SIST EN 17277:2019

SLOVENSKI STANDARD
oSIST prEN 17277:2018
01-september-2018
+LGURPHWULMD0HULOQH]DKWHYHLQUD]YUVWLWHYLQVWUXPHQWRY]DPHUMHQMHPRþL
SDGDYLQ
Hydrometry - Measurement requirements and classification of rainfall intensity
measuring instruments
Messung der Regenintensität - Messbedingungen und Klassifizierung für auffangende
Regenmesser
Ta slovenski standard je istoveten z: prEN 17277
ICS:
07.060 Geologija. Meteorologija. Geology. Meteorology.
Hidrologija Hydrology
oSIST prEN 17277:2018 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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oSIST prEN 17277:2018

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oSIST prEN 17277:2018


DRAFT
EUROPEAN STANDARD
prEN 17277
NORME EUROPÉENNE

EUROPÄISCHE NORM

August 2018
ICS 07.060
English Version

Hydrometry - Measurement requirements and
classification of rainfall intensity measuring instruments
 Messung der Regenintensität - Messbedingungen und
Klassifizierung für auffangende Regenmesser
This draft European Standard is submitted to CEN members for enquiry. It has been drawn up by the Technical Committee
CEN/TC 318.

If this draft becomes a European Standard, CEN members are bound to comply with the CEN/CENELEC Internal Regulations
which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.

This draft European Standard was established by CEN in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC
Management Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and United Kingdom.

Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are
aware and to provide supporting documentation.

Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change without
notice and shall not be referred to as a European Standard.


EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2018 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN 17277:2018 E
worldwide for CEN national Members.

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Contents Page
European foreword . 3
Introduction . 5
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 6
4 User requirements for RI measurements . 9
5 Measurement of RI . 9
5.1 General. 9
5.2 RI measurement accuracy . 9
5.3 Types of rain gauge . 10
6 Classification of RI gauges . 11
6.1 Criteria for rain gauge classification. 11
6.2 Check of the balancing of the buckets . 14
6.3 Consistency of information . 15
6.4 Characteristics of the device used to generate the equivalent reference intensity . 15
6.4.1 Overview . 15
6.4.2 Metrological characteristics . 15
6.5 Testing procedure . 15
6.5.1 Dynamic calibration . 15
6.5.2 Step response (time constant) for weighing rain gauges . 16
6.5.3 Balancing of the buckets . 16
6.5.4 Re-calibration. 17
6.5.5 Verification of the calibration in the field . 18
6.5.6 Characteristics of the Class Attribution Certificate . 18
Bibliography . 19

