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Integrated hydrological modeling of the No-Name watershed, Medicine Bow Mountains, Wyoming (USA)Modélisation hydrologique intégrée du bassin versant de No-Name, Montagnes de Medicine Bow, Wyoming (Etats-Unis d’Amérique)Modelización hidrológica integrada de una cuenca en Medicine Bow Mountains, Wyoming (EE.UU.)美国怀俄明州Medicine Bow山脉无名流域的综合水文模拟Modelagem hidrológica integrada da bacia hidrográfica No-Name, Montanhas Medicine Bow, Wyoming (EUA)
Abstract
Integrated modeling of headwater watersheds in mountain environments is often limited by the lack of hydrological characterization and monitoring data. For the No-Name watershed in the Medicine Bow Mountains in Wyoming (USA), this research integrates regional surface and subsurface hydrological and geophysical measurements to create three-dimensional integrated hydrological models with which interactions between surface water, soil water, and groundwater are elucidated. Data used to build and calibrate the integrated model include a digital elevation model (DEM), stream discharge at the outlet of the watershed, soil-moisture data, weather data, and geophysical surveys including seismic refraction, airborne resistivity, and nuclear magnetic resonance (NMR). Based on interpretation of geophysical measurements, subsurface hydrostratigraphy consists of a top unconsolidated layer, a middle layer of fractured granite and metamorphic bedrock, and a lower protolith. Given that both measurements and interpretations have uncertainty, a sensitivity analysis was carried out to evaluate conceptual model uncertainty, which suggests the following: (1) for predicting stream discharge at the No-Name outlet, the most influential parameters are the Manning coefficient, DEM, hydrostratigraphy and hydraulic conductivity, and land cover. Compared to a lower-resolution DEM, a LiDAR DEM can lead to more accurate predictions of the stream discharge and stream elevation profile. (2) For predicting soil moisture, the most influential parameters are hydrostratigraphy and the associated hydraulic conductivities and porosities. (3) Based on a calibration exercise, the likely values for subsurface hydraulic conductivity at No-Name are ~10–5 m/s (the unconsolidated layer), ~10–6 m/s (fractured bedrock), and ~10–6 m/s or lower (protolith).
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Highly simplified approaches continue to underpin hydrological climate change impact assessments across the Earth's mountainous regions. Fully‐integrated surface‐subsurface models may hold far greater potential to represent the distinctive regimes of steep, geologically‐complex headwater catchments. However, their utility has not yet been tested across a wide range of mountainous settings. Here, an integrated model of two adjacent calcareous Alpine headwaters that accounts for two‐dimensional surface flow, three‐dimensional (3D) variably‐saturated groundwater flow, and evapotranspiration is presented. An energy balance‐based representation of snow dynamics contributed to the model's high‐resolution forcing data, and a sophisticated 3D geological model helped to define and parameterize its subsurface structure. In the first known attempt to calibrate a catchment‐scale integrated model of a mountainous region automatically, numerous uncertain model parameters were estimated. The salient features of the hydrological regime could ultimately be satisfactorily reproduced – over an 11‐month evaluation period, the Nash‐Sutcliffe efficiency of simulated streamflow at the main gauging station was 0.76. Spatio‐temporal visualization of the forcing data and simulated responses further confirmed the model's broad coherence. Presumably due to unresolved local subsurface heterogeneity, closely replicating the somewhat contrasting groundwater level signals observed near to one another proved more elusive. Finally, we assessed the impacts of various simplifications and assumptions that are commonly employed in physically‐based modeling – including the use of spatially uniform forcings, a vertically limited model domain, and global geological data products – on key simulated outputs, finding strongly affected model performance in many cases. Although certain outstanding challenges must be overcome if the uptake of integrated models in mountain regions around the world is to increase, our work demonstrates the feasibility and benefits of their application in such complex systems.
Mountain-block recharge (MBR) is the subsurface inflow of groundwater to lowland aquifers from adjacent mountains. MBR can be a major component of recharge but remains difficult to characterize and quantify due to limited hydrogeologic, climatic, and other data in the mountain block and at the mountain front. The number of MBR-related studies has increased dramatically in the 15 years since the last review of the topic was conducted by Wilson and Guan (2004), generating important advancements. We review this recent body of literature, summarize current understanding of factors controlling MBR, and provide recommendations for future research priorities. Prior to 2004, most MBR studies were performed in the southwestern United States. Since then, numerous studies have detected and quantified MBR in basins around the world, typically estimating MBR to be 5–50% of basin-fill aquifer recharge. Theoretical studies using generic numerical modeling domains have revealed fundamental hydrogeologic and topographic controls on the amount of MBR and where it originates within the mountain block. Several mountain-focused hydrogeologic studies have confirmed the widespread existence of mountain bedrock aquifers hosting considerable groundwater flow and, in some cases, identified the occurrence of interbasin flow leaving headwater catchments in the subsurface—both of which are required for MBR to occur. Future MBR research should focus on the collection of high-priority data (e.g., subsurface data near the mountain front and within the mountain block) and the development of sophisticated coupled models calibrated to multiple data types to best constrain MBR and predict how it may change in response to climate warming.
Widespread irrigated agriculture and a growing population
depend on the complex hydrology of the San Joaquin River basin in
California. The challenge of managing this complex hydrology hinges, in
part, on understanding and quantifying how processes interact to support the
groundwater and surface water systems. Here, we use the integrated
hydrologic platform ParFlow-CLM to simulate hourly 1 km gridded hydrology
over 1 year to study un-impacted groundwater–surface water dynamics in the
basin. Comparisons of simulated results to observations show the model
accurately captures important regional-scale partitioning of water among
streamflow, evapotranspiration (ET), snow, and subsurface storage. Analysis
of this simulated Central Valley groundwater system reveals the seasonal
cycle of recharge and discharge as well as the role of the small but
temporally constant portion of groundwater recharge that comes from the
mountain block. Considering uncertainty in mountain block hydraulic
conductivity, model results suggest this component accounts for 7–23 %
of total Central Valley recharge. A simulated surface water budget guides a
hydrograph decomposition that quantifies the temporally variable
contribution of local runoff, valley rim inflows, storage, and groundwater
to streamflow across the Central Valley. Power spectra of hydrograph
components suggest interactions with groundwater across the valley act to
increase longer-term correlation in San Joaquin River outflows. Finally,
model results reveal hysteresis in the relationship between basin streamflow
and groundwater contributions to flow. Using hourly model results, we
interpret the hysteretic cycle to be a result of daily-scale fluctuations
from precipitation and ET superimposed on seasonal and basin-scale recharge and discharge.
