Patterns Of Groundwater Discharge Research Articles

Groundwater discharge and saltwater-freshwater mixing in a mangrove wetland over tidal cycles: A field and modeling study

Material transport and transformations in mangrove wetlands are closely related to seawater-groundwater mixing processes, which can influence mangrove growth and development. Therefore, a clearer understanding of groundwater and salt transport dynamics in aquifers beneath mangrove wetlands is needed to accurately estimate chemical fluxes to mangrove ecosystems. We established a multi-level groundwater monitoring profile in a mangrove wetland and constructed a numerical model of tidally-driven groundwater flow and salt transport that was calibrated to salinity and pore water pressure. Model results indicated that 99% of groundwater discharge to the mangrove wetland was saltwater and that this discharge accounted for 33% of total discharge over a spring-neap tidal cycle. Tidal creeks that cross-cut the wetland received the remaining 67% of total discharge. Seawater infiltration from tidal action across the wetland formed a subsurface saltwater-freshwater mixing zone with a geometry that followed the undulation of a high permeably layer at depth. Model sensitivity tests showed that salinity distributions and groundwater discharge patterns were controlled strongly by wetland topography and anisotropy. Model cases with a gently sloping and flat mangrove platform led to substantially less salt mass in the subsurface compared to the base case with mangrove topographic relief. Models with lower anisotropy allowed salt to penetrate through the high-permeability layer to the base of the aquifer. The spatial patterns of fresh and saline groundwater discharge and mixing processes within mangrove wetlands as shown in this study may have implications for mangrove tree nutrient availability and for the prevalence and spatial distribution of mangroves along coastlines.

Aquifer-Stream Exchanges and the Importance of Groundwater Discharge for Maintaining Instream Habitat and Benthic Macroinvertebrates

Periods of streamflow cessation and stream drying are increasing in frequency and intensity in many regions of the world due to climate change, land-use change, and increased groundwater pumping. Thus, streams with intermittent flow will likely continue to grow in their global extent, which is thought to already surpass that of perennial streams. Yet, variability in streamflow and aquifer-stream exchange associated with flow cessation and drying is not well understood, nor is the influence of hydrological variability on aquatic communities and their habitat. This study aimed to characterize the heterogeneity of physical and chemical characteristics of habitat and the associated variability in benthic macroinvertebrate (BMI) communities in relation to spatial and temporal variability in streamflow and aquifer-stream exchange. Habitat conditions and BMI communities varied between individual sites, riffle and pool habitat, and between an intermittent riffle and a perennial riffle maintained by groundwater discharge. Unlike other studies, BMI richness, diversity, and abundance were consistently higher at intermittent riffle sites than perennial riffle sites, which were similar in BMI community composition to pool sites. High velocity distinguished the intermittent riffle sites, while two flow-loving BMI taxa distinguished their communities. This suggests that site-specific heterogeneity in habitat conditions was a key control on spatial and temporal variability in BMI communities due to the habitat preferences of certain BMI taxa. Low richness, diversity, and high dominance of highly tolerant taxa across the site suggested long-term patterns of extreme variability in habitat conditions associated with flow cessation and drying were the primary control on BMI communities. Thus, the interactive effects of these controls may produce unexpected patterns in BMI community richness, diversity, and composition in relation to longitudinal patterns of groundwater discharge and flow persistence. These findings demonstrate the importance of considering the influence of heterogeneity in environmental conditions at multiple spatial and temporal scales when interpreting relationships between hydrological and abiotic or biotic ecological variable in streams with intermittent flow.

