new perspectives for ecological flow determination in semi-arid regions: a preliminary approach

REGULATED RIVERS: RESEARCH & MANAGEMENT

Regul. Ri6ers: Res. Mgmt. 15: 221–229 (1999)

NEW PERSPECTIVES FOR ECOLOGICAL FLOW DETERMINATIONIN SEMI-ARID REGIONS: A PRELIMINARY APPROACH

J.M. BERNARDOa,* AND M.H. ALVESb

a Departamento de Ecologia, Uni6ersidade de E6ora, 7000 E6ora, Portugalb Instituto da Agua, A6. Almirante Gago Coutinho 30, 1000 Lisboa, Portugal

ABSTRACT

Streams in semi-arid regions, such as the south of Portugal, have highly seasonal flow regimes with a marked patternof low or zero flow during summer and early autumn. As a result, throughout these months, most rivers have longdry reaches with occasional pools. The biotic communities have evolved specific adaptive strategies to face thosealternating lotic–lentic conditions, but when the dry season becomes longer and/or more extreme, the environmentalstress can be lethal for important groups of the biota. Most ecological flow methodologies have been developed forpermanent streams, namely salmonid streams. Ecological flow determination for temporary rivers requires a differentapproach, as an essential issue is the maintenance of the pools until the end of the dry season in such conditions thatmake possible the survival of the aquatic communities. Groundwater plays an important role in this process becauseof the hydric connection with the surface water, and so it should be considered. In this paper, the limitations of themost common ecological flow methods to the unique Mediterranean temporary rivers are discussed, particularly forthe summer period. Because of the utmost importance of summer conditions, an empirical approach was developed.Three types of procedures were involved: (1) characterisation of fish assemblages along the river in order to identifythe most important reaches, (2) analysis of the existing aerial photography to identify interannual variation ofsummer water availability for the river in general and especially for the more relevant reaches, (3) development of aprecipitation–run-off model to relate river run-off to the persistence of the summer pools. The purpose was to definethe lowest possible flow regime compatible with favourable water conditions during summer. Copyright © 1999 JohnWiley & Sons, Ltd.

KEY WORDS: ecological flow; temporary rivers; semi-arid regions

INTRODUCTION

Ecological flow methods and Mediterranean ri6ers

Most methods for estimating quantitative ecological flows were developed for perennial rivers in USA,Canada and northern Europe. They are generally divided into three major categories, based on similarprinciples and assumptions: historic flow methods, hydraulic rating methods, and habitat rating methods.

The historic flow methods define the ecological flow based on the recorded or estimated naturalhydrological river regime. These methods assume that a percentage of the mean flow, or a chosen flowbased on the flow duration curve, will guarantee a certain level of ecosystem protection. One mainlimitation of these methods is that the river systems should be morphologically similar to those for whichthe method was evaluated. These methods tend to be community type and site specific (Loar and Sale,1981; Gordon et al., 1992; Jowett, 1997).

Hydraulic methods relate various parameters of the hydraulic geometry of the river channels, such aswetted perimeter, to discharge. The ecological flow is defined as the flow that allows only a certainreduction of that hydraulic parameter for a selected section (e.g. Jowett, 1997).

Habitat methods use habitat/flow relationships to define the flow that provides maximum habitat, theoptimum flow below which the area of suitable habitat begins to decrease rapidly. The most used habitat

* Correspondence to: Departamento de Ecologia, Universidade de Evora, 7000 Evora, Portugal.

CCC 0886–9375/99/010221–09$17.50Copyright © 1999 John Wiley & Sons, Ltd.

Recei6ed 20 December 1997Re6ised 8 May 1998

Accepted 25 August 1998

J.M. BERNARDO AND M.H. ALVES222

method is the instream flow incremental methodology (IFIM; Bovee, 1982). This methodology involvestransect analysis and is based on the evaluation of suitable habitat availability for particular aquaticspecies, mainly fish, as a function of the stream flow. The major general limitations are (e.g. Arthingtonand Pusey, 1994): (1) the assumption that stream channels are stable and will remain so; (2) requirementof detailed information on the habitat of relevant species at each life history stage; (3) the complexity ofmodelling cover.

The applicability of these methods to Mediterranean rivers is questionable, because of some particularfeatures of these ecosystems and biota.

The south of Portugal, and a great part of the Iberian Peninsula, has a homogenous dry Mediterraneanclimate, with hot summers, high isolation and a mean annual potential evapotranspiration of 900 mm.According to the climatic classification of Thornthwaite (1948), most of these areas have a semi-aridclimate.

