Hydrological Sciences—Journal—des Sciences Hydrologiques, 43(3) June 1998
409
Interannual variability of annual streamflow and
the Southern Oscillation in Costa Rica
RICHARD K. GEORGE
United States Defense Attache Office, US Embassy, Santiago, Chile
PETER WAYLEN
Department of Geography, University ofFlorida, Gainesville, Florida 32611-7315,'USA
e-mail: [email protected]
SADÎ LAPORTE
Department ofHydrology, Institut o Costarricense de Electricidad, Apdo 10 032,
1000 San José, Costa Rica
Abstract This study illustrates the association between annual and seasonal
streamflow characteristics on six Costa Rican rivers and the Southern
Oscillation Index (SOI). Annual discharge from rivers within the Pacific
watershed are clearly positively associated with contemporary values of the SOI
and experience significant reductions in both mean and variance in El Nine
years. The considerable practical implications of this finding to a country in
which over 60% of national electrical power comes from hydroelectric schemes
is illustrated using quantile estimates from various models. Rivers draining
towards the Caribbean show less clear and coherent patterns of associations.
The observed associations with seasonal flows on some rivers appear to be the
opposite of those on the Pacific, and may even vary during the course of a year
at a site. The exact nature of the response seems to be closely related to the
elevation of the gauge site. The larger the proportion of the basin at elevations
above about 500-1000 m the greater the similarity to the Pacific pattern,
suggesting that the marked topographic divide between the two coastal
watersheds does not correspond to the divide in associations between streamflow
and the SOI.
Relation entre l'oscillation australe et la variabilité interannuelle des débits
au Costa Rica
Résumé Cette étude illustre la relation entre les caractéristiques des débits
annuels et saisonniers de six rivières costaricaines et l'indice de l'oscillation
australe (IOS). Les débits moyens annuels des rivières du versant Pacifique sont
nettement corrélées positivement avec les valeurs correspondantes de l'IOS, et
présentent à la fois une diminution de leur moyenne et de leur variance pour les
années El Nino. Les importantes implications pratiques de ces résultats dans un
pays dont 60% de la production électrique est d'origine hydraulique sont
illustrées par l'estimation de quantiles issus de différents modèles. Les rivières
du versant Caraïbe ne présentent pas de corrélation aussi nette. Certaines
rivières ont un comportement qui s'oppose à celui des rivières du versant
Pacifique et qui peut même varier au cours de l'année pour un même site. La
nature de la relation semble être étroitement liée à l'altitude de la station de
jaugeage. Plus la portion du bassin située au dessus de 500-1000 m est grande,
plus la similitude avec les bassins du versant Pacifique est importante, ce qui
suggère que la division topographique très accusée entre les deux versants ne
correspond pas à une limite de la corrélation entre les débits et l'IOS.
Open for discussion until 1 December 1998
410
Richard K. George et al.
INTRODUCTION
Considerable research has investigated the links between causes of climatic variability, particularly the El Nino-Southern Oscillation (ENSO), and precipitation in the
tropics. Several investigators (e.g. Hastenrath, 1990; Poveda & Mesa, 1997) have
noted the advantages of seeking regional associations through the analysis of
streamflow data. The ability of drainage basins to integrate the potentially noisy at-apoint estimates of precipitation provides both modelling and logistic advantages in
areas where rainfall records may be sparse. From an applied perspective, streamflow
may have greater direct consequences, at a variety of social and economic levels,
than rainfall. Many countries in the tropics have invested heavily in hydroelectric
power in the last three decades and correct operation of reservoirs is critical. Costa
Rica derives over 60% of its national power from water, half of that coming from a
single project, the Arenal. For example, Colombia endured rotating daily blackouts
of 16 h during 1992, incurring considerable losses. If the effects of climate
variability are regional in their scale, little relief will be gained from the proposed
creation of a Central American electricity grid. Other dimensions of consequences
may include: disruption of irrigation, increased occurrence of disease, deterioration
of water supply for drinking and sanitation, and losses of freshwater fisheries and
lucrative tourism.
