of vegetation type, ranged from 0·67 to 1·0 and 0·12 to 0·15 s/m^1/3, respectively. The solute transport model calibration for four monitoring sites agreed well with the measured annual variations in chloride concentration with an average percent model error of about 15%. The longitudinal dispersivity was estimated to be about 2 m and was more than an order of magnitude higher than the transverse one. This study is expected to play the role of a stepping stone for future modelling efforts on the development and application of more advanced flow and transport models applicable to a variety of constructed wetland systems, as well as to the Everglades stormwater treatment areas in operation or in preparation. Copyright © 2010 John Wiley & Sons, Ltd.     

Response of water and energy exchange to the environmental variable in a desert‐oasis wetland of Northwest China

A case study on a desert‐oasis wetland ecosystem in the arid region of Northwest China measured the seasonal and interannual variation in energy partitioning and evapotranspiration to analyse the response of water and energy exchange on soil moisture, groundwater, and environmental variables. Energy partitioning showed a clear seasonal and interannual variability, and the process of water and energy exchange differed significantly in the monthly and interannual scales. The net radiation was 7.31 MJ m^?2· day^?1, and sensible heat flux accounted for 50.42% of net radiation in energy fluxes, 40.56% for latent heat flux, and 9.02% for ground heat flux. The parameters in energy fluxes were best described by a unimodal curve, whereas sensible heat flux followed a bimodal curve. Variations in the daily evapotranspiration and crop evapotranspiration also exhibited a single peak curve with annual values of 569.84 and 644.47 mm, respectively. Canopy conductance averaged 20.77 ± 13.75 mm s^?1 and varied from 0.16 to 83.96 mm s^?1 during the two hydrological years. The variation in water and energy exchange reflected environmental conditions and depended primarily on vapour pressure deficit, net radiation, soil moisture, and water depth. Although the effects of precipitation on evapotranspiration showed that the response of this ecosystem to climate changes was not obvious, the variation of air temperatures had a strong influence on evapotranspiration, resulting in a significant increase in evapotranspiration (R = 0.730; P < 0.01). Copyright © 2013 John Wiley & Sons, Ltd.     

Characterizing solute transport with transient storage across a range of flow rates: The evidence of repeated tracer experiments in Austrian and Italian streams

Solute transport in rivers and streams with hyporheic zone exchange and/or in-stream storage is typically affected by the prevailing flow rate. The research reported here focuses on stream tracer experiments repeated many times along the same Austrian (Mödlingbach) and Italian (Torrente Lura) channel reaches to characterize parameter dependency on flow rate. Both groups of data sets showed an increase of storage zone area and main stream area with discharge. In either case, a strong negative correlation was obtained between storage zone residence time and flow rate. From the Mödlingbach data, no clear relationship with Q emerged for the dispersion coefficient and the dead zone ratio, whereas Torrente Lura showed a clear positive correlation of the dispersion coefficient with the flow rate and a slightly negative Q-dependency for the dead zone ratio. Mödlingbach and Torrente Lura results are presented against the background of other repeat experiments reported in literature.     

Large seasonal changes in the recharge of seawater in a subterranean estuary revealed by a radon tracer

Activities of radon (²²²Rn) in groundwater were continuously monitored in a saline aquifer from September 2010 to July 2011. The activities of ²²²Rn ranged from 200 to 4300 Bq m^?3, with a large seasonal variation. Because the activity of ²²²Rn in seawater is low, ²²²Rn in saline groundwater must be produced in the aquifer from radium (²²⁶Ra) in rocks and sediments. The ²²²Rn activities were higher in the warm‐dry seasons (September–November 2010 and April–May 2011) when the saline aquifer was stable. In contrast, the lowest ²²²Rn activities were observed in the cold‐dry season (December 2010 and January–March 2011), because of the effective exchange between groundwater and seawater. In addition, sudden decreases of ²²²Rn activities coincided with episodic drops in groundwater temperatures. These results reveal that lower seawater temperature in winter may result in density‐driven seawater intrusion. During the wet season (June–July 2011), ²²²Rn activities were more clearly affected by semi‐monthly and diurnal tidal pumping, showing higher ²²²Rn activities during low and spring tides. Such a tidal effect was not clearly observed during the warm‐dry and cold‐dry seasons. This result implies that the residence time of SGD in coastal zones is significantly affected by seasonal changes in driving forces such as tidal pumping and seawater intrusion. Copyright © 2016 John Wiley & Sons, Ltd.     