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European foreword
This document (prEN 17277:2018) has been prepared by Technical Committee CEN/TC 318
“Hydrometry”, the secretariat of which is held by BSI.
This document is currently submitted to the CEN Enquiry.
The Executive Council of the World Meteorological Organization (WMO), noting the working
arrangements between the ISO and WMO formally adopted on 16 September 2008, recognized the
wide-ranging benefits to National Meteorological and Hydrological Services and user communities
resulting from the implementation of common Standards relevant for meteorology and hydrology. This
included the need to establish the benefit/cost implication to WMO Members of elevating an existing
Technical Regulation/Manual/Guide to a common Standard. The EC finally approved procedures to be
followed in proposing common technical standards (Resolution 8, Abridged Final Report of the sixty-
first session of the WMO Executive Council).
The WMO Commission on Instruments and Methods of Observation during its fifteen session (2010)
encouraged the development of standards related to rainfall intensity measurements (Final Report of
the fifteen Session of CIMO, par. 4.16, 2-8 September 2010, Helsinki, Finland).
The Commission for Hydrology (CHy) of the WMO recalled the importance of maintaining links with
relevant regional standardization bodies, such as the European Committee for Standardization (CEN), in
order to contribute to the development of quality management framework objectives and for the
benefit of all WMO Members (Abridged Final Report, CHy-14, par. 6.13, Geneve, Switzerland, 6-12
November 2012). The CHy made a specific note of a European initiative to develop standards and
guidance material on rainfall intensity and metrological aspects, at the level of CEN/TC 318
Hydrometry, and requested that the products and information be shared with English-speaking
member countries of the Commission (Abridged Final Report, CHy-14, par. 7.14, Geneve, Switzerland, 6
- 12 November 2012).
This document is a standard focusing on the accuracy of the measurement of rainfall intensity (RI). It is
developed from existing European standards and technical reports and through their harmonization:
CEN/TR 16469:2013 (Measurement of the rainfall intensity: requirements, calibration methods and field
measurements), the Italian standard UNI 11452:2012 (Hydrometry - Liquid precipitation intensity:
measurements requirements and calibration methods for catching-type gauges), and the British Standard
BS 7843-3:2012 (Code of practice for the design and manufacture of storage and automatic collecting
rain gauges). The concepts expressed in this document about RI measurement requirements and/or
instrument calibration also takes into account the findings from the international RI gauge
intercomparison organized by the WMO. Specifically, the calibration procedures described in this
standard have been approved and adopted for rain gauge calibration by the CIMO, as reported in the
regulatory guide WMO no.8 ed. 2008 updated 2010 (otherwise referred as “CIMO Guide”).
This document has been prepared in collaboration with the WMO Lead Centre on Precipitation
Intensity “B. Castelli” (Italy) which has been designated by the WMO Commission of Instruments and
Methods of Observation (CIMO General Summary of the fifteen Session, Helsinki, Finland, 2 - 8
September 2011) to consider the requirement for general standardization and homogeneity of
precipitation intensity measurements. The Lead Centre is a Centre of Excellence for instrument
development and testing, established with the purpose of providing:
— guidance and standard procedures for rain gauge calibration;
— the achievable calibration uncertainty;
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— laboratory and field tests;
— the intercomparison of instruments;
— and technical development for the measurement of precipitation intensity and the related data
analysis and interpretation.
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Introduction
Precipitation gauges are one of the basic components of world hydro-meteorological networks. Liquid
atmospheric precipitation is a fundamental variable of the natural hydrological cycle. A requirement for
increasingly more accurate and reliable measurements is becoming crucial for water resources
management, public safety and disaster mitigation. Consistent rainfall intensity (RI) measurements
contribute throughout Europe for improvement in the mitigation of hydrological risk and flooding
events, flood warnings and the analysis of climatic variations. This standard provides a consistent
process for classification of catching-type rainfall intensity gauges throughout Europe.
The intercomparisons of RI gauges organized by WMO [12, 16] recommended that RI measurements
should be covered by International Standards. These standards should be developed based on the
results of that work and other research and good practice. The focus should be on standard procedures
for carrying out calibration tests in the laboratory and on a classification of the instrument performance
to help users select the most appropriate rain gauges for their specific applications.
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1 Scope
This document considers liquid atmospheric precipitation and defines the procedures and equipment
to perform laboratory and field tests, in steady-state conditions, for the calibration, check and
metrological confirmation of liquid precipitation measurement instruments. It provides a classification
