Groundwater-surface Water Interface Research Articles

Reservoir sediments: a sink or source of chemicals at the surface water-groundwater interface.

This study delineates the physical, chemical, and biological effects resulting from anthropogenic and endogenic activities in a sensitive dammed reservoir situated in a semi-arid region. The reservoir is characterized by two major flow regimes: a wet fill hydrologic regime and a dry spill one. A seasonal sampling campaign was carried out over a period of 2 years (2011-2013) where water samples were collected across the water column and from piezometers just outside the perimeter of the reservoir. Similarly, sediments were collected from the corresponding areas beneath the water column. The water samples were analyzed for environmental isotopic ratios, elemental composition, and physical, biological and chemical parameters, whereas the sediment and algal samples were subjected to physical, mineralogical, spectroscopic, and microscopic analyses. This investigation indicated that the dam had resulted in the alteration of the biogeochemical cycle of nutrients as well as the degradation of the sediment and water quality. The hydrological and biogeochemical processes were found to induce vertical downward transport of chemicals towards the fine grained calcareous sediments during the fill mode, whereas the sediments acted as a source of a chemical flux upward through the water column and downward towards the groundwater during the spill mode. The geomorphological characteristics of the reservoir enhanced the strong hydrological connectivity between the surface water and the groundwater where the reservoir responded quickly to natural and anthropogenic changes in the upper watershed. The water and sediments in the sensitive spill mode were of poor quality and should receive more attention due to the potential hazard for the associated hydro-project and the sustainability of the agricultural soil in the long term. Thus, a safe water and sediment management plan should be implemented in order to improve the dam functionality and to safeguard the precious water resources.

Detecting groundwater discharge dynamics from point-to-catchment scale in a lowland stream: combining hydraulic and tracer methods

Abstract. Detecting, quantifying and understanding groundwater discharge to streams are crucial for the assessment of water, nutrient and contaminant exchange at the groundwater–surface water interface. In lowland agricultural catchments with significant groundwater discharge this is of particular importance because of the risk of excess leaching of nutrients to streams. Here we aim to combine hydraulic and tracer methods from point-to-catchment scale to assess the temporal and spatial variability of groundwater discharge in a lowland, groundwater gaining stream in Denmark. At the point-scale, groundwater fluxes to the stream were quantified based on vertical streambed temperature profiles (VTPs). At the reach scale (0.15–2 km), the spatial distribution of zones of focused groundwater discharge was investigated by the use of distributed temperature sensing (DTS). Groundwater discharge to the stream was quantified using differential gauging with an acoustic Doppler current profiler (ADCP). At the catchment scale (26–114 km2), runoff sources during main rain events were investigated by hydrograph separations based on electrical conductivity (EC) and stable isotopes 2H/1H. Clear differences in runoff sources between catchments were detected, ranging from approximately 65% event water for the most responsive sub-catchment to less than 10% event water for the least responsive sub-catchment. This was supported by the groundwater head gradients, where the location of weaker gradients correlated with a stronger response to precipitation events. This shows a large variability in groundwater discharge to the stream, despite the similar lowland characteristics of sub-catchments indicating the usefulness of environmental tracers for obtaining information about integrated catchment functioning during precipitation events. There were also clear spatial patterns of focused groundwater discharge detected by the DTS and ADCP measurements at the reach scale indicating high spatial variability, where a significant part of groundwater discharge was concentrated in few zones indicating the possibility of concentrated nutrient or pollutant transport zones from nearby agricultural fields. VTP measurements confirmed high groundwater fluxes in discharge areas indicated by DTS and ADCP, and this coupling of ADCP, DTS and VTP proposes a novel field methodology to detect areas of concentrated groundwater discharge with higher resolution.

From submarine to lacustrine groundwater discharge

Abstract. Submarine groundwater discharge (SGD) and its role in marine nutrient cycling are well known since the last decade. The freshwater equivalent, lacustrine groundwater discharge (LGD), is often still disregarded, although first reports of LGD are more than 50 years old. We identify nine different reasons why groundwater has long been disregarded in both freshwater and marine environments such as invisibility of groundwater discharge, the size of the interface and its difficult accessibility. Although there are some fundamental differences in the hydrology of SGD and LGD, caused primarily by seawater recirculation that occurs only in cases of SGD, there are also a lot of similarities such as a focusing of discharge to near-shore areas. Nutrient concentrations in groundwater near the groundwater–surface water interface might be anthropogenically enriched. Due to spatial heterogeneity of aquifer characteristics and biogeochemical processes, the quantification of groundwater-borne nutrient loads is challenging. Both nitrogen and phosphorus might be mobile in near-shore aquifers and in a lot of case studies large groundwater-borne nutrient loads have been reported.