Where groundwater seeps: Evaluating modeled groundwater discharge patterns with thermal infrared surveys at the river-network scale

Predicting baseflow dynamics, protecting aquatic habitat, and managing legacy contaminants requires explicit characterization and prediction of groundwater discharge patterns throughout river networks. Using handheld thermal infrared (TIR) cameras, we surveyed 47 km of stream length across the Farmington River watershed (1,570 km2; CT and MA, USA), mapping locations of bank and waterline groundwater discharges based on their thermal signature. Using the observed groundwater discharge locations and predicted groundwater discharge rates from 6 variations of a numerical groundwater-flow model (MODFLOW-NWT), we compared 1) predicted groundwater-discharge rates in areas with and without observed groundwater discharge, 2) spatial patterns of observed and predicted groundwater discharge locations, and 3) density of observed groundwater discharge locations with predicted discharge rates. Five of six models reasonably predicted the spatial patterns of discharge locations along the 5th order mainstem, but fewer models predicted groundwater discharge patterns in smaller streams. Our results highlight 1) the feasibility of using TIR observations to evaluate groundwater models, 2) model parameters that influence discharge prediction accuracy (riverbed sediment and bedrock hydraulic conductivity and river-aquifer connections), and 3) current strengths and future opportunities for improved modeling of groundwater-discharge patterns.

Three-dimensional hydrostratigraphy and groundwater flow models in complex Quaternary deposits and weathered/fractured bedrock: evaluating increasing model complexity

Greater complexity in three-dimensional (3D) model structures yields more plausible groundwater recharge/discharge patterns, especially in groundwater/surface-water interactions. The construction of a 3D hydrostratigraphic model prior to flow modelling is beneficial when the hydraulic conductivity of geological units varies considerably. A workflow for 3D hydrostratigraphic modelling with Leapfrog Geo and flow modelling with MODFLOW-NWT was developed. It was used to evaluate how the modelling results for groundwater flow and recharge/discharge patterns differ when using simple or more complex hydrostratigraphic models. The workflow was applied to a study site consisting of complex Quaternary sediments underlain by fractured and weathered crystalline bedrock. Increasing the hydrostratigraphic detail appeared to improve the fit between the observed and simulated water table, and created more plausible groundwater flow patterns. Interlayered zones of low and high conductivity disperse the recharge/discharge patterns, increasing the vertical flow component. Groundwater flow was predominantly horizontal in models in which Quaternary sediments and bedrock were simplified as one layer per unit. It appears to be important to define the interlayered low-conductivity units, which can limit groundwater infiltration and also affect groundwater discharge patterns. Explicit modelling with Leapfrog Geo was found to be effective but time-consuming in the generation of scattered and thin-layered strata.

Enhancing SWAT+ simulation of groundwater flow and groundwater-surface water interactions using MODFLOW routines

This paper presents a version of SWAT + that uses MODFLOW to simulate groundwater flow and groundwater-surface interactions within a watershed system. The modeling code is applied to the 470 km2 Middle Bosque River Watershed (Texas, USA) to demonstrate accuracy and differences with SWAT+. The model is tested against field-measured stream discharge, groundwater levels and fluctuation patterns, runoff, baseflow, recharge, watershed boundary groundwater flow, groundwater-surface water interactions, and evapotranspiration. The model captures the dominant baseflow patterns throughout the watershed and the key patterns of water table dynamics and relation to nearby streams. Whereas annual rates of runoff, lateral flow, and annual percolation are unchanged from a stand-alone SWAT + simulation, annual groundwater discharge rates increase by 50% (37 mm–54 mm). In particular, temporal patterns of groundwater discharge are governed by the physically-based rise and fall of groundwater near streams, rather than storm-induced recharge pulses as in the SWAT + simulation.

SEASONAL PATTERNS OF SPRING DISCHARGE AT SILVER FALLS, CROSBY COUNTY, TEXAS

Abstract Beneath the high plains of the Llano Estacado lies the southern extension of the Ogallala aquifer, which provides a key source of groundwater for the region. Along the eastern fringes of the Llano Estacado one can find numerous natural springs that discharge from the Ogallala formation and provide a valuable source of freshwater. Large-scale irrigation has altered hydrological conditions, which has influenced the flow of springs along the eastern escarpment. In late 2012, the author began a study of one such spring located at Silver Falls in Blanco Canyon, east of Crosbyton, Texas. The goal was to monitor the discharge of this naturally flowing spring over a period of many years to establish temporal patterns. Measurements of spring flow rates over a seven-year period did not show an appreciable reduction associated with the depletion of the Ogallala aquifer; however, discharge was found to follow a seasonal pattern of declining flow during the summer followed by a recovery starting in late fall and reaching maximum discharge during winter and early spring. Whereas seasonal variations of spring discharge can be measured precisely, the cause of these seasonal patterns is less certain. It is likely that the combined effects of seasonal groundwater extraction for irrigation and the growth and transpiration of natural vegetation contribute to the observed seasonal patterns of groundwater discharge at Silver Falls.