Short‐ and long‐term evapotranspiration rates at ecological restoration sites along a large river receiving rare flow events

Many large rivers around the world no longer flow to their deltas, due to ever greater water withdrawals and diversions for human needs. However, the importance of riparian ecosystems is drawing increasing recognition, leading to the allocation of environmental flows to restore river processes. Accurate estimates of riparian plant evapotranspiration (ET) are needed to understand how the riverine system responds to these rare events and achieve the goals of environmental flows. In 2014, historic environmental flows were released into the Lower Colorado River at Morelos Dam (Mexico); this once perennial but now dry reach is the final stretch to the mighty Colorado River Delta. One of the primary goals was to supply native vegetation restoration sites along the reach with water to help seedlings establish and boost groundwater levels to foster the planted saplings. Patterns in ET before, during, and after the flows are useful for evaluating whether this goal was met and understanding the role that ET plays in this now ephemeral river system. Here, diurnal fluctuations in groundwater levels and Moderate Resolution Imaging Spectroradiometer (MODIS) data were used to compare estimates of ET specifically at 3 native vegetation restoration sites during 2014 planned flow events, and MODIS data were used to evaluate long‐term (2002–2016) ET responses to restoration efforts at these sites. Overall, ET was generally 0–10 mm d^?1 across sites, and although daily ET values from groundwater data were highly variable, weekly averaged estimates were highly correlated with MODIS‐derived estimates at most sites. The influence of the 2014 flow events was not immediately apparent in the results, although the process of clearing vegetation and planting native vegetation at the restoration sites was clearly visible in the results.     

Evapotranspiration partitioning using a simple isotope‐based model in a semiarid marsh wetland in northeastern China

Partitioning evapotranspiration (ET) into evaporation (E) and transpiration (T) in wetlands is important for understanding the hydrological processes in wetlands and the contribution of wetland ET to local and regional water cycling and for designing effective wetland management strategies. Stable water isotopes are useful in the application of ET partitioning through the evaluation of the isotopic compositions of E (δ_E), T (δ_T), and ET (δ_ET) obtained from observation or modelling methods. However, this approach still suffers from potentially large uncertainties in terms of estimating the isotopic endmembers. In this study, we modified the traditional isotope‐based ET partitioning methods to include leaf‐level biological constraints to separately estimate the relative contributions of T from Scirpus triqueter and Phragmites australis and the relative contributions of E from the standing surface water in a semiarid marsh wetland in northeastern China. The results showed that although the δ_T values of S. triqueter and P. australis were rather similar, the mean δ_T values of the 2 species were different from the values of δ_E, making it possible to distinguish the relative contributions of E and T through the use of isotopes. The simulation of leaf water using a non‐steady‐state model indicated obvious deviations in leaf water enrichment (δ_Lb) from isotopic steady states for both species, especially during early mornings and evenings when relative humidity was highest. The isotopic mass balance showed that E accounted for approximately 60% of ET, and T from S. triqueter and P. australis each contributed approximately 20% to ET; this implied that the transpiration of one reed was equivalent to that of 5.25 individuals of S. triqueter. Using the estimated ratio of T to ET and the measured leaf transpiration, the total ET was estimated to be approximately 10 mm day^?1. Using the NSS‐Tr method, the estimated ET was higher than the water loss calculated from the water level gauge. This indicated that the river water and surrounding groundwater were the sources of the marsh wetland, with a supply rate of 8.3 mm day^?1.     