Modelling hyporheic processes for regulated rivers under transient hydrological and hydrogeological conditions

Abstract. Understanding the effects of major hydrogeological controls on hyporheic exchange and bank storage is essential for river water management, groundwater abstraction, restoration and ecosystem sustainability. Analytical models cannot adequately represent complex settings with, for example, transient boundary conditions, varying geometry of surface water–groundwater interface, unsaturated and overland flow, etc. To understand the influence of parameters such as (1) sloping river banks, (2) varying hydraulic conductivity of the riverbed and (3) different river discharge wave scenarios on hyporheic exchange characteristics such as (a) bank storage, (b) return flows and (c) residence time, a 2-D hydrogeological conceptual model and, subsequently, an adequate numerical model were developed. The numerical model was calibrated against observations in the aquifer adjacent to the hydropower-regulated Lule River, northern Sweden, which has predominantly diurnal discharge fluctuations during summer and long-lasting discharge peaks during autumn and winter. Modelling results revealed that bank storage increased with river wave amplitude, wave duration and smaller slope of the river bank, while maximum exchange flux decreased with wave duration. When a homogeneous clogging layer covered the entire river–aquifer interface, hydraulic conductivity positively affected bank storage. The presence of a clogging layer with hydraulic conductivity < 0.001 m d−1 significantly reduced the exchange flows and virtually eliminated bank storage. The bank storage return/fill time ratio was positively related to wave amplitude and the hydraulic conductivity of the interface and negatively to wave duration and bank slope. Discharge oscillations with short duration and small amplitude decreased bank storage and, therefore, the hyporheic exchange, which has implications for solute fluxes, redox conditions and the potential of riverbeds as fish-spawning locations. Based on these results, river regulation strategies can be improved by considering the effect of certain wave event configurations on hyporheic exchange to ensure harmonious hydrogeochemical functioning of the river–aquifer interfaces and related ecosystems.

Fully integrated surface–subsurface flow modelling of groundwater–lake interaction in an esker aquifer: Model verification with stable isotopes and airborne thermal imaging

Water resources management is moving towards integration, where groundwater (GW), surface water (SW) and related aquatic ecosystems are considered one management unit. Because of this paradigm shift, more information and new tools are needed to understand the ecologically relevant fluxes (water, heat, solutes) at the GW–SW interface. This study estimated the magnitude, temporal variability and spatial distribution of water fluxes at the GW–SW interface using a fully integrated hydrological modelling code (HydroGeoSphere). The model domain comprised a hydrologically complex esker aquifer in Northern Finland with interconnected lakes, streams and wetlands. The model was calibrated in steady state for soil hydraulic conductivity and anisotropy and it reproduced the hydraulic head and stream baseflow distribution throughout the aquifer in both transient and steady state modes. In a novel analysis, model outputs were compared with the locations and magnitude of GW discharge to lakes estimated using field techniques. Spatial occurrence of GW–lake interaction was interpreted from airborne thermal infrared imaging. The observed GW inflow locations coincided well with model nodes showing positive exchange flux between surface and subsurface domains. Order of magnitude of simulated GW inflow to lakes showed good agreement with flux values calculated with a stable water isotope technique. Finally, time series of GW inflow, extracted as model output, showed moderate annual variability and demonstrated different interannual inflow changes in seepage and drainage lakes of the aquifer. Overall, this study demonstrated the ability of a fully integrated numerical model to reproduce observed GW–SW exchange processes in a complex unconfined aquifer system. The model-based estimates obtained for GW influx magnitude and spatial distribution, along with information on GW quality can be used to estimate ecologically relevant fluxes in future water resources management.

