Water balance

Water balance

A water balance assessment is estimated for each catchment to understand how water travels through the catchment.
The assessment consists of a calculation that accounts for all significant inputs and outputs of water to and from the surface water and groundwater systems and any interactions between them.

The water balance assumes steady state conditions. The water balance allows us to test the integrity of the data used for the catchment assessment and to identify possible data gaps.

The process for developing the water balance assessment follows the European Commission Guidance on the application of water balances for supporting the implementation of the WFD, Version 6.1 (EU Commission, 2015).

Components of the water balance assessment 

1. INPUTS


Effective Rainfall (or precipitation) is a difference between total rainfall and actual evapotranspiration.
Surface water run-off, where effective rainfall joins the surface water system directly by overland flow and rejected recharge from a groundwater aquifer.
Groundwater recharge is the remaining portion of the effective rainfall that enters the groundwater system. 
Surface water imports from other catchments, e.g. the import of surface water through a pipeline to a reservoir.
Groundwater inflows from other catchment through the topographical divides that form the catchment boundaries, e.g. through karst conduits, faults, etc.
Anthropogenic discharges to surface water and groundwater, e.g. discharge of public and industrial waste water, storm water, leakage from pipe, etc.

2. GROUNDWATER – SURFACE WATER INTERACTIONS

Surface water discharge to groundwater through e.g. losing streams or swallow holes. 
Groundwater discharge to surface water through seeps and springs (base flow component of surface water flows).

3. OUTPUTS

Groundwater outflow to other aquifer through the topographical divides that set the catchment boundaries, e.g. through karst conduits, faults, etc.
Surface water and groundwater abstractions including abstractions for drinking water, irrigation, industrial activities, etc.
Surface water outputs from catchment represents the natural outflow of all surface water and groundwater and groundwater  discharges to surface water (as baseflow) from the catchment.