Over the past 15 years a growing number of countries have invested in Reference Hydrometric Networks (RHNs) (Whitfield et al., 2012), to collect streamflow data that is minimally impacted by confounding human influences (Stahl et al., 2010). RHNs provide a more reliable basis for detecting climate variability and change and are increasingly employed in trend analyses (e.g., Lins and Slack, 1999; Douglas et al., 2000; Zhang et al., 2001; Burn and Hag Elnur, 2002; Yue and Wang, 2002; Hannaford and Marsh 2008; Hannaford and Buys, 2012). In addition, RHNs facilitate more focused and strategic investment in monitoring, increased understanding of hydrological change and a heightened awareness of the importance of long streamflow series for contextualising trends from recent decades.
As part of the HydroDetect project (Murphy et al., 2013a; Murphy et al., 2013b), the Irish Reference Network (IRN) of streamflow stations was identified by screening the national hydrometric network against a set of criteria based on best international RHN
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standards, most notably the UK Benchmark Network and is described in detail in Murphy et al. (2013a) and Murphy et al., (2013b). The key criteria for streamflow stations to be included in the IRN include:
Good and consistent hydrometric data quality (particularly at extreme flow ranges), as determined by hydraulic conditions at each site (stable control and accurate rating curves) and by expert judgement from hydrometric agencies.
Near natural flow regime - zero or stable water abstractions and discharges (impact less than 10% of flow at or in excess of the low flow parameter Q95, i.e. the 5th percentile).
Long record length (minimum 25 years).
Limited land-use influence (≤ 2.5 % of catchment area developed). Stations subject to arterial drainage were excluded where possible, otherwise post- drainage records were used if spatial coverage was lacking.
Stations must be representative of Irish hydrological conditions and climatic regions with good geographical coverage, ensuring that stations from each of the eight Water Framework Directive (WFD) River Basin Districts (RBDs) are included.
In some circumstances it was necessary for rules to be relaxed in order to capitalise on the existing network. For example, stations that have good hydrometric quality across the full flow regime are difficult to obtain (Hannaford and Marsh, 2008) particularly in terms of quality ratings at high and low flow extremes. Avoidance of arterial drainage was particularly challenging given such widespread installation across the island to improve agricultural land drainage and reduce flood risk. This has resulted in the deepening and widening of river channels to improve their discharge conveyance resulting in changes in catchment response (Bhattarai and O’Connor 2004). For Irish conditions Lynn (1981) notes an acceleration of catchment response to rainfall, with increased intensity of flood peaks and more rapid recessions post-drainage. This would clearly confound detection and attribution of trends. Where a station was found to be drained, the post drainage record was included for consideration and close attention paid to the comparison of trends from other non-drained stations to check for consistency. Only where there were no obvious inconsistencies with trends identified
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for non-drained stations were these stations included in the IRN. While attention was paid to ensuring homogeneity of records, a lack of metadata on land-use change means that such influences cannot be entirely ruled out. Similar challenges have been noted by Hannaford and Marsh (2008).
Stations from Northern Ireland that are part of the UK Benchmark Network (Bradford and Marsh, 2003; Hannaford and Marsh, 2006) were also included to facilitate an all- Island analysis. Thirty-five stations were identified for inclusion in the IRN plus a further 8 from the UK Benchmark Network (Murphy et al., 2013a; Murphy et al., 2013b). However, not all stations were used in this analysis. First, all IRN catchments that are nested (i.e. a smaller catchment contained within a larger catchment area) were removed as this could lead to misleading results through, for example, ‘double counting’ any statistically significant trends. This dropped the available stations from 43 to 32 non-nested catchments. Second, both the number and length of records for streamflow stations is much less than for precipitation, so selection of common periods to assess changes in both extreme precipitation and floods is limited to the number and length of record of streamflow stations. Only stations with greater than 30 years of data were selected for the analysis. This reduced the number of streamflow stations to 29.
