A water balance assessment is estimated for each catchment to understand surface water and groundwater split. 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 uses long term average records. 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 runoff, where effective rainfall joins the surface water system directly by overland flow and rejected recharge from an aquifer.
Groundwater recharge is the proportion of rainfall that effectively enters the phreatic (saturated) zone of the (bedrock) aquifer.
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 aquifers 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.
