Water Budgets
A common approach to investigating seepage flux between a stream and underlying aquifer is to measure stream flow at specific points. These measurement sites subdivide the stream into reaches and a water budget is estimated for each reach, accounting for inputs such as tributary flows and outputs such as evaporative losses and diversions. The difference between inflows and outflows is then attributed to the interaction between the stream and the underlying aquifer. When applied to a defined reach, the groundwater flux (Qgw) is estimated from:
Equation 1
where Qdn is the flow at the downstream end of the reach, Qup is the flow at the upstream end, Qout are outputs from the reach (such as distributaries, evaporation, extraction) and Qin are inputs to the reach (such as direct rainfall, runoff, tributaries, irrigation drainage, sewage outfall). This follows the convention that a positive Qgw indicates a net input of groundwater to the reach. A negative Qgw indicates a net loss of surface water to the groundwater system and is commonly termed a transmission loss.
Although the method is simple, involving the quantification of all fluxes to or from the river, it is difficult to apply in many cases. The method relies on the accurate measurement of surface water flow, as well as appropriately accounting for all other gains and losses evident for the reach. The uncertainties associated with the flow measurements and estimates for water balance components such as unmetered extraction, evaporation, ungauged tributary flows, overbank flooding losses and flood return flows can often exceed the magnitude of the seepage flux being estimated. Stream flow measurement errors can be +25% during high flow conditions, and from -50 to +100% for flash floods in semiarid catchments (Lerner et al, 1990). This means that the reach must be relatively long so that the cumulative volume of seepage exceeds the errors in the water balance.
Further Information
A specific type of water balance technique called a pondage test is commonly used for man-made structures such as irrigation supply channels. A reach of the channel is isolated by placing embankments at each end, and filled with water at or higher than the operating level. After correcting for rainfall and evaporation, the subsequent decline in water level is attributed to seepage losses to the underlying aquifer. Alternatively, water is added to maintain a constant water level, and the added volume used in the seepage calculations.
Advantages
- Provides an average net seepage flux for the entirety of the surface water feature being considered eg for a defined stream reach. This is in contrast to other methods, such as seepage meters, which only measure seepage at a point.
- Relies on stream flow data that is commonly collected and publicly available. As stream flow gauging is typically time-series data, temporal changes in seepage can be estimated, highlighting seasonal or longer-term variations.
- Simple water balances can be estimated relatively quickly and cheaply (using existing stream flow monitoring) to derive an initial rough estimate of the direction and magnitude of seepage. Stream flow measurements at various points along a stream can help target hotspots in terms of seepage losses or gains.
- Is simple in concept.
Disadvantages
- Measurement errors in stream flow measurements can exceed the magnitude of the seepage flux. This is particularly the case for relatively short reaches of streams or channels.
- Seepage estimates may be misleading if a component of the water balance (eg extraction, evaporation) may not be adequately accounted for or effectively measured.
- Can be time consuming and expensive depending on methodologies used to measure water balance components.
- Does not provide information on the spatial variability of seepage along the reach being investigated.
Data Availability
The main data requirement for the water budget approach is reliable stream flow gauging data. The significant surface water monitoring databases in Australia are maintained by the State and Territory agencies involved in water management, refer Hydrology Data.
Case Studies
Relevant Links
ANCID Channel Seepage Management Tool
ANCID Know the Flow
BOM Water Resource Station Catalogue
Channel Geometry Analysis Program (CGAP): USGS software for the analysis, interpretation, and quantification of physical properties of an open-channel reach
NCALC: USGS Manning's n value calculation program
Slope Area Computation (SAC): USGS standardised procedure for computing discharge by the slope-area method
References
Lerner DN, Issar AS, Simmers I, 1990. Groundwater recharge, a guide to understanding and estimating natural recharge. International Association of Hydrogeologists, Kenilworth.