of catching-type measurement instruments based on their laboratory performance. The classification
does not relate to the physical principle used for the measurement, nor does it refer to the technical
characteristics of the instrument assembly, but is solely based on the instrument calibration.
Attribution of a given class to an instrument is not intended as a high/low ranking of its quality but
rather as a quantitative standardized method to declare the achievable measurement accuracy in order
to provide guidance on the suitability for a particular purpose, while meeting the user’s requirements.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
CEN/TR 16469:2013, Hydrometry — Measurement of the rainfall intensity (liquid precipitation):
requirements, calibration methods and field measurements
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— IEC Electropedia: available at http://www.electropedia.org/
— ISO Online browsing platform: available at http://www.iso.org/obp
3.1
precipitation (snowfall and rainfall)
Precipitation is defined as the liquid or solid product of the condensation of water vapour falling from
clouds or deposited from air onto the ground. It includes rain, hail, snow, dew, rime, hoar frost and fog
precipitation
The total amount of precipitation that reaches the ground in a stated period is defined “rainfall” when
precipitation is liquid and “snowfall” when the precipitation is snow
Rainfall (total amount of liquid precipitation) is expressed in terms of the vertical depth of water
(usually in millimetres, mm) to which it would cover a horizontal projection of the Earth’s surface
Snowfall (total amount of snow) is expressed in terms of the vertical depth of water equivalent to which
it would cover a horizontal projection of the Earth’s surface. Snowfall is also expressed by the depth of
fresh, newly fallen snow covering an even horizontal surface
[SOURCE: WMO no.8 “CIMO Guide” Part I Chap. 6 new edition 2014]
3.2
rainfall intensity (RI)
Rainfall intensity (RI) is defined as the amount of liquid precipitation (rainfall) collected per unit time
interval. Due to its variability from minute to minute, RI is measured or derived (from the measurement
of the amount) over 1 minute time intervals and the measurement units are vertical depth of water per
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−1
hour, usually in millimetres per hour or mm h . The RI is derived or measured directly using only
rainfall intensity gauges (see definition 3.4)
[SOURCE: WMO no.8 “CIMO Guide” Part I Chap. 6 new edition 2014]
3.3
catching type rain gauge
a rain gauge which collects precipitation through an orifice, often a funnel, of well-defined size and
measures its water equivalent, volume, mass or weight that has been accumulated in a certain amount
of time
Note 1 to entry: This type of gauge includes storage, level monitoring, tipping bucket and weighing rain gauges.
These are the most common type of recording rain gauge in use in operational networks at the time of preparing
this text.
3.4
rainfall intensity gauge (RI gauge)
automatic recording rain gauge which measures RI at a resolution of at least one minute
3.5
delay time of the output of a RI gauge
possible time delay between the output signal of a RI gauge and the time when the measurement was
performed
Note 1 to entry: This delay is usually due to internal calculations of the rain gauge.
Note 2 to entry: The internal calculation of the rainfall intensity in some rain gauges can cause a delay of the
output data message (e.g. 1 min) that can easily be shifted automatically to the correct time without any
degradation in measurement accuracy. This is typical of software corrected tipping bucket rain gauges through
embedded electronic chips or interfaces. The delay time should not be confused with the time constant. If real-
time output is not needed, software induced delay times are less critical than longer time constants or any other
effects, because delay times can easily be corrected to retrieve the original RI information.
[SOURCE: WMO IOM – 99]
3.6
measurand
quantity intended to be measured
[SOURCE: VIM 3rd edition, JCGM 200:2012]
3.7
measurement uncertainty
non-negative parameter characterizing the dispersion of the quantity values being attributed to a
measurand, based on the information used
[SOURCE: VIM 3rd edition, JCGM 200:2012]
Note 1 to entry: The parameter may be, for example, a standard deviation called standard measurement
uncertainty (or a specified multiple of it), or the half-width of an interval, having a stated coverage probability.
Note 2 to entry: Measurement uncertainty comprises, in general, many components. Some of these may be
evaluated by Type A evaluation of measurement uncertainty from the statistical distribution of the quantity values
from series of measurements and can be characterized by standard deviations. The other components, which may
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be evaluated by Type B evaluation of measurement uncertainty, can also be characterized by standard deviations,
evaluated from probability density functions based on experience or other information.
Instrumental measurement uncertainty (VIM 3rd edition, JCGM 200:2012): component of measurement
uncertainty arising from a measuring instrument or measuring system in use
Instrumental uncertainty is used in a Type B evaluation of measurement uncertainty
Achievable measurement uncertainty (WMO no. 8, Part I Annex 1.B): it is intended as the measurement
uncertainty achievable in field and/or operational conditions