The ability to improve estimates of likely streamflow characteristics is therefore
of considerable practical importance. This paper considers the annual streamflow
characteristics from six basins in Costa Rica, and attempts to establish the nature and
timing of their association to the ENSO phenomenon and any patterns of spatial
variability therein. Despite its limited areal extent, the topographic complexity of
Costa Rica produces a variety of precipitation regimes which are reflective of the
much broader, and frequently less well monitored region of Central America (Portig,
1965; Hastenrath, 1967). The practical implications of the findings are discussed in
terms of flow quantile estimation and potential sites of future hydroelectric power
generating schemes.
ENSO AND STREAMFLOW IN THE REGION
The regional response of annual precipitation within the Caribbean and Central
America to a warming of the Equatorial Pacific (El Nino) has been considered to be
one of drought (Ropelewski & Halpert, 1987; Hastenrath, 1988; Rogers, 1988), and
excess rains during periods of cooler sea surface temperatures. More detailed
analyses of records from within particular countries (Estoque et al., 1985; Cavazos
& Hastenrath, 1990; Fernandez & Ramirez, 1991; Herrera, 1994), reveal a contrast
between locations on either side of the Cordillera which divide the isthmus. The
response is far less marked on the Caribbean side of Costa Rica, where precipitation
during the differing seasons indicates opposing associations (Waylen et al., 1996b).
Hastenrath (1990) sought an ENSO signal in the streamflow records of several
rivers in northern South America. Of those records studied, outflows from the
Interannual variability of annual streamflow and the Southern Oscillation in Costa Rica
411
Madden Reservoir, Panama, were the closest to Costa Rica. This, and records from
other rivers in Colombia and Venezuela, appeared to confirm the regional association
of drought to higher sea surface temperatures in the Pacific. The outflows displayed
a bi-modal regime, but only the flow in the earlier (July-August) peak showed
significant association to the Southern Oscillation Index (SOI), which is used as one
measure of ENSO. The larger November-December peak was associated with zonal
winds in the 10-20°N region. These are dominantly northeast trades originating over
the northern equatorial Atlantic and Caribbean. This inconsistency of associations
within the regime, and their varying provenances, may also be indicative of the
hydrometeorological complexity of the area.
Poveda & Mesa (1997) have proposed a scheme by which the displacement of the
eastern equatorial Pacific inter-tropical convergence zone (ITCZ) during years of a
warm Pacific (Pulwarty & Diaz, 1993) decreases the number of storms which
penetrate into the Colombian and Brazilian Amazon from the Pacific (Velasco &
Frisch, 1987). The resulting reduction in évapotranspiration from the continent
diminishes the regional intensity of the ITCZ, thereby diminishing the pressure
gradient to the North Atlantic anticyclone, and weakening the northeast trades. Costa
Rica, the southern isthmus of Central America and northwestern South America may
therefore lie at the intersection of various interrelated and lagged climatological
influences which will be reflected in regional streamflow.
STUDY AREA AND DATA
Figure 1 shows the general physiographic setting and extent of the six river basins,
all of whose flows are unmodified by major hydraulic structures. Data are collected
by Instituto Costarricense de Electricidad (ICE), the national electricity generating
company. The basins are broadly representative of both flanks of the cordillera and
have reliable daily data (Table 1). Available records cover the last three decades, but
those from the more remote southeast are shorter. Together the gauged areas represent about 15% of total national territory, extending from near the Nicaragua border
in the northwest to the Panamanian border in the south. In 1970 the gauge on the
Colorado River was moved downstream, increasing the basin area from 125.2 km2 to
128.2 km2, otherwise gauge locations have remained fixed. Descriptive statistics of
Table 1 Basin, gauge and historic record information for the six rivers in the study.
