HYDROGRAPH SEPARATION OF THE AMAZON RIVER USING 18O AS AN ISOTOPIC TRACER

HYDROGRAPH SEPARATION OF THE AMAZON RIVER USING ¹⁸O AS AN ISOTOPIC TRACER

J. MORTATTI; J.M. MORAES; J.C.RODRIGUES, JR; R.L. VICTORIA; L.A. MARTINELLI

Centro de Energia Nuclear na Agricultura/USP - C.P. 96, CEP: 13400-970, Piracicaba, SP.

ABSTRACT: The ¹⁸O content of rain and river waters was used as an isotopic tracer in order to carry out the hydrograph separation of the Amazon river, during the 1973-1974 hydrological years, and to estimate the contributions of the surface runoff (event water) and baseflow (pre-event water) components to the total river flow. The average surface runoff and baseflow contributions were 30.3 and 69.7% respectively. At peak discharge, the mean contribution of the baseflow was about 57%. The results of the isotopic separation model were compared with the filter-separation autoregressive method, showing similar behavior and magnitude.

Key Words: isotopic tracers, hydrograph separation, Amazon river

SEPARAÇÃO DE HIDRÓGRAFAS DO RIO AMAZONAS USANDO ¹⁸O COMO TRAÇADOR ISOTÓPICO

RESUMO: Os teores de ¹⁸O em águas de chuva e amostras de água de rio foram utilizados como traçadores num modelo isotópico de separação de hidrógrafas do rio Amazonas durante o período 1973-1974, com o intuito de se estimar as contribuições do escoamento superficial rápido e escoamento de base para o escoamento total da bacia de drenagem. As contribuições médias desses reservatórios foram de 30,3 e 69,7% respectivamente. Durante o período de pico de cheia, o fluxo de base contribuiu com cerca de 57% do volume total de água escoada. Os resultados do modelo de separação isotópico foram comparados com o método estatístico de filtros auto-recursivos, mostrando comportamentos similares.

Descritores: traçadores isotópicos, separação de hidrógrafas, rio Amazonas

INTRODUCTION

In a drainage basin, the total river flow may be considered in a simple model as the sum of two components: the surface runoff and the baseflow (Pilgrim et al.,1979; Hooper & Shoemaker, 1986; Robson & Neal, 1990). The surface runoff is related to event waters and is generated immediately after the rainfall. This flow component, basically responsible for the mechanical erosion in the drainage basin, can be separated in three subcomponents: direct, subsurficial and overland flows, the last being specific in floodplain areas (Martinelli et al., 1989). The baseflow, with a slower circulation and responsible for chemical erosion processes, is derived from several sources occuring generally in deeper soil layers and groundwater. The knowledge of these contributions to the total river flow is a key step to assess the erosion balance. Most of previous studies about mechanical erosion relate the river suspended sediment load to the total river discharge and not to the surface runoff component. This type of erosion that takes place in the soil surface, corresponds to the loss in suspension of clays, secondary minerals and organic matter, and also hydrolysed primary minerals of bedrock (Tardy, 1990).

Several methods were described for separating river flow hydrographs in two or more components. Some of them use the graphical procedures, but for large basins the major difficulty is to estimate the maximum baseflow discharge during the high water periods. In addition, due to the smoothness of the hydrogram curves in large rivers, the precision for this kind of approach is low (Barnes, 1939; Schoeller, 1962; Castany, 1963; Roche, 1963; Linsley & Franzini, 1964; Chernaya, 1964; Chow, 1964; Rambert, 1971; Tardy, 1986; Probst & Bazerbachi, 1986; Etchanchu & Probst, 1986; Kattan et al., 1987). Others procedures are related to chemical and mixing models between reservoirs. These classical methods use the concentration of conservative elements as tracers to identify the routing of water in the drainage basin, (Lasala, 1967; Pinder & Jones, 1969; Pilgrim et al., 1979; Takeuchi et al., 1984; Hooper & Shoemaker, 1986; Kattan & Probst, 1986; Robson & Neal, 1990; Neal et al., 1992; O'Brien & Hendershot, 1993; Mortatti et al., 1994; Tardy et al., 1995). All these methods use direct measurements mainly carried out in small watersheds and generally are not applicable to large drainage basins. Statistical approaches using spectral analysis and filter-separation autoregressive methods are also used (Jackson, 1974; Mangin, 1981; Probst & Sigha, 1989; Mortatti et al., 1992; Hino & Hasebe, 1981, 1986; Araújo & Dias, 1995).

The use of stable isotopes as conservative tracers, mainly HDO and H₂¹⁸O is reported by several authors as being the only way to know the origin and amounts of components mixing in surface water systems (Dincer et al., 1970; Mook et al., 1974; Fritz et al., 1976; Sklash & Farvolden, 1979).