3.8
non-catching rain gauge
rain gauge where the rain is not collected in a container/vessel
Note 1 to entry: The rainfall intensity or amount is either determined by a contact-less measurement using
optical or radar techniques or by an impact measurement. This type of gauge includes optical disdrometers,
impact disdrometers, microwave radar disdrometers, optical/capacitive sensors.
3.9
resolution
smallest change in a quantity being measured that causes a perceptible change in the corresponding
indication
[SOURCE: VIM 3rd edition, JCGM 200:2012]
3.10
step function or unit step function
input signal that switches on at a specified time and stays switched on indefinitely for determining the
response (output) of a dynamic instrument system
[SOURCE: CEN/TR 16469:2013]
3.11
step response
time-varying response of an instrument system to a step function (heaviside step function)
[SOURCE: CEN/TR 16469:2013]
3.12
step response time
duration between the instant when an input quantity value of a measuring instrument or measuring
system is subjected to an abrupt change between two specified constant quantity values and the instant
when a corresponding indication settles within specified limits around its final steady value
[SOURCE: VIM 3rd edition, JCGM 200:2012]
3.13
time constant
rise time characterizing the response of an instrument classified as a system of first order response (the
way the system responds is approximated by a first order differential equation)
Note 1 to entry: It represents the time that the step response of an instrument system takes to reach the (1–
1/e)•100[%] approximately 63 % of the final or asymptotic value.
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[SOURCE: CEN/TR 16469:2013]
4 User requirements for RI measurements
This standard defines three classes of RI gauges according to their calibration. The standard will
describe the three classes, the laboratory calibration methods and the requirements for checking the
calibration in the field. The user shall determine what class of rain gauge to use for any given purpose,
based on the local hydro-geological and meteorological conditions. The network/instrument manager
shall declare the classification at the applicable RI ranges. Data from unclassified rain gauges shall be
used with caution.
5 Measurement of RI
5.1 General
Rainfall intensity (RI) is defined as the amount of liquid precipitation (rainfall) collected per unit time
interval. Due to its variability from minute to minute, there is an agreement of measuring RI over 1 min
time intervals and then RI in mm/hour is derived from the measurements taken in 1 min. RI is
measured directly using rainfall intensity gauges, for instance, using a gauge and measuring the flow of
the captured water, or the increase in collected water as a function of time. A number of measurement
techniques for the determination of the amount of precipitation are based on these direct intensity
measurements by integrating the measured intensity over a certain time interval.
Traditionally, the volume of liquid precipitation received by a collector through an orifice of known
surface area in a given period of time is assumed as the reference quantity, namely the rainfall amount.
Under the restrictive hypothesis that rainfall is constant over the accumulation period, a derived
quantity – the rainfall rate or intensity – can be easily calculated. The shorter the time interval used for
the calculation, the nearer to the real rate of precipitation reaching the ground. This approximate
measure of the rainfall intensity has been accepted for a long time as sufficiently accurate to meet the
requirements of both scientific and technical applications. Reasons for this are on the one hand that
most traditional applications in hydrology operate at the basin scale, thus dealing with a process of
rainfall aggregation on large space and time scales, while on the other hand the available technology of
measurement instruments, especially in terms of data storage and transmission capabilities, was lower
than is currently available.
Rainfall data requirements have become tighter and applications increasingly require enhanced quality
in RI measurements. The interpretation of rainfall patterns, rainfall event models and forecasting
efforts, everyday meteorological and engineering applications, etc., are all based on the analysis of
rainfall intensity arrays that are recorded at very fine intervals in time. The importance of RI
measurement is dramatically increased and very high values of RI are recorded, due to the shortening
of the reference period.
It is worth noting that the time scales required for calculation of RI at the ground are now much shorter
than in traditional applications. The design and management of urban drainage systems, flash flood
forecasting and mitigation, transport safety measures, and in general most of the applications where
rainfall data are sought in real-time, call for enhanced resolution in time (and space), even down to the
scale of one minute in many cases (1-MIN RI).
5.2 RI measurement accuracy
According to [17], the WMO “CIMO Guide” (Annex 1.D), the following values of expanded uncertainty
apply for precipitation intensity (liquid) measurements, in laboratory (calibration in constant flow
conditions) and in field conditions:
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Table 1 — Uncertainty of precipitation measurements according to WMO
Under constant flow conditions in laboratory 5 % above 2 mm/h
2 % above 10 mm/h
In field conditions 5 mm/h, and
5 % above 100 mm/h