Temporal variation of over‐bank flooding of Wei River and its impact on a riparian wetland in Xi'an,China

Over‐bank flooding is one of the driving forces controlling ecological integrity of riparian wetlands. Indentifying natural over‐bank flooding regime and its temporal variations is crucial for developing conservation and restoration plans and making water resources management policies for these ecosystems. Along the midstream of the Wei River in Xi'an, China lies the Jingwei riparian wetland, which was well preserved until the 1970s. Based on historical record of hydrological and morphological data of the Wei River from 1951 to 2000, we analysed temporal variations of over‐bank flooding frequency, duration, and timing in this paper. The natural annual over‐bank flooding regime was identified as having an occurrence frequency of 2·2 times a year and average duration of 5·3 days; these flooding events typically occur between June and September with occasional occurrence in late spring and late autumn. Over‐bank flooding occurrence frequency and duration decreased significantly during the 1990s, seasonal events of over‐bank floods were changed through reduced flooding frequency during summer and disappearing flooding events in late spring and late autumn. Further investigations showed that reduced discharge in the Wei River was the principal cause for these changes in over‐bank flooding dynamics. Our analysis also showed that decreased discharge of the Wei River during the 1990s was attributed near equally to disturbances from human activities and decreased regional precipitation. Results from this study may help reestablish natural over‐bank flooding dynamics in order to ensure successful restoration of Jingwei riparian wetland. Copyright © 2010 John Wiley & Sons, Ltd.     

Total solar eclipse effects on evapotranspiration captured by groundwater fluctuations in a Southern Appalachian fen

Solar eclipses provide opportunities to observe unusual natural phenomena. This paper presents data showing that diurnal groundwater fluctuations were briefly interrupted in a Southern Appalachian fen following a total solar eclipse on August 21, 2017. Groundwater levels, which were rapidly declining in the middle of a summer afternoon, nearly stabilized following totality. This anomaly in the water‐level record was caused by reduced evapotranspiration (ET). ET was significantly lower (36% ± 11%, p < 0.02) over a 45‐min time period following the eclipse when compared with other days within the study period, although the effects were short lived and did not have a significant impact on the daily total ET (p > 0.05). These findings may prompt evaluation of existing groundwater records along the path of totality from past eclipses or plan for studies ahead of future eclipses.     

Beaver dams along an agricultural stream in southern Ontario,Canada: their impact on riparian zone hydrology and nitrogen chemistry

The hydrology and nitrogen biogeochemistry of a riparian zone were compared before and after the construction of beaver dams along an agricultural stream in southern Ontario, Canada. The beaver dams increased surface flooding and raised the riparian water table by up to 1·0 m. Increased hydraulic gradients inland from the stream limited the entry of oxic nitrate‐rich subsurface water from adjacent cropland. Permeable riparian sediments overlying dense till remained saturated during the summer and autumn months, whereas before dam construction a large area of the riparian zone was unsaturated in these seasons each year. Beaver dam construction produced significant changes in riparian groundwater chemistry. Median dissolved oxygen concentrations were lower in riparian groundwater after dam construction (0·9–2·1 mg L^?1) than in the pre‐dam period (2·3–3·9 mg L^?1). Median NO₃‐N concentrations in autumn and spring were also lower in the post‐dam (0·03–0·07 mg L^?1) versus the pre‐dam period (0·1–0·3 mg L^?1). In contrast, median NH₄‐N concentrations in autumn and spring months were higher after dam construction (0·3–0·4 mg L^?1) than before construction (0·13–0·14 mg L^?1). Results suggest that beaver dams can increase stream inflow to riparian areas that limit water table declines and increase depths of saturated riparian soils which become more anaerobic. These changes in subsurface hydrology and chemistry have the potential to affect the transport and transformation of nitrate fluxes from adjacent cropland in agricultural landscapes. Copyright © 2009 John Wiley & Sons, Ltd.     