The mean annual precipitation is 560 mm with a very irregular distribution pattern within the year. Theinterannual distribution is also very irregular, with 350 mm in dry years and 900 mm in wet years, andthe region is subject to long random series of drought years. The annual streamflow distribution isstrongly determined by the rainfall distribution, with 80% of the streamflow concentrated betweenNovember and April. The mean annual run-off is approximately 100 mm, and the coefficient of variationhas a mean value of 0.8 (Veiga da Cunha et al., 1974).

Any methodology for the assessment of the environmental flow requirements in Mediterranean rivers,particularly temporary rivers, i.e. flowing 20–80% of the year (Davies et al., 1994), should consider:

� Highly variable flow regime and a heterogeneous geomorphology that do not allow a transect-basedcharacterisation at a few points along the river channels;

� Periods of prolonged low flows and droughts;� Floods promote post-summer recolonisations and spawning migrations up the river systems;� Seasonally, high flow events maintain channel morphology and substrate;� There are no economically important fish species, but many species are Iberian endemics with a high

conservation value;� There is a lack of quantitative information on endemic fish species, namely on their habitat

requirements, but apparently many species are generalists, occupying wider ranges of environmentaldimensions than cold water biota;

� Riparian vegetation has a high ecological and landscape value in many sectors, even in first-orderstreams, where fish assemblages may not exist or are very simple and with no conservation value;

� On a larger scale, there is a lack of information on the functioning of Mediterranean fluvialecosystems, especially on the effects of floods, droughts, and flow variability on the structure of thecommunities;

� There is a limited understanding of flow regulation effects on those ecosystems.

On the meaning of ecological flow in semi-arid regions

Extreme seasonal variation of the flow regimes, with a marked pattern of zero or low flow, and thereduction of the water surface to isolated pools along the river when the flow ceases, submit the biota toalternating lotic and lentic conditions during the year.

Biota may be subject to a high level of environmental stress and developed adaptive strategies. As theflow reduces and the level of water decreases, some species, namely fish, migrate to deeper zones, wherethe probability of water persistence is higher. The remaining pools become refugia for resident individuals,and the abundance and diversity of species in the pools seem to be related to its depth, area and degreeof isolation. The declining volume and wetted area of those pools, together with the concomitant increasesin temperature, changing of chemical characteristics, and the increased predation pressure by otter,determine the success of populations occupying the pools, until recharge and reconnection occur duringthe following run-off period.

Copyright © 1999 John Wiley & Sons, Ltd. Regul. Ri6ers: Res. Mgmt. 15: 221–229 (1999)

ECOLOGICAL FLOW DETERMINATION 223

When the streams again start to flow, the surviving organisms recolonise the river system. In general,recolonisation takes place from downstream to upstream, from pools located in the same stream or fromthird- and/or fourth-order streams to first- and/or second-order streams. Therefore, after summerextinction, comes a recolonisation/migration period characterised by population expansion.

Some endemic fish species, like rheophilous cyprinids (Chondrostoma willkommii and Barbus spp.),require different habitat conditions during different life history stages. Spawning habitats are riffles witha cobble–pebble substrate and those are also the habitats preferred by juveniles. In non-reproductiveseasons, adults are usually found in deeper zones with lower velocities. Reproduction of rheophils occursfrom late winter to mid-spring (Bernardo, 1996) and their spawning migrations contribute to therecolonisation of the river system.

The existence of a pattern of habitat use, related to different flow velocities for the rheophilous species,reflects the significance of flow, especially during the reproduction and the juvenile stages.

Dam construction and water pumping change the natural hydrological regime, by reducing the annualrun-off and modifying the temporal flow patterns, as well as the duration, timing, frequency, magnitudeand the rate recession of floods. Consequently, the dry season begins earlier and may last for almost 8months, very frequently with a critical decrease in water quality.

An ecological flow regime should consider the essential features of the natural flow regime. Theincorporation of those features into the modified flow regime should maintain the structure and thefunctional integrity of the riverine ecosystem. Special features of the natural hydrological regime (seasonalpatterns of flow, annual and interannual flow variability, flood flow of different magnitudes, duration andrecurrence intervals, low flows and periods of natural cessation of flow, perenniality or non-perennialityof flow) certainly have a determinant role on the fluvial ecosystem characteristics of semi-arid rivers(Arthington, 1994; Arthington and Pusey, 1994; Richter et al., 1997).