Buried particulate organic carbon stimulates denitrification and nitrate retention in stream sediments at the groundwater–surface water interface

AbstractThe interface between ground water and surface water in streams is a hotspot for N processing. However, the role of buried organic C in N transformation at this interface is not well understood, and inferences have been based largely on descriptive studies. Our main objective was to determine how buried particulate organic C (POC) affected denitrification and NO3− retention in the sediments of an upwelling reach in a sand-plains stream in Wisconsin. We manipulated POC in mesocosms inserted in the sediments. Treatments included low and high quantities of conditioned red maple leaves (buried beneath combusted sand), ambient sediment (sand containing background levels of POC), and a control (combusted sand). We measured denitrification rates in sediments by acetylene-block assays in the laboratory and by changes in N2 concentrations in the field using membrane inlet mass spectrometry. We measured NO3−, NH4+, and dissolved organic N (DON) retention as changes in concentrations and fluxes along groundw...

An open loop equilibrator for continuous monitoring of radon at the groundwater–surface water interface

A continuous monitoring system (CMS) using an open loop equilibrator for assessment of 222Rn at the groundwater–surface water interface was developed and tested. For the characterization and validation of the system, three air loops (open loop, closed loop, and open bubble loop) were tested in relation to high and precise count rates, rapid response, and equilibration of radon. The water and air stream is fed to the equilibrator by an experimental setup with a commercial submersible water pump and the internal pump with built-in radon-in-air detector. Efficiency calibration of the CMS is done by simultaneous determination of a groundwater sample using liquid scintillation counting, and the RAD7 accessories RAD-H2O, BigBottle RAD-H2O. The higher count rates are provided by the closed loop. However, the open loop with bubbler (open bubble loop) provides the best precision count rates, rapid response, and equilibration time. The CMS allows radon determination in discrete water samples as well as continuous water streams.

Impact of microforms on nitrate transport at the groundwater–surface water interface in gaining streams

Small streambed structures (or microforms, 0.01–1m in length) exist ubiquitously in riverbed systems. Small-scale topography is potentially important in controlling hyporheic exchange flow and transport of conservative and reactive solutes at the groundwater–surface water interface. The role of microforms on NO3- transfer in a riffle-scale (macroforms of >1m length) hyporheic zone within a gaining river setting is investigated using a 2-D flow and transport model which accounts for both nitrification and denitrification. Results show that the short pathlines caused by microforms lead to more NO3- discharge to the river compared with a macroform-only condition due to shortened residence times of both surface water and groundwater in mixing zones. Short hyporheic exchange flow pathways caused by microforms could remain oxic along their entire length or switch from nitrate producing to nitrate consuming as oxygen concentrations decline. Microforms affect net NO3- flux by the combined effect of introducing more stream mass flux and reducing their residence time in mixing zones under different hydrological and biogeochemical conditions. Our findings underscore that ignoring microforms in river beds may underestimate NO3- load to the river and have practical implications for pore water sampling strategies in groundwater–surface water studies.

A method for automated, daily, temperature-based vertical streambed water-fluxes

Heat is increasingly used as a natural tracer to quantify water fluxes at the groundwater-surface waterinterface. We present a systematic approach to monitor and evaluate stream and streambed temperatures to derive daily-updated temperature-based water exchange fluxes between the stream and the streambed. Specifically designed multi-level temperature sensors coupled with a data logger and GSM modem are used to monitor temperature in the stream and streambed and transfer this data daily to a database. A suite of MATLAB scripts with structured query language (SQL) commands is applied to extract the data for processing using an inverse numerical model to estimate water flow based on the measured temperatures. Compared to common analytical approaches, which typically require sinusoidal diurnal temperature pattern, our numerical model can utilize temperature records without daily variations. Temperature-based calculations to quantify vertical water fluxes at the stream-groundwater interface can provide a supplement to, or even a replacement of, calculations based on vertical hydraulic gradients and Darcy’ law.

Seasonal and spatial variations in microbial activity at various phylogenetic resolutions at a groundwater - surface water interface.

