Natural flow statistics

Within the Republic of Ireland, Qube utilises the Environmental Protection Agency (EPA) hydrological modelling system for estimation of annual flow duration curves, known as HydroTool. The methods from HydroTool have been updated and expanded for use within Qube.

Details of the method updates are available within the following report to the EPA: Thomas Bree, December 2018, 'Annual and Monthly Flow Duration Curves and Mean Flows for Ungauged Catchments in Ireland'.

Subsequent use of the Qube Local Data Algorithm within natural flow estimation in Ireland is detailed here.

Influenced flow statistics

Incorporation of artificial influences in Qube is described in the following pages:

• Quantifying Artificial Influences
• Estimation of Artificially Influenced Flows

Uncertainty in natural flow estimation

Table 1 presents the three descriptor Region of Influence Model used in Qube, with the associated annual FDC Bias and FSE. The estimated FDC FSE error is shown below where Q95 has FSE 56%. This will be a conservative estimate in most applications. The estimated mean flow, calculated from the estimated FDC, has an FSE of 16%.

These uncertainty estimates for natural annual flow statistics are prior to incorporation of the Qube Local Data Algorithm.

Table 1: FDC Three Descriptor Model, Bias and FSE (Extract from Thomas Bree, December 2018, 'Annual and Monthly Flow Duration Curves and Mean Flows for Ungauged Catchments in Ireland')

Percentile	Descriptor 1	Descriptor 2	Descriptor 3	Bias	FSE
Q0.1	SAAR	PDP	RKcRk	2%	49%
Q01	SAAR	PDP	RKcRk	1%	31%
Q05	SAAR	PDP	RKcRk	1%	22%
Q10	SAAR	PDP	RKcRk	0%	18%
Q20	SAAR	PDP	RKcRk	1%	17%
Q30	SAAR	PDP	RKcRk	1%	17%
Q40	SAAR	PDP	FARL	1%	18%
Q50	SAAR	PDP	FARL	1%	20%
Q60	SAAR	PDP	FARL	2%	22%
Q70	SAAR	PDP	PEAT	2%	25%
Q80	SAAR	PDP	PEAT	0%	32%
Q90	SAAR	PDP	PEAT	1%	44%
Q95	SAAR	PDP	PEAT	4%	56%
Q99	SAAR	PDP	PEAT	9%	92%
Q99.9	SAAR	PDP	PEAT	2%	205%