Basin
area
(km2)
661.4
Candelaria at El Rey
Terraba at Palmar
4766.7
125.2
Colorado at Colorado
128.2
Colorado at Coyolar
Estrella at Pandora
634.5
367.4
Pacuare at Bajo Pacuare
Sarapiqui at Puerto Viejo 820.9
River basin and location
Station
latitude
(N°)
9.40
8.59
10.43
10.42
9.45
9.49
10.28
Station
longitude
(W°)
84.16
83.29
85.29
85.30
82.59
83.30
84.00
Period of
record
1964-1992
1963-4992
1952-1969
1971-1992
1974-1992
1959-1991
1969-1992
Number
of years
of record
29
30
18
22
19
33
24
Elevation
(m a.s.l.)
175
65
320
315
17
608
75
412
Richard K. George et al.
85°W
84°W
83°W
Fig. 1 Topography of Costa Rica at contour intervals of 500 m, showing the
locations of the six drainage basins, their gauge sites and the locations of the two
major hydroelectric power generating schemes.
the two periods on the Colorado actually suggest a decrease in mean, variance and
skewness in the latter period. This is statistically significant at the 0.05 level for the
variance only. On the basis of these observations it was concluded that the two series
could be combined.
Details of the precipitation regimes of the area are available elsewhere
(e.g. Portig, 1965; Hastenrath, 1967) and are summarized by Waylen et al. (1996b),
into five regions (Fig. 2). The cordillera runs orthogonal to the path of the prevailing
northeast trades, producing heavy precipitation along the Caribbean coast, particularly during the boreal summer, and rainshadow along the Pacific. The ITCZ in the
eastern equatorial Pacific generally migrates between a southerly winter extreme of
3°N and a summer position of 10°N. It is associated with a broad band of unstable
air, bringing heavy rainfall to the Pacific coast and cross-equatorial westerlies during
the period May-November. This rainy season is shorter to the north, and is interrupted by a drier period, the Veranillos de San Juan, in July, corresponding to an
intensification of the trades over the Caribbean. Tropical cyclones in the North
Interannual variability of annual stream/low and the Southern Oscillation in Costa Rica
h
413
1
nil n1 1
r
2
...,_
i
•
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_L_L...
--il 111
Fig. 2 A regionalization of monthly precipitation regimes displaying typical regimes
as monthly percentages of annual total rainfall (numbered 1-5), and the mean
monthly streamflow regimes of the six rivers (letters A-F).
Atlantic and Caribbean, which have peak occurrences in September and October,
disrupt the trades, reverse the direction of the pressure gradient across the isthmus,
and are frequently associated with copious rainfalls along the Pacific (Vargas &
Trejos, 1994). During the boreal winter the Caribbean coast is also subject to rainfall
from cold fronts at the leading edge of outbursts of cold air, or nortes, originating
over the continental North America (Klaus, 1973; Reding, 1992; Schultz et al.,
1998). These events have been recorded from September through to May, but have
the greatest historic frequency in November and December.
Patterns of precipitation are clearly reflected in the mean monthly hydrographs of
the six rivers (Fig. 2). Pacific coast rivers are distinctly bi-modal, with the first peak
corresponding to the rainy season before the Veranillos, followed by much a larger
response which declines throughout November and December. Bi-modality corresponding to the intensification of the trades and the season of cold outbursts can be
clearly detected on the Sarapiqui and Estrella. It is less pronounced than on the
Pacific rivers due to the absence of truly "dry" seasons, and is also out of phase with
them. On the basis of the hydrographs and the regional climatology, each year is
divided into five seasons (as detailed in Table 2) for seasonal analysis.
The Southern Oscillation Index (SOI) is a standardized difference of the atmospheric pressures of Tahiti and Darwin, representing the South Pacific anticyclone and
the Indonesian low respectively. It is frequently used as an indicator of atmosphereocean conditions throughout the South Pacific. Negative values (low phase) are
generally associated with warmer ocean waters in the eastern Pacific and an enhanced
El Nino current along the western coast of South America. Positive values (high
phase) are associated with colder waters and are frequently termed "La Nina" or
414
Richard K. George et al.