The Colorado River has been identified as the most overallocated river in the world. Considering predicted future imbalances between water supply and demand, and the growing recognition that baseflow (a proxy for groundwater discharge to streams) is critical for sustaining flow in streams and rivers, there is a need to develop methods to better quantify present-day baseflow across large regions. We adapted and applied the spatially referenced regression on watershed attributes (SPARROW) water quality model to assess the spatial distribution of baseflow, the fraction of streamflow supported by baseflow, and estimates of and potential processes contributing to the amount of baseflow that is lost during in-stream transport in the Upper Colorado River Basin (UCRB). On average, 56% of the streamflow in the UCRB originated as baseflow, and precipitation was identified as the dominant driver of spatial variability in baseflow at the scale of the UCRB, with the majority of baseflow discharge to streams occurring in upper elevation watersheds. The model estimates an average of 1.8x1010 m3/yr of baseflow in the UCRB; greater than 80% of which is lost during in-stream transport to the Lower Colorado River Basin via processes including evapotranspiration and water diversion for irrigation. Our results indicate that surface waters in the Colorado River Basin are dependent on baseflow, and that management approaches that consider groundwater and surface water as a joint resource will be needed to effectively manage current and future water resources in the Basin. This article is protected by copyright. All rights reserved.
Interactions between surface and groundwater systems are well-established
theoretically and observationally. While numerical models that solve both
surface and subsurface flow equations in a single framework (matrix) are
increasingly being applied, computational limitations have restricted their
use to local and regional studies. Regional or watershed-scale simulations
have been effective tools for understanding hydrologic processes; however,
there are still many questions, such as the adaptation of water resources to
anthropogenic stressors and climate variability, that can only be answered
across large spatial extents at high resolution. In response to this grand
challenge in hydrology, we present the results of a parallel, integrated
hydrologic model simulating surface and subsurface flow at high spatial
resolution (1 km) over much of continental North America (~ 6.3 M km2). These simulations provide integrated predictions of
hydrologic states and fluxes, namely, water table depth and streamflow, at
very large scale and high resolution. The physics-based modeling approach
used here requires limited parameterizations and relies only on more
fundamental inputs such as topography, hydrogeologic properties and climate
forcing. Results are compared to observations and provide mechanistic
insight into hydrologic process interaction. This study demonstrates both
the feasibility of continental-scale integrated models and their utility for
improving our understanding of large-scale hydrologic systems; the
combination of high resolution and large spatial extent facilitates analysis
of scaling relationships using model outputs.
Although we have an intuitive understanding of the behavior and functions of groundwater in the Earth's critical zone at the scales of a column (atmosphere-plant-soil-bedrock), along a toposequence (ridge to valley), and across a small catchment (up to third-order streams), this paper attempts to assess the relevance of groundwater to understanding large-scale patterns and processes such as represented in global climate and Earth system models. Through observation syntheses and conceptual models, evidence are presented that groundwater influence is globally prevalent, it forms an environmental gradient not fully captured by the climate, and it can profoundly shape critical zone evolution at continental to global scales. Four examples are used to illustrate these ideas: (1) groundwater as a water source for plants in rainless periods, (2) water table depth as a driver of plant rooting depth, (3) the accessibility of groundwater as an ecological niche separator, and (4) groundwater as the lower boundary of land drainage and a global driver of wetlands. The implications to understanding past and future global environmental change are briefly discussed, as well as critical discipline, scale, and data gaps that must be bridged in order for us to translate what we learn in the field at column, hillslope and catchment scales, to what we must predict at regional, continental, and global scales.
The techniques and standards for making discharge measurements at streamflow gaging stations are described in this publication. The vertical axis rotating-element current meter, principally the Price current meter, has been traditionally used for most measurements of discharge; however, advancements in acoustic technology have led to important developments in the use of acoustic Doppler current profilers, acoustic Doppler velocimeters, and other emerging technologies for the measurement of discharge. These new instruments, based on acoustic Doppler theory, have the advantage of no moving parts, and in the case of the acoustic Doppler current profiler, quickly and easily provide three-dimensional stream-velocity profile data through much of the vertical water column. For much of the discussion of acoustic Doppler current profiler moving-boat methodology, the reader is referred to U.S. Geological Survey Techniques and Methods 3–A22 (Mueller and Wagner, 2009).
Personal digital assistants (PDAs), electronic field notebooks, and other personal computers provide fast and efficient data-collection methods that are more error-free than traditional hand methods. The use of portable weirs and flumes, floats, volumetric tanks, indirect methods, and tracers in measuring discharge are briefly described.
Multi-Resolution Land Characterization 2001 (MRLC 2001) is a second-generation Federal consortium designed to create an updated pool of nation-wide Landsat 5 and 7 imagery and derive a second-generation National Land Cover Database(NLCD 2001). The objectives of this multi-layer, multi-source database are two fold: first, to provide consistent land cover for all 50 States, and second, to provide a data framework which allows flexibility in developing and applying each independent data component to a wide variety of other applications. Components in the database include the following: (1) normalized imagery for three time periods per path/row, (2) ancillary data, including a 30 m Digital Elevation Model(DEM) derived into slope, aspect and slope position, (3) per-pixel estimates of percent imperviousness and percent tree canopy, (4) 29 classes of land cover data derived from the imagery, ancillary data, and derivatives, (5) classification rules,
confidence estimates, and metadata from the land cover classification. This database is now being developed using a Mapping Zone approach, with 66 Zones in the continental United States and 23 Zones in Alaska. Results from three initial mapping Zones show single-pixel land cover accuracies ranging from 73 to 77 percent, imperviousness accuracies ranging from 83 to 91 percent, tree canopy accuracies ranging from 78 to 93 percent, and an estimated 50 percent increase in mapping efficiency over previous methods. The database has now entered the production phase and is being created using extensive partnering in the Federal government with planned completion by 2006.
A distributed physically based model incorporating novel approaches for
the representation of surface-subsurface processes and interactions is
presented. A path-based description of surface flow across the drainage
basin is used, with several options for identifying flow directions, for
separating channel cells from hillslope cells, and for representing
stream channel hydraulic geometry. Lakes and other topographic
depressions are identified and specially treated as part of the
preprocessing procedures applied to the digital elevation data for the
catchment. Threshold-based boundary condition switching is used to
partition potential (atmospheric) fluxes into actual fluxes across the
land surface and changes in surface storage, thus resolving the exchange
fluxes, or coupling, between the surface and subsurface modules. Nested
time stepping allows smaller steps to be taken for typically faster and
explicitly solved surface runoff routing, while a mesh coarsening option
allows larger grid elements to be used for typically slower and more
compute-intensive subsurface flow. Sequential data assimilation schemes
allow the model predictions to be updated with spatiotemporal
observation data of surface and subsurface variables. These approaches
are discussed in detail, and the physical and numerical behavior of the
model is illustrated over catchment scales ranging from 0.0027 to 356
km2, addressing different hydrological processes and
highlighting the importance of describing coupled surface-subsurface
flow.