Mapping spatial patterns of groundwater discharge in a deep lake using high-resolution temperature sensors

ABSTRACTIdentifying groundwater discharge zones to lakes is important because the groundwater system can contribute a significant flux of water and solutes and potentially influence lake chemistry and ecology. We present a new instrument, Temperature Mapper (TM), to map groundwater discharge zones in lakes via temperature anomalies along the lake bottom, tested in Lake Constance at 2 former sand and gravel extraction sites. TM is essentially a 4 m wide sled with an array of 19 high-resolution sensors attached to an aluminum lattice towed along the lake bed behind a boat. It is able to map spatial temperature patterns representing groundwater discharge zones at a resolution of 20 cm laterally and 2.5 m in path direction. We conducted 2 field campaigns in February 2016 and February 2017, during which TM identified several temperature anomalies interpreted as groundwater discharging zones. Radon-222 was also analyzed at selected locations as an independent indicator of groundwater discharge. Overall, when comparing the different methods and different years, we observed similar spatial patterns of preferential groundwater discharge. TM offers a useful qualitative tool to identify and map potential groundwater discharge zones in lakes or coastal areas that can then be further studied by more quantitative, small-scale methods such as chemical tracers or seepage meters.

Evaluating long-term patterns of decreasing groundwater discharge through a lake-bottom permeable reactive barrier

Identifying and quantifying groundwater exchange is critical when considering contaminant fate and transport at the groundwater/surface-water interface. In this paper, areally distributed temperature and point seepage measurements are used to efficiently assess spatial and temporal groundwater discharge patterns through a glacial-kettle lakebed area containing a zero-valent iron permeable reactive barrier (PRB). Concern was that the PRB was becoming less permeable with time owing to biogeochemical processes within the PRB. Patterns of groundwater discharge over an 8-year period were examined using fiber-optic distributed temperature sensing (FO-DTS) and snapshot-in-time point measurements of temperature. The resulting thermal maps show complex and uneven distributions of temperatures across the lakebed and highlight zones of rapid seepage near the shoreline and along the outer boundaries of the PRB. Repeated thermal mapping indicates an increase in lakebed temperatures over time at periods of similar stage and surface-water temperature. Flux rates in six seepage meters permanently installed on the lakebed in the PRB area decreased on average by 0.021 md−1 (or about 4.5 percent) annually between 2004 and 2015. Modeling of diurnal temperature signals from shallow vertical profiles yielded mean flux values ranging from 0.39 to 1.15 md−1, with stronger fluxes generally related to colder lakebed temperatures. The combination of an increase in lakebed temperatures, declines in direct seepage, and observations of increased cementation of the lakebed surface provide in situ evidence that the permeability of the PRB is declining. The presence of temporally persistent rapid seepage zones is also discussed.

A Modified Water-Table Fluctuation Method to Characterize Regional Groundwater Discharge

A modified Water-Table Fluctuation (WTF) method is developed to quantitatively characterize the regional groundwater discharge patterns in stressed aquifers caused by intensive agricultural pumping. Two new parameters are defined to express the secondary information in the observed data. One is infiltration efficiency and the other is discharge modulus (recurring head loss due to aquifer discharge). An optimization procedure is involved to estimate these parameters, based on continuous groundwater head measurements and precipitation records. Using the defined parameters and precipitation time series, water level changes are calculated for individual wells with fidelity. The estimated parameters are then used to further address the characterization of infiltration and to better quantify the discharge at the regional scale. The advantage of this method is that it considers recharge and discharge simultaneously, whereas the general WTF methods mostly focus on recharge. In the case study, the infiltration efficiency reveals that the infiltration is regionally controlled by the intrinsic characteristics of the aquifer, and locally distorted by engineered hydraulic structures that alter surface water-groundwater interactions. The seasonality of groundwater discharge is characterized by the monthly discharge modulus. These results from individual wells are clustered into groups that are consistent with the local land use pattern and cropping structures.