In this study we will apply the ¹⁸O as an isotopic tracer in order to separate the hydrograph of the Amazon river and to estimate the surface runoff and baseflow components to the total river flow. The results will be compared with the filter-separation autoregressive method, developed by Hino & Hasebe (1981), applied to the data for the same period.

MATERIAL AND METHODS

The Amazon river drains the largest contiguous tropical rain forest area of the world with a basin of about 6 x 10⁶ km² and discharge of 5.5 x 10³ km³ of water per year. According to Oltman et al. (1964), this value represents 15% of the total global runoff. Approximately 60% of the rainfall (2000-2400 mm/y) return to the atmosphere via evapotranspiration (Salati et al., 1979; Salati & Marques, 1984). Three major morphostructural zones characterize the diversity of the geological formation in the Amazon basin: (i) the Precambrian Shields (metamorphic and igneous rocks); (ii) the Andean Cordillera (carbonate rocks and evaporites); and (iii) the Amazon Trough (Pleistocene fluvial deposits).

Water samples from the main channel for the upper Solimões river at Benjamin Constant and for the lower Amazon river near Obidos, were collected monthly during the 1973-1974 period. The river isotopic data are from Salati et al. (1979) and Reis et al. (1977), synthesized by Mortatti et al. (1985). The monthly isotopic composition for precipitation are from IAEA (1983).

The ¹⁸O analysis were performed at the Stable Isotopes Laboratory of the Centro de Energia Nuclear na Agriculatura, University of São Paulo, according to the CO₂ equilibration method (Epstein & Mayeda, 1953), modified by Matsui (1980). The d ¹⁸O values are given as relative deviations from the SMOW (Standard Mean Ocean Water), defined by Craig (1961) as:

(1)

The precision of d¹⁸O measurements was less than 0.2 ^o/_oo.

At the mouth of the drainage basin, the isotopic composition is imposed by the mixing of waters from different components of the total flux (Qt), with different origins and routing. In order to distinguish pre-event water (Qb, baseflow) from event water (Qr, surface runoff) using ¹⁸O measurements, a two-component mass balance equation can be used:

Qt = Qr + Qb

(2)

Qt d¹⁸ t = Qr d¹⁸ r + Qb d¹⁸ b

(3)

where Q is the discharge measurements (m³/s) and d¹⁸ is the isotopic composition of each flow component (^o/_oo).

The determination of Qr and Qb components requires, according to Sklash & Farvolden (1979), the following assumptions:

(1) The ¹⁸O content of the surface runoff component must to be significantly different from that of the baseflow.

(2) The ¹⁸O content of intrastorm rainfall has no spatial and temporal variations.

(3) The ¹⁸O content of the baseflow remains constant during the flood period.

Combining the equations (2) and (3), yields

(4)

According to equation (4), if the streamflow is measured and the concentrations of ¹⁸O in the river water before and during of the high precipitation period are known, it is possible to identify the contributions of the surface runoff and baseflow to the total river flow.

In order to compare the results obtained by the isotopic hydrograph separation for the Amazon river, it was applied a statistical separation method using a numerical filter, in two components, for the same period. The separation frequency was determined from the order of magnitude of autoregressive coefficients (AR), in a monthly base data. This technique, studied for daily and hourly data, developed by Hino & Hasebe (1981) and used by Araújo & Dias (1995), was chosen due to its simplicity and physically based calculation of some parameters, differently of the classical methods found in the literature. The method is based in a high-frequency cut-off filter which allows the passage of only the low frequency component of the total river flow, corresponding to the baseflow. The high-frequency cut-off filtering of the total river flow time series Q(t), for the first order filter equation is calculated by:

Qb(t) = a [B Qb (t-1)] + A Q(t) (5)

where, Qb(t) is the filtered baseflow, a is the weighting factor chosen to satisfy the condition that the filtered and residual outputs should not be negative, A and B are constants given by:

(6) (7)

where D t is the time interval and T_c = f_c^-1 is the separation period which is the inverse of the cut-off frequency.

RESULTS AND DISCUSSION

The seasonal variation of the ¹⁸O content in the precipitation near the Marajó Island coast and in the Amazon river, at mouth, for 1973-1974 mean monthly data period, can be observed in Figure 1. During the high water period, the ¹⁸O content in the river reached minimum values ranging between -6.0 and -6.5 ^o/_oo, while the precipitation shows a minimun value around -8.0 ^o/_oo. If the river water at the peak stage was almost entirely derived from the precipitation, the ¹⁸O value of the river water should be equal or less than -8.0 ^o/_oo. These measured values (-6.0 and -6.5 ^o/_oo) suggest that part of the peak runoff was derived from pre-event water in the basin. The pre-event water or baseflow was characterized by the ¹⁸O values (d¹⁸b) obtained during the low water period which corresponds to -3.5 and -4.1^o/_oo for 1973 and 1974 periods respectively.