The understanding of streamflow generation processes is vitally
important in the management of water resources. In the absence of the
data required to achieve this, Integrated Surface-Subsurface
Hydrological Models (ISSHM) can be used to assist with the development
of this understanding. However, the standard outputs from these models
only enable elicitation of information about hydrological drivers and
hydrological responses that occur at the same time. This generally
limits the applicability of ISSHMs for the purposes of obtaining an
improved understanding of streamflow generation processes to catchment
areas that do not exhibit significant storage, travel times or flow
depletion mechanisms. In order to overcome this limitation, a previously
published Hydraulic Mixing-Cell (HMC) method is improved so that it can
be used to follow surface water derived from direct rainfall and
groundwater discharge to the stream and adjacent overland flow areas.
The developed approach was applied to virtual experiments (based on the
Lehstenbach catchment in southeastern Germany), which are composed of
two ISSHMs of contrasting scales: (1) a riparian wetland of area 210
m2 and (2) a catchment of area 4.2 km2. For the
two models, analysis of modeling results for a large storm event showed
complex spatiotemporal variability in streamflow generation and surface
water-groundwater interaction. Further analysis with the HMC method
elucidated in-stream and overland flow generation mechanisms. This study
showed within a modeling framework that identification and
quantification of in-stream and overland flow generation better informed
understanding of catchment functioning through decomposition of
streamflow hydrographs, and analysis of spatiotemporal variability of
flow generation mechanisms.
Previous studies indicate predominantly increasing trends in precipitation across the Western United States, while at the same time, historical streamflow records indicate decreasing summertime streamflow and 25th percentile annual flows. These opposing trends could be viewed as paradoxical, given that several studies suggest that increased annual precipitation will equate to increased annual groundwater recharge, and therefore increased summertime flow. To gain insight on mechanisms behind these potential changes, we rely on a calibrated, integrated surface and groundwater model to simulate climate impacts on surface water/groundwater interactions using 12 general circulation model projections of temperature and precipitation from 2010 to 2100, and evaluate the interplay between snowmelt timing and other hydrologic variables, including streamflow, groundwater recharge, storage, groundwater discharge, and evapotranspiration. Hydrologic simulations show that the timing of peak groundwater discharge to the stream is inversely correlated to snowmelt runoff and groundwater recharge due to the bank storage effect and reversal of hydraulic gradients between the stream and underlying groundwater. That is, groundwater flow to streams peaks following the decrease in stream depth caused by snowmelt recession, and the shift in snowmelt causes a corresponding shift in groundwater discharge to streams. Our results show that groundwater discharge to streams is depleted during the summer due to earlier drainage of shallow aquifers adjacent to streams even if projected annual precipitation and groundwater recharge increases. These projected changes in surface water/groundwater interactions result in more than a 30% decrease in the projected ensemble summertime streamflow. Our findings clarify causality of observed decreasing summertime flow, highlight important aspects of potential climate change impacts on groundwater resources, and underscore the need for integrated hydrologic models in climate change studies.
In semiarid climates, a significant component of recharge to basin aquifers oc- curs along the mountain front. Traditionally called "mountain-front recharge" (MFR), this process has been treated by modelers of basins as a boundary condi- tion. In general, mountain-front recharge estimates are based on the general pre- cipitation characteristics of the mountain (as estimated, e.g., by the chloride mass balance and water balance methods), or by calibration of a basin groundwater model. These methods avoid altogether the complexities of the hydrologic sys- tem above the mountain front, or at best consider only traditional runoff process. Consequently hydrology above the mountain front is an area ripe for significant scientific advancement. A complete view would consider the entire mountain block system and examine hydrologic processes from the slope of the highest peak to the depth of the deepest circulating groundwater. Important aspects above the mountain front include the partitioning of rainfall and snowmelt into vegetation-controlled evapotranspiration, surface runoff, and deep infiltration through bedrock, especially its fractures and faults. Focused flow along mountain stream channels and the diffuse movement of groundwater through the underly- ing mountain block would both be considered. This paper first defines some key terms, then reviews methods of studying MFR in arid and semiarid regions, dis- cusses hydrological processes in the mountain block, and finally addresses some of the basic questions raised by the new mountain-block hydrology approach, as well as future directions for mountain-block hydrology research.
The influence of the vadose zone, land surface processes, and
macrodispersion on the shape and scaling behavior of residence time
distributions of baseflow is studied using a fully coupled watershed
model in conjunction with a Lagrangian, particle-tracking approach.
Numerical experiments are used to simulate groundwater flow paths from
recharge locations along the hillslope to the streambed. These
experiments are designed to isolate the influences of topography, vadose
zone/land surface processes, and macrodispersion on subsurface transport
of tagged parcels of water. The results of these simulations agree with
previous observations that such distributions exhibit a power law form
and fractal behavior, which can be identified from plots of the
residence time distribution and the power spectra. It is shown that
vadose zone/land surface processes significantly affect both the
residence time distributions and their spectra.
Time-lapse resistivity imaging is increasingly used to monitor hydrologic processes. Compared to conventional hydrologic measurements, surface time-lapse resistivity provides (1) superior spatial coverage in two or three dimensions, (2) potentially high-resolution information in time, and (3) information in the absence of wells. However, interpretation of time-lapse electrical tomograms is complicated by the ever increasing size and complexity of long-term, three-dimensional time-series conductivity datasets. Here, we use three-dimensional (3D) surface time-lapse electrical imaging to monitor subsurface electrical conductivity variations associated with stage-driven groundwater/surface-water interaction along a stretch of the Columbia River adjacent to the Hanford 300 Area, Hanford WA, USA. We reduce the resulting 3D conductivity time series using both correlation and time-frequency analysis to isolate a paleochannel causing enhanced groundwater/river-water interaction. Correlation analysis on the time-lapse imaging results concisely represents enhanced ground water/surface-water interaction within the paleochannel, and provides information concerning groundwater flow velocities. Time-frequency analysis using the Stockwell (S) Transform provides additional information by 1) identifying the stage periodicities driving ground water/river-water interaction due to upstream dam operations, 2) identifying segments in time-frequency space when these interactions are most active. These results provide new insight into the distribution and timing of river water intrusion into the Hanford 300 area, which has a governing influence on the behavior of a uranium plume left over from historical nuclear fuel processing operations.