Table 2 Definition of flow seasons employed in the analyses. (These lag a little behind the patterns of
precipitation along both the Caribbean and Pacific slopes of Costa Rica).
Season
April, May, June
July, August
Caribbean precipitation
Generally drier, although some
precipitation associated with
trade winds, alisios.
Rainfall increase associated with
intensified northeast trades.
Pacific precipitation
Early, smaller peak in rainy season as ITCZ
moves northwards in Pacific. More marked
in south.
Slight decrease in precipitation, the
Veranillos de San Juan, due to rainshadow
effect of trades.
Annual maximum as ITCZ and westerlies
reach maximum extent. Height of tropical
storm activity in N. Atlantic encouraging
westerlies across the isthmus.
Dry season (particularly in north), as ITCZ
retreats southwards. Region entirely in
rainshadow.
September, October,
November
Trade winds relax, precipitation
diminishes.
December, January
Period of frequent but milder
cold fronts, nortes, generally
associated with high pressure
over North America originating
over the North Pacific
Infrequent but severe cold
Continued dry season along entire Pacific
fronts, which may bring copious slope, although some rains persist in the
extreme south.
rains. High pressure originates
over Canadian Plains.
February, March
"anti-El Nino". Monthly SOI are published by the US Department of Commerce
(1996) and may be downloaded from the Internet from http:Wwww.nic.fb4.noaa.gov.
METHODS
Data were tested for normality using a modified Kolmogorov-Smirnov
goodness-of-fit test (Lilliefors, 1967) at the 0.20 level to try to minimize the chances
of committing type II errors, while testing the null hypothesis that there was no
significant difference between the observed and fitted distributions of streamflow. In
addition, the probability plot correlation coefficient test for normality (Filliben,
1975) was applied to further strengthen the test of normality. Pearson product
moments were used to estimate correlations between measures of the SOI and
streamflow variables over various lagged time periods. Analysis of precipitation data
(Waylen et al., 1996a) suggested that lags from -1 to +2 years needed to be
considered. Historic series were subdivided into three sub-populations corresponding
to warm, cold and other years according to Table 3. Probability distributions, F, , of
annual flows under separate sets of conditions (z = 1, 2, 3) were weighted according
to their relative frequency of occurrence in the historic record, p,. The overall mixed
probability distribution, FT, was obtained by summing the individual weighted
probabilities (Waylen & Caviedes, 1990) as follows:
Table 3 Years of warm and cold events during period of historic streamflow records, 1952-1992.
Warm Pacific: 1953; 1957; 1958; 1965; 1972; 1973; 1977;1978; 1982; 1983; 1986; 1987; 1991; 1992
Cold Pacific: 1954; 1960; 1963; 1964; 1966; 1967; 1968; 1970, 1979; 1980; 1988; 1989
Interannual variability of annual streamflow and the Southern Oscillation in Costa Rica
FT=I.rlFl
415
(1)
7=1
In this case each sub-population was considered to be normally distributed.
Comparisons of variances in each sub-population were completed using F-tests, and
means were compared using the appropriate r-test. All tests of significance of
correlations and statistics were completed at the 0.05 level.
RESULTS
Correlation analyses
Figure 3 shows the correlation between annual discharge and the SOI for all six
rivers. The pattern along the Pacific coast is consistent and statistically significant at
lag 0. These clearly display the anticipated positive correlation to the SOI (droughts
in years of warmer eastern Pacific), which appear to be foreshadowed in the previous
year, although only the observation on the Colorado can be considered to be
significant at that lag. There is also a consistent, but not significant, tendency for
negative correlations to occur in the two following years. The larger Terraba basin in
the south consistently yields higher values of the coefficient. By contrast, the
Caribbean rivers show little coherence in their responses. The centrally located
Pacuare mimics the pattern of correlations of the Pacific coast, although none of the
coefficients are significant. The more southerly Estrella is in complete opposition,
Estrella
0.6 -
0.4
o
O
0.2
0.0
-0.2
-0.4
Lag (Years)
Fig. 3 Lag cross-correlations between annual streamflow and the SOI, from
precipitation one year preceding the SOI to two years after it (filled symbols
represent significant correlations at the 0.05 level).
