Drought Monitoring using the Island of Ireland long term precipitation series (IIP).

Introduction

The team at ICARUS have been monitoring the monthly rainfall across Ireland to identify the onset of drought. The results showed that October, November, December and January were unusually dry months at many  locations. Although, rainfall totals during February and March returned to near normal levels across the island of Ireland some regions are still experiencing considerable rainfall deficits. Available monthly rainfall data was used to update 12 stations in the Island of Ireland precipitation series (IIP) (1850-2016) to end of March 2017. Figure 1 shows the location of all 12 IIP stations. The monthly rainfall for October to March at each location was compared with the long-term average (LTA) of the same months over the period 1850-2017. The average of all 12 stations was also established and presented as the Island of Ireland (IoI) series. Finally using the Standard Precipitation Index (SPI) the 6-month accumulative rainfall anomalies (SPI-6) for all the series January 1850 - March 2017 was produced.

Figure 1 map showing the location of the 12 update IIP stations.

Results

October 2016 rainfall

Figure 2 presents the results for October 2016 and show a particularly dry month when compared with the October LTA rainfall. Markree station located in the west only received 25% of the LTA rainfall in October 2016. The station at Malin head located in the north received 35% of LTA rainfall. Similarly, during October Shannon airport located in the south west received less than 40% of LTA rainfall. Other stations such as Armagh, Galway, Mullingar, Cork Airport and Roches Point received less than 50% of LTA rainfall in October 2016. However, some stations were not as dry with Dublin Airport and Valentia receiving 81% and 122% of LTA rainfall respectively. But overall the average rainfall across all stations (IoI series) was 54% below LTA rainfall in October 2016.

November 2016 rainfall

Rainfall totals during November 2016 were also well below LTA at most stations (See Figure 2). The lowest rainfall occurred across the south with Cork Airport and Roches Point receiving less than 35% of LTA rainfall in November 2016. Similarly, Athboy, UC Galway, Armagh, Valentia, Phoenix Park and Dublin Airport all received less than 60% of LTA rainfall. Conversely, Malin Head and Markree received near normal rainfall in November 2016 when compared with the LTA.

December 2016 rainfall

December saw a return to near normal conditions with all stations receiving rainfall greater than 60% of LTA (see Figure 2). The overall average rainfall (IoI series) was 68% of LTA in December 2016. Mullingar experienced 83% of LTA rainfall in December 2016 which is a stark contrast to December 2015 when 325% of LTA rainfall fell at Mullingar. Most of the IIP stations experienced over 2.5 times the LTA rainfall in December 2015.

January 2017 rainfall

Results shows that much dryer conditions returned to most stations in January 2017 (see Figure 2) with the exception of those located in the south and south west of Ireland (Valentia, Cork Airport, Roches Point). The driest region was along the east with Dublin Airport and Phoenix Park receiving only 35% of LTA rainfall in January 2017. Markree and UC Galway in the west also experienced a relatively dry January with less than 50% of LTA rainfall. Armagh, and Athboy located in the east and north east also saw less than 45% LTA rainfall in January.

February 2017 rainfall

The rainfall during the month of February 2017 returned to near normal conditions mainly due to Storm Doris which brought heavy rainfall to most areas on the 22^nd February. All stations received greater than 80% of LTA during February 2017(see Figure 2).  Mullingar and Phoenix Park received 20% more than average rainfall in February 2017.

March 2017 rainfall

The month of March was also a much wetter month than previous months when compared with the LTA (see Figure 2). The results show that all IIP stations received over 20% more than average rainfall in March 2017. However, Shannon Airport experienced double the amount of average rainfall and UC Galway received more than 1.5 times the average rainfall in March 2017.

Figure 2 October 2016-March 2017 monthly rainfall as a percentage of the long-term average rainfall of each month over the period 1850-2017.

SPI-6 results

Although, we experienced near or above normal rainfall in February and March most stations across Ireland are still showing some significant rainfall deficits over the past 6 months. According to the World Meteorological Organisation (WMO) SPI values between 0.99 and -0.99 are near normal conditions, while SPI values of -1.00 to -1.49 is moderate drought, -1.50 to -1.99 is severe drought, and less than -2.00 is extreme drought (WMO, 2012). The SPI-6 analysis (1850-2017) results suggest that ten IIP stations are currently experiencing moderate drought.  All stations except Shannon Airport and Valentia show less than -1.00 SPI-6 month accumulative anomalies for March 2017. The results also indicate that the IoI series (12 station average) is experiencing severe drought with SPI-6 values less than -1.50 for March 2017. 

Figure 3 Results at four IIP stations with SPI-6 values less than -1.50 for March 2017 indicating severe drought.

 Results also suggest that a severe drought is currently present at four IIP stations (Armagh, Markree, Athboy and UC Galway), with SPI-6 values that range from -1.66 to 1.83 for March 2017. During November and December 2016 four stations (Armagh, Markree, Athboy and Roches Point) experienced SPI-6 values less than -2.00 indicating extreme drought. Figure 3 presents the SPI-6 values for the past 15 months at the four stations (Armagh, Markree, Athboy and UC Galway) currently experiencing severe drought. The most striking result is the contrast between SPI-6 values for the first 4 months of 2016 and the SPI-6 values for first 4 months of 2017. Figure 4 map shows the extent of the current severe drought as indicated by the SPI-6 values for march 2017 at Armagh, Markree, Athboy and UC Galway.

Figure 4 Shows the area that is experiencing severe drought as at end of March 2017 based on the SPI-6 values calculated over the period 1850-2017. Blue shaded area indicates extent of current severe drought.

April rainfall.

Dryer conditions have returned for the month of April. So far up to the 12^th April rainfall has been very low across Ireland with the next 10 days also expected to be dryer than normal. All stations have only received less than 15% of LTA rainfall so far this month, except for Valentia which has received 25% of LTA rainfall. The station at UC Galway and Roches Point have seen only 4% of LTA rainfall during the first 12 days of April. Phoenix park, Dublin Airport and Shannon Airport have received less than 10% of LTA rainfall.

October 2016 -April 12th 2017 rainfall 

Figure 5 shows the current rainfall deficits from the start of the hydrological year from October 2016 up to April 12^th 2017 when compared with the LTA rainfall 1850-2017 for the same period. Most stations  have only received two thirds of the LTA rainfall up to 12^th April for the period October 2016 to April 2017.

Figure 5 Current rainfall deficits for October 2016 -April 12^th 2017 as a percentage of long term average for the months of October-April 1850-2016.

 Figure 6 shows the amount of rainfall needed before the end of April at each station in order bring the rainfall totals back to the LTA normal amounts. UC Galway and Cork Airport will need over 250 mm and Markree over 200 mm of rainfall over the next 18 days to bring the total rainfall to the normal LTA. In addition, Athboy, Malin Head, and Armagh will need over 150 mm and all the other the stations will need a further 100 mm by the end of this month to attain LTA rainfall totals.

Figure 6 Current rainfall deficits in (mm) for October 2016 -April 12^th 2017 of long term average for the months of October-April 1850-2016.

Final remarks.

The results of the drought monitoring indicate that some areas of Ireland (west, midlands and north-east) are currently experiencing drought conditions.This situation needs ongoing monitoring as many Irish rivers are groundwater dominated catchments and require winter rainfall to replenish storage. The results are suggesting that there are considerable rainfall deficits accumulated in many areas of Ireland due to a unusually dry winter. Therefore, if spring rainfall continues to be dry with below average rainfall certain regions of Ireland could be heading for a water shortages during the summer months.