The proposal of a modified flow regime should consider all the individual water allocations necessaryto guarantee fundamental features of the natural hydrological regime on which important ecologicalprocesses depend, as already suggested by some authors (King and O’Keeffe, 1989; Arthington et al.,1991; Arthington, 1994; Arthington and Pusey, 1994; King and Tharme, 1996; Richter et al., 1997) Theecological flow regime should be presented on a monthly basis, or at least seasonally, and shouldincorporate the interannual variation with special values for dry years.

As the greater constraints (limiting environmental factors and loss of habitats) in these ecosystems arerelated to the summer period, and water abstraction aggravates these constraints, pool persistence is oneof the main issues for the definition of an ecological flow regime. A new methodological approach wasdeveloped for the River Enxoe in order to assess the flow requirements for the maintenance of suitablewater availability during the dry period.

CASE STUDY—THE ENXOE RIVER

A dam for urban supply was built by the Water Institute (INAG, Instituto da Agua), in the River Enxoe,15 km distant from the headwaters.

The dam is 20.5 m high and, at the full supply level, will create a reservoir with 10.4×106 m3 ofcapacity and a surface area of 2.05 km2 (Hidrotecnica Portuguesa, 1994). The dam drainage basin is 60.8km2, which represents 26.4% of the river watershed.

The Enxoe, a second-order stream, is a tributary of the River Guadiana, the largest river in the southof Portugal. The River Enxoe is 34 km long, and its drainage basin is 230 km2. The mean annualprecipitation in its basin is 602 mm, with a minimum value around 380 mm, and a maximum ofapproximately 890 mm. The mean annual runoff is 32.66×106 m3 and most of the flow is concentratedfrom November to April.

The dam basin mean annual run-off is 8.63×106 m3. In a dry year, with a probability of occurrenceof 20%, this value is 2.24×106 m3.



There is no significant point source pollution in the drainage basin, although there is a traditional oliveoil mill and a small village with an inefficient treatment plant. However, no signs of eutrophication or lowwater quality in general were noticed, even by the end of the summer.

INAG, the governmental agency for water resources planning and management, recognising theopportunity to evaluate environmental effects and to define a water release strategy to minimise flowregulation, promoted, for the first time, a long-term study for the development of an ecological flowrelease strategy.

In this paper only preliminary results on the persistence of water during summer, a major constraint forthe whole ecosystem, are presented. Other issues are also being considered in the general program, suchas the requirements for cyprinid reproduction, maintenance of riparian vegetation, channel morphologyand substrate.

GENERAL METHODOLOGY

The proposed methodology for ecological flow assessment has a preliminary and exploratory character.It is composed of the following procedures:

(1) Characterisation of fish assemblages along the river, in order to detect any possible longitudinaldistribution pattern and to identify the more important reaches to the fish fauna;

(2) Analysis of aerial photography from several different hydrological years to identify persistentsummer pools in distinct hydrological conditions;

(3) Use of a precipitation–run-off model to allow the analysis of the hydrological regime for the yearsof the aerial photographs;

(4) Based on the run-off data from the model, definition of the ecological flow that should allow thepersistence of the summer pools in the conditions considered as favourable for the fish fauna.

APPLICATION OF THE METHODOLOGY

Characterisation of fish communities

Sampling locations A, C, D, E, F and G (near the river mouth) are 21.5, 24.9, 25.8, 26.5, 30.0, and 35.1km, respectively, from the river headwaters. Sampling (electrofishing) took place during November 1996,before the rainy period.

The fish fauna of Enxoe is composed of seven species (Table I), which is a relatively low number whencompared with the Guadiana (16 freshwater species). A possible explanation for this fact is the slope andthe natural accidents (small water falls) in the final reach of Enxoe river, near the mouth, which may actas a physical barrier. The apparent reduction of migratory flow from the River Guadiana probably causes

Table I. Fish species of Enxoe river and conservation status

Species Type Conservation statusa

CyprinidaeBarbus microcephalus Almaca, 1967 Endemic Rare

EndemicChondrostoma lemmingii (Steindachner, 1866) RareLeuciscus pyrenaicus Gunther, 1868 Native Not threatenedRutilus alburnoides (Steindachner, 1866) Native Not threatened

Not threatenedIntroducedCyprinus carpio Linnaeus, 1758IntroducedCarassius auratus (Linnaeus, 1758) Not threatened

PoecilidaeNot threatenedGambusia holbrooki Girard, 1859 Introduced

a According to SNPRCN (1991).