Table 4.1 provides details of the 29 IRN station sub-set used here (note: the use of IRN henceforth also includes Northern Irish UK Benchmark catchments). The average record length is 41 years (minimum of 32 years and longest 63 years), with an average catchment size of 661 km2 (smallest 64.8 km2 and largest 2418.3 km2). All stations have data up to the end of 2009. Missing data is an important consideration for analysis of trends. Stations within the IRN sub-set have on average ~3.8 % of record missing. Stations have been infilled, where possible, using a hydrological modelling approach (see Murphy et al. (2013a) for details). The exceptions are the Northern Irish stations as it was not necessary due to the limited degree of missing data (< 1 %), and St26029 and St27002 (both < 2.5 % missing) due to the inability to fit an acceptable hydrological model to these stations. Visual inspections of St26019 and St27002 were made to ensure calculation of flood indices was not heavily impacted.
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Table 4.1: Station detials with flood relavent Physical Catchment Descriptors (PCDs) for 29 IRN catchments.
Station Station name River Easting Northing Org. ERR Start Year AREA (km2) QMED (m2 s-1) SAAR (mm) ALTBAR
(m a. s. l.) FLATWET FARL BFI RBMED
6013 Charleville Weir Dee 304411 290763 OPW IE 1977 309.1 25.97 873 82 0.60 0.971 0.617 0.165
6014 Tallanstown Weir Glyde 295298 297888 OPW IE 1977 270.4 21.47 928 82 0.62 0.927 0.634 0.112
12001 Scarawalsh Slaney 298380 145014 OPW IE 1956 1030.8 133.05 1167 160 0.54 0.999 0.716 0.179
14019 Levitstown Barrow 270623 187609 OPW IE 1955 1697.3 99.21 862 93 0.58 1 0.624 0.149
15006 Brownsbarn Nore 261699 139098 OPW IE 1973 2418.3 260.02 942 137 0.58 0.999 0.633 0.183
16009 Caher Park Suir 205297 122870 OPW CU 1955 1582.7 157.05 1079 140 0.59 0.998 0.667 0.140
18002 Ballyduff Blackwater 196410 99140 OPW CU 1956 2333.7 331.27 1201 166 0.62 0.999 0.622 0.215
19001 Ballea Owenboy 170971 63276 OPW CU 1973 103.3 19.15 1176 99 0.66 1 0.64 0.199
21002 Coomhola Coomhola 99825 54901 EPA SW 1976 64.8 42.30 2580 245 0.68 0.982 0.373 0.753
23002 Listowel Feale 99700 133295 OPW CU 1961 646.8 200.23 1346 197 0.63 1 0.312 0.511
25002 Barrington's Br. Newport 167908 154908 OPW CU 1954 221.6 40.57 1298 189 0.59 0.999 0.542 0.337
25006 Ferbane Brosna 211536 224406 OPW IE 1956 1162.8 76.50 936 87 0.63 0.955 0.708 0.129
25030 Scarriff Graney 164180 184277 OPW CU 1973 280 42.05 1185 135 0.61 0.85 0.542 0.146
26009 Bellantra Bridge Black 212848 289416 OPW IE 1973 90.5 11.43 1019 91 0.68 0.936 0.538 0.353
26021 Ballymahon Inny 216107 256987 OPW IE 1977 1098.8 64.75 945 88 0.66 0.807 0.828 0.068
26029 DOWRA Shannon 199064 326947 EPA CU 1976 116.9 45.82 1759 217 0.70 0.988 0.463 0.628
27002 Ballycorey Fergus 134431 180323 OPW CU 1955 511.4 34.61 1337 71 0.62 0.835 0.697 0.061
34001 Rahans Moy 124367 317782 OPW CU 1975 1974.8 178.66 1323 80 0.73 0.825 0.776 0.083
35005 Ballysadare Ballysadare 166832 329046 OPW CU 1947 639.7 70.47 1198 99 0.71 0.898 0.609 0.146
36010 Butlers Bridge Annalee 240817 310466 OPW IE 1972 771.7 66.47 968 122 0.66 0.861 0.632 0.129