Richard K. George et al.
416
with positive correlations at lags one and two years (droughts following a warm
episode), a pattern that is repeated, but not as markedly, in the Sarapiqui.
Frequency distributions
Normal distributions fitted to the historic data using the parameter estimates listed in
Table 4 lead to rejection of the null hypothesis of normality on all Pacific rivers and
on the Pacuare. The test statistics are equally poor for the Estrella and Sarapiqui
Rivers, but their small sample sizes increase the critical value. In light of the
skewness of the data sets (see, for example, Fig. 4(a)), the general inadequacy of the
normal distribution and the proven association of flow with the SOI, each time series
is sub-divided according to conditions in the Pacific, and statistical procedures
repeated on each sub-sample. None of the sub-populations fail to meet the
Kolmogorov-Smirnov test for normality, and only the Candelaria River during warm
years fails Filliben's (1975) test for normality. Table 4 details the descriptive
statistics of the sub-populations and summarizes comparisons of variances and means
between each. The expected pattern of decreased (increased) means and variances in
warm (cold) years is particularly apparent along the Pacific, although the response is
once again inconsistent along the Caribbean. The Colorado River shows significant
increases in sample variance during cold years in comparison to both warm and other
Table 4 Estimates of sample mean and standard deviations for historic records, and for separate subpopulations based upon ENSO conditions. Superscripts indicate statistically significant differences
between sub-population parameter estimates.
Candelaria
Colorado
Terraba
All years:
30
40
Sample size
29
30.75
4.67
322.67
Mean
2.31
8.99
70.12
Std Dev.
Other years:
16
10
Sample size
10
33.90*
4.56*
351.09*
Mean
1.64
68.07
7.93
Std Dev.
Warm years:
10
13
Sample size
10
24.11"
3.27"
268.55"
Mean
1.29*
7.93
49.57
Std Dev.
Cold years:
10
11
Sample size
9
34.61
6.40
348.38
Mean
9.07
2.98*
62.18
Std Dev
* significantly different from warm years (means);
" significantly different from cold years (means);
* significantly different from cold years (variances);
f
significantly different from other years (variances).
Estrella
Pacuare
Sarapiqui
19
40.29
8.30
33
33.49
6.81
24
117.32
16.44
8
42.92
9.98
13
33.36
5.55
10
118.76
16.29
7
39.07
8.37
9
29.28**
3.86*
9
111.80"
10.79
4
37.15
2.391
11
36.66
8.73
5
124.39
24.27
Interannual variability of annual streamflow and the Southern Oscillation in Costa Rica
417
12
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10
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Fig. 4(a) Examples of the frequencies of annual flows from the Colorado and
Sarapiqui Rivers. Historic records are classified into sub-populations according to
atmosphere-ocean conditions in the south Pacific and the fitted normal and mixed
normal distributions are shown; and (b) corresponding probability plots of each subpopulation and their fitted normal distributions.
years, while the Pacuare reveals greater variances in cold years than warm years, and
the Estrella greater variances in other years than cold years. All three Pacific stations
have significant differences in their means between warm years and both cold and
other years. Differences are also significant on the Pacuare between warm and cold
years.
Figure 4(a) provides examples of the observed distributions of annual flows on
both the Colorado and Sarapiqui Rivers and the fitted normal probability distribution.
The various normal distributions fitted to each sub-population and described by their
respective estimates of mean and variance, are plotted in Fig. 4(b). Each is then
substituted into equation (1) to provide the fit of the mixture distribution shown in
418
Richard K. George et al.
Fig. 4(a). With only the noted exception, the normal distributions fitted to each subpopulation cannot be considered significantly different from the observed data, nor
are any of the mixed distributions different from the combined annual series.