A hydrographic method to identify the groundwater net recharge,barometric effect and evapotranspiration from a complicated semidiurnal water table fluctuation

A hydrographic method was proposed to separate out the hourly scaled groundwater level changes caused by net recharge, barometric effects and evapotranspiration from a semidiurnal water table fluctuation. A characteristic midnight time, with a turning point of the barometric pressure change and high relative air humidity, which meant that neither the barometric effect nor groundwater evapotranspiration occurred, was proposed for determining the net recharge rate r_net. Then, the barometric efficiency f_bar was estimated using the other time period without evapotranspiration, and the evapotranspiration rate r_ETG was finally obtained using the remainder of the water level changes. A case example illustrated that estimation of f_bar using the proposed method was more accurate than that using the traditional error analysis method, which may result in a significant underestimation under the condition of the present water level changes. Additionally, the abnormal semidiurnal fluctuations, more specifically, two step-down fluctuations, which may be a common pattern when the groundwater level is controlled by net recharge, barometric effects and evapotranspiration, can be well understood using the three components separated out. The results also showed that nighttime groundwater evapotranspiration, accounting for an average of 23% of that during the daytime, cannot be ignored.     

Groundwater dynamics in a till hillslope: flow directions,gradients and delay

Knowledge of groundwater dynamics is important for the understanding of hydrological controls on chemical processes along the water flow pathways. To increase our knowledge of groundwater dynamics in areas with shallow groundwater, the groundwater dynamics along a hillslope were studied in a boreal catchment in Southern Sweden. The forested hillslope had a 1‐ to 2‐m deep layer of sandy till above bedrock. The groundwater flow direction and slope were calculated under the assumption that the flow followed the slope of the groundwater table, which was computed for different triangles, each defined by three groundwater wells. The flow direction showed considerable variations over time, with a maximum variation of 75°. During periods of high groundwater levels the flow was almost perpendicular to the stream, but as the groundwater level fell, the flow direction became gradually more parallel to the stream, directed in the downstream direction. These findings are of importance for the interpretation of results from hillslope transects, where the flow direction usually is assumed to be invariable and always in the direction of the hillslope. The variations in the groundwater flow direction may also cause an apparent dispersion for groundwater‐based transport. In contrast to findings in several other studies, the groundwater level was most responsive to rainfall and snowmelt in the upper part of the hillslope, while the lower parts of the slope reached their highest groundwater level up to 40 h after the upper parts. This can be explained by the topography with a wetter hollow area in the upper part. Copyright © 2011 John Wiley & Sons, Ltd.     

Soil pipe flow tracer experiments: 1. Connectivity and transport characteristics

Much debate has occurred in catchment hydrology regarding the connectivity of flow paths from upslope areas to catchment outlets. This study was conducted in two catchments, one with three upper branches, in a loess soil with a fragipan that fosters lateral flow and exhibits an extensive distribution of soil pipe collapse features. The study aimed to determine the connectivity of multiple soil pipe networks as well as determine pipe flow velocities during storm events. Fluorescein dye was injected directly into soil pipes at the upper most pipe collapse feature of four different hillslopes. Breakthrough curves (BTC) were determined by sampling multiple pipe collapse features downslope. The BTCs were used to determine the ‘average’ (centre of mass) and ‘maximum’ (first arrival) flow velocities. This study confirmed that these catchments contain individual continuous soil pipe networks that extend over 190 m and connect the upper most hillslopes areas with the catchment outlet. While the flow paths are continuous, the individual pipe networks consist of alternating reaches of subsurface flow through soil pipes and reaches of surface flow through gullies formed by pipe collapses. In addition, flow can be occurring both through the subsurface soil pipes simultaneous with surface flow generated by artesian flow from the soil pipes. The pipe flow velocities were as high as 0.3 m/s, which was in the range of streamflow velocities. These pipe flow velocities were also in the range of velocities observed in pinhole erosion tests suggesting that these large, mature soil pipes are still actively eroding. Copyright © 2015 John Wiley & Sons, Ltd.