Climate change and climate variability, population growth, and land use change drive the need for new hydrologic knowledge and understanding. In the mountainous West and other similar areas worldwide, three pressing hydrologic needs stand out: first, to better understand the processes controlling the partitioning of energy and water fluxes within and out from these systems; second, to better understand feedbacks between hydrological fluxes and biogeochemical and ecological processes; and, third, to enhance our physical and empirical understanding with integrated measurement strategies and information systems. We envision an integrative approach to monitoring, modeling, and sensing the mountain environment that will improve understanding and prediction of hydrologic fluxes and processes. Here extensive monitoring of energy fluxes and hydrologic states are needed to supplement existing measurements, which are largely limited to streamflow and snow water equivalent. Ground-based observing systems must be explicitly designed for integration with remotely sensed data and for scaling up to basins and whole ranges.
Scientists from several institutions and with different research backgrounds have worked together to develop a prototype modular land model for weather forecasting and climate studies. This model is now available for public sue and further development.
1] Mountains are important sources of freshwater for the adjacent lowlands. In view of increasingly scarce freshwater resources, this contribution should be clarified. While earlier studies focused on selected river systems in different climate zones, we attempt here a first spatially explicit, global typology of the so-called ''water towers'' at the 0.5° Â 0.5° resolution in order to identify critical regions where disproportionality of mountain runoff as compared to lowlands is maximum. Then, an Earth systems perspective is considered with incorporation of lowland climates, distinguishing four different types of water towers. We show that more than 50% of mountain areas have an essential or supportive role for downstream regions. Finally, the potential significance of water resources in mountains is illustrated by including the actual population in the adjacent lowlands and its water needs: 7% of global mountain area provides essential water resources, while another 37% delivers important supportive supply, especially in arid and semiarid regions where vulnerability for seasonal and regional water shortage is high.
Small mountain lakes function as temporary storage basins for rain and snowmelt-derived water. Many small lakes lose water seasonally, but questions remain about the processes involved and effects on watershed hydrology. Evaporation and groundwater outflow from lakes may influence baseflow in streams, hydrologic connections among lakes, and water fluxes from a watershed. To evaluate the role of small mountain lakes in watershed hydrology and the dominant pathways of water loss, we studied the water balances of four shallow, closed-basin, subalpine lakes in southern Wyoming that lose up to 99% of their volumes between early summer and late fall. We tested the performance of seven evaporation models, compared observed rates of water loss with simulations of evaporation and drainage, and conducted geophysical surveys to evaluate the hydrologic environment between lakes. Our results show that groundwater outflow, rather than evaporation, can dominate water loss and cause closed-basin mountain lakes to be ephemeral. Groundwater fluxes may contribute to varied rates and timing of water loss from the lakes. Evaporation accounted for 14% of water loss in a lake that overlays thin (<0.5 m) sediments and fractured bedrock and 83% in a lake underlain by >3 m of sediments and clay. Gradual recharge of groundwater (<18,000 m³·km⁻²·day⁻¹) from each study lake likely helps sustain baseflow in streams once snowmelt has subsided. Total water loss from closed-basin, subalpine lakes may therefore help to maintain baseflow of rivers in late summer, but their impact varies based on geological context and snowmelt availability.
In subalpine watersheds of the intermountain western United States, snowpack melt is the dominant water input to the hydrologic system. The primary focus of this work is to understand the partitioning of water from the snowpack during the snowmelt period and through the remainder of the growing season. We conducted a time-lapse electrical resistivity tomography (ERT) study in conjunction with a water budget analysis to track water from the snow-on through snow-off season (May-August 2015). Seismic velocities provided an estimate of regolith thickness while transpiration measurements from sap flow in conifer trees provided insight into root water uptake. We observed four hydrologic process-periods and found that deep flow and tree water fluxes are the primary pathways through which water moves off of the hillslope. Overland flow and interflow were negligible. We observed temporal changes in vadose zone water content more than 3.0 m below the surface. Our results show that vertical flow through the thin soil mantle overlaying coarse colluvial regolith was the primary pathway to a local unconfined aquifer.
Estimates of groundwater circulation depths based on field data are lacking. These data are critical to inform and refine hydrogeologic models of mountainous watersheds, and to quantify depth- and time-dependencies of weathering processes in watersheds. Here we test two competing hypotheses on the role of geology and geologic setting in deep groundwater circulation and the role of deep groundwater in the geochemical evolution of streams and springs. We test these hypotheses in two mountainous watersheds that have distinctly different geologic settings (one crystalline, metamorphic bedrock and the other volcanic bedrock). Estimated circulation depths for springs in both watersheds range from 0.6 to 1.6 km and may be as great as 2.5 km. These estimated groundwater circulation depths are much deeper than commonly modeled depths suggesting that we may be forcing groundwater flowpaths too shallow in models. In addition, the spatial relationships of groundwater circulation depths are different between the two watersheds. Groundwater circulation depths in the crystalline bedrock watershed increase with decreasing elevation indicative of topography-driven groundwater flow. This relationship is not present in the volcanic bedrock watershed suggesting that both the source of fracturing (tectonic versus volcanic) and increased primary porosity in the volcanic bedrock play a role in deep groundwater circulation. The results from the crystalline bedrock watershed also indicate that relatively deep groundwater circulation can occur at local-scales in headwater drainages less than 9.0 km2 and at larger fractions than commonly perceived. Deep groundwater is a primary control on streamflow processes and solute concentrations in both watersheds. This article is protected by copyright. All rights reserved.
Land cover change due to drought and insect-induced tree mortality or altered vegetation succession is one of the many consequences of anthropogenic climate change. While the hydrologic response to land cover change and increases in temperature have been explored independently, few studies have compared these two impacts in a systematic manner. These changes are particularly important in snow-dominated, headwaters systems that provide streamflow for continental river systems. Here we study the hydrologic impacts of both vegetation change and climate warming along three transects in a mountain headwaters watershed using an integrated hydrologic model. Results show that while impacts due to warming generally outweigh those resulting from vegetation change, the inherent variability within the transects provides varying degrees of response. The combination of both vegetation change and warming results in greater changes to streamflow amount and timing than either impact individually, indicating a nonlinear response from these systems to multiple perturbations. The complexity of response underscores the need to integrate observational data and the challenge of deciphering hydrologic impacts from proxy studies.
Groundwater flow drives partitioning
Soil evaporation and plant transpiration together contribute a substantial proportion of terrestrial freshwater fluxes. Land surface models are used to understand the partitioning of these fluxes on a continental scale; however, model outputs are often inconsistent with stable isotope observations. Maxwell and Condon incorporated dynamic groundwater flow into an integrated hydrologic model simulation for the entire United States. The model showed that water table depth and lateral flow strongly affect transpiration partitioning, thus explaining the inconsistencies between observations and models.