Estimates of flow quantités
Some of the potential practical implications of these findings may be gauged by
viewing the estimated quantiles of the annual flows with exceedance probabilities of
0.01, 0.05. 0.50, 0.95 and 0.99 (Table 5). A comparison may be made between
estimates based on the normal distribution and the mixed distribution, which might
be used for longer term predictions of likely flows, for such purposes as design
discharges. On all rivers, estimates of the smaller quantiles (<2001 and <2005) are
higher using the mixed model than the normal distribution. This is also true of the
larger quantiles (<2095 and Q099) with the exception of the Sarapiqui and Estrella
Rivers. Estimates of median annual flows (<2050) are lower than through the normal
model. These illustrate the curvilinear nature of the mixed model when plotted on a
probability scale (Fig. 4(b)). Differences between either coast and the unexpected
behaviour of the Pacuare are again evident. Standardized measures of these
differences are derived by expressing them as percentages of the historic median flow
on each river (Table 5). The differences are generally only in the order of less than
20% of the median annual flow, and most marked on the Colorado.
A second application of this information would be to use one sub-population
function to provide a forecast for flows likely to be encountered in the following year
based upon anticipated sea surface temperatures in the Pacific, which are now readily
available (see for example Climate Diagnostics Bulletin, Department of Commerce,
1996). Similar quantile estimates are provided in Table 5 for each sub-population.
Selecting warm and cold years as representing the two extreme sets of flow
distributions (Table 4), differences in flow quantiles are likewise standardized.
Pacific coast rivers generally showed the most marked differences, ranging between
30-150% at smaller quantiles, 25-70% for median flows and 10-20% for the larger
quantiles. It would appear that the incorporation of likely future atmosphere-ocean
conditions could greatly improve flow estimation in the coming year, thereby
influencing the management decisions concerning water storage for such uses as
power, irrigation and fisheries.
Seasonal analyses
The complexity of responses in basins of similar areas and topographic setting,
particularly within the Caribbean watershed, requires investigation at a finer temporal resolution. Seasonal lag 0 correlations (Fig. 5) indicate the strongest positive
association between flow in the Pacific during (July-August) and immediately
following the Veranillos (September-October-November). Significant correlations in
these rivers are entirely restricted to these seasons with the addition of a positive
Interannual variability of annual stream/low and the Southern Oscillation in Costa Rica
ON ON
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Richard K. George et al.
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0.8
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0.6
0.4
SE
0.2 -
O
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0.0
/ \
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/
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Fig. 5 Cross-correlations between seasonal streamflows and the SOI (filled
symbols represent significant correlations at the 0.05 level).
correlation on the Colorado in February-March. The Caribbean rivers continue
to display small and differing seasonal associations to the SOI, with only the
Pacuare producing a significant positive association in July-August. Both the
Estrella and Sarapiqui report opposing signs of correlation in the two rainy
seasons (negative in July-August and positive in December-January) in keeping
with the reported associations in precipitation, however, contrary to the notion
that drainage basins act as filters of "noisy" precipitation signals, these are not
statistically significant.
Statistical comparisons of seasonal variances and means should be viewed with
some caution, particularly in small basins during dry seasons, when the assumption
of normally distributed data may be violated. The Colorado shows the greatest
number of statistically significant seasonal differences between parameters in each
sub-population. In all but January-February there are differences in variances:
generally variances are greatest in the cold sub-population, followed by other
years, and with the lowest variances in the warm sub-population. A similar
relationship is apparent amongst the variances in September-October-November
and December-January on the Candelaria, while only April-May-June returns
differences in variances on the Terraba. Means in warm years are significantly
lower in all seasons on all three Pacific rivers, with the exception of DecemberJanuary on the Colorado and Candelaria. Variances and means in warm years are
significantly different (lower) in April-May-June and February-March on the
Pacuare, whereas differences in the other Caribbean basins are restricted to
variances in cold years during September-October-November on the Estrella and
December-January on the Sarapiqui.
