Reference number  
RS 188: 2018  
© RSB 2018  
 
 
 RWANDA 
STANDARD  DRS  
188 
 
Second  edition  
2018 -mm-dd 
(Reaffirmed yyyy)  
Water quality — Irrigation water — Tolerance 
limits  
 
 
  DRS  188: 2018  
©RSB 2018  - All rights reserved                  ii 
 In order to match with technological development and to keep continuous progress in industries, 
standards are subject to periodic review. Users shall ascertain that they are in possession of the latest 
edition  
 
 
 
  
 
 
 
 
© RSB 2018  
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized 
in any form or by any means, electronic or mechanical, including photocopying and microfilm, without 
prior written permission from R SB. 
Requests for permission to reprodu ce this document should be addressed to : 
Rwanda Standards  Board  
P.O Box 7099 Kigali -Rwanda  
KK 15 Rd, 49  
Tel. +250 788303492  
Toll Free: 3250  
E-mail: info@rsb.gov.rw  
Website: www.rsb.gov.rw   
ePortal: www.portal.rsb.gov.rw   DRS  188: 2018  
iii                ©RSB 2018  - All rights reserved  
 Contents  Page 
Foreword  ................................ ................................ ................................ ................................ ...................  iv 
1 Scope  ................................ ................................ ................................ ................................ .............  1 
2 Normative references  ................................ ................................ ................................ ...................  1 
3 Terms and definitions  ................................ ................................ ................................ ..................  1 
4 Requirements  ................................ ................................ ................................ ................................  2 
4.1 Requirements for irrigation water quality  ................................ ................................ ..................  2 
Annex  A (normative)  Precautions for sampling  ................................ ................................ ....................  3 
Annex  B (normative)  Recommended maximum concentrations of trace e lements in irrigation 
water  ................................ ................................ ................................ ................................ ..............  4 
Annex  C (normative)  Irrigation water quality and wastewater re -use ................................ .................  5 
 DRS  188: 2018  
©RSB 2018  - All rights reserved                  iv 
 Foreword   
Rwanda Standard s are prepared  by Technical Committees and approved by R wanda Standards Board (RSB) 
Board  of Directors in  accordance with the procedures of RSB, in compliance wit h Annex 3 of the WTO/TBT 
agreement on the preparation, adoption and application of standards.  
The main task of technical committees is to prepare national standards.  Final Draft Rwanda S tandards adopted 
by Technical committ ees are ratified  by me mbers of RSB Board of Director s for publication and gazettment as 
Rwanda Standards.  
DRS S 188 was prepared by Technical Committee RBS/TC 013, on Water and Sanitation .  
In the preparation of this standard, reference was made to the following standards:  
MS 714:2005 Occupational safety and health management systems — Specification   
BS 18001:2007 Occupational health and safety management systems — Requirements  
RS ISO 9001:2008 Quality management systems — Requirements  
The assistance derived from the above source is here by acknowledged with thanks.  
This second edition cancels and replaces the first. edition (RS  188: 2013), which has been technically revised  
  DRS  188: 2018  
v                ©RSB 2018  - All rights reserved  
 Committee membership  
The following organizations were represented on the Technical Committee on Water and Sanitation  (RSB/TC 
013) in the preparation of this standard.  
AQUASAN Ltd  
ELECTROMAX  
Entreprise U RWIBUTSO  
Inyange Industries Ltd  
JIBU Corporate  
Ministry in charge of Emergency management  
ROTO Tank Ltd  
RULIBA Clays  
Rwanda Mines, Petroleum and gas Board  (RMB)  
Rwand a Utility Regulatory Authority ( RURA ) 
SONATUBE  
SULFO Industries  
University of Rwanda ( UR-CST) 
Water and Sanitation Corporation (WASAC)  
Rwanda  Standards Board  (RSB) – Secretariat  DRS  188: 2018  
©RSB 2018  - All rights reserved                  vi 
 Introduction   
With a growing world population and the need of food secu rity, irrigation practices have a vital role to play 
worldwide. Climate changes with rainfall variations during all or part of the year require irrigation water 
conservation practices. Despite being a country of great lakes and the headwaters of two greate st African rivers, 
the Nile and the Congo, Rwanda is a water scarce country. Rwanda has about 165 000 hectares of marshlands 
in which only 94 000 hectares are in use, but some of them are not used wisely. About 90 % of the rural 
populations are farmer; how ever, they do not produce enough food to satisfy their needs. This is due to the 
continuous use of traditional, subsistence and inefficient farming practices which are mainly rain fed. Irrigation 
is practiced mainly in the marshlands using the inflows into  the marshlands. Over the years, the marshlands are 
becoming less productive, with visible signs of fertility decline. This trend could be attributed to irrigation 
practices in the marshlands, which tend to drain the marshlands instead of conserving the wa ter.  
The objective of this standard is to contribute to Government of Rwanda Agriculture program to avoid 
dependence on rain fed agriculture, assure stable, affordable and nutritionally adequate food supplies for its 
citizens. In addition, it plans to pen etrate the markets by superseding the pace of the turbulent national, regional 
and global financial situations.  
Irrigated agriculture is the largest consumer of available water in Rwanda.  Since many irrigation schemes are 
situated at the lower end of dra inage basins, they are often at the receiving end of upstream water quality 