We investigated the seasonal and spatial variation in activity and density of the metabolically active in situ microbial community (AIMC) at a landfill leachate-impacted groundwater - surface water interface (GSI). A series of AIMC traps were designed and implemented for AIMC sampling and microbial activity and density examinations. Measurements were made not only at the level of bacterial domain but also at the levels of alphaproteobacterial Rhizobiales order and gammaproteobacterial Pseudomonas genus, both of which included a large number of iron-oxidizing bacteria as revealed from previous analysis. Consistently higher microbial activities with less variation in depth were measured in the AIMC traps than in the ambient sediments. Flood disturbance appeared to control AIMC activity distributions at the gradually elevated GSI. The highest AIMC activities were generally obtained from locations closest to the free surface water boundary except during the dry season when microbial activities were similar across the entire GSI. A clone library of AIMC 16S rRNA genes was constructed, and it confirmed the predominant role of the targeted alphaproteobacterial group in AIMC activity and composition. This taxon constituted 2%-14% of all bacteria with similar activity distribution profiles. The Pseudomonas group occupied only 0.1‰-0.5‰ of the total bacterial density, but its activity was 27 times higher than the bacterial average. Of the 16S rRNA sequences in the AIMC clone library, 7.5% were phylogenetically related to putative IOB, supporting the occurrence and persistence of active microbial iron oxidation across the studied iron-rich GSI ecosystem.

Discharge of landfill leachate to streambed sediments impacts the mineralization potential of phenoxy acid herbicides depending on the initial abundance of tfdA gene classes

To understand the role of abundance of tfdA gene classes belonging to β- and γ-proteobacteria on phenoxy acid herbicide degradation, streambed sediments were sampled around three seepage meters (SMs) installed in a landfill-impacted groundwater–surface water interface. Highest herbicide mass discharge to SM3, and lower herbicide mass discharges to SM1 and SM2 were determined due to groundwater discharge rates and herbicide concentrations. SM1-sediment with the lowest abundance of tfdA gene classes had the slowest mineralization, whereas SM2- and SM3-sediments with more abundant tfdA genes had faster mineralization. The observed difference in mineralization rates between discharge zones was simulated by a Monod-based kinetic model, which confirmed the role of abundance of tfdA gene classes. This study suggests presence of specific degraders adapted to slow growth rate and high yield strategy due to long-term herbicide exposure; and thus groundwater–surface water interface could act as a natural biological filter and protect stream water quality.

Ground Water-Surface Water Interface (GWSWI) Modeling: Recent Advances and Future Challenges

Ground water-surface water interface (GWSWI) represents the interconnection of ground water and surface water in the hydrologic continuum. Interactions between these two water masses result in unique gradients and/or transitions of contaminant concentration profiles, biological populations, chemistry, flow, mixing characteristics, redox potential, dissolved oxygen, organic content, and thermal properties across GWSWI. In addition, many unique biogeochemical processes occur in this zone that can have significant impact on the fate and transport of contaminants across the GWSWI. Prediction of the distribution and concentrations of contaminants across the GWSWI is challenging, but is essential for evaluating human health and environmental risk, including remedial alternatives at contaminated sites. The objective of this paper is to review and assess existing modeling capabilities to recognize (a) the need for development of new modeling tools, (b) knowledge gaps, and (c) challenges required to address contaminant transport within GWSWI. Currently, the most important needs for future model development are, (a) linkage between ground water and surface water models; (b) integration of process-level models within the framework of large-scale models; and (c) understanding ground water-surface water interactions within a spatial and temporal framework.

A Novel Method Using a Silicone Diffusion Membrane for Continuous 222Rn Measurements for the Quantification of Groundwater Discharge to Streams and Rivers

²²²Rn is a natural radionuclide that is commonly used as tracer to quantify groundwater discharge to streams, rivers, lakes, and coastal environments. The use of sporadic point measurements provides little information about short- to medium-term processes (hours to weeks) at the groundwater-surface water interface. Here we present a novel method for high-resolution autonomous, and continuous, measurement of ²²²Rn in rivers and streams using a silicone diffusion membrane system coupled to a solid-state radon-in-air detector (RAD7). In this system water is pumped through a silicone diffusion tube placed inside an outer air circuit tube that is connected to the detector. ²²²Rn diffuses from the water into the air loop, and the ²²²Rn activity in the air is measured. By optimizing the membrane tube length, wall thickness, and water flow rates through the membrane, it was possible to quantify radon variations over times scales of about 3 h. The detection limit for the entire system with 20 min counting was 18 Bq m⁻³ at the 3σ level. Deployment of the system on a small urban stream showed that groundwater discharge is dynamic, with changes in ²²²Rn activity doubling on the scale of hours in response to increased stream flow.

