263CHAPTER 10HYDROLOGICAL REGIMESTheanalysisandunderstandingofhydrologicalregimesoccupiesanimportantplace in the hydrological cycle. Although the hydrological regime is not explicitly partof the water cycle process, it plays an integrating role in the sense that on the one hand,discharge is an integrating measurement of the conditions that generate surface runoff,and on the other hand the analysis of hydrological regimes is based on large data setsand requires a number of statistical tools. Thus, the hydrological regime can be viewedasalinkingelementbetweenhydrologicalspaceandtime,andsuppliesinvaluableinformation for anyone interested in the behavior of a river or a region, whether it isfor hydrological, geographical or climatic purposes. In this chapter we will cover someof the essential concepts, followed by some classification options and a description ofspecific regimes in our latitudes.10.1 INTRODUCTIONRecords of a river's discharge over a number of years reveal systematic seasonalvariations(positionsofhighandlowflows)asafunctionofthemainfactorsinfluencingflow:theprecipitationregime,thenatureofthewatershed,itsgeographicalsituation,theinfiltrationcapacity,etcThehydrologicalregimeofariver summarizes all its hydrological characteristics and its variations, and is definedby its discharge variations. The concept of hydrological regime is fundamental becauseit links hydrological processes to the seasons in which they occur, as Lambert noted(1996).Consequently, we can characterize a watershed and its flow by adopting a regimeclassificationforitsriversaccordingtothesystematicseasonalfluctuationsindischarge they show, and their mode of alimentation. A monthly flow distribution isthen used to classify the flow regime of a river.10.1.1 Monthly Discharge CoefficientThe monthly discharge coefficient is defined by the ratio of the average monthlydischarge(calculatedoveranumberofyears)calledtheinter-monthlydischarge orinter-annualmodule.Theinter-annualmoduleiscalculated(arithmeticmean)foraseries of observations over a number of years. This gives us the distribution expressedas a percentage of the monthly discharge over the year:(10.1)Cm(%)=monthly dischargeinter-annual module.100 2011 by Taylor and Francis Group, LLCHydrological regimes 264The annual runoff coefficient is expressed by the following ratio:(10.2)where, Le is the average depth of runoff and P is the average annual rainfall.This coefficient is of great interest because it allows us to estimate the volume ofseasonalrunoffvolumesinordertodesignareservoir.Thiscoefficientvariesaccording to the following elements: nature of the soil and vegetal cover, surface area of the watershed, depth of annual precipitation, yearly rainfall distribution.Annual runoff coefficients are highly variable. The curve of the monthly dischargecoefficientsforanaverageyearallowsustoestimatethesystematiccharacterofseasonalvariations,andtocomparethehydrologicalbehaviorofdifferentrivers.Figure 10.1 shows some examples of the monthly discharge coefficients for variousrivers.In the same way, the discharge relative frequency curves over a number of yearsshow the seasonal variations in the discharge quantiles (Figure 10.2) . Curves markedas 10%, 25% 90% indicate a 10, 25,..90 percent chance thatthe monthly depthof water (or discharge) will be reached or exceeded.CLPae(%) = 1000.00.51.01.52.02.53.03.54.0Jan March May July Sept NovMonthly discharge coefficientBlackVoltaNorth SaskatchewanCongoLoireNileRhineFeb April June Aug Dec OctFig. 10.1 : Example of monthly discharge coefficients. 2011 by Taylor and Francis Group, LLCHydrology: A Science of Nature 26510.1.2 Classification of Hydrological RegimesTheclassificationofthehydrologicalregimesofriversrepresentstheseasonalvariationsoftheirdischarges.Ontheonehand,theregimeclassificationshowsthecomplexityofthedischargedistributionoverthecourseoftheyear(meanannualvariations of monthly discharge coefficients). On the other hand, the classification alsotakesintoaccountthemodeofwatersupply,i.e.thenatureandtheoriginofwater(rain, snow or glacial). According to Pard (1955), there are three basic types of regimes:simple regime:characterizedbyonemaximum,oneminimum, and usuallyonlyonemodeofalimentation;thisregimedisplaystheexistenceoftwodistinct hydrological seasons,mixedregime:2maximumsand2minimumsperyear,correspondingtoseveral alimentation modes, complex mode: several extremaand modes of alimentation.Thisclassificationmightonoccasionbeadjustedduetoice/woodjammingorbreakup phenomena that cause the appearance of high water.Jamming designates an accumulation of ice or wood in a river, creating a dam, dueto the presence of a bridge, a dam, a narrowing or bend in the waterway, or any otherobstacles. The breakup follows the jamming. It involves the breakup of the ice layeronriversthat resultsfromthawingeffects. A breakup produces aflow of relativelylarge chunks of ice, which can in turn pile up and create ice dams due to a hydrographicCmtime90%50%10%Fig. 10.2 : Example of frequency curves. 