Hydrogeological controls on spatial patterns of groundwater discharge in peatlands

Abstract. Peatland environments provide important ecosystem services including water and carbon storage, nutrient processing and retention, and wildlife habitat. However, these systems and the services they provide have been degraded through historical anthropogenic agricultural conversion and dewatering practices. Effective wetland restoration requires incorporating site hydrology and understanding groundwater discharge spatial patterns. Groundwater discharge maintains wetland ecosystems by providing relatively stable hydrologic conditions, nutrient inputs, and thermal buffering important for ecological structure and function; however, a comprehensive site-specific evaluation is rarely feasible for such resource-constrained projects. An improved process-based understanding of groundwater discharge in peatlands may help guide ecological restoration design without the need for invasive methodologies and detailed site-specific investigation. Here we examine a kettle-hole peatland in southeast Massachusetts historically modified for commercial cranberry farming. During the time of our investigation, a large process-based ecological restoration project was in the assessment and design phases. To gain insight into the drivers of site hydrology, we evaluated the spatial patterning of groundwater discharge and the subsurface structure of the peatland complex using heat-tracing methods and ground-penetrating radar. Our results illustrate that two groundwater discharge processes contribute to the peatland hydrologic system: diffuse lower-flux marginal matrix seepage and discrete higher-flux preferential-flow-path seepage. Both types of groundwater discharge develop through interactions with subsurface peatland basin structure, often where the basin slope is at a high angle to the regional groundwater gradient. These field observations indicate strong correlation between subsurface structures and surficial groundwater discharge. Understanding these general patterns may allow resource managers to more efficiently predict and locate groundwater seepage, confirm these using remote sensing technologies, and incorporate this information into restoration design for these critical ecosystems.

Combining Remote Temperature Sensing with in-Situ Sensing to Track Marine/Freshwater Mixing Dynamics.

The ability to track the dynamics of processes in natural water bodies on a global scale, and at a resolution that enables highly localised behaviour to be visualized, is an ideal scenario for understanding how local events can influence the global environment. While advances in in-situ chem/bio-sensing continue to be reported, costs and reliability issues still inhibit the implementation of large-scale deployments. In contrast, physical parameters like surface temperature can be tracked on a global scale using satellite remote sensing, and locally at high resolution via flyovers and drones using multi-spectral imaging. In this study, we show how a much more complete picture of submarine and intertidal groundwater discharge patterns in Kinvara Bay, Galway can be achieved using a fusion of data collected from the Earth Observation satellite (Landsat 8), small aircraft and in-situ sensors. Over the course of the four-day field campaign, over 65,000 in-situ temperatures, salinity and nutrient measurements were collected in parallel with high-resolution thermal imaging from aircraft flyovers. The processed in-situ data show highly correlated patterns between temperature and salinity at the southern end of the bay where freshwater springs can be identified at low tide. Salinity values range from 1 to 2 ppt at the southern end of the bay to 30 ppt at the mouth of the bay, indicating the presence of a freshwater wedge. The data clearly show that temperature differences can be used to track the dynamics of freshwater and seawater mixing in the inner bay region. This outcome suggests that combining the tremendous spatial density and wide geographical reach of remote temperature sensing (using drones, flyovers and satellites) with ground-truthing via appropriately located in-situ sensors (temperature, salinity, chemical, and biological) can produce a much more complete and accurate picture of the water dynamics than each modality used in isolation.