Figure 1
- Seasonal variation of the monthly ¹⁸O content of the Amazon river and rain water at mouth, related with the measured total river flow(Qt), during the 1973-1974 hydrological period.

The ¹⁸O values of the event water (d¹⁸r), which characterize the surface runoff component, was estimated to be -10 ^o/_oo which corresponds to the weighted average of the rain samples collected during the maximun rainfall of the same period at Benjamin Constant, Manaus and Marajó (Salati et al.,1979 and Mortatti et al.,1985), with the respective ¹⁸O values: -13.0, -9.0 and -7.8 ^o/_oo.

The results of the isotopic hydrograph separation of the Amazon river are shown in TABLE 1, where the behavior of the surface runoff (Kr) and baseflow (Kb) coefficients, calculated in a model of two reservoirs, can be observed. The average Kr and Kb were 30.3 and 69.7% respectively. These values are very similar to previous estimations carried out by Mortatti et al. (1994) for the same river, using a chemical method during 1982-1984 period. At peak discharge, the mean contribution of the pre-event water (baseflow) to the total river flow was 57%. Similar values were observed in other drainage basins reported in the literature (Fritz et al., 1976; Sklash & Farvolden, 1979; Leopoldo et al., 1987; Wels et al., 1990; Mortatti et al., 1994). If the isotopic compositions of rain and river waters shows the same seasonal and spatial patterns along the hydrological years, it is possible to estimate the contribution of the surface runoff as a function of the total river discharge, using the relationship obtained during the modeling period. From TABLE 1, the linear correlation between Qr and Qt was delineated by the following equation:

Qr = 0.7085 Qt - 62709 (8)

r = 0.9764

The results obtained from the isotopic hydrograph separation were compared with the filter-separation AR method applied for the same period. The cut-off frequency was estimated from the lag time where the auto-correlation coefficient have an inflection point. This value was around 0.5, giving a recession constant of 0.6065. The average Kr and Kb coefficients were 37.9 and 62.1%. At peak discharge the mean contribution of baseflow to the total river flow was 55.5%, very close to the value obtained by the isotopic separation model. Figure 2 shows the hydrograph separation of the Amazon river by both methods, it can be observed that isotopic and numerical filter method are comparable, mainly during the high water period. At low water, the numerical filter method overestimate the surface runoff contribution. This fact was already observed for other drainage basin by Araújo & Dias (1995). The overestimation could be probably due to the uncertainty of the cut-off frequency calculations during the low water period.

Figure 2
- Comparison of the isotopic and numerical filter hydrograph separation for the Amazon river, in terms of baseflow (Qb), during 1973-1974 hydrological period.

CONCLUSIONS

The isotopic method for hydrograph separation of the Amazon river showed good results in spite of the little available data. Two main reservoirs or components were identified, the surface runoff, represented by the contribution of the rainwater to the basin during the high precipitation period (event water) and the baseflow, characterized by the water that was present before the precipitation (pre-event water). The method highlighted the baseflow contribution during the peak discharge, suggesting that the groundwater plays a much more active and important role in the storm dynamics. For the Amazon river basin, the mean contribution of the baseflow to the total river flow was at peak discharge, 55%. The mean surface runoff contribution, which represents the water volume related to the efective mechanical erosion in a drainage basins, expressed in terms of Kr, was 30.3%. The results obtained by the isotopic separation method shown to be of the same order of magnitude when compared with the statistical method using numerical filter separation.

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Recebido para publicação em 14.08.96