Table II. Number of species, relative abundance, number of threatened species, presenceof large barbel specimens and Ichthyological Index for the fish assemblages in thesampled locations of the River Enxoe

Parameters Sampling locations

E GFA C D5 4 3Number of species 5 5 4

12 4 8Relative abundance 8 5 1212 121Threatened species 1

No Yes Yes No YesLarge individuals, TL\25 cm No1111 10Ichthyological Index 11 10 12

some degree of isolation for Enxoe fish assemblages. As a consequence, persistence of the Enxoe fishcommunity depends entirely or strongly depends on local reproduction and non-critical environmentalconditions. If environmental conditions become lethal to fish, recolonisation from downstream probablycannot occur. The apparent isolation of the Enxoe fish fauna is a relevant issue and enhances theimportance of the ecological flow implementation.

For the evaluation of fish assemblages several parameters were used:

1. Number of species;2. Relative abundance as catch per unit effort (CPUE, 1 min electrofishing as the unit effort);3. Number of threatened species from the Red List of Portuguese Freshwater Fish (SNPRCN, 1991);4. Occurrence of medium- and large-sized specimens (total length, TL\25 cm) of long-living species;5. Ichthyological Index (II), a simple biotic index incorporating five metrics, such as the above referred

number of species and relative abundance and, also, percent intolerant individuals and number ofendemic species; for the calculation of the Ichthyological Index, which measures the relative quality offish assemblages, six classes (0–5) were considered for each of the metrics and the value of the indexis the sum of the scores (Bernardo, 1997).

Actual ocupation of habitats by fish was also evaluated but those results will be presented elsewhere.Species with long life-spans, such as barbel Barbus microcephalus, were considered as indicators of low

hydrological constraints along the years. The occurrence of barbels of medium- and large-size in some ofthe reaches suggests the persistence, throughout the years, of summer favourable conditions withpersistent pools with depth usually greater than 0.8 m. During summer, large fish are only found in thedeeper habitats. Although larger barbels are tolerant to environmental degradation (low water qualityduring summer) they are actually quite vulnerable (e.g. predation) if exposed in shallow, small poolsduring the dry periods. Therefore, the presence of older individuals in certain reaches seems to beindicative of local low hydrological stress and high biotic value.

The values for all the parameters, presented in Table II, emphasise the higher importance of locationsE and D, which should be considered as the most valuable.

Analysis of aerial photography

The analysis of all available aerial photography was performed in order to detect the presence ofsummer pools downstream of the dam.

The following photographs were used: July 1958, May 1970, August 1978, June 1984, June 1988,August 1990 and September 1995. All the prints were in black and white, except the last two. Badlyprinted photographs, well-developed canopies of riparian vegetation and emergent aquatic macrophytesdecreased the ability to detect the presence of water. The presence of water was represented as apercentage of the reaches or river length on a linear basis (100% means no discontinuity in the riverwater).



Table III. Precipitation in the hydrological years and presence of water during the dry period in the River Enxoe,as percentage of the river length, evaluated from the aerial photographs

Aerial WaterPrecipitation (mm)Hydrologicalyear in thephotograph

river(%)

July Aug. Sept.Oct. Nov. Dec. Jan. Feb. March April May June

July 5857/58 49 76 40 63 24 51 19 6 28 2523969/70 70 May 70105 40 224 15 43 13 46

0 0 Aug. 7877/78 120 91 166 34 103 38 3589 49 16June 8483/84 56 179 85 34 14 53 120 52 29 36June 8887/88 67 41114 12 44 44 35 91 79 31

0 1 Aug. 9089/90 164 160 53273 45 7 58 132 12 20 17 Sept. 9594/95 38 44 37 33 53 31 34 16 8 1 12

The years of the available aerial photography are representative of the irregular interannual distributionof precipitation. The hydrological years of 1957–1958 and 1994–1995 were dry years and 1989–1990 wasa relatively humid year. The precipitation during spring–early summer was higher in 1978 and 1984.

Water availability observed in the photographs is correlated (r=0.87, PB0.01) to the accumulatedprecipitation from the beginning of the hydrological year, October, until the month of the aerialphotograph (Figure 1, Table III).

Figure 1. Relation of the river water availability during the dry period, expressed as percentage of the river length with water, andthe precipitation in the hydrological year



Table IV. Average values of water in the reaches from the dam to the river mouth, evaluated from the aerialphotographs of July 1958, May 1970, August 1978, June 1984, June 1988, August 1990, September 1995

GEReach Dam FA C D

44.7Percentage of the reach with water 16.4 34.7 15.0 35.3 39.3

Making interannual comparisons, and considering average values for the river from the dam to themouth, 1958 and 1995 were the worst years (Figure 1), which is consistent with the precipitation thatoccurred in those dry hydrological years (Table III).