2011 by Taylor and Francis Group, LLCHydrological regimes 266accident or other situation, and this often leads to flooding. Inaddition,geologycanappreciablymodifyflowsandbeyondthat,theinputregime of waterways. This is particularly true in karstic areas (e.g. in the Jura).10.1.3 Other Types of Hydrological RegimesAlthough Pard's regime classification is used in French-speaking countries, it isnot the only one, and it is also hard to translate it into a mathematical equation. Lvovich(1973) proposed a classification of hydrological regimes based on the main input modeand the distribution of the volume of flow throughout the year.His system considersfour principal flow modes (groundwater, glacial, rain and snow) and four seasons thatare dispersed according to the distribution of runoff volume. This produces a matrix of144possibleregimes.Inpractice,only38ofthesesituationshavebeenformallyidentified (Sauquet, 2000). Although this classification is interesting in many ways, itdepends on knowledge of the processes of flow generation in order to be applied, andso using this classification system is relatively difficult.A second "family" of classification is the Swedish classification system. It is basedon distinguishing the periods of high and low water, each of which is divided into threedistinct types. The first type displays low water due to nival retention (the lowest meaninter-annual discharge appears in spring or the beginning of the summer), an interme-diatetypeinvolvestwolowestmeanmonthlyinter-annualdischargesthatdonotsucceed each other, and the third is characterized by low water due to a dominatingprocess of evapotranspiration or when summer precipitation is very low. In the samemanner, the discharges of floods or high water periods are divided into three types: atype in which the flood is due to snowmelt, a transition type tending towards a pluvialregime,andatypewheremaximummonthlydischargetakeplaceintheautumn(supplied primarily by rain). Usually, these types are labeled L1, L2 and L3 for the lowflows and H1, H2, H3 for the floods.Basedonthisclassification,someauthorshaveidentifiedsixcombinationsrepresentingsixhydrologicalregimesintheScandinaviancountries.ItshouldbeaddedthatthisnorthernEuropeanclassificationsystemhasbeenadoptedbythecountriesparticipatingintheFRIENDproject(FlowRegimesfromInternationalExperimental and Network Data) , an international regional hydrology project that ispart of phase five of UNESCO's international hydrological program.10.1.4 Quantitative AspectsFromaquantitativepointofview,wenotedpreviouslythatitisdifficulttoautomaticallyexploitseriesofdischargesinordertodeterminethehydrologicalregime. Haines et al. (1988) proposed constructing a typology based on an index ofsimilaritybetweenmeanmonthlyinter-annualdischarges.Thisclassification,validated a posteriori with 913 flow series measurements on all continents, makes itpossible to automatically attribute a measurement station to a hydrological regime. Theindex of similarity used is: 2011 by Taylor and Francis Group, LLCHydrology: A Science of Nature 267(10.3)where Isis the index of similarity, QmX1,i is the mean monthly inter-annual dischargefor the ithmonth of the year at point X1 and QmX2, Iis the mean monthly inter-annualdischarge for the ithmonth of the year at point X2.KrasovskaaandGottschalk(1996)tookadifferentapproach,proposingananalytical determination of the hydrological regime by calculating a stability index ofoccurrence of low and high flow periods. This index is based on the concept of datacontent and entropy and is calculated as follows:(10.4)where H is the entropy of the system and pi is the probability of occurrence of the statei of the system which includes a maximum of n states. From this definition, the authorspropose a classification based on an iterative procedure that makes it possible to assigna series of observations from a particular measurement station to a particular regime.Finally, in addition to the relatively complex classifications we have just mentioned,there are some rather more empirical methods of regime classification that are basedessentially on climatic classifications.10.2 SIMPLE REGIMEAccording to Pard (1955), a simple regime is characterized by only one maximumandoneminimumofannualcoefficientofflowandusuallyinvolvesonlyonealimentation or water supply mode (glacial, nival or pluvial regime). There are severaltypes of simple regimes.10.2.1 Glacial RegimeIn temperate climates such as Switzerland's, a glacial regime is characterized by: Very high discharge in summer after the ice melt; in our region, the uniqueannual maximum and very discernable discharge occurs in July-August. Very low discharge at the end of the