Assessing regional‐scale spatio‐temporal patterns of groundwater–surface water interactions using a coupled SWAT‐MODFLOW model

AbstractInteraction between groundwater and surface water in watersheds has significant impacts on water management and water rights, nutrient loading from aquifers to streams, and in‐stream flow requirements for aquatic species. Of particular importance are the spatial patterns of these interactions. This study explores the spatio‐temporal patterns of groundwater discharge to a river system in a semi‐arid region, with methods applied to the Sprague River Watershed (4100 km2) within the Upper Klamath Basin in Oregon, USA. Patterns of groundwater–surface water interaction are explored throughout the watershed during the 1970–2003 time period using a coupled SWAT‐MODFLOW model tested against streamflow, groundwater level and field‐estimated reach‐specific groundwater discharge rates. Daily time steps and coupling are used, with groundwater discharge rates calculated for each model computational point along the stream. Model results also are averaged by month and by year to determine seasonal and decadal trends in groundwater discharge rates. Results show high spatial variability in groundwater discharge, with several locations showing no groundwater/surface water interaction. Average annual groundwater discharge is 20.5 m3/s, with maximum and minimum rates occurring in September–October and March–April, respectively. Annual average rates increase by approximately 0.02 m3/s per year over the 34‐year period, negligible compared with the average annual rate, although 70% of the stream network experiences an increase in groundwater discharge rate between 1970 and 2003. Results can assist with water management, identifying potential locations of heavy nutrient mass loading from the aquifer to streams and ecological assessment and planning focused on locations of high groundwater discharge. Copyright © 2016 John Wiley & Sons, Ltd.

Seawater intrusion in fractured coastal aquifers: A preliminary numerical investigation using a fractured Henry problem

Despite that fractured coastal aquifers are widespread, the influence of fracture characteristics on seawater intrusion (SWI) has not been explored in previous studies. This research uses numerical modelling in a first step towards understanding the influence of fracture orientation, location and density on the extent of seawater and accompanying patterns of groundwater discharge in an idealised coastal aquifer. Specifically, aquifers containing single fractures or networks of regularly spaced fractures are studied using modified forms of the Henry SWI benchmark problem. The applicability of equivalent porous media (EPM) models for representing simple fracture networks in steady-state simulations of SWI is tested. The results indicate that the influence of fractures on SWI is likely to be mixed, ranging from enhancement to reduction in seawater extent and the width of the mixing zone. For the conceptual models considered here, vertical fractures in contact with the seawater wedge increase the width of the mixing zone, whereas vertical fractures inland of the wedge have minimal impact on the seawater distribution. Horizontal fractures in the lower part of the aquifer force the wedge seaward, whereas horizontal fractures located within the zone of freshwater discharge enhance the wedge. Inclined fractures roughly parallel to the seawater-freshwater interface increase the landward extent of seawater and fractures perpendicular to the interface inhibit the wedge. The results show that EPM models are likely inadequate for inferring salinity distributions in most of the fractured cases, although the EPM approach may be suitable for orthogonal fracture networks if fracture density is high and appropriate dispersivity values can be determined.

Studying groundwater and surface water interactions using airborne remote sensing in Heihe River basin, northwest China

Abstract. Managing surface water and groundwater as a unified system is important for water resource exploitation and aquatic ecosystem conservation. The unified approach to water management needs accurate characterization of surface water and groundwater interactions. Temperature is a natural tracer for identifying surface water and groundwater interactions, and the use of remote sensing techniques facilitates basin-scale temperature measurement. This study focuses on the Heihe River basin, the second largest inland river basin in the arid and semi-arid northwest of China where surface water and groundwater undergoes dynamic exchanges. The spatially continuous river-surface temperature of the midstream section of the Heihe River was obtained by using an airborne pushbroom hyperspectral thermal sensor system. By using the hot spot analysis toolkit in the ArcGIS software, abnormally cold water zones were identified as indicators of the spatial pattern of groundwater discharge to the river.

