Journal of Environmental Protection, 2014, 5, 71-79
Published Online January 2014 (
The Mexican Environmental Flow Standard: Scope,
Application and Implementation
María Antonieta Gómez-Balandra, María del Pilar Saldaña-Fabela, Maricela Martínez-Jiménez
Hydrobiology and Environmental Assessment Department, Mexican Institute of Water Technology, Jiutep ec, México .
Email: magomez@tlalo
Received November 8th, 2013; revised December 11th, 2013; accepted January 5th, 2014
Copyright © 2014 María Antonieta Gómez-Balandra et al. This is an open access article distributed under the Creative Commons
Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is
properly cited. In accordance of the Creative Commons Attribution License all Copyrights © 2014 are reserved for SCIRP and the
owner of the intellectual property María Antonieta Gómez-Balandra et a l. All Copyright © 2014 are guard ed by law and by SCIRP
as a guardi an.
With the i mplementatio n of the O fficia l Mexican Standa rd NOM -011-CONAGUA-2000 [1], the water balance of
730 ba sins has been calcula ted and its water availability agreement is publishe d. This rule points out to allocate
water for the environment only as an annual volume since methods for estimating environmental flows were not
standardized in the country. For this reason, The Water Agency (CONAGUA) issued the standard NMX-AA-
159-SCFI-2012 [2], to assess environmental flows needed both, at the strategic level in Integrated Water Re-
sources Management (IWRM), or as part of the Environmental Impact Assessment (EIA) of large hydraulic
projects. For over ten years, this standard was developed and finally published in September 201 2 [3]. It explains
different methods from hydrological to holistic approaches, with examples for the country. Its application will
cover the urgent need to preserve water for ecosystems in watersheds with high ecological importance and low
stress for water use. In this paper, an analysis of the environmental flow standard and examples of the suggested
hydrological methods are presented. For its implementation, some steps are taking place, mainly establishing
environmental water reserves and building capacities. In addition, environmental allocations are becoming a
common practice for all water projects, as well as setting limits to hydrological alterations by hydroelectric dams.
The sta ndard pr omotes the use of technical integra tion tools to analyze the responses of ecosystems to changes in
the flo w regime and adapt ive manage ment under diffe rent scena rios of wa ter use. Although t he main steps hav e
been take n, i ts implementation as mandatory rule will take time.
Environmental Flows; Hydrological Methods; Mexican Standard
1. Introduction
Environmental, ecological or in stream flow (EF) is the
amount of water that is kept flowing down in order to
maintain the river in a desired environmental condition in
terms of its spatial and timing distribution required to
maintain the components, functions and aquatic ecosys-
tems processes from which people obtain benefits ([4-10].
The concept has evolved to recognize the natural varia-
bility of flow regimes which consider: magnitude, dura-
tion, timing, frequency and rate of change associated to
environmental services and e cosystems resilience [11].
Environmental flow assessment (EFA) is based on a
scientific process to deal with basin features, proba b ilis-
tic variability of natural flows and cross section hydro-
morphology models. These issues determine habitat
connectivity and availability to be used by species at
several stages of their life cycle. EFA is also a social
process since water regulators and users define the level
of stress that can be supported by a river in terms of their
quantity, quality and ecosystem services, although not
many of these decisions have been properly taken. Then,
generalized flow standards can be used to allocate water
for the environment [12] but, every river can have an
environmental flow regime according to what people
want from a river in t erms of conservation and u s e [13].