autumn, in winter, and in early spring (afew l/s /km2). Amplitude of monthly variation of discharge greater than 25. Very high daily variability in discharge during the year (in the order of 1 to20). Great inter-annual regularity of the regime because temperature is the mostii1122112( ) =( )==i( )=2112IsQmX1,iQmX2,i;QmX1,iQmX2,i.QmX1,iQmX2,iH p pi iin= ( )=ln1piin==11with 2011 by Taylor and Francis Group, LLCHydrological regimes 268regular of all the weather parameters. High flow (several hundred l/s/km2). This regime is found in high-altitude watersheds (average altitude higher than 2500meters) that are partially covered by glaciers. There is also a transition glacial regime which differs from a glacial regime in thefollowing ways: Average altitude of 2300 to 2600 meters. Monthly minimal discharge coefficient ranging from 0.8 to 0.12. Amplitude of monthly discharge varies from 12 to 35.10.2.2Nival RegimeA pure nival (snow) regime presents in attenuated form the same characteristics asthe glacial regime. However, maximum flow takes place earlier (in June). It is subdi-vided into mountain and plain nival. The plain nival regime is of concern to the low-altitude continental and maritimeareas of northern Europe. Its characteristics are:ShortandviolentfloodinApril-Mayfollowingmassivespringthawingofwintersnows;atthesamelatitude,plainfloodoccursearlierthaninthemountain regime. Great daily variability. Very great variability over the course of the year. Great inter-annual variability.Significant flow.There is also a transition snow regime which is found in watersheds at an average0.00.51.01.52.02.53.0Jan March May July Sept Nov Feb April June Aug Dec OctMonthly discharge coefficient [-]Fig. 10.3 : Monthly discharge coefficient for glacial regime (Rhne at Brigue). 2011 by Taylor and Francis Group, LLCHydrology: A Science of Nature 269altitude of between 1200 and 1600 meters. The transition nival regime is in fact closertoacomplexregimeduetothepresenceoffourhydrologicalseasons.Atransitionnival regime is characterized by: The curve of monthly discharge coefficients shows two maximums and twominimums. A minimum coefficient, in January, of about 0.2 to 0.5. After relatively low flow in October, one sees a slight rise in November dueto rain, which creates a secondary maximum with a coefficient of less than 1.10.2.3Pluvial RegimeAlthoughthepluvial(orrainfall)regimeisasimpleregime,itpresentssomedifferent characteristics from the preceding regimes. It is characterized by: High water(witha more or less conspicuous maximum) in winter and lowwater in summer; evaporation plays a big role here because it is common thatthe rainfall in low water season equals or exceeds the rainfall in the season ofhigh water. Great inter-annual variability; the moment of maximum high water can varyappreciably from one year to another depending on the variability of the rains. Flow is generally rather weak (Seine: 6 l/s/km2).This regime is typical of rivers at low to moderate altitude (500-1000 meters).10.2.4Tropical Pluvial RegimeThe tropical rainfall regime can be distinguished by the shape of the Cm variationcurves, which look like those of glacial regimes. Tropical regimes have the followingcharacteristics: 0.00.51.01.52.02.5Monthly discharge coefficient [-]Jan March May July Sept Nov Feb April June Aug Dec OctFig. 10.4 : Monthly discharge coefficient for a nival regime (Simme at Oberwil). 2011 by Taylor and Francis Group, LLCHydrological regimes 270 Drought in the cold season and abundant rainfall in the warm season (fromJune to September); the maximum occurs at the end of summer. Minimumcanshowverylowvalues;inKoulikoro(Guinea),forexample,discharge can exceed 8000 m3/s in September but remain below 100 m3/s atthe end of spring. This indicates great variability of discharge during the year. Relatively regular from one year to another; however some years are markedby a net rainfall deficit (as was the case in the years 1971 and 1973) in sub-Saharan regions.The Niger River is classified as a tropical pluvial regime. 0.00.20.40.60.81.01.21.41.61.82.0Jan March May July Sept Nov Feb April June Aug Dec OctMonthly discharge coefficient [-]Fig. 10.5 : Monthly discharge coefficient for a pluvial regime (Sane at Saint-Albin).0.00.51.01.52.02.53.03.54.0Monthly discharge coefficient [-]Jan March May July Sept Nov Feb April June Aug Dec OctFig. 10.6 : Monthly discharge coefficient for a tropical regime (Niger at Banankoro). 