Combining isotopic tracers (222Rn and δ13C) for improved modelling of groundwater discharge to small rivers

AbstractIn regions where aquifers sustain rivers, the location and quantification of groundwater discharge to surface water are important to prevent pollution hazards, to quantify and predict low flows and to manage water supplies. 222Rn is commonly used to determine groundwater discharge to rivers. However, using this isotopic tracer is challenging because of the high diffusion capacity of 222Rn in open water. This study illustrates how a combination of isotopic tracers can contribute to an enhanced understanding of groundwater discharge patterns in small rivers. The aim of this paper is to combine 222Rn and δ13CDIC to better constrain the physical parameters related to the degassing process of these tracers in rivers. The Hallue River (northern France) was targeted for this study because it is sustained almost exclusively by a fractured chalk aquifer. The isotopes 222Rn, δ13CDIC, δ2H and δ18O were analysed along with other natural geochemical tracers. A mass balance model was used to simulate 222Rn and δ13CDIC. The results of δ2H and δ18O analyses prove that evaporation did not occur in the river. The calibration of a numerical model to reproduce 222Rn and δ13CDIC provides a best‐fit diffusive layer thickness of 3.21 × 10−5 m. This approach is particularly useful for small rivers flowing over carbonate aquifers with high groundwater DIC where the evolution of river DIC reflects the competing processes of groundwater inflow and CO2 degassing. This approach provides a means to evaluate groundwater discharge in small ungauged rivers. Copyright © 2014 John Wiley & Sons, Ltd.

Stratigraphic controls on fluid and solute fluxes across the sediment—water interface of an estuary

Shallow stratigraphic features, such as infilled paleovalleys, modify fresh groundwater discharge to coastal waters and fluxes of saltwater and nutrients across the sediment—water interface. We quantify the spatial distribution of shallow surface water—groundwater exchange and nitrogen fluxes near a paleovalley in Indian River Bay, Delaware, using a hand resistivity probe, conventional seepage meters, and pore‐water samples. In the interfluve (region outside the paleovalley) most nitrate‐rich fresh groundwater discharges rapidly near the coast with little mixing of saline pore water, and nitrogen transport is largely conservative. In the peat‐filled paleovalley, fresh groundwater discharge is negligible, and saltwater exchange is deep (∼1 m). Long pore‐water residence times and abundant sulfate and organic matter promote sulfate reduction and ammonium production in shallow sediment. Reducing, iron‐rich fresh groundwater beneath paleovalley peat discharges diffusely around paleovalley margins offshore. In this zone of diffuse fresh groundwater discharge, saltwater exchange and dispersion are enhanced, ammonium is produced in shallow sediments, and fluxes of ammonium to surface water are large. By modifying patterns of groundwater discharge and the nature of saltwater exchange in shallow sediments, paleovalleys and other stratigraphic features influence the geochemistry of discharging groundwater. Redox reactions near the sediment—water interface affect rates and patterns of geochemical fluxes to coastal surface waters. For example, at this site, more than 99% of the groundwater‐borne nitrate flux to the Delaware Inland Bays occurs within the interfluve portion of the coastline, and more than 50% of the ammonium flux occurs at the paleovalley margin.

Geologic effects on groundwater salinity and discharge into an estuary

Summary Submarine groundwater discharge (SGD) can be an important pathway for transport of nutrients and contaminants to estuaries. A better understanding of the geologic and hydrologic controls on these fluxes is critical for their estimation and management. We examined geologic features, porewater salinity, and SGD rates and patterns at an estuarine study site. Seismic data showed the existence of paleovalleys infilled with estuarine mud and peat that extend hundreds of meters offshore. A low-salinity groundwater plume beneath this low-permeability fill was mapped with continuous resistivity profiling. Extensive direct SGD measurements with seepage meters (n = 551) showed fresh groundwater discharge patterns that correlated well with shallow porewater salinity and the hydrogeophysical framework. Small-scale variability in fresh and saline discharge indicates influence of meter-scale geologic heterogeneity, while site-scale discharge patterns are evidence of the influence of the paleovalley feature. Beneath the paleovalley fill, fresh groundwater flows offshore and mixes with saltwater before discharging along paleovalley flanks. On the adjacent drowned interfluve where low-permeability fill is absent, fresh groundwater discharge is focused at the shoreline. Shallow saltwater exchange was greatest across sandy sediments and where fresh SGD was low. The geologic control of groundwater flowpaths and discharge salinity demonstrated in this work are likely to affect geochemical reactions and the chemical loads delivered by SGD to coastal surface waters. Because similar processes are likely to exist in other estuaries where drowned paleovalleys commonly cross modern shorelines, the existence and implications of complex hydrogeology are important considerations for studies of groundwater fluxes and related management decisions.