Science , this issue p. 377
Light Detection and Ranging (LiDAR), is relatively inexpensive, provides high spatial resolution sampling at great accuracy, and can be used to generate surface terrain and land cover datasets for urban areas. These datasets are used to develop high-resolution hydrologic models necessary to resolve complex drainage networks in urban areas. This work develops a five-step algorithm to generate indicator fields for tree canopies, buildings, and artificial structures using Geographic Resources Analysis Support System (GRASS-GIS), and a common computing language, Matrix Laboratory. The 54 km2 study area in Parker, Colorado consists of twenty-four 1,500 × 1,500 m LiDAR subsets at 1 m resolution with varying degrees of urbanization. The algorithm correctly identifies 96% of the artificial structures within the study area; however, application success is dependent upon urban extent. Urban land use fractions below 0.2 experienced an increase in falsely identified building locations. ParFlow, a three-dimensional, grid-based hydrological model, uses these building and artificial structure indicator fields and digital elevation model for a hydrologic simulation. The simulation successfully develops the complex drainage network and simulates overland flow on the impervious surfaces (i.e., along the gutters and off rooftops) made possible through this spatial analysis process.
Background: Snow and ice melt provide sensitive indicators of climate change and serve as the primary source of stream flow in alpine basins.
Aims: We synthesise the results of hydrological and hydrochemical studies during the period 1995–2014, building on a long history of earlier work focused on snow and water on Niwot Ridge and the adjacent Green Lakes Valley (GLV), which is part of the Niwot Ridge Long Term Ecological Research site (NWT LTER).
Methods: These studies are discussed in the context of how snow, snowmelt and runoff reflect changing local climate. We review recent results of snow, snowmelt, hydrology and hydrochemistry from the plot to the basin scale, utilising new tools such as continuous global positioning system (GPS) measurements of snow depth, along with remotely-sensed measurements of snow-covered area and melt, combined with long-term measurements of snow properties, discharge and solute and isotopic content of water.
Results and Conclusions: Surface–groundwater interactions are important components of water quantity and quality in alpine basins. Some or most snowmelt infiltrates underlying soils and bedrock, transporting soil and bedrock products to surface waters. Infiltrating snowmelt, along with increased melt of stored ice, increases the hydrologic connectivity between the terrestrial and aquatic systems. Alpine basins are being impacted by increases in atmospheric nitrogen deposition, which has caused changes in soil microbial processes such as nitrification. Nitrate, dissolved organic carbon and dissolved organic nitrogen are thus flushed from soils and talus to streams. Our long-term results show that alpine catchments, such as Green Lake 4 at NWT LTER+, have the greatest sensitivity and least resilience to climate warming, with any warming leading to increased water yields.
A one-dimensional vertical numerical model for coupled water flow and heat transport in soil and snow was modified to include all three phases of water: vapor, liquid, and ice. The top boundary condition in the model is driven by incoming precipitation and the surface energy balance. The model was applied to three different terrestrial systems: A warm desert bare lysimeter soil in Boulder City, NV; a cool mixed-grass rangeland soil near Laramie, WY; and a snow-dominated mountainous forest soil about 50 km west of Laramie, WY. Comparison of measured and calculated soil water contents with depth yielded modeling efficiency (ME) values (maximum range: -∞< ME ≤ 1) of 0.32 ≤ ME ≤ 0.75 for the bare soil, 0.05 ≤ ME ≤ 0.30 for the rangeland soil, and 0.06 ≤ ME ≤ 0.37 for the forest soil. Results for soil temperature with depth were 0.87 ≤ ME ≤ 0.91 for the bare soil, 0.92 ≤ ME ≤ 0.94 for the rangeland soil, and 0.85 ≤ ME ≤ 0.88 for the forest soil. The model described the mass change in the bare soil lysimeter due to outgoing evaporation with moderate accuracy (ME = 0.41, based on 4 yr of data and using weekly evaporation rates). Snow height for the rangeland soil and the forest soil was captured reasonably well (ME = 0.57 for both sites based on 5 yr of data for each site). The model is physics based, with few empirical parameters, making it applicable to a wide range of terrestrial ecosystems.
This textbook provides an introduction to the study of hydrogeology, and maintains the process oriented approach of the earlier edition. The introduction is followed by chapters on: the origin of porosity and permeability; groundwater movement; equations of flow, boundary conditions and flow nets; groundwater in the basin hydrologic cycle; hydraulic testing; groundwater resources; stress, strain and pore fluids; heat transport in groundwater flow; solute transport; aqueous geochemistry; chemical reactions; colloids and microorganisms; mass transport equations; mass transport in natural groundwater systems and groundwater flow; contaminant hydrology; modelling of dissolved contaminant transport; multiphase fluid systems; remediation; and in situ destruction and risk assesment.
SkyTEM is a time-domain, helicopter electromagnetic system designed for hydrogeophysical and environmental investigation. Developed as a rapid alternative to ground-based, transient electromagnetic measurements, the resolution capabilities are comparable to that of a conventional 40 x 40 m(2) system. Independent of the helicopter, the entire system is carried as an external sling load. In the present system, the transmitter, mounted on a lightweight wooden lattice frame, is a four-turn 12.5 x 12.5 m(2) square loop, divided into segments for transmitting a low moment with one turn and a high moment with all four turns. The low moment uses about 30 A with a turn-off time of about 4 mu s; the high moment draws approximately 50 A, and has a turn-off time of about 80 mu s. The shielded, overdamped, multi-turn receiver loop is rigidly mounted on the side of the transmitter loop. This is essentially a central-loop configuration with a 1.5 m vertical offset. In vertical hover mode the SkyTEM responses were within 2% of those from a conventional ground-based system. Instrument bias level is not a concern as high-altitude tests showed that the background noise level is higher than the instrument bias level. By inverting a sounding from a test site to a standard model and then applying the SkyTEM system parameters to compute the forward response, conventional measurements were within 5% of SkyTEM responses for flight heights of 7.25, 10, and 20 m. Standard field operations include establishment of a repeat base station in the survey area where data are acquired approximately every 1.5 hours, when the helicopter is refuelled, to monitor system stability. Data acquired in a production survey were successful in detecting and delineating a buried-valley structure important in hydrogeophysical investigations.
Surface water and ground water always influence each other in nature, both of which constitute an organic unity. Because of the gradual understanding of the impact of this interaction, surface water and groundwater coupled models are mostly used for the simulation and analysis of the process and its impact. This article concludes the research progress and focuses the coupled system of surface and subsurface flow to describe. However, the interaction between surface water and groundwater is still hot and difficult in today's research, because the interaction law is very complex. The application of MODHMS which is a coupled model for evaluating surface and subsurface flow is used for simulating non-point source pollution. Advantages and disadvantages of the model are analyzed for the future hydrology application.