degradation activities. It is therefore likely that many irrigation farmers will, in the future, have to contend with 
both deteriorating quality and a diminishing supply of irrigat ion water.   
As irrigation water is used to supply the water requirements of a wide variety of plants, under widely varying 
degrees of intensification, with a range of different distribution and application systems, to a wide range of soils 
over all climat ic ranges in Rwanda, a wide spectrum of problems may be encountered where water does not 
meet requirements.  
Sources of Water supplies for irrigation may originate from large reservoirs, farm dams, rivers, ground water, 
municipal supplies and industrial ef fluent.  
As such, irrigation water supplies span a wide range. On irrigation schemes, irrigators mostly rely on an 
adequate supply of fairly good water quality from untreated water, while home gardeners mostly receive a supply 
of conventionally treated wat er of high quality.  
The quality and quantity of water from rivers is highly variable, and is due to seasonal droughts or floods. The 
quality of ground water also varies greatly.  
Irrigation water users may experience a range of impacts as a result of chan ges in water quality. These may be 
categorised as follows:  
a) reduced crop yield (as a result of increased salinity or the presence of constituents that are toxic to plants);  
b) impaired crop quality (this may result in inferior products or pose a health risk to consumers);  
c) impairment of soil suitability (as a result of the degradation of soil properties and accumulation of 
undesirable constituents or toxic constituents); and  
d) damage to irrigation equipment (corrosion or encrustation).  DRS  188: 2018  
vii                ©RSB 2018  - All rights reserved  
 The water quality problems and issues listed above can often be linked to the constituents that cause them. 
Some water quality problems are associated not only with the presence of constituent, but with the interactions 
between constituents, for example, i nfiltration of water into the soil which is affected by both the sodium 
adsorption ratio and the total dissolved solids of the water.  
There have been calls to establish standards as a guide for judging the suitability of water for irrigation. Any 
classifi cation should be based on the total concentration and the composition of salts. However, the suitability 
of water for irrigation also depends on other associated factors, such as the crop, soil, climate and management 
practices.  
 DRS  188: 2018  
1                ©RSB 2018  - All rights reserved  
 Water quality — Irrigation water — Tolerance limits   
1 Scope   
The standard specifies essentially a user needs of the quality of water required for different irrigation uses. It is 
intended to provide the information to make judgements on the fitness of water to be used for irrigation purposes, 
primarily for crop production. The standard is applicable to any water tha t is used for irrigation purposes, 
irrespective of its source (municipal supply, borehole, river, etc.) or whether or not it has been treated . 
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 .  
ASTM D 5907 Standard test methods for filte rable matter (Total Dissolved Solids) and non -filterable matter 
(total suspended solids) in water   
ISO 9297, Water quality — Determination of chloride — Silver nitrate titration with chromate indicator (Mohr’s 
method)   
ISO 11885,  Water quality — Determinat ion of selected elements by inductively coupled plasma optical emission 
spectrometry  
ISO 7890, Water quality — Determination of nitrate —Part 3: Spectrometric method using sulfosalicylic acid   
ISO 10523, Water quality — Determination of pH   
ISO 9963, Wate r quality — Determination of alkalinity   
ISO 7888, Water quality — Determination of electrical conductivity  
ISO 9964, Water quality — Determination of sodium and potassium  
3 Terms and definitions  
For the purposes of this standard, the following terms and  definitions apply.  
3.1  
toxicity   
toxicity problem occurs when certain constituents in the water are taken up by the crop and accumulate in 
mounts that result in a reduced yield  and crop quality . This is usually related to one or more specific ions in the 
water namely boron, chloride and sodium.    
©RSB 2018  - All rights reserved                  2 
 3.2  
salinity  
salinity problem related to  water quality occurs if the total quantity of salts in the irrigation water is high enough 
that salts accumu late in the crop root zone to the extent that yields are affected.  
3.3  
electrical conductivity (EC i),  
measure of the total soluble salts in the water. EC i may be measured and reported in deciSiemens per metre 
(dS/m) or microSiemens per centimetre (µS/cm ).   
4 Requirements  
4.1 Requirements for irrigation water quality   
The following table illustrates major characteristics for irrigation water.  
Table 1 — Irrigation water characteristics  
Characteristics  Degree of restriction of use  
 Test methods  
No Problem  Increasing 
problem  Severe 
problem  
 Salinity (affects crop water availability)   
ECw (mmhos/cm)  < 0.75  0.75 - 3.0 > 3.0   ISO 7888  
TDS (mg/l)  < 450  450 - 2000  > 2000   ASTM D 5907  
Permeability/Filtration (affects 
infiltration rate into soil)  
ECw (mmhos/cm)  > 0.5  0.5 - 0.2 < 0.2   - 
 Specific ion toxicity (affects sensitive crops)   
Sodium (Na) _4/ 5/ (adj. SARO  < 3 3 - 9 > 9  ISO 9964  
Chloride (Cl) (meq/l)"  < 4 4 - 10 > 10  ISO 9297  
Boron (B) (mg/l)  < 0.75  0.75 - 2.0 > 2.0   ISO 11885  
 Miscellaneous effects (affects susceptible crops)   
(2) NO3-N (or)NH4 -N (mg/l  < 5 5 - 30 > 30  ISO 7890  
HCO3 (meq/l) [overhead 
sprinkling] I  < 1.5  1.5 - 8.5 > 8.5   ISO 9963  
pH < 6.5  6.5 - 8.4 >8.4  ISO 10523  
Note 1  Most tree crops and woody ornamentals are sensitive to sodium and chloride (sensitive crops)  
Note 2  NO3 -N means nitrate nitrogen reported in terms of elemental nitrogen (NH4 -N and Organic -N should be 
included when wastewater is being tested . DRS  188: 2018  
3                ©RSB 2018  - All rights reserved  
 Annex  A 
(normative)  
 