2011 by Taylor and Francis Group, LLCHydrology: A Science of Nature 27110.3 MIXED REGIME A mixed regime is characterized by two maximums and two minimums, which areinfluencedbyseveralinputparameters(nivo-glacial,glacio-nival,nivo-pluvial,pluvio-nival).10.3.1 Nivo-glacial RegimeThis regime has the following features: Onlyonerealmaximumwhichoccursratherearly(May-June-July),corresponding to the snow melt followed by the glacial melt. Relatively high diurnal variations during the hot season. Greatvariationsfromoneyeartoanother,althoughlessthaninthesnowregime. Significant flow.10.3.2Nivo-Pluvial RegimeThis regime is actually subdivided into upper nivo-pluvial (average altitude of between1000 and 1200 meters) and lower nivo-pluvial (average altitude between 750 and 1000meters) and is characterized by: Twomaximums,onehighlypronouncedaroundApril-May,andtheotherabout November; depending on rainfall, the second maximum can be weak(with a coefficient lower than 1). A main low water in October and a secondary low water in January, both withcoefficientsof 0.6 to 0.8.0.00.20.40.60.81.01.21.41.61.82.02.22.42.6Monthly discharge coefficient [-]Jan March May July Sept Nov Feb April June Aug Dec OctFig. 10.7 : Monthly discharge coefficient for anivo-glacial regime (Albula at Tiefencastel). 2011 by Taylor and Francis Group, LLCHydrological regimes 272 The amplitude, defined as the relationship between the extreme monthly co-efficients, is between 2 and 5. Significant inter-annual variations.10.3.3Pluvio-nival regimeOverall,rainfalltendstobehigherinwatershedsatloweraltitudes(650to750meters). The pluvio-nival regime is characterized by: AperiodofrainfallinJanuary,followedbyalightincreaseinFebruary-March. A principal or single low water in October, with a coefficient of 0.7 to 0.85. Low amplitude (1.4 to 2).0.00.20.40.60.81.01.21.41.61.8Monthly discharge coefficient [-]Jan March May July Sept Nov Feb April June Aug Dec OctFig. 10.8 : Monthly discharge coefficient curve for a pluvio-nival regime (Mississippi at Alton).012345Monthly discharge coefficient [-]Jan March May July Sept Nov Feb April June Aug Dec OctFig. 10.9 : Example of a monthly discharge coefficent curve in a nivo-pluvial regime (Emme at Emmenmat). 2011 by Taylor and Francis Group, LLCHydrology: A Science of Nature 27310.4 COMPLEX REGIMEThe complex regime is generally found at large rivers, where the tributaries, fromupstream to downstream, influence the main flow in very diverse ways. The regimesof large rivers are basically a synthesis of the regimes of itsconstituent watersheds,which can vary enormously in termsof altitude, climate, etc. Usually, these variousinfluences tend to attenuate extreme discharges and to increase the annual regularityof the mean monthly discharge from upstream to downstream.10.5 HYDROLOGICAL REGIMES IN SWITZERLANDIn Switzerland, hydrological regimes were classified and regionalized following astudy of 95 representative watersheds chosen from the federal and provincial networksusingwell-definedcriteria:thenaturalflowregime,withseriesofhomogenousmeasurements over a number of years, a surface area of between 10 and 500 km2, andflows not influenced by large lakes or dams. This study, which was carried out by theInstituteofGeographyattheUniversityofBern(AschwandenandWeingartner,1985), led to the drawing of a map of natural hydrological regimes with average inter-annual discharges. Three classes of hydrological regime were distinguished: the alpine regime, the Jura Plateau regime, the southern Alps regime. Thesethreeprincipalclassificationswereinturnsubdividedinto16typesofregimes, assigned as a function of spatial parameters (average altitude, surface area ofglaciers) orfollowingstatisticalanalyses. Itshouldbenoted thatthe regimesdeter-minedinthisprocesswereforwatershedsundernaturalconditions,andacertainnumber of rivers have had their behavior modified by anthropogenic activities.0.00.51.01.52.02.53.0Monthly discharge coefficient [-]Jan March May July Sept Nov Feb April June Aug Dec OctFig. 10.10 : Monthly discharge coefficientfor a pluvio-nival regime (Ishikari at Ishikari-Ohashi Japan). 2011 by Taylor and Francis Group, LLCHydrological regimes 274Basedontheseanalyses,Switzerlandrefinedanddevelopedanautomatedprocedure for attributing a measurement station to one of the sixteen regimes of classi-fication. Other more recent developments have since taken place and can be consultedin the Hydrological Atlas of Switzerland (2002).10.6CONCLUSIONSThis chapter summarizes some of the typologies of hydrological regimes of rivers.ItisworthnotingthatthereisatrendawayfromPard'sapproachtoclassificationbased on climatic similarities, and towards automatic "objective" classifications basedon algorithms. What we should take away from this chapter is the idea that a hydrological regimeisacomplexconsequenceoftheinteractionofseveralfactors,fromthephysicalstructure of the watershed, to its alimentation regime, and touching upon the topologyof its rivers and the structure of its geological substrate. Finally, as a result of humaninterventions in the form of increasingly larger installations, a hydrological regime canchange not just over the course of geological time, but in a century or even a decade. 2011 by Taylor and Francis Group, LLC