Alpine watersheds represent an important source of fresh water in western Canada. Understanding of hydrological processes in headwater environments is crucial to effectively manage and allocate alpine water resources. Since rainfall, snowmelt and glacier melt make minimal contributions to streamflow during winter months, essentially all winter flow in unregulated streams is provided by groundwater discharge. Therefore, the analysis of winter flow provides critical information regarding the magnitude of groundwater discharge and its relation with the physiographical characteristics of watersheds such as climate, geology and topography. Streamflow records of 18 watersheds (21–3900 km2) in the Canadian Rocky and Columbia mountain ranges were analyzed. Winter flows were in a narrow range (0.2–0.6 mm d–1) throughout the study area, and had a relatively small inter-annual variability, while the flows during spring, summer and fall had a large inter-annual variability. This suggests that the groundwater storage is filled up to the maximum capacity every year, and the groundwater discharge in winter is mostly controlled by the stationary factors such as the spatial variability of geology, topography and climatic variables. Precipitation inputs, reflected in the long-term mean annual flow, had a strong correlation with the long-term mean winter flow, indicating the dominant effect of the climatic gradient. Bedrock geology influenced winter flows, where the watersheds underlain by younger rocks having generally higher permeability and porosity had higher winter flow, while the topographic effects were not discernible. These suggest that the local effect of topography on the hydraulic gradient is averaged out, but the geological influence on the aquifer hydraulic property operates at a scale comparable to the size of the study watersheds.
Understanding of hydrological processes in wetlands may be complicated by management practices and complex groundwater/surface-water interactions. This is especially true for wetlands underlain by permeable geology, such as chalk. In this study, the physically based, distributed model MIKE SHE is used to simulate hydrological processes at the CEH River Lambourn Observatory, Boxford, Berkshire, UK. This comprises a 10 ha lowland, chalk valley bottom, riparian wetland designated for its conservation value and scientific interest. Channel management and a compound geology exert important, but to date not completely understood, influences upon hydrological conditions. Model calibration and validation was based upon comparisons of observed and simulated groundwater heads and channel stages over an equally split 20-month period. Model results are generally consistent with field observations and include short-term responses to events as well as longer-term seasonal trends. An intrinsic difficulty in representing compressible, anisotropic soils limited otherwise excellent performance in some areas. Hydrological processes in the wetland are dominated by the interaction between groundwater and surface water. Channel stage provides head boundaries for broad water levels across the wetland, whilst areas of groundwater upwelling control discrete head elevations. A relic surface drainage network confines flooding extents and routes seepage to the main channels. In-channel macrophyte growth and its management have an acute effect on water levels and the proportional contribution of groundwater and surface water. The implications of model results for management of conservation species and their associated habitats are discussed. This article is protected by copyright. All rights reserved.
Hydrological models for headwater catchments have typically excluded deep groundwater flow based on the assumption that it is a negligible component of the water budget. This study tests this assumption using a coupled surface water-groundwater model to explore the potential contribution of deep groundwater recharge to the bedrock in a snowmelt-dominated headwater catchment (Upper Penticton Creek 241) in the Okanagan Basin, British Columbia. Recharge to the bedrock is estimated at similar to 27% of the annual precipitation over the period 2005-2010, recognizing the uncertainty in this estimate due to data limitations, parameter uncertainty and calibration errors. A specified outward flux from the catchment boundary within the saturated zone, representing similar to 2% of the annual water budget, was also included in the model. This outward flux contributes to cross-catchment flow and, ultimately, to groundwater inflow to lower elevation catchments in the mountain block. This modeling exercise is one of the first in catchment hydrologic modeling within steep mountainous terrain in which the bedrock is not treated as impermeable, and in which recharge to the bedrock and discharge to the surrounding mountain block were estimated.
Initial conditions have been shown to have a strong effect on outputs of surface water models, but their impact on integrated hydrologic models is not well documented. We investigated the effects of initial conditions on an integrated hydrologic model of a 5632km2 basin in the northeastern U.S. Simulations were run for the year 1980 using four initial conditions spanning a range of average depth to water table, including 1m (“wet”), 3m, 5 m, and 7m (“dry”) below land surface. Model outputs showed significant effects of initial conditions on basin-averaged variables such as subsurface storage, surface storage, and surface runoff, with the greatest impact observed on surface storage and runoff. Effects of initial conditions were related to meteorological conditions, with precipitation reducing the effects of initial conditions on surface storage and runoff. Additionally, feedbacks between soil moisture and land-energy fluxes affected the impacts of initial conditions: higher temperatures magnified the differences in storage, recharge and discharge among the four initial-condition scenarios. 10-year recursive runs were conducted for the wet and dry scenarios. Spin-up times varied by model components and were considerably smaller for land-surface states and fluxes. Spin-up for dry initial conditions was slower than for wet initial conditions, indicating longer system memory for dry initial conditions. These variations in persistence of initial conditions should be taken into consideration when designing model initialization approaches. More broadly, this behavior is indicative of increased persistence of the effects of dry years as opposed to wet years in hydrologic systems. This article is protected by copyright. All rights reserved.
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.
The impact of shallow tile drainage networks on groundwater flow patterns, and associated transport of chemical species or sediment particles, needs to be quantified to evaluate the effects of agricultural management on water resources. A current challenge is to represent tile drainage networks in numerical models at the scale of a catchment for which proper simulation and reproduction of subsurface drainage water dynamics are essential. This study investigates the applicability of various tile drainage modeling concepts by comparing their results to a reference model. The models were setup for a 3.5 ha regularly monitored tile-drained area located in the clayey till Lillebæk catchment in Denmark, where the main input of water to the streams originates from subsurface drainage. Several simulations helped identify the most efficient way of modeling tile-drained areas using the integrated surface water and groundwater HydroGeoSphere code, which also simulates one-dimensional water flow in tile drains. The aim was also to provide insight on the design of larger scale models of complex tile drainage systems, for which computational time can become very large. A reference model that simulated coupled surface flow, groundwater flow and flow in tile drains was setup and showed a rapid response in drain outflow when precipitation is applied. A series of additional simulations were performed to test the influence of flow boundary conditions, temporal resolution of precipitation data, conceptual representations of clay tills and drains, as well as spatial discretization of the mesh. For larger scale models, the simulations suggest that a simplification of the geometry of the drainage network is a suitable option for avoiding highly discretized meshes. Representing the drainage network by a high-conductivity porous medium layer reproduces outflow simulated by the reference model. This approach greatly reduces the mesh complexity and simulation time and thus represents an alternative to a discrete representation of drains at the field scale, but specifying the hydraulic properties of the layer requires calibration against observed drain discharge.