Precautions for sampling  
This is intended to be a very brief non -technical discussion to obtain more reliable water samples for analysis.  
A laboratory analysis is no better than the sample submitted for analysis. The sample should be as 
repre sentative of the conditions of use as it is reasonably possible to make it.  
a) sample bottles should be clean. If possible rinse a clean bottle, at least three times with the water to be 
sampled. If samples are to be analysed for boron, plastic bottles (not glass) should be used;  
b) size of sample: one quart or one litre is usually ample; and  
c) a representative sample. Take time to think about the reasons for the sample.  
Get a sample or series of samples that will be representative of the conditions of use. For surface waters, decide 
where to take the sample - surface, below the surface, near the bottom, mid -stream or edge. In taking samples 
representative of the water div erted for irrigation, will one sample be adequate or are differences expected in 
quality due to flow rate, drainage return -flow fluctuations, etc. that indicate a series of samples that w ill be 
needed to show changes.   
If a series is necessary, over what time interval - one day, one week, one month, one year, or several years? A 
choice should be made based on types and numbers of samples needed to be representative of true conditions.  
For well water pumped from the underground sampling is simpler. Be sure  the pump has been delivering water 
for at least 30 minutes. If a new well, a sample taken after surging or well development and after several hours’  
delivery at designed capacity should be more representative than samples taken earlier.  
Samples should be  kept cool until analyzed. If samples cannot be analyzed immediately storage near Z~°C is 
ideal. Samples for nitrates, ammonia or organic substances will need to be kept frozen or near freezing (~+°C). 
This is to prevent utilization or depletion of these c onstituents from the sample by growth of organisms (bacteria, 
algae, etc.). Freezing is a very satisfactory method of holding samples prior to analysis but remember that water 
expands on freezing and the container must be less than full to allow for expans ion 
 