EFA as an escalated set of methodologies is becoming
The Mexican Environmental Flow Standard: Scope, Application and Implementation
an important toolbox to assess sequentially from rivers’
natural flow regime to its ecological integrity (structure,
function, processes and alterations). Methods have been
classified as : hydr olo gical, hydraulics, for habitat simula-
tion and holistic [9,14-17]). Hydrological approaches are
the basis to identify the natural flow regime and its alte-
rations caused by water subtractions or hydraulic infra-
structure [11,18]. Results are complemented with hy-
draulic measurements and associated to habitat availabil-
ity and connectivity among aquatic, riparian and coastal
communities. In addition, holistic approaches consider
environmental services for people and comparison of
water allocation strategies or altered scenarios by infra-
structure [19]. Due to the condition of many rivers which
are over-allocated, fragmented, polluted and losing bio-
diversity, it is recognized that EFA is needed as a part o f
approaches both at strategic level in the integrated water
resource management IWRM and as a part of EIA for
large hydraulic projects [20-22]
The EF implementation is on progress and overcoming
several drags, since hydrological records are sometimes
the only available data for many regions, whereas hy-
draulic studies are really scarce and its development is
time and cost consuming [9]. Besides, these methods
need to broaden their scope towards habitat availability
for species, not only as hydraulics research. This ap-
proach is under development since several disciplines are
involved and need to formulate flow alterationhabitat
availability—ecolog ical responses hypothesis to be tested
through differe nt i ndicator s. The final goal is to dea l with
an adaptive approach both to operate hydraulic infra-
structure and produce conditions for preserving species,
maybe in the year by year basis. Thus the final outp ut is
to reduce the effects of anthropogenic activities on aqua-
tic ecosystems. Changes can be followed by hydrological
parameters, habitat availability, species presence—ab-
sence, condition factor, weight—length relationship and
food webs, among others [23]
2. EFA Mexican Approach
The “environmental water use or use for ecological con-
servation” is defined by the National Water Law as “the
minimum flow or volume required in receiving water
bodies, including streams or reservoirs, or the minimum
flow of natural discharge from one aquifer that must be
maintained to protect environmental conditions and the
ecological balance.
The specific definition of Ecological Flow in the
Mexican Standard [2], which establishes the procedure to
determine the environ me ntal flo w is: the quantity, qualit y,
and flow variations or water levels required to preserve
environmental services, components, functions, pro-
cesses and the resilience of aquatic and terrestrial eco-
systems. They depend on the hydrological, geomorpho-
logical, ecological and social processes. This implies that
in addition to provide water for domestic, urban public,
livestock and agriculture uses, it is possible to maintain
flows from both runoff and aquifer outcropto preserve-
rivers (perennial, intermittent and ephemeral), lenticwa-
ter bodies (lakes, ponds, and wetlands), and riparian
The need for ecological flows was a pendant issue
since the Water Act publication in 1992 and re -emerged
with the application o f an Official National Standa rd [1],
to set basins’ water availability. This availability com-
prises a committed natural discharge as a fraction of the
hydrogeological unit of natural discharge, committed as
superficial water to diverse uses or to be preserved to
prevent negative impacts on ecosystems or the migratio n
of bad quality water into the aquifer. The estimated water
availability is then published in each hydrometric gauge
of the basin and as a total volume.
The Mexican Standard to estimate the EF is a technical
guideline rather than a compulsory limit [2]. Its main
scientific principles are: the recognition of natural hy-
drological regime and the gradient of the biological con-
dition. Therefore, any methodology is valid for the stan-
dard as long as it focuses on understanding the ecological
significance of each component of the natural flow re-
gime, and it generates proposals for its conservation or
restoration in whole or partially, from the functional
point of view.
Following a top-down initiative and based on the gra-
dients of ecological importance and water scarcity, an
environmental o bjective ( E O ) was assigned such as water
availability is issued for each basin or sub-basin in the
country (Fig ur e 1). The Rio Verde sub-basins were
numbered for fur ther analysis .
The EF Mexican standard calls for the application of
hydrological methods as the basis to reserve or allocate
water for the environment. Therefore, according to the
EO’s, reference values (% of medium annual volume or
flow) as well as the components of a seasonal flow re-
gime are stated as follows:
1) Under a modified Tennant approach (García et al.
a) Percentage of mean annual flow (MAF) with dif-
ferent seasonal percentages (Reference values).
b) Percentage of monthly mean flows MMF.
c) A b ase flo w dur ing dr y seaso n not le ss tha n the his-
torical minimum monthl y flow (MinF).
2) Under WWF Mexico approach.
a) A total volume associated to an ordinary regime
(TVOR), considering wet, medium and dry years.
b) A total volume related to the frequency of floods’
regime (TVFR) with a return period of 1, 1.5 and 5 years
to be reproduced within 10 years return period according
to EO’s.
3) Under TNC approach [14].
a) Intra and inte r-annua l natural variability.
The Mexican Environmental Flow Standard: Scope, Application and Implementation
Figure 1 . Envir onmental objectives for the Mex ican basins a nd t he Ri o Verde Basin (M odified from WWF, [24]).
b) Thresholds for hydrological alterations on hydro-
logical parameters with ecological importance (monthly
flows, 1, 7, 30 and 90 days minimum and maximum
These approaches can be applied at basins, sub-basins
or river reaches as it is pointed out in Figure 2. At least
20 years of hydrological data are recommended for all
these ana lysis.