Groundwater flow in karst includes exchange of water between large fractures, conduits, and the surrounding porous matrix, which impacts both water quality and quantity. Electrical resistivity tomography combined with End-Member Mixing Analysis (EMMA) and numerical flow and transport modeling was used to study mixing of karst conduit and matrix waters to understand spatial and temporal patterns of mixing during high flow and baseflow conditions. To our knowledge this is the first time EMMA and synthetic geophysical simulations have been combined. Here, we interpret an eight-week time-lapse electrical resistivity data set to assess groundwater-surface mixing. We simulate flow between the karst conduits and the porous matrix to determine fractions of water recharged to conduits that has mixed with groundwater stored in the pore space of the matrix using a flow and transport model in a synthetic time-lapse resistivity inversion. Comparing the field and synthetic inversions, our results enable us to estimate exchange dynamics, spatial mixing, and flow conditions. Results showed that mixing occurred at a volumetric flux of 56 m3/d with a dispersivity around 1.69 m during the geophysical experiment. For these conditions it was determined that conduit water composition ranged from 75% groundwater during baseflow conditions to less than 50% groundwater in high flow conditions. Though subject to some uncertainties, the time-lapse inversion process provides a means to predict changing hydrologic conditions, leading to mixing of surface water and ground water and thus changes to water quantity and quality, as well as potential for water-rock reactions, in a semi-confined, sink-rise system.
A terrain-following grid formulation (TFG) is presented for simulation
of coupled variably-saturated subsurface and surface water flow. The TFG
is introduced into the integrated hydrologic model, ParFlow, which uses
an implicit, Newton Krylov solution technique. The analytical Jacobian
is also formulated and presented and both the diagonal and non-symmetric
terms are used to precondition the Krylov linear system. The new
formulation is verified against an orthogonal stencil and is shown to
provide increased accuracy at lower lateral spatial discretization for
hillslope simulations. Using TFG, efficient scaling to a large number of
processors (16,384) and a large domain size (8.1 Billion unknowns) is
shown. This demonstrates the applicability of this formulation to
high-resolution, large-spatial extent hydrology applications where
topographic effects are important. Furthermore, cases where the
analytical Jacobian is used for the Newton iteration and as a
non-symmetric preconditioner for the linear system are shown to have
faster computation times and better scaling. This demonstrates the
importance of solver efficiency in parallel scaling through the use of
an appropriate preconditioner.
Interest in groundwater (GW)-surface water (SW) interactions has grown steadily over the last two decades. New regulations such as the EU Water Framework Directive (WFD) now call for a sustainable management of coupled ground- and surface water resources and linked ecosystems. Embracing this mandate requires new interdisciplinary research on GW-SW systems that addresses the linkages between hydrology, biogeochemistry and ecology at nested scales and specifically accounts for small-scale spatial and temporal patterns of GW-SW exchange. Methods to assess these patterns such as the use of natural tracers (e.g. heat) and integrated surface-subsurface numerical models have been refined and enhanced significantly in recent years and have improved our understanding of processes and dynamics. Numerical models are increasingly used to explore hypotheses and to develop new conceptual models of GW-SW interactions. New technologies like distributed temperature sensing (DTS) allow an assessment of process dynamics at unprecedented spatial and temporal resolution. These developments are reflected in the contributions to this Special Issue on GW-SW interactions. However, challenges remain in transferring process understanding across scales.
[1] We know little regarding how geomorphological features along the surface-groundwater interface collectively affect water quality and quantity. Simulations of surface water-groundwater exchange at increasing scales across bed forms, bars and bends, and basins show that groundwater has a power-law transit time distribution through all these features, providing a purely mechanistic foundation and explanation for temporal fractal stream chemistry. Power-law residence time distributions are almost always attributed to spatial variability in subsurface transport properties- something we show is not necessary. Since the different geomorphological features considered here are typical of most landscapes, fractal stream chemistry may be universal and is a natural consequence of water exchange across multifaceted interfaces.
Methodology was developed to estimate surface water-groundwater interactions in a changing climate in cold, snow-dominated regions and tested on an unconfined esker aquifer in northern Finland. The Watershed Simulation and Forecasting System (WSFS) model was used to simulate surface water levels. A coupled heat and mass transfer model for soil-plant-atmosphere systems (CoupModel) was used to simulate recharge. The CoupModel is able to simulate water flow through frozen soil. The simulated surface water level and recharge data were linked to the Groundwater Modelling System (GMS) version 6.5, and the three-dimensional groundwater flow model MODFLOW was used to study the impact of climate variability on groundwater levels and groundwater-surface water interactions. In addition, under A1B climate change scenario changes in aquifer storage and groundwater-surface water interactions were studied through the end of the 21st century. Recharge was shown to be sensitive to changes in soil frost and hence modelling approaches should include a soil frost component to account for the impact of frost on hydraulic conductivity. Under A1B climate change scenario study, groundwater recharge is projected to increase in winter months due to increase in snowmelt and decrease in soil frost depth. The spring snowmelt peak in late spring will decrease. This will reduce aquifer storage in early spring, increasing the vulnerability to summer droughts.
The feasibility of using the transient electromagnetic sounding (TS or TDEM) method for groundwater exploration can be studied by means of numerical models. As examples of its applicability to groundwater exploration, we study four groundwater exploration problems: (1) mapping of alluvial fill and gravel zones over bedrock; (2) mapping of sand and gravel lenses in till; (3) detection of salt or brackish water interfaces in freshwater aquifers; and (4) determination of hydrostratigraphy. These groundwater problems require determination of the depth to bedrock; location of resistive, high-porosity zones associated with fresh water; determination of formation resistivity to assess water quality; and determination of lithology and geometry, respectively. The TS method is best suited for locating conductive targets, and has very good vertical resolution. Unlike other sounding techniques where the receiver-transmitter array must be expanded to sound more deeply, the depth of investigation for the TS method is a function of the length of time the transient is recorded. Present equipment limitations require that exploration targets with resistivities of 50 Ω m or more be at least 50 m deep to determine their resistivity. The maximum depth of exploration is controlled by the geoelectrical section and background electromagnetic (EM) noise. For a particular exploration problem, numerical studies arc recommended to determine if the target is detectable.
