Welcome to the Hydro-Data Web-Site
provided by the
Office of Public Works

1. INTRODUCTION

There is a range of hydrometric data types, including water levels, estimated flows and summary statistics, available from almost 300 locations on rivers and lakes around the Republic of Ireland.

If you require data at a location that is not available on this Web-Site, then you might contact the Environmental Protection Agency (EPA - www.epa.ie) which also maintains a database of hydrometric information from stations at other locations.

It is important that you read carefully all of the notes provided below. Having read the information provided, you may proceed to viewing and downloading the data by acknowledging the notes and agreeing to certain conditions of use by clicking the ‘Yes’ button provided at the bottom of the page.

If you have any queries or comments concerning the use of the site or the data provided, please contact our staff in the Hydrology and Hydrometric Section of the OPW, by Email at hydrometric@opw.ie, or by telephone on: 093 35456 / 35603.

To help us to continue to improve the quality and standard of the service we provide to you, we would be grateful if, after using the Hydro-Data web-site, you could complete the on-line questionnaire to provide us with some feedback. This can be accessed via a hyper-link at the top of the Search Query, Map-Finder and data pages.

2. HOW TO USE THE SITE

Once you have read the notes below and agreed to the conditions of use of the data, you will be able to view or download data by first selecting a particular hydrometric station, and then specifying the type of data and period for which it is required.

To select a station you will need to enter the station name or number on the Station Selection Page. If this is not known, you will be able to search for the relevant station using the map-finder to locate your site graphically, or use the search and query options provided to find relevant sites by river name, county, nearby towns or villages.

Once the station has been selected, you will need to specify the data parameter required (water level or flow), the data type required (e.g., summary statistics or daily mean values) and the period of record required. You will then be able (depending on the data type selected) to view and print a graph, to view and print a report, or to download the data to a textfile. Please note that the last of these options may take some time depending on the period of record selected and the speed of your Internet connection.

3. DATA COLLECTION PROCESS

3.1 Water Level Recording

The water level data is collected using either autographic recorders or data loggers.

An autographic recorder provides a continuous trace of water level on weekly or monthly charts (piece of graph paper approximately A3 in size) using a float and weight system which moves a pen on a spindle via a pulley and gearing mechanism. The water level trace on the charts is then digitised to provide a ‘time-series’ of water level against time.

A data logger is a more recent method of recording water level. The OPW use these devices to digitally record the water level at set time intervals using either a float system (as described above) or a pressure transducer.

The water levels are recorded as water depths (stage) above a staff gauge zero level. A staff gauge is a graduated plate that allows our Field Technicians to manually check that the correct water level is being displayed on the recording devices, and hence validate the data recorded. The zero level of the staff gauges have been surveyed to either Poolbeg or Malin Head Ordnance Datum levels, and these heights are added to the recorded levels (stage) to provide water levels to an Ordnance Datum.

The water level data provided on this Web-Site are given to either Poolbeg or Malin Head datum levels. It is important that you check which datum level is being used for each site accessed.

The equipment can malfunction or breakdown, and there are therefore gaps in the records provided. These are indicated by a certain quality code (see below), and must be considered when evaluating the data, and in particular peak or low water levels or flows.

3.2 Flow Estimation

The OPW does not continuously record river flow, but rather estimates it from ratings (stage-discharge relationships) and the recorded water levels.

A rating is a relationship that equates a given water level to a given flow at a particular location. It is developed from a series of flow gaugings taken at the monitoring sites (see below). By applying the rating to the water level time-series it is possible to estimate a flow time-series which can then be manipulated to provide data such as daily mean flows, or flow duration statistics.

During a flow gauging, the velocity is measured at a series of points across the river from which the flow is calculated for the water level at that time. This is the velocity-area method of measurement (see ISO748 / BS3680). A flow gauging is hence a spot calculation of flow (discharge) for a given level (stage). Using a series of such gaugings over a range of water levels, it is possible to derive an equation for the line-of-best-fit through the scatter of gaugings, which effectively describes the relationship at that location between the stage and discharge, i.e., a stage-discharge relationship, or rating as they are commonly known.

A fundamental assumption of the use of ratings to generate flow estimates is that the relationship between stage and discharge remains constant in time. This is not always the case. Seasonal weed-growth, sedimentation, erosion, drainage, variable structures (such as locks or sluices) or construction work in the channel or floodplain can all change the control that determines level for a given flow. Other water bodies downstream, such as lakes, larger rivers and indeed the sea can backwater the monitoring site and so the level is dependent on factors other than the flow at the gauging location. At fully tidal sites, the level is generally independent of the flow and so there is effectively no relationship between level and flow. These effects and phenomena have a number of impacts:

• it is not possible to derive an adequate rating at some stations, and so while water levels are recorded and provided on this Web-Site, flow data will not be available at all stations,
• the ratings at most sites can change with time, and so ratings can therefore be extrapolated in time in the absence of recent flow gaugings, and might be changed retrospectively by the OPW should further data come to light, and so flow estimates for a given date can be
subject to revision,
• ratings at stations subject to frequent but minor changes in control can have a significant degree of scatter of gauging points around the rating, and hence a high margin of error, and so estimated flow data at these sites should be treated with caution (see quality codes below).

It is important to remember that the flow data provided is estimated and can contain a significant degree of uncertainty due to the scatter in flow gaugings around the rating equation and changes of control in time. The flow data provided should therefore only be used with due consideration given to the above warnings and the quality codes attached.

You might note that a range of flow data statistics (low flow statistics in particular) for a number of stations around Ireland are available from the EPA website.

4. DATA TYPES

4.1 Average Annual Mean Values (Summary Statistics)

These values are the mean values of flow or water level as calculated over a period of full years.