Interannual variability of annual streamflow and the Southern Oscillation in Costa Rica
421
DISCUSSION AND CONCLUSIONS
The simple coastal differentiation observed in the analysis of precipitation (Waylen et
al., 1996b) is insufficient to explain these patterns of streamflow. Nor does there
appear to be an obvious latitudinal progression in rivers along the Caribbean coast as
the more northern and southern sites behave differently to the Pacuare. A closer
examination of basin locations reveals that elevation may be an important controlling
factor in governing the association to ENSO as the Pacuare is gauged at approximately
600 m (a.s.l.), while both the Sarapiqui and Estrella are gauged at below 100 m.
The dependence of precipitation quantity upon elevation in such areas of high
relief is a well established fact (see, for example Barros & Lettenmaier, 1994, and
locally Fernandez et al., (1996) however it would appear that the association of
rainfall to ENSO is also controlled in a similar fashion. Figure 6 portrays the
changing nature of correlations of seasonal precipitation with the SOI for 18
meteorological stations located in and near the Pacuare basin. It suggests a transition
between a Caribbean pattern of responses (negative correlations in July-August and
February-March, positive associations in December-January) and a Pacific pattern
(positive associations throughout) between 500 and 1000 m a.s.l. The Pacuare basin
therefore drains a zone in which rainfall shows a typical Pacific association, while
over 60% of the gauged areas of the Estrella and Sarapiqui lie below this elevation.
Although none of these basins is particularly large on either a national scale, or with
SEASON
PERCENTAGE OF BASIN AREA
Fig. 6 (Lower left) Generalized correlations (iso-correlation interval 0.05) between
seasonal precipitation and the SOI at meteorological stations located at various
elevations in and near the Pacuare River basin; (upper left) correlations of seasonal
streamflow and SOI; and (lower right) hypsometric curves for the three basins within
the Caribbean watershed.
422
Richard K. George et al.
regard to mountainous basins, such as the Magdalena used by Hastenrath (1990),
they appear to encompass mixed responses to ENSO, thereby eliciting apparently
weaker regional associations than may actually exist at various points along their
lengths. Drainage basin boundaries, even in areas of high topographic contrast, may
not correspond to the limits of a consistent ENSO signal.
A less marked regional association with ENSO along the Caribbean is found
between December and April, arising from the period of norte rainfalls. Within this
season the earlier period shows a positive association while the later season is
slightly negative. This is in keeping with the findings of Schultz et al. (1998), who
have re-analysed data on the frequency and extent of cold surges over Central
America (Klaus; 1973; Reding, 1992). Those surges reaching Costa Rican latitudes
appear to have two different origins over North America. The milder surges, which
have a propensity to occur more frequently and earlier in the year, result from
anticyclonic cells crossing the Rockies from the Pacific into the interior of the United
States. The less frequent, colder surges originate from Arctic anticyclones forming
over Canada, moving southwards later in the season. A composite of hemispherical
circulation patterns at the time of historic events, reveals that the colder later surges
are more likely to develop in conjunction with a strong meridional component to the
polar jet over North America and a sub-tropical jet passing over central Mexico to
converge with the polar jet over the southeastern United States. These latter
conditions are fairly typical of boreal winters following a warming of the equatorial
Pacific (Douglas & Englehart, 1981; Yarnal & Diaz, 1986; Cavazos & Hastenrath,
1990). Therefore, during these winters the Sarapiqui and Estrella are likely to
experience a reduction in December-January flows, but increased chances of rare
late season events. The meeting of these Arctic and Tropical air masses, sometimes
enhanced by the presence of early-season tropical waves, is responsible for the
largest daily rainfall totals recorded in Costa Rica (Ramirez, personal
communication), and associated flooding, during the generally drier month of March.
Annual streamflow within Costa Rica is therefore associated with atmosphereocean conditions in the south Pacific. This is most obvious in rivers draining the
Pacific watershed and should be incorporated into procedures for the estimation of
flow frequencies in this region in order to account for the types of flow variability
that have been experienced thus far in this decade. The effects on flows within the
Caribbean watershed are more complex, both temporally and spatially. The