Nitrogen Dynamics at the Groundwater–Surface Water Interface of a Degraded Urban Stream

Few studies have quantified the impact of urbanization on the biogeochemistry of streams at the groundwater-surface water interface, a zone that may be critical for managing nitrogen transformations. We investigated the groundwater ecosystem of Minebank Run, a geomorphically degraded urban stream near Baltimore, Maryland in the Chesapeake Bay watershed. Our objectives were to identify the spatial and temporal extent of chemical, microbial, and hydrological factors known to influence denitrification, a microbial process that removes nitrate nitrogen (NO(3)(-)). Measurements of denitrification enzyme activity confirmed that subsurface sediments at Minebank Run, especially those with high concentrations of organic carbon, have the capacity to denitrify NO(3)(-). Levels of NO(3)(-) in groundwater were lower where more dissolved organic carbon (DOC) was available, suggesting that denitrification and removal of NO(3)(-) in groundwater were limited by DOC availability. Groundwater NO(3)(-) was highest when groundwater levels were highest, which, in turn, corresponded to high oxidation-reduction potential (ORP), indicative of high groundwater-surface water exchange. Stream flow patterns controlled stream bank and bed infiltration and, subsequently, dictated groundwater levels. Declines in water levels likely increased subsurface mixing, which led to low ORP conditions that sustained NO(3)(-) removal via denitrification. The groundwater-surface water interface is a zone of active nitrogen transformation. Management efforts that increase DOC availability to denitrifiers, reduce stream-flow velocity and flashiness, and increase groundwater residence time will likely improve the nitrogen removal capacity of urban stream channels.

Seasonal variability of groundwater—surface exchange and its implications for riparian groundwater nitrate retention at the Havel River

The IWAN (Integrated Water Balance and Nutrient Dynamic) model was used to quantify the spatially and temporally variable impact of groundwater—surface water interactions on the groundwater nitrate dynamics in a riparian floodplain of the Havel River, Germany. The model coupled simulation of the floodplain water balance and groundwater nitrate dynamics to quantify the exchange fluxes and the nitrate transport between the river and the riparian groundwater. Investigations of the riparian water balance indicated extensive heterogeneity of process interactions between the groundwater and surface water, which have a temporally and spatially variable impact on the floodplain water balance, groundwater dynamics and river discharge. The simulated transport of nitrate across the groundwater—surface water interface was transient, varying both in intensity and direction. The floodplain acted as a nitrate sink or source with a seasonal periodicity. Riparian groundwater contributed with seasonally variable nitrate loads of the Havel River. The simulated annual groundwater nitrate contributions of the research area represented on average 1 % of the observed nitrate loads of the Havel River. However, in summer the groundwater nitrate contributions were significantly higher, representing more than 20% of the river loads. The simulation results indicated that 93% of the nitrate input into the riparian zone was attenuated within the groundwater, highlighting the role of the riparian floodplain as an important retention filter for diffuse nutrient pollution.

Presence, distribution, and diversity of iron-oxidizing bacteria at a landfill leachate-impacted groundwater surface water interface

We examined the presence of iron-oxidizing bacteria (IOB) at a groundwater surface water interface (GSI) impacted by reduced groundwater originating as leachate from an upgradient landfill. IOB enrichments and quantifications were obtained, at high vertical resolution, by an iron/oxygen opposing gradient cultivation method. The depth-resolved soil distribution profiles of water content, Fe(2+), and total Fe indicated sharp gradients within the top 10 cm sediments of the GSI, where the IOB density was the highest. In addition, the vertical distribution of iron-reducing bacteria at the same sampling site mirrored the IOB distribution. Clone libraries from two separate IOB enrichments indicated a stratified IOB community with clear differences at short vertical distances. Alpha- and Betaproteobacteria were the dominant phylotypes. Clones from the near-surface sediment (1-2 cm below ground surface) were dominated by members of the Bradyrhizobiaceae and Comamonadaceae; clones from the deeper sediments were phylogenetically more diverse, dominated by members of the Rhodocyclaceae. The iron deposition profiles indicated that active iron oxidation occurred only within the near-to-surface GSI sediments. The match between the iron deposition profiles and the IOB abundance profiles strongly hints at the contribution of the IOB community to Fe oxidation in this Fe-rich GSI ecosystem.