38001 Clonconwal Ford Owenea 176584 392714 OPW CU 1973 111.2 40.76 1752 183 0.70 0.922 0.285 0.555
39006 Claragh Leannan 220215 420084 EPA CU 1978 245.1 47.99 1527 132 0.70 0.842 0.329 0.180
201005 Camowen Camowen Terr. 246071 373048 NI IE 1973 276.6 58.01 1144 157 0.64 0.989 0.514 0.391
201008 Castlederg Derg 226512 384216 NI CU 1977 335.4 117.30 1558 176 0.62 0.914 0.504 0.508
202002 Drumahoe Faughan 246411 415098 NI CU 1977 273.1 61.30 1219 188 0.61 1 0.426 0.425
203028 Whitehill Agivey 288337 419362 NI CU 1973 100.5 27.95 1270 181 0.61 0.999 0.404 0.550
204001 Seneirl Bridge Bush 294191 436250 NI CU 1973 299.2 52.96 1116 119 0.61 0.992 0.561 0.354
205008 Drumiller Lagan 323635 352493 NI IE 1975 84.6 21.80 1015 168 0.53 0.992 0.403 0.427
206001 Mountmill Bridge Clanrye 308536 331026 NI IE 1975 120.3 17.91 975 96 0.53 0.972 0.568 0.270
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As discussed in the previous chapter, the objective of deriving ERRs (Extreme Rainfall Regions) was borne from the need to understand the climatic drivers of floods. However, it is also recognised that streamflow is controlled by many more factors than just climate, and every catchment will translate inputs of precipitation into streamflow differently depending on physiographic characteristics. Figure 4.1 maps 6 flood relevant Physical Catchment Descriptors (PCDs) along with median flood magnitude (QMED) and streamflow flashiness (RBMED). The PCDs have been derived for catchments in Ireland as part of the recent Flood Studies Update (FSU; OPW, 2014) and the UK Flood Estimation Handbook (FEH; Institute of Hydrology, 1999) for Northern Ireland. PCDs for Ireland and Northern Ireland were derived from two separate projects, hence 4 of the 6 PCDs (SAAR, FLATWET, FARL, and BFISOILS) were based on similar, but not necessarily the same, data and methods. Figure 4.2 shows the relationship (based on the non-parametric Spearman’s rho correlation coefficient, 𝜌) between each of the six PCDs, QMED, and RBMED. Details on the calculation of these variables and their relevance to understanding floods are summarised below:
General descriptors - Catchment area (AREA), median flood magnitude (QMED), mean altitude (ALTBAR), and Standard-period Average Annual Rainfall (SAAR): Catchment area (AREA in km2) and mean catchment altitude (ALTBAR in m a. s. l.) were calculated using a 90m Digital Elevation Model (DEM) for the Island of Ireland. QMED is the median of the annual maximum streamflow calculated using Daily Mean Flows (DMFs) rather than instantaneous (15-min) peak flow data as only 22 of the 29 stations have instantaneous peak flows available. QMED is the magnitude of the 1 in 2 year flood, and as expected is very strongly positively correlated to catchment area (𝜌 = 0.83), that is, larger floods occur in larger catchments. SAAR is the Standard-period Average Annual Rainfall over the catchment for 1961-1990 in mm and catchments at higher altitude (ALTBAR) receive considerably more precipitation (𝜌 = 0.6) due to the effect of orography. For example, St21002 is the only catchment situated within the most extreme ERR in the southwest (ERR-SW) and has the largest SAAR (2580 mm) and ALTBAR (245 m a. s. l.)
