Framework

Hydrometric Analysis

The underlying principle of hydrometric methods is Darcy's Law, which defines the flow of water in a porous medium as:

(Equation 1)

Q=A(dh/dl)K

where Q is the flux of water (volume per unit time), A is the cross-sectional area of the porous medium through which flow occurs, dh/dl is the hydraulic gradient where dh is the change in hydraulic head along the distance dl of the groundwater flow line, and K is the hydraulic conductivity of the material (Figure 1). The parameter K of the aquifer material can vary significantly with the direction of water flow. The hydraulic conductivity in the vertical direction (Kv) in aquifers can be several orders of magnitude lower than that in the horizontal direction (Kh). This is because the interlayering of finer-grained clays and silts impedes vertical water movement, while laterally extensive sand and gravel deposits enable high rates of horizontal flow.

Darcy's Law can be applied to the situation where flow between surface water and groundwater systems are assumed to be entirely vertical (Figure 1). If the water level in the surface water body is higher than the groundwater level in the underlying aquifer than the hydraulic gradient is downwards. There is then the potential for the stream to lose water to the underlying groundwater system. In contrast, potential upwards groundwater flow towards the stream is indicated by a groundwater level that is higher than the water level in the stream. The magnitude of the actual water flow depends on the vertical hydraulic conductivity (Kv) of the stream bed and aquifer material. If this material is coarse grained such as gravel or sand than the flow rate is expected to be high, in contrast to much lower flow rates in finer grained material such as silt or clay. Thin deposits of low permeability material on the stream bed such as mud veneers or algal mats can significant impede the flow of groundwater.

Hence, measuring the rate and direction of vertical water flow between a surface water feature and an aquifer involves:

  1. Measuring Head Difference, which is the difference in the surface water level and the groundwater level (dh)
  2. Measuring vertical distance between the measuring point in the aquifer and the bed of the surface water feature (dl), and
  3. Measuring Hydraulic Conductivity(K) of the material along the flow path between the groundwater measuring point and the bed of the surface water feature.

This requires access to the aquifer to be able to measure the groundwater level. This is usually done by constructing a piezometer, which is a cased hole with a screen at a fixed interval down the hole.

Piezometers

In reality, groundwater flow paths near streams and lakes can be significantly more complex than simple vertical movement as represented in Figure 1. Even at the metre-scale of the near-stream environment, the groundwater flow in channel deposits can be a complex pattern of upward, downward and lateral movement (Vaux, 1968; Woessner, 2000).

This complexity of groundwater flow direction becomes more evident when the level of investigation broadens to the kilometre-scale. For this reason, flow net analysis is a hydrometric method that can be applied to the watertable contours surrounding a surface water feature. However, predictive models, such as analytical solutions or numerical models can better accommodate these complexities and tend to be used rather than simply applying Darcy's Law. These methods still require the basic understanding of the groundwater flow system and robust estimates of the hydraulic gradient and hydraulic conductivity.

Flow Net Analysis
Predictive Models

diagram representing the configuration of minipiezometer and stilling well for hydrometric measurement of seepage flux
Figure 1: Configuration of minipiezometer and stilling well for hydrometric measurement of seepage flux. In this example the groundwater level is lower than the surface water level and seepage flux is negative and downwards.

Advantages

  1. Based on fundamental principles for groundwater flow in porous medium.
  2. Hydrometric investigations in beds of surface water features give valuable information of the water/sediment interface. Many minipiezometers can be quickly and cheaply installed to provide information on the areal distribution of seepage.
  3. Comparison of surface water levels and groundwater levels are a simple guide to the direction of potential seepage, and can be used in quickly targeting losing or gaining conditions. Comparison of hydrographs from stream gauging sites with piezometers can indicate temporal changes in seepage direction.
  4. Flow nets can be generated for various geometrical and physical configurations to define key processes. This approach was used to generate a framework for determining groundwater-surface water interactions for lakes and other surface water bodies (Nield et al,1994).
  5. Flow net analysis can provide a simple, quick and cost-effective way of estimating first-order approximation of seepage flux. This allows flux conditions (i.e. losing, gaining, through-flow) to be predicted using geometrical and physical parameters, and flux boundary conditions.

Disadvantages

  1. Depends on a reasonable estimate for hydraulic conductivity (K) which has a potential range over several orders of magnitude. Measurement of hydraulic conductivity tends to be point estimates and may not reflect the average conditions for the groundwater flow path considered. This is particularly true if thin clay veneers in bed deposits are the main control for water flow.
  2. Assumes simple groundwater flow conditions (ie vertical) that may not reflect actual conditions. This assumption is typically invalid when using piezometers for a long distance from the surface water feature.
  3. Can be expensive if monitoring bores need to be drilled and constructed.
  4. Flow net analysis cannot account for spatial variability and other local groundwater factors.

Data Availability

Hydrometric analysis of seepage flux in streams requires data on stream water levels, nearby groundwater levels and estimates of aquifer properties (mainly hydraulic conductivity). The State and Territory agencies involved in water management are the main custodians for monitoring stream water levels, refer Hydrology Data. These agencies also tend to be involved in the collection of groundwater information (such as borehole databases).

Case Studies

Lower Richmond minipiezometers
Border Rivers hydrographs

Relevant Links

Aquifer Test Forum: Information on the design, performance and analysis of aquifer tests
ILRI Analysis and evaluation of pumping test data [PDF 15.4MB]
ILRI Software for aquifer test evaluation [PDF 6.4MB]
LWBC Minimum construction requirements for water bores [PDF 2MB]
US Geological Survey Spreadsheets for analysis of aquifer-test and slug-test data

References

Nield SP, Townley LR and Barr AD, 1994. A framework for quantitative analysis of surface water - groundwater interaction: Flow geometry in a vertical section. Water Resources Research 30(8): 2461-2475.

Vaux WG, 1968. Intragravel flow in interchange of water in a streambed. US Fish Wildlife Service Fishery Bulletin 66:479-489.

Woessner WW, 2000. Stream and fluvial groundwater interactions: rescaling hydrogeological thought. Ground Water 38(3): 423-429.