Pedotransfer functions (PTFs) are becoming a more common way to predict soil hydraulic properties from soil texture, bulk density, and organic matter content. Thus far, the calibration and validation of PTFs has been hampered by a lack of suitable databases. In this paper we employed three databases (RAWLS, AHUJA, and UNSODA) to evaluate the accuracy and uncertainty of neural network-based PTFs. Sand, silt, and clay percentages and bulk density were used as input for the PTFs, which subsequently provided retention parameters and saturated hydraulic conductivity, Ks as output. Calibration and validation of PTFs were carried out on independent samples from the same database through combination with the bootstrap method. This method also yielded the possibility of calculating uncertainty estimates of predicted hydraulic parameters. Calibration and validation results showed that water retention could be predicted with a root mean square residual (RMSR) between 0.06 and 0.10 cm3 cm-3; the RMSR of log(Ks) was between 0.4 and 0.7 log (cm day-1). Cross-validation was used to test how well PTFs that were calibrated for one database could predict the hydraulic properties of the other two databases. The results showed that systematically different predictions were made when the RMSR values increased to between 0.08 and 0.13 cm3 cm-3 for water retention and to between 0.6 and 0.9 log(cm day-1) for log(Ks). The uncertainty in predicted Ks was one-half to one order of magnitude, whereas predicted water retention points had an uncertainty of about 0.04 to 0.10 cm3 cm-3. Uncertainties became somewhat smaller if the PTFs were calibrated on all available data. We conclude that the performance of PTFs may depend strongly on the data that were used for calibration and evaluation.
We characterize the seasonal variation in the geochemical and isotopic content of the outflow of the Green Lake 5 rock glacier (RG5), located in the Green Lakes Valley of the Colorado Front Range, USA. Between June and August, the geochemical content of rock glacier outflow does not appear to differ substantially from that of other surface waters in the Green Lakes Valley. Thus, for this alpine ecosystem at this time of year there does not appear to be large differences in water quality among rock glacier outflow, glacier and blockslope discharge, and discharge from small alpine catchments. However, in September concentrations of Mg2+ in the outflow of the rock glacier increased to more than 900 µeq L−1 compared to values of less than 40 µeq L−1 at all the other sites, concentrations of Ca2+ were greater than 4,000 µeq L−1 compared to maximum values of less than 200 µeq L−1 at all other sites, and concentrations of SO reached 7,000 µeq L−1, compared to maximum concentrations below 120 µeq L−1 at the other sites. Inverse geochemical modelling suggests that dissolution of pyrite, epidote, chlorite and minor calcite as well as the precipitation of silica and goethite best explain these elevated concentrations of solutes in the outflow of the rock glacier. Three component hydrograph separation using end–member mixing analysis shows that melted snow comprised an average of 30% of RG5 outflow, soil water 32%, and base flow 38%. Snow was the dominant source water in June, soil water was the dominant water source in July, and base flow was the dominant source in September. Enrichment of δ18O from −10‰ in the outflow of the rock glacier compared to −20‰ in snow and enrichment of deuterium excess from + 17.5‰ in rock glacier outflow compared to + 11‰ in snow, suggests that melt of internal ice that had undergone multiple melt/freeze episodes was the dominant source of base flow. Copyright © 2005 John Wiley & Sons, Ltd.
Estimating the impacts of climate change on groundwater represents one of the most difficult challenges faced by water resources specialists. One difficulty is that simplifying the representation of the hydrological system often leads to discrepancies in projections. This study provides an improved methodology for the estimation of the impacts of climate change on groundwater reserves, where a physically-based surface–subsurface flow model is combined with advanced climate change scenarios for the Geer basin (465 km2), Belgium. Coupled surface–subsurface flow is simulated with the finite element model HydroGeoSphere. The simultaneous solution of surface and subsurface flow equations in HydroGeoSphere, as well as the internal calculation of the actual evapotranspiration as a function of the soil moisture at each node of the defined evaporative zone, improve the representation of interdependent processes like recharge, which is crucial in the context of climate change. More simple models or externally coupled models do not provide the same level of realism. Fully-integrated surface–subsurface flow models have recently gained attention, but have not been used in the context of climate change impact studies. Climate change simulations were obtained from six regional climate model (RCM) scenarios assuming the SRES A2 emission (medium–high) scenario. These RCM scenarios were downscaled using a quantile mapping bias-correction technique that, rather than applying a correction only to the mean, forces the probability distributions of the control simulations of daily temperature and precipitation to match the observed distributions. The same corrections are then applied to RCM scenarios for the future. Climate change scenarios predict hotter and drier summer and warmer and wetter winters. The combined use of an integrated surface–subsurface modelling approach, a spatial representation of the evapotranspiration processes and sophisticated climate change scenarios improves the model realism and projections of climate change impacts on groundwater reserves. For the climatic scenarios considered, the integrated flow simulations show that significant decreases are expected in the groundwater levels (up to 8 m) and in the surface water flow rates (between 9% and 33%) by 2080.
Interactions between surface and groundwater are a key component of the hydrologic budget on the watershed scale. Models that honor these interactions are commonly based on the conductance concept that presumes a distinct interface at the land surface, separating the surface from the subsurface domain. These types of models link the subsurface and surface domains via an exchange flux that depends upon the magnitude and direction of the hydraulic gradient across the interface and a proportionality constant (a measure of the hydraulic connectivity). Because experimental evidence of such a distinct interface is often lacking in field systems, there is a need for a more general coupled modeling approach.
Results are presented from the multi-institution partnership to develop a real-time and retrospective North American Land Data Assimilation System (NLDAS). NLDAS consists of (1) four land models executing in parallel in uncoupled mode, (2) common hourly surface forcing, and (3) common streamflow routing: all using a 1/8° grid over the continental United States. The initiative is largely sponsored by the Global Energy and Water Cycle Experiment (GEWEX) Continental-Scale International Project (GCIP). As the overview for nine NLDAS papers, this paper describes and evaluates the 3-year NLDAS execution of 1 October 1996 to 30 September 1999, a period rich in observations for validation. The validation emphasizes (1) the land states, fluxes, and input forcing of four land models, (2) the application of new GCIP-sponsored products, and (3) a multiscale approach. The validation includes (1) mesoscale observing networks of land surface forcing, fluxes, and states, (2) regional snowpack measurements, (3) daily streamflow measurements, and (4) satellite-based retrievals of snow cover, land surface skin temperature (LST), and surface insolation. The results show substantial intermodel differences in surface evaporation and runoff (especially over nonsparse vegetation), soil moisture storage, snowpack, and LST. Owing to surprisingly large intermodel differences in aerodynamic conductance, intermodel differences in midday summer LST were unlike those expected from the intermodel differences in Bowen ratio. Last, anticipating future assimilation of LST, an NLDAS effort unique to this overview paper assesses geostationary-satellite-derived LST, determines the latter to be of good quality, and applies the latter to validate modeled LST.