Aceito para publicação em 21.02.97

References

• ARAÚJO, A.A.M.; DIAS, N.L. Separaçăo do hidrograma de cheias através de filtros auto-recursivos. In: Simpósio Brasileiro de Recursos Hídricos, 11.; Simpósio de Hidráulica e Recursos Hídricos dos Países de Língua Oficial Portuguesa, 2., Recife, 1995. p.267-272.
• BARNES, B.S. The structure of discharge recession curves. Transactions of the Amererican Geophysical Union, v.20, n.4, p.721-725, 1939.
• CASTANY, G. Traité pratique des eaux souterraines. Paris: Dunod, 1963. 686p.
• CHERNAYA, T.M. Comparative evaluation of graphical methods of separation of groundwater components of streamflow hydrographs. Soviet Hydrology, v.5, p.454-465, 1964.
• CHOW, V.T. Handbook of applied hydrology. New York: Mc Graw-Hill, 1964. 1400p.
• CRAIG, H. Standard for reporting concentrations of deuterium and oxygen-18 in natural waters. Science, v.133, p.1833-1834, 1961.
• EPSTEIN, S.; MAYEDA, T. Variation of ¹⁸O content of water from natural sources. Geochimica Cosmochimica Acta, v.4, p.213-224, 1953.
• ETCHANCHU, D.; PROBST, J.L. Erosion et transport des matičres en suspension dans un bassin versant en region agricole. Methode de mesure du ruisselement superficiel, de sa charge et des deux composantes du transport solide dans un cours d'eau. Comptes Rendus de l'Academie des Sciences. Serie 2, v.302, n.17, p.1063-1068, 1986.
• FRITZ, P.; CHERRY, J.A.; WEYER, K.U.; SKLASH, M.G. Storm runoff analysis using environmental isotopes and major ions. In: INTERNATIONAL ATOMIC ENERGY AGENCY. Interpretation of environmental isotope and hydrochemical data in groundwater hydrology. Vienna: IAEA, 1976, p.111-131.
• HINO, M; HASEBE, M. Analysis of hydrologic characteristics from runoff data - A hydrologic inverse problem. Journal of Hydrology, v.49, p.287-313, 1981.
• HINO, M; HASEBE, M. Separation of a storm hydrograph into runoff components by both filter-separation method and environmental isotope tracers. Journal of Hydrology, v.85, p.251-264, 1986.
• HOOPER, R.P.; SHOEMAKER, C.A. A comparison of chemical and isotopic hydrograph separation. Water Resources Research, v.22, p.1444-1454. 1986.
• INTERNATIONAL ATOMIC ENERGY AGENCY. World survey of isotope concentration in precipitation (1976-1979) Vienna: IAEA, 1983.
• JACKSON, R.E. Time serie analysis of groundwater hydrograph for surficial deposits of Canadian shield. Canadian Journal of Earth Science, v.11, p.177-188, 1974.
• KATTAN, Z.; PROBST, J.L. Transports en solution et en suspension par la Moselle en périodes de crues. In: JOURNESS D'HYDROLOGIE, Strasbourg, 1986. Crue et inondations; actes. Strasbourg, 1986. p.143-167.
• KATTAN, Z.; GAC, J.Y.; PROBST, J.L. Suspended sediment load and mechanical erosion in the Senegal basin, estimation of the surface runoff concentration and relative contributions of channel and slope erosion. Journal of Hydrology, v.92, p.59-76. 1987.
• LASALA, A.M. Jr. New approaches to water-resources investigations in upstate New York. Ground Water v.5, n.1, p.6-11, 1967.
• LEOPOLDO, P.R.; MARTINEZ, J.C.; MORTATTI, J. Runoff hydrograph analysis in agricultural watersheds by oxygen-18. In: International Symposium on the Use of Isotope Techniques in Water Resources Development, Vienna. 1987. Vienna: IAEA, 1987. p.30-32.
• LINSLEY, R.K.; FRANZINI, J.B. Water-resources engineering. New York: Mc Graw Hill, 1964.
• MANGIN, A. Apports des analyses corrélatoires et spectrale croisées dans la connaissance des systémes hydrologiques. Comptes Rendus de l'Academie des Sciences. Serie 2. v.293, p.1011-1014, 1981.
• MATSUI, E. A simple method using a disposable syringe to prepare samples for d¹⁸O measurements in water samples. Analytica Chimica Acta, v.120, p.423-425, 1980.
• MOOK, W.G.; GROENEVELD, D.J.; BROWN, A.E.; VAN GANSWIJK, A.J. Analysis of a runoff hydrograph by means of natural ¹⁸O. In: SYMPOSIUM ON Isotope Techniques in Groundwater Hydrology, Vienna, 1974. Proceedings. Vienna: IAEA, 1974. p.145.
• MORTATTI, J.; SALATI, E.; VICTORIA, R.L.; RIBEIRO, M.N.G. Analysis of the isotopic behaviour of hydrogen and oxygen of water in the main channel of the river Solimőes/Amazon. Mitteilungen aus dem Geologisch-Palaontologischen Institut der Universitat Hamburg. v.58, p.259-266, 1985.
• MORTATTI, J.; PROBST, J.L.; FERREIRA, J.R. Hydrological and geochemical characteristics of the Jamari and Jiparana river basin (Rondonia, Brazil). Geojournal, v.26, n.3, p.287-296, 1992.
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Publication Dates

• Publication in this collection
  03 Feb 1999
• Date of issue
  Sept 1997

History

• Accepted
  21 Feb 1997
• Received
  14 Aug 1996