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Measuring Groundwater–Surface Water Exchange

Every model needs ground-truth. The field methods below each measure a different aspect of GW–SW exchange — at different scales, with different uncertainties. Picking the right method (and combining methods) is the standard practice.

SGMA Depletion Series
Page 07 of 07
Bedrock Stream ① Stream gaging (USGS / state) 1 ② Seepage meter 2 ③ Streambed temperature 3 ④ Piezometer + head screen 4 ⑤ Pumping/aquifer test P 5 ⑥ Resistivity / EM survey 6 ⑦ ET / phreatophyte flux 7 water table
Figure 1. Field methods for characterizing groundwater–surface water exchange, with measurement locations. Click any method below for details. After Rosenberry & LaBaugh (2008).
Method-by-method

① Stream gaging & differential gaging

Continuous stage and discharge records at one or more points. Differential gaging — measuring Q at the top and bottom of a reach and computing the difference — gives the net gain or loss to/from the aquifer.

  • Scale: reach (10s of m to 10s of km).
  • Pros: direct, integrative, well-established (USGS NWIS).
  • Cons: high uncertainty when gain/loss is < 5% of Q; biased by ungaged tributaries and abstractions.
  • Best practice: dry-season synoptic; multiple stations; combine with seepage runs.

② Seepage meter

A bottomless drum pressed into the streambed, connected to a collapsible bag. Captures all upward (gaining) or downward (losing) flow through the streambed area beneath the drum over a measurement interval.

  • Scale: point (≈ 0.1 m²).
  • Pros: direct flux measurement; inexpensive.
  • Cons: highly local; bag inefficiency and friction biases (must be calibrated — Rosenberry & Menheer 2006); poor in coarse cobble streambeds.
  • Best practice: deploy in arrays; replicate; calibrate each unit.

③ Streambed temperature (heat-as-tracer)

Vertical arrays of thermistors at multiple depths in the streambed. The diel and seasonal surface-water temperature signal propagates down differently depending on vertical flux direction and magnitude (1‑D analytical or numerical inverse solution).

  • Scale: point to short reach.
  • Pros: non-invasive, long deployments, captures variability.
  • Cons: requires temperature gradient (less informative in cool gaining systems); inverse solution sensitive to thermal-property assumptions.
  • Tools: VFLUX (R/Matlab), 1DTempPro (USGS).

④ Piezometer / minipiezometer + Darcy

Drive a small-diameter piezometer below the streambed to measure the head under the stream. Compare to stream stage → vertical hydraulic gradient (VHG). Combined with streambed K (slug test), gives a Darcy flux.

  • Scale: point.
  • Pros: direct head data; mature.
  • Cons: point-K is hard to upscale; only valid when streambed K is reasonably uniform under the piezometer.

⑤ Pumping/aquifer test interpreted for depletion

A constant-rate pumping test analyzed with stream-aquifer drawdown solutions (Hunt 1999, Hantush 1965). Observation wells between the well and the stream reveal stream influence as a divergence from Theis drawdown.

  • Scale: reach (~100s of m to km).
  • Pros: estimates the depletion parameters that matter for prediction (T, S, λ).
  • Cons: expensive; requires sufficient pumping duration; non-unique inversions (Lough & Hunt 2006).

⑥ Geophysics (ERT, EM, fiber-optic DTS)

Resistivity surveys reveal the geometry of saturated zones, clay lenses, and stream connection. Distributed temperature sensing (DTS) with a fiber-optic cable laid in the streambed maps high-resolution temperature and inferred flux along an entire reach.

  • Scale: reach to basin.
  • Pros: spatial coverage; non-invasive; identifies disconnection zones.
  • Cons: inversion non-unique; requires geophysics expertise.

⑦ Groundwater ET measurement

Eddy-covariance flux towers, sap-flow sensors, or analysis of nighttime water-table fluctuations (White method) quantify phreatophyte use of shallow groundwater. Critical for the "captured ET" term in the depletion budget (page 03).

  • Scale: patch to landscape.
  • Pros: closes the water budget; constrains numerical models.
  • Cons: instrumentation cost; siting requirements; partitioning between SW-ET and GW-ET is hard.

⑧ Tracers (isotopes, dye, age)

Stable isotopes (²H, ¹⁸O, ³H, ¹⁴C) distinguish stream water from groundwater of different ages. Fluorescent dye traces direction and travel time. Useful for confirming connectivity and source mixing in complex reaches.

  • Scale: reach to basin.
  • Pros: fingerprints sources; complements physical measurements.
  • Cons: lab cost; equifinality in mixing models.
Best practices for a defensible field campaign

Combine methods

  • Differential gaging + seepage meters → reconciles reach-scale and point-scale flux.
  • Temperature + piezometers → independent measurement of the same flux.
  • Pumping test + monitoring well network → estimates parameters and validates conceptual model.
  • Geophysics first → identifies where to deploy point measurements.

Sample across time

  • Wet vs. dry season — exchange direction can reverse.
  • Diel cycle — ET drawdown and stream-stage diel signals reveal connection state.
  • Pre- and post-irrigation season — to capture pumping effects.
  • Wet-year and dry-year deployments to capture climate variability.

Where field methods are non-negotiable

Key references in the project library

  1. Rosenberry, D.O. & LaBaugh, J.W., eds. (2008). Field techniques for estimating water fluxes between surface water and groundwater. USGS Techniques and Methods 4-D2. (Canonical reference.)
  2. Rosenberry, D.O. & Menheer, M. (2006). A system for calibrating seepage meters. USGS SIR 2006-5053.
  3. Hunt, B., Lough, H. & Smith, B. (1999). A stream depletion field experiment. Groundwater 37(3): 364–372.
  4. Lough, H. & Hunt, B. (2006). Pumping test evaluation of stream depletion parameters. Groundwater 44(4): 540–546.
  5. Majcher, E.H. et al. (2007). Preferential groundwater seepage from a chlorinated hydrocarbon plume to West Branch Canal Creek. USGS SIR 2006-5233.
  6. Thomle, J.N. et al. (2020). A flux detection probe to quantify dynamic groundwater–surface water exchange.
  7. Niswonger, R. & Fogg, G. (2008). Influence of perched groundwater on base flow. WRR.
  8. Malama, B. et al. (2021). Assessing stream-aquifer connectivity in a coastal California watershed.
  9. Maroney, S. (2017). Flow and transport from a stream to a well in an unconfined aquifer.
  10. Constantz, J. (2008). Heat as a tracer to determine streambed water exchanges. WRR. (Foundational reference for method ③.)
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