Catchment News

Hydrometric update: River flows and water levels in relation to the drought

During the week of July 16th, 10-20mm of rain fell on the south western and eastern-midlands-border regions, with lower rainfall observed elsewhere across the country. This caused a slight increase in river flows in these regions, but the increased flows were only observed for a few days before dropping back to pre-rainfall flows.

Overall, the river flows, lake and groundwater levels remain low and continue to fall across the country following the sustained period of dry weather. River flow and water level data gathered by the EPA and OPW indicate that approximately 61% (115 gauges) of monitoring gauges are currently at or below their 95%ile flow.

Of the remaining 72 gauges that have been assessed, 16 are within 10% of their 95%ile flow. This represents a 5% increase in gauges that are at or below their 95%ile flow over the past seven days.

• The EPA hydrometric data are available via the HydroNet portal http://www.epa.ie/hydronet/
• The OPW water level data can be accessed at https://waterlevel.ie/
• ESB hydrometric briefings can be accessed at https://www.esb.ie/our-businesses/generation-energy-trading-new/hydrometric-information/overview

The map below shows a nationally widespread distribution of gauges with flow at or below their 95%ile flow. This 95%ile flow statistic is the percentage of time that flow is exceeded at a monitoring gauges i.e. the current flow has historically been reached fewer than 5% of the time at these 115 gauges.

The 95%ile flow statistic is an important low flow metric as the assimilative capacity assessments for wastewater and other discharge licences are derived from this flow statistic i.e. discharges take account of the 95%ile flow when considering the available dilution. Therefore, the risk of impact on the receiving river ecology is greater when flows fall below the 95%ile. Similarly, if water is abstracted from a river when the flow is below the 95%ile then the assimilative capacity reduces and therefore the risk of impact on the river ecology is greater.

Lake and groundwater levels are not unusually low for this time of year although levels continue to fall towards their seasonal low.

Figures 1 – 5 show river flows across the country since the start of July 2018. The graphs highlight the relative lack of impact the rainfall of July 16th and 20th had on the river flows in the south western and east-midlands regions respectively. Typically, river flows returned to their previous low levels within two-five days.

Figures 1 (Tolka, Dublin) and 2 (Triogue, Portlaoise) indicate that current flows (July 23rd) in the eastern and midlands regions are dropping back to the flows recorded prior to the rainfall on the 20th July. Figures 3 (Feale, Kerry) and 4 (Deel, Limerick) show that the 20th July rain did not reach the south west, but a comparable rainfall event happened on July 16th. In both catchments, river flows dropped back to pre-July 16th flows within a few days.

Figure 5 offers a comparison between the adjacent Feale and Deel catchments in the south west, with the larger Feale catchment showing a slower flow recession than the Deel catchment. Consequently, flows in the Feale catchment roughly returned to their pre-16th July rainfall flow approximately five days after the rainfall event, whereas the flows in the Deel catchment had returned to their pre-16th July rainfall flow within two days and have continued to drop further in recent days.

Figure 6 shows that the mean river flow for the River Deel was at or above long term monthly levels until May 2018. However, although the River Deel experiences its lowest flows in June and July, the 2018 flows are considerably lower than the long term mean monthly flows and the long term monthly flows in August and September are not especially high. Therefore, it is unlikely that flow in the River Deel will increase substantially above current levels unless there is above average rainfall in the coming months.

Figure 7 shows that, since May 2018, the flow in the River Deel has been below long term mean levels and the flow in July 2018 will shortly reach the lowest flow on record for that river.

Update by Matthew Craig and the EPA Hydrometrics Unit

http://www.epa.ie/water/wm/hydrometrics/network/