It should be noted that the values might be skewed under the following circumstances:

• the period of record was abnormally wet or dry,
• significant gaps in the records have occurred during low or high flow periods,
• inappropriate extrapolation of ratings has been used (flow data only).

These are values of flow or water level that are equalled or exceeded for the given percentage of time.

It should be noted that these values can be skewed for the same reasons as those given under 4.1, and for extreme percentiles, the last of the reasons given is particularly relevant.

4.3 Daily Mean Values

These are the mean values of flow or level as calculated for each day.

4.4 Annual Maxima Values

These are the highest recorded water levels or estimated flows in each available hydrometric year of record. A hydrometric year runs from 1^st October in the given year to the 30^th September the following year, i.e., the hydrometric year 2000 runs from 1^st October 2000 to 30^th September 2001.

As noted above, there are gaps in the record due to recorder malfunction, missing charts, etc. It is therefore possible that the water levels (and estimated flows) in any given year may have exceeded the maximum value given for that year, and indeed that the maximum given for the period of record may have been exceeded during that period.

The majority of estimated annual maxima flows have been derived from extrapolated ratings (see 3.2). Where this is the case, the data may be erroneous and should be treated with caution. Note however that quality codes are not attached to annual maxima data, and reference should therefore be made to the ‘reliable limit’ given in the notes under each series.

5. DATA

Quality codes provide an indicative measure of the reliability and confidence placed on each item of data, and can appear numerically or as a symbol, depending on the format of the data. It should be noted that the majority of the codes are allocated following certain criteria but are subjective, and are not associated with any given confidence limits in terms of percentage, absolute value or standard error. The codes should therefore be used only as an indication of the reliability of the data, and must not be used in any way as justification to waive or forgo any independent verification process appropriate to the proposed use of the data.

5.2 Table of Quality Codes

Code	Symbol	Description
WATER LEVEL DATA
1	*	Unchecked digitised water level data – Data is provisional only and must be used with caution
31		Inspected water level data – Data may contain some error, but has been approved for general use
32	C	As per Code 31, but where the digitised water level data has been corrected
99	*	Unchecked imported water level data – Data is provisional only and must be used with caution
145	<	Data is below prescribed data range and must only be used with caution
146	>	Data is above prescribed data range and must only be used with caution
150	I	Partial statistic – Data has been derived from records that are incomplete and do not necessarily represent the true value
101	!	Unreliable water level data – Data is suspected of being erroneous or is artificially affected (e.g., during drainage works) and must only be used with caution
>150	Various	Data is not available as it is missing, erroneous or of unacceptable quality
ESTIMATED FLOW DATA
31		Flow data estimated using a rating curve that it is considered to be of good quality and inspected water level data – Data may contain some error, but is considered to be of acceptable quality for general use
32	C	As per Code 31, but using water level data of Code 32
36	F	Flow data estimated using a rating curve that it is considered to be of fair quality and inspected or corrected water level data – Data may contain a fair degree of error and should therefore be treated with some caution
46	B	Flow data estimated using a rating curve that it is considered to be of poor quality and inspected or corrected water level data – Data may contain a significant degree of error and should therefore be used for indicative purposes only
56	X	Flow data estimated using an extrapolated rating curve (see Section 3.2) and inspected or corrected water level data – Reliability of data is unknown and it should therefore be treated with caution
99	*	Flow data that has been estimated using unchecked water level data – Data is provisional only and must be used with caution
101	!	Flow data that has been estimated using unreliable water level data – Data is suspected of being erroneous and must only be used with caution
145	<	Data is below prescribed data range and must only be used with caution
146	>	Data is above prescribed data range and must only be used with caution
150	I	Partial statistic – Data has been derived from records that are incomplete and do not necessarily represent the true value
>150	Various	Data is not available as it is missing, erroneous or of unacceptable quality

Please note that quality codes are only associated with daily mean values text files and reports, and will be displayed either as a code (text files) or a symbol (reports). The quality codes are not displayed on hydrograph plots, and so reference should be made to either text files or reports for an indication of data quality. The reliability of annual maxima and summary statistic data is indicated on the relevant page.

5.3 Independent Verification

The operations of the Hydrology and Hydrometric Section of the OPW are quality assured and have been certified under ISO 9001: 2000 by the National Standards Authority of Ireland. While the quality management procedures include quality control of data, this does not guarantee accuracy, and you must undertake any independent quality control and verification procedures appropriate to the proposed use of any data taken from this Web-Site. To this end, unprocessed and source data are available for inspection on prior arrangement at our Dublin offices. To arrange to inspect the data, you can contact our staff using the Email address or telephone numbers given under section 1 above.

6. USERS DECLARATION

I have read in full, understand and accept all of the above notes and warnings concerning the source, reliability and use of the data available on this Web-Site.

I agree that the Commissioners of Public Works have the absolute right to reprocess, revise, add to, or remove any data available on this Web-Site as they deem necessary, and that I will in no way hold the Commissioners of Public Works liable for any damage or cost incurred as a result of such acts.

I will use any data available on this Web-Site in an appropriate and responsible manner and in accordance with the above notes, warnings and conditions.

I understand that the Commissioners of Public Works do not guarantee the accuracy of any data available on this Web-Site, and it is my responsibility to independently verify and quality control any of the data used and ensure that it is fit for use.

I will not pass on any data used from this Web-Site to any third party without ensuring that said party is fully aware of the notes, warnings and conditions of use.

I accept all responsibility for the use of any data available on this Web-Site that is downloaded, read or interpreted or used in any way by myself, or that is passed to a third party by myself, and will in no way hold the Commissioners of Public Works liable for any damage or loss howsoever arising out of the use of interpretation of this data.