The definitions introduced by the WMO and the corresponding values of the maximum acceptable
measurement uncertainties are adopted by this standard and, therefore, they shall be taken into
consideration for any catching type RI gauge.
The compliance to this standard does not include further sources of instrumental errors such as
sampling errors in tipping-bucket rain gauges.
5.3 Types of rain gauge
Rain gauges can be categorized in two main groups: (a) catching, and (b) non-catching types of rainfall
intensity measurement instruments ([16]). Gauges of the first group collect precipitation through an
orifice of well-defined size and measure its water equivalent volume, mass or weight that has been
accumulated in a certain amount of time. At present, catching type gauges are widely used in
operational hydro-meteorological networks to measure rainfall amount and intensity. Instruments of
the second group determine the rainfall amount or intensity either by a contactless measurement using
optical or radar techniques or by an impact measurement. A standardized procedure for the calibration
of non-catching rain gauges is not yet available.
Catching type rain gauges can be characterized as follows:
— they can be calibrated in the laboratory;
— they are able to measure RI within sampling time intervals ranging from a few seconds to several
minutes;
— they have finite resolution ranging from 0,001 mm to 1 mm;
— they have reasonably good reproducibility and long-term stability;
— they are widely used in operational practice and are cost effective;
— they are prone to wind-induced catching losses (depending on appropriate wind shielding);
— they are prone to wetting and evaporation losses, especially in low RI;
— regular maintenance, annual calibration and servicing, is needed to obtain high quality
measurements.
The majority of catching type gauges used in operational networks are weighing gauges (WGs) and
tipping bucket rain gauges (TBRGs) (see [16] for details).
In weighing gauges, precipitation is collected and continuously weighed. The WGs are those
instruments where the volume of water is derived by using the gravitational acceleration and the
density of water. These rain gauges do not use any moving mechanical parts in the weighing
mechanism, only elastic deformation occurs. Therefore, mechanical degradation and consequently the
need for maintenance are significantly reduced. The weighing is accomplished by various methods, e.g.
a frequency measurement of a string suspension, a strain gauge, or load cells measuring collected
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precipitation as change of measured weight increase by method of Wheatstone bridge. The digitized
output signal is generally averaged and filtered.
A tipping bucket rain gauge uses a metallic or plastic twin bucket balance to measure the incoming
water in portions of equal weight. When one bucket is full, its centre of mass is outside the pivot and the
balance tips, dumping the collected water and bringing the other bucket into position to collect. The
water mass content of the bucket is constant (m [g]), therefore by using the density of water
3 3
(ρ = 1 g/cm ) the corresponding volume (V [cm ]) is derived from the weight of the water and,
consequently the corresponding precipitation amount is retrieved in terms of vertical depth (h [mm])
2
by using the surface of the area collector (Ω [cm ]). The equation will be: V = mass/ρ = h*Ω and, by using
2
the density of water, h is calculated, where 1mm corresponds to 1g of water over 10 cm of surface.
Recently, the operational networks, specifically those with AWS – Automatic Weather Stations, have
been increasingly equipped with non-catching type gauges, such as optical or disdrometers (see [16] for
details on such gauges).
6 Classification of RI gauges
6.1 Criteria for rain gauge classification
Rain gauges shall be attributed a suitable class, based on the maximum observed deviations of the
measurement with respect to a known, constant reference intensity at the temporal resolution of one
minute. For weighing type gauges the time constant of the instrument shall be contained within the
same one minute interval.
The reference to be used in assessing the instrument’s performance shall be a known steady rate
continuous/discontinuous volumetric flow of purified water named “reference flow rate”, Q . This is
ref
described further in part 6.4. Each level, or state, is equivalent to a reference liquid precipitation
intensity, which depends on the physical characteristics of an individual rain gauge, i.e. on the surface
area of the catching device, or collector, Ω. The reference equivalent rainfall intensity I associated with
ref
a given value Q is obtained as:
ref
IQ= /Ω (1)
ref ref
Within the range of intensity values appropriate to a given Class, the performance of RI gauges can be
determined by calculating the percentage relative deviation, e [%], that shall be calculated as:
rel
II−
mis ref
e [%] ⋅ 100 (2)
rel
I
ref
where
I is the measured liquid precipitation intensit
...