   
©RSB 2018  - All rights reserved                  4 
 Annex  B 
(normative)  
 
Recommended maximum concentrations of trace elements in irrigation 
water  
Table 1 — Maximum concentrations of trace elements in irrigation water.  
Constituents  Long term use (mg/l)  Short term use (mg/l)  
Aluminium (Al)  5.0   20  
Arsenic (As)   0.10  2.0  
Beryllium (Be)  0.10  0.5  
Cadmium (Cd)   0.01  0.05  
Chromium (Cr)  0.1  1.0  
Cobalt (Co)  0.05  5.0  
Copper (Cu)  0.2   5.0  
Fluoride (F –)  1.0  15.0  
Iron (Fe)  5.0   20.0  
Lead (Pb)   5.0  10.0  
Lithium (Li)  2.5   2.5  
Manganese (Mg)   0.2  10.0  
Molybdenum (Mo)   0.01  0.05  
Nickel (Ni)   0.2  2.0  
Selenium (Se)   0.02  0.02  
Vanadium (V)   0.1  1.0  
Zinc (Zn)   2.0  10.0  
NOTE The maximum concentration is based on a water application rate which is consistent with good irrigation practices (10 000 
m3 per hectare per year). If the water application rate greatly exceeds this, the maximum concentrations should be adjusted 
downward accordingly. No adjustment should be made for application rates less than  10 000 m3 per hectare per year. The values 
given are for water used on a continuous basis at one site  DRS  188: 2018  
5                ©RSB 2018  - All rights reserved  
 Annex  C 
(normative)  
 
Irrigation water quality and wastewater re -use 
Agriculture is the major user of water and can accept lower quality water than domestic and industrial users. It 
is therefore inevitable that there will be a growing tendency to look toward irrigated agriculture for solutions to 
the overall effluent dispos al problem. Because wastewater contains impurities, careful consideration must be 
given to the possible long -term effects on soils and plants from salinity, sodicity, nutrients and trace elements 
that occur normally manageable if associated problems with t hese impurities are understood and allowances 
made for them.  
Effluent irrigation may also lead to microbial contamination of air, soils and plants in the vicinity of the irrigation 
site. The extent of such contamination depends upon the degree of treatment provided, the prevailing climatic 
conditions, nature of the crop being irrigated and the design of the irrigation system. Where the terrain and the 
crop type are suitable, effluents may be applied.  
When considering the use of effluents for irrigation, their microbial and biochemical properties should be 
evaluated.  These values should then be compared with the public health standards, taking into consideration 
the crop, soil and irrigation system and consumption of the produce, and only when the effluent meets these 
standards should it be evaluated in terms of chemi cal criteria such as dissolved salts, relative sodium content 
and specific toxic ions.  
 DRS  188: 2018  
ICS 13.060.01  
 
 
©RSB 2018  - All right s reserved