Tool for assessment of process importance at the groundwater/surface water interface

The groundwater/surface water interface (GWSWI) represents an important transition zone between groundwater and surface water environments that potentially changes the nature and flux of contaminants exchanged between the two systems. Identifying dominant and rate-limiting contaminant transformation processes is critically important for estimating contaminant fluxes and compositional changes across the GWSWI. A new, user-friendly, spreadsheet- and Visual Basic-based analytical screening tool that assists in evaluating the dominance of controlling kinetic processes across the GWSWI is presented. Based on contaminant properties, first-order processes that may play a significant role in solute transport/transformation are evaluated in terms of a ratio of process importance ( P i) that relates the process rate to the rate of fluid transfer. Besides possessing several useful compilations of contaminant and process data, the screening tool also includes 1-D analytical models that assist users in evaluating contaminant transport across the GWSWI. The tool currently applies to 29 organics and 10 inorganics of interest within the context of the GWSWI. Application of the new screening tool is demonstrated through an evaluation of natural attenuation at a site with trichloroethylene and 1,1,2,2-tetrachloroethane contaminated groundwater discharging into wetlands.

Transient or steady‐state? Using vertical temperature profiles to quantify groundwater–surface water exchange

AbstractHeat is recognized as a natural tracer to identify the exchange of water between the groundwater and surface water compartment. One‐dimensional (1D) heat transport models have the ability to obtain quantitative estimates of vertical fluxes through the sediment matrix. Input to these models can come from temperatures observed in the surface water and in the bed material of rivers and lakes. The upper thermal boundary condition at the groundwater–surface water interface is affected by seasonal and diurnal temperature variations. We hypothesize that effects of these transient influences are negligible at certain times of the year, such that the vertical temperature distribution can be approximated to be at steady state. Temperature time series observed over a year in the surface water and at several depths below a river in Belgium and in sediments of an acid mine lake in Eastern Germany were simulated with a heat balance model implemented in FEMME and the water and energy model VS2DH to obtain seepage fluxes. Temperature variations throughout the year at all depths could be adequately reproduced with the transient models. Vertical temperature profiles at several measuring times during the year were also fitted with an analytical, steady‐state solution for 1D heat transport and the obtained fluxes compared to the results from transient simulations. Fluxes obtained from the much simpler steady‐state solution were compared well with the flux rates from transient simulations for moments between mid and late summer, as well as during the winter. During transitional seasons (fall and spring), the fluxes from the steady‐state solution deviated significantly from the transient estimates with a tendency to underestimate at the beginning and to overestimate towards the end of those seasons. We conclude that fitting a simple analytical solution for 1D vertical heat transport to temperature data observed at particular well‐selected times of the year can provide an inexpensive, simple method to obtain accurate point estimates of groundwater–surface water exchange in rivers and lakes. Copyright © 2009 John Wiley & Sons, Ltd.

Pollutant attenuation at the groundwater–surface water interface: A classification scheme and statistical analysis using national-scale nitrate data

Summary A classification scheme for pollutant natural attenuation potential at the groundwater–surface water interface is presented, and its predictive power for explaining baseflow river nitrate concentration investigated. Both the classification scheme and statistical analysis are undertaken at Water Framework Directive surface water body scale for England and Wales, in baseflow conditions when relative groundwater contribution to rivers is greatest. The results of multiple regression analyses demonstrate statistically significant relationships between the classification of natural attenuation potential, its component properties, and baseflow river nitrate concentration. Natural attenuation at the groundwater–surface water interface is shown to be a significant control on observed river nitrate concentrations, albeit less influential than land-use descriptors. The results indicate that natural attenuation processes have a measurable impact on baseflow river chemistry at surface water body scale, and that consideration of natural attenuation processes at the groundwater–surface water interface would improve regional and catchment-scale risk prediction, and could help in the design of more sustainable catchment management strategies.

Coupling FEFLOW and MIKE11 to optimise the flooding system of the Lower Havel polders in Germany

Based on the software MIKE 11 and FEFLOW, a coupling surface water-groundwater interface has been developed to investigate the interaction for river branches, polders and wetlands. Its functionality covers the exchange of boundary conditions during runtime and the synchronisation of the temporal discretisation. The system has been applied to simple benchmark cases and complex field conditions. One project focused on the optimisation of a polder flooding system which prevents flooding in the downstream regions of the river Elbe. Due to the coupling, the influence of groundwater storage could be estimated and an optimised flooding strategy could be developed.