Crouching enemy, hidden ally: the decisive role of groundwater discharge features in two major British battles, Flodden 1513 and Prestonpans 1745

AbstractGeomorphological features associated with groundwater discharge zones can affect ground conditions so greatly that they determine the outcomes of battles. Two cases in point are found in Scottish history: (i) despite outnumbering their English foes, the Scots lost the Battle of Flodden Field (9 September 1513), largely due to failing to identify the presence of marshy ground associated with an area of groundwater discharge; (ii) on 21 September 1745, the Jacobites defeated the Hanoverian army at Prestonpans by finding a way around marshland corresponding to a regional groundwater discharge zone, upon which the Hanoverian commander had been relying as a natural defensive feature. Analysis of both battlefields drawing upon present-day understanding of local stratigraphy and hydrogeological conditions allows identification of the specific groundwater discharge patterns that largely determined the outcomes of these two emblematic battles. At Flodden, the proximal source of groundwater discharge is Quaternary outwash gravels, distally fed from sedimentary strata (Cementstones) of lowermost Dinantian age. In the case of Prestonpans, the groundwater emerges from particularly arenaceous coal-bearing strata of Namurian age. Both case studies suggest that military commanders selecting advantageous terrain could benefit from consulting hydrogeologists who are familiar with the intricacies of groundwater geomorphology.

Uncertainty in 1D Heat-Flow Analysis to Estimate Groundwater Discharge to a Stream

Temperature measurements have been used by a variety of researchers to gain insight into groundwater discharge patterns. However, much of this research has reduced the problem to heat and fluid flow in one dimension for ease of analysis. This approach is seemingly at odds with the goal of determining spatial variability in specific discharge, which implies that the temperature field will vary in more than one dimension. However, it is unclear how important the resulting discrepancies are in the context of determining groundwater discharge to surface water bodies. In this study, the importance of these variations is examined by testing two popular one-dimensional analytical solutions with stochastic models of heat and fluid flow in a two-dimensional porous medium. For cases with low degrees of heterogeneity in hydraulic conductivity, acceptable results are possible for specific discharges between 10(-7) and 10(-5) m/s. However, conduction into areas with specific discharges less than 10(-7) m/s from adjacent areas can lead to significant errors. In some of these cases, the one-dimensional solutions produced estimates of specific discharge of nearly 10(-6) m/s. This phenomenon is more likely in situations with greater degrees of heterogeneity.

Designing a conservation area network that supports the representation and persistence of freshwater biodiversity

Summary1. The progression of approaches in systematic conservation planning from representation to representation and persistence has greatly enhanced its potential applicability to freshwaters. However, conceptual frameworks that consolidate principles for incorporating persistence into freshwater conservation planning are still lacking.2. We present four key principles to consider when planning for the persistence of freshwater biodiversity: selecting ecosystems of high ecological integrity; incorporating connectivity; incorporating areas important to population persistence; and identifying additional natural processes that can be mapped.3. The practicalities of gathering data and conducting the conservation plan to address these principles are explored here using a case study in the Olifants/Doorn Water Management Area, South Africa. Spatial layers are developed for depicting ecological integrity, sub‐catchment boundaries, riparian zones and wetlands, high water‐yield areas and patterns of groundwater discharge and recharge.4. These data are used to develop a conservation area network that supports both representation and persistence of freshwater biodiversity. Although the planning region is relatively data rich by global standards, several data deficiencies were identified. We suggest ways of using environmental surrogates to address data deficiencies, improving confidence in these surrogates by combining them wherever possible with existing field data and expert knowledge.5. We also recommend methods to achieve spatial efficiency by simultaneously designing for representation and persistence of freshwater biodiversity. Spatial efficiency can be achieved in different ways when using a conservation planning algorithm and a multiple‐use zoning strategy.6. The allocation of multiple‐use zones aligns closely with the objectives of integrated water resources management and land use planning. Given the practicalities at local levels of planning, we recommend using multiple‐use zones in the design phase, rather than merely at the end once the design is complete.