The Mexican flow standard also describes some initial
expe riences i n usin g habitat simulat ion with BB M build-
ing blocks methodology [24] as holistic approach and
recommends the use of habitat simulation methods like
PHabSim or others.
2.1. EFA Hydrological Methods Application
The Environmental objectives for the Rio Verde Basin
and sub-basins located in the hydrological region 20
named Costa Chica de Guerrero were stated as is pointed
out in Table 1.
The hydrological gauge Paso de la Reina was chosen
to show result from the standard application (Fig ure 3)
in this site a proposed hydro project is under planning
2.2. Modified Tennant Approach ([5] )
General steps:
1) Selection of the site of study.
2) Analysis of the monthly dataset.
3) Determination o f monthl y and a nnual flow re gime.
4) Formulation of proposals for monthly and annual
environmental flow r egime.
As mentioned above, the selected site was the Paso de
la Reina hydrometric gauge, this gauge has 45 years of
records. For this approach, the start of the monthly series
begin s in May b ec aus e is the mo nt h whe n the rain season
begins. The monthly flow regime (MMF) was stated as
well as the mean annual flow (MAF) as the reference
started points (Figure 4).
The Mexican Environmental Flow Standard: Scope, Application and Implementation
According to environmental objectives (EO), Table 2
sho ws rec omme nded seasonal flow percentages. Tennant
modified by García et al. [5,26], and proposed by [25].
As Paso de la Reina site was classified as Environ-
mental Objective “A” [2], then annual and monthl y allo-
cated percentages are shown in Figure 5.
After setting percentages it must be reviewed that en-
vironmental flow in dry season should never be greater
than t he mon thl y a ver a ge flow (M MF ) no r le sse r t han the
base flow.
The environmental flow proposal is based on monthly
percentages (light green area in Figure 6 and values of
Table 3). For dry years the proposal can be adjusted up
to seasonal thresholds (30% and 60% of MAF blueline).
For rain season these limits can be set between percen-
tages o f mean a nnual flow MAF (blue line) and monthly
mean flow (dark green area). For each stream, a base
flow must be e stimated (b la c kline).
2.3. WWF Approach
According to the World Wildlife Fund Mexico approach
a reference volume as percentage of the medium annual
flow s hould be equal or greater than 40% (Table 4).
The next step is to id entify alt erations in the hydrolog-
ical regime trough dividing time series data in two pe-
riods. For the first set of 20 years (natural hydrological
regimen RHN) obtain percentiles 10 and 90 and check if
the number of the monthly means of the second period
(present hydrological regime RHA) that are within these
limits. If the fulfillment of the actual regime (RHA) is
<50% in relation to percentiles (RHN) the river is consi-
dered altered. The Figures 7 and 8 pointed out that the
Rio Verde at Paso de la Reina hydrometric gauge is not
For seasonal environmental flow estimation, the Total
Ordinary Volume Regime (TOVR) and the Total Volume
for Flood Regime (TVFR) were obtained using the pro-
cedure proposed by World Wildlife Fund Mexico at the
study basin level. TOVR was obtained multiplying the
frequency by volume for each year condition to be re-
produced with regards to the environmental objective
(Table 5).
The two steps to calculate TVFR that try to resemble
natural pattern, according to the conservation objectives,
1) Classification and characterization of peaks: To
separate floo d types initiall y used the criterio n of mag ni-
tude, maximum flow and probability of occurrence over
time (frequency). Analysis of statistical distributions are
needed to determine magnitude of floods associated re-
Figure 2. EFA hydrological approaches in the Mexican
Figure 3. Water availability issued for the basin. CFR: 05/
07/2013 [25].
Table 1. Environmental obje ctives by sub-basin.
Ri o V erde Sub-basin s 1 2 3 4 5
Annual Availa bility (Mm³/year) 92.4 481.1 3173.8 5430.3 5873.3
Ecological Importance High High High Medium High
Water use stress Medium Medium Low Low Low
Present/Desired Condition Good Good V e r y Goo d Goo d V e r y Goo d
Environmental Objective B B A B A
The Mexican Environmental Flow Standard: Scope, Application and Implementation
Figure 4 . Monthly fl ow re gime and mean annual flow that determin e s th e dry and ra iny seasons.
Figure 5 . Annual an d monthl y alloc at e d pe rce nt ag es.
turn periods as follows:
a) Runoff with return period of 1 year (Category I). b) Runoff wi th return per iod of 1.5 years (Categor y II).
c) Runoff with return period of 5 years (Category III).