The best water availability was observed in 1988 (June) and 1990 (August), which could be related tothe precipitation in May and June 1988 (79 and 31 mm, respectively) and in April 1990 (132 mm).Considering the precipitation in 1969–1970, 1983–1984 and 1987–1988, the river water availability wasexpected to be lower than was observed. However, the aerial photograph in 1970 was taken in May andin 1984 and 1988 was taken in June, i.e. before the hottest and driest months (July and August), whichhave a strong effect on the decreasing availability of river water.

In order to evaluate the spatial distribution of summer surface water along the river, average values forall the photographs were estimated. Reaches from D to the river mouth have the highest values (TableIV). As previously mentioned (Table II), best scores for fish fauna were observed in D, E and G, and onepossible explanation is the persistence of favourable conditions during the dry period. Those are the morevaluable reaches and the ecological flow should sustain this biotic richness.

In a general way, a relatively high summer water availability was observed in the River Enxoe, whencompared with similar small rivers in identical climatic conditions. This emphasises the importantcontribution of the groundwater to the river during the summer.

Precipitation–run-off model

The original hydrological estimates were based on the Enxoe dam’s pre-feasibility study (HidrotecnicaPortuguesa, 1994). The mean annual runoff for the Enxoe river basin was based on a precipitation–run-off relation from a similar basin in this region (River Limas).

This model estimated a mean flow of zero in July and August for all the hydrological year. During fieldwork, performed in the summer of 1996 for the ecological characterisation, the existence of flow in theriver downstream the dam location was noticed. This situation suggests the lack of validity of theprecipitation–run-off model used, and stresses the importance of the groundwater contribution for riverflow and for summer pool persistence.

In fact, the Enxoe dam was constructed near the boundaries of two aquifer systems, Moura-Ficalhoand Beja garboard-strake. Furthermore, several fissures cross different sections of the river, where somelarge pools remain during the summer. Other geological features, namely contact zones between differentgeological formations, may be the cause for the presence of springs on the river bed. On the other hand,the high values of potential evapotranspiration of this region and the small contribution of Enxoetributaries to main river flow corroborate this hypothesis.

It will be necessary to use a precipitation–run-off model that considers groundwater, such as the modelof Temez (1977), which includes soil moisture, soil maximum infiltration capacity and the dischargecoefficient of the aquifer.

With this model it will be possible (1) to know the within the year run-off distribution and (2) toestablish a relationship between run-off and pool availability during summer.

FUTURE RESEARCH AND MONITORING

Several procedures are presently taking place for a better understanding of the factors related to poolpersistence and the links between surface and groundwater:



– Geological characterization, including the interpretation of the aerial photography of the Enxoe riverbasin and the analysis of the information provided by wells;

– Monitoring in wells and in nearby pools of (1) the water levels and (2) several water chemicalparameters, as tracers, for the evaluation of groundwater–surface water connections;

– Water balance of the summer pools;– Calibration and application of the Temez model.

Meanwhile, a tentative value for the ecological flow will be defined and a monitoring program of theriver ecosystem is being designed. It will be implemented after the start of the reservoir filling, and willassess the short- and long-term effects on fish fauna, riparian vegetation, channel morphology andsubstrate.

FINAL CONSIDERATIONS

The main constraints and the general consequences of water abstraction in the temporary river ecosystemare identified. However, the lack of precise information on the functioning of these ecosystems and on theimpacts of their regulation are obvious. Sound hydrological and ecological knowledge is lacking, whichseems to justify empirical/experimental approaches.

At this stage, we consider the summer environmental conditions as a key feature of the river ecology.Groundwater plays an important role in the maintenance of summer pools. The understanding of theinteractions between river and groundwater, and the need to consider it in the precipitation–run-offmodels, seem to be essential for the ecological flow assessment in semi-arid regions.

We hope that, in the near future, a better understanding of the functioning of temporary riverecosystems will make possible the development of adequate methodologies for determining ecologicalflow. The development of river ecology studies over appropriate time scales, experimentation withdifferent water release patterns, and the implementation of long-term monitoring programs on the impactsof river regulation and water allocation strategies are particularly important.

ACKNOWLEDGEMENTS

The authors wish to thank Dr James B. Layzer and two anonymous reviewers for their comments andsuggestions and to Eng. Maria Ilheu for technical assistance.

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new perspectives for ecological flow determination in semi-arid regions: a preliminary approach

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