Catchment wetness (FLATWET): Prior catchment wetness provides antecedent conditions for flooding, and determines how receptive soils are to absorbing precipitation by infiltration. This is captured using the FLATWET index (PROPWET used
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Figure 4.1: Maps of the 29 non-nested IRN catchments used in the analysis, 6 Physical Catchment Descriptors (PCDs), QMED-DMF, and RBMED.
for Northern Irish stations) derived using Soil Moisture Deficit (SMD) data. The index is defined as the proportion of time for which soils can be expected to be typically quite wet, and values represent the broad variation in soil wetness across the Island. While
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moderately positively correlated to SAAR (𝜌 = 0.47), it is not influenced by orographic precipitation (i.e. no correlation with ALTBAR, 𝜌 = -0.055), lending to the ‘FLAT’ in the naming of ‘FLATWET’. There is a clear spatial pattern to catchment wetness with the driest (FLATWET < 55 %) catchments distributed along the eastern seaboard, compared to catchments in the western half of the country that receive more reliable frontal precipitation and lower evapotranspiration rates. For example, while St12001 is within the coastal and upland ERR (ERR-CU) and receives a high annual precipitation receipt, it is located in the east so the prevailing SMD is larger and will inhibit flood generation more of the time.
Streamflow flashiness (RBMED), base flow index (BFISOILS), and flood attenuation by reservoirs and lakes (FARL): A catchments streamflow regime reflects all the components affecting a catchment such as the climate, topography, geology, soils, vegetation, catchment size (as shown above), as well as many non-natural influences. Flashiness is defined as the frequency and rapidity of short term changes in streamflow (Baker et al., 2004). This means catchments that translate precipitation quickly to streamflow are ‘flashier’ than those that tend to filter the inputs into the slower groundwater pathways and/or are attenuated by reservoirs and lakes. The baseflow index (BFI) is a widely used measure of catchment permeability. It is measured on a scale between 1 and 0 with a value close to 1 indicating a highly permeable catchment in which baseflow fed from groundwater is the dominant contributor to streamflow (low flashiness) and a value ~0.3 represents an impervious surface dominated catchment that will produce a more immediate rainfall-runoff response (high flashiness). Soil information was used during the FSU and FEH projects to generalise the BFI index for extrapolation to ungauged catchments (BFISOILS and BFIHOST, respectively). There are many ways of performing baseflow separation (defining the proportion of streamflow between peaks that is not attributed to storm precipitation), and no single accepted standard. However, when observations are available a remarkably simple measure of streamflow flashiness is the Richards-Baker Flashiness Index (RB Index; Baker et al., 2004). It is calculated by dividing the pathlength of flow oscillations for a time interval (i.e., the sum of the absolute values of day-to-day changes in DMF) by total streamflow during that time interval. Here, the RB Index was calculated on DMFs for each Water Year (1st October – 30th September) over the
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period 1978-2009. The median over this period is taken to give a single measure of streamflow flashiness (RBMED).
Figure 4.2 Scatterplot matrix of 8 FSU/FEH PCDs, QMED based on DMFs, and RBMED using Spearman’s Rho correlation.
Figure 4.3: Scatterplot of ALTBAR and RBMED (𝝆 = 0.81). Catchments with areas < 650 km2 and an average altitude > 150 m a. s. l. are highlighted in red.
The relationship between RBMED and BFI is physically consistent as they are very strongly negatively correlated (𝜌 = -0.84). Catchments with a high BFI have lower
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flashiness values. Interestingly, RBMED is also very strongly positively correlated with ALTBAR (𝜌 = 0.81) and strongly positively correlated with SAAR (𝜌 = 0.52), suggesting that floods in flashy catchments are driven by orography. There does appear however to be a physical threshold for streamflow flashiness based on catchment size, rather than a linear relationship. Catchments with an area above the average (661 km2) have an RBMED < 0.25, whereas smaller than average catchments have an RBMED between 0.061 and 0.753. Hence, large catchments located even within ERR-CU (e.g. St34001), or at high elevations, do not have a flashy response; essentially large precipitation inputs are dampened with increased catchment area. It is not obviously clear then what controls the degree of flashiness in less than average sized catchments. With further investigation it was found that high streamflow flashiness (RBMED > 0.4) occurred only in smaller (< 650 km2) upland (ALTBAR > 150 m a. s. l.) catchments (Figure 4.3). Finally, catchments with reservoir and lake attenuation (FARL) tend to be wetter (FLATWET), lowland catchments (ALTBAR) with less flashy streamflow response (RBMED).