The Mexican Environmental Flow Standard: Scope, Application and Implementation
Figure 6 . Envir onmental flow proposal.
Figure 7. M ont hly pr esent mean and RHN limits.
Figure 8. Annual present mean and RHN li mits.
2) Duration in days of each runoff period is then ob-
tained and the product of this duration, frequency and
volume resulted in the Total Volume for Flood Regime
for three different runoffs within 10 years. Table 6
shows the results for these calculations.
The integratio n of the final reserved volume (FRV) or
environmental flow to be considered in the annual water
balance of the basin and its water availability is the sum
of total ordinary volume regime (TOVR) with the total
volume of flood regime (TVFR), as follows:
The Mexican Environmental Flow Standard: Scope, Application and Implementation
Table 2. R ef erence values for each EO.
Perio d
Dry Rain
% MAF % MMF % MAF % MM F
A 30 100 60 50
B 20 80 40 40
C 15 60 30 30
D 5 40 10 20
Table 3. Monthly environmental flow regime in cubic me-
ters per sec ond (m³/s).
Month E nvironmenta l Flow
January 48.3
February 37.8
Ma rch 28.3
April 24.2
May 37
June 80.4
July 111.9
August 138.7
Septemb er 204.8
Oct o ber 139.2
November 48.3
December 48.3
Table 4 . Reference values for specific environmental objec-
Objective Conservation
Environmental Flow (% MAF )
Perennial streams
A Very good >40
> 64.6 m ³ /s or >2172 Mm3/ year
Table 5. Total Ordinary Volu me Regime.
Type of Yea r Very dry Dry Average Wet
Percent ile P0 P10 P25 P75
Ordinary volume regime
(OVR Mm3/year ) 2052 2862 3611 6066
% Mean Annual Runoff 40 56 71 119
Frequency 0.2 0 .3 0.4 0 .1
Total Ordinary
Volume Regime 3319 Mm3/yea r
FRV = 3319 + 198 = 3517 Mm 3/year
Table 6. T otal Vol ume of Floo d Re gime.
Attribute of
hydrological regime C at egory I Category II Category III
1 yea r
return 1.5 years
return 5 years
m3/s 303 843 1874
Mm3/day 26 73 162
Frequency 10 6 2
Dura tion 3 2 1
Timing Jul-Oct
Rate of
Rise 75
Fall 40
Total Volume for
Flood Regi me
in 10 Ye ars 1980 Mm3
TVFR/Year 198 Mm3
MAR= 5430 Mm3/year
FRV = 3517 Mm 3/Year = 64% of MAR
3. Conclusions
Although the Mexican Water Act states the environmen-
tal use and the Availabilit y Standar d points out an annual
volume to the environment in each basin, for more than
fifteen years, the minimum flow as much as 10% of
mont hly flo ws have pre vai led to d eal with e nviro nme nta l
allocation of water for projects with consumptive uses
(water supply and irrigation). Availability agreements
have been issued and are going to be reviewed every two
years. Therefore, impleme nt in g E F A is t he next chal le ng e
to be included in these agreements and then gives some
priority of reserving water for the enviro nment [27].
The Mexican environmental flow standard as a tech-
nical procedure gives the support to move ahead from
this 10% minimum approach and consider the natural
flow variability. Through the recognition of this natural
variability and applying at least hydrological methods,
twofold protection goals can be achieved to: 1) Repro-
duce seasonal and even monthly variability and 2) Con-
sider thres holds for alteratio n either seaso nal, monthly or
inter-annually when ecological process can be vulnera-
The hydrological methods are also the basis for t he re-
gional water resources management under frameworks
like the integrated watershed resource management
IWRM and the ecological limits of hydrological altera-
tion (ELOHA) [28]
The application of habitat simulation and holistic me-
thods is under development in Mexico. Therefore, expe-
riences at regional and basin level as well as for large
infrastructure projects are an urgent need to protect the
The Mexican Environmental Flow Standard: Scope, Application and Implementation
ecological integrity of ecosystems. These approaches
should be integrative and under an interdisciplinary ap-
As a result of the application of habitat simulation or
holistic methods, a set of ecological and social indicators
can be obtained to follow dur ing the E F imple mentatio n.
For Mexico, the regional ecosystem changes and social
responses will be valuable when the adaptive approach
has to consider tradeoffs between ecological conserva-
tion and water use tightening furt her hydraulic develop-
ment .
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