Groundwater Fluxes Across Interfaces (2004) / Chapter Skim
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4 Spatial and Temporal Scales of Recharge and Discharge
4 Spatial and Temporal Scales of Recharge and Discharge
Pages 42-55
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From page 42... ...
NEXUS OF TECHNOLOGY AND NEED Evaluations of groundwater fluxes often are based on observations at the scale needed for wetland delineation, seep and spring identification, recharge area identification for groundwater protection, or the identification of gaining and losing streams. Estimations of fluxes at larger scales are typically in the form of water budgets on a basin scale for river management.
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From page 43... ...
Averaged over the Mississippi River basin, and weighted by mean annual precipitation, the range is 29.6 cm. The upper Mississippi River basin, the Red-Arkansas basin and the eastern side of the Rocky Mountain divide are areas of large interannual variation in water storage, and therefore expected large natural variability in groundwater levels and recharge.
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From page 44... ...
Does the surface modeling approach, which ignores groundwater interactions, tend to bias the estimated moisture storage excursions downward? Restated, do groundwater-surface water interactions exert a significant influence on variations in subsurface storage, and hence evapotranspiration, over areas as large as the Mississippi River basin.
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From page 45... ...
~3 Jan Feb Mar Apt May Jun Jul Month Aug Sep Oct Nov Dee FIGURE 4-3 Mean annual cycles of monthly changes in terrestrial water storage and its components over Illinois. RS: reservoir water, SN: snow water, SM: soil moisture, GW: groundwater, IZ: intermediate zone storage.
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From page 46... ...
46 Groundwater Fluxes Across Interfaces make it possible to test our understanding in new ways. Finally, the last ten years have brought forth a host of low-cost sensors that can collect data with unprecedented temporal resolution.
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From page 47... ...
. By examining the relative magnitude and variability of groundwater fluxes at increasing spatial and
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From page 48... ...
48 hA'rro~.onin ~ Mo:~rQSCoo1C In As o CL o Groundwater Fluxes Across Interfaces , Heterogeneous c~ - Homogeneous O 9, 92 93 ~ A Volume y mm m B Vs ~ inter basin aquifer In K alluvial basin Fluvial . aqul er core ,'— ~ Earth 1 1 1 1 1 1\ ,, ?
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From page 49... ...
Of course, for the scaling process to make sense knowledge of the underlying physical processes is needed; the averaging would not be simple statistical interpolation as is the case for many upscaling problems. EXPECTED SCALING BEHAVIOR OF RECHARGE/DISCHARGE FLUXES Understanding and measuring groundwater fluxes across interfaces along a continuum of temporal and spatial scales is important for determining water and solute budgets.
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From page 50... ...
These methods produce estimates of flux that reflect different spatial and temporal scales; have different data requirements, strengths and limitations; and have varying sensitivity to water flux in fractures. Recharge varies spatially owing to variations in precipitation, surface microclimates, thickness of alluvial deposits, faults and fractures, and thickness and hydrologic properties of geologic strata in the unsaturated zone.
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From page 51... ...
Spatial and Temporal Scales of Recharge and Discharge at o IL A o 6 en o o in: IL Q U]
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From page 52... ...
and how the observing system will continuously update the model and model forecasts. The issues of scale relative to recharge and discharge fluxes involve understanding the interdependence between variability in fluxes and the physical and chemical characteristics of a site.
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From page 53... ...
represent the subsurface as one or more soil "slabs", with finest vertical resolution of depth typically a meter or two. Land surface models can be run in so-called "off-line" mode, that is, forced with observed precipitation and other surface atmospheric variables (e.g., downward radiation, wind, surface air temperature, vapor pressure deficit)
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From page 54... ...
In addition, the common practice of estimating recharge using numerical flow models is subject to an implied scaling due to the inabiTity of models to reproduce all surface-water features. Springs and seeps- As point locations of groundwater discharge, springs are often attractive Tocations for flow measurements, yet the proportion of groundwater discharge that a particular point spring represents is usually unmown, and may be small.
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From page 55... ...
classification (Lilly et al., 1998) , originally developed to predict river flows for ungaged catchrnents and later used within other models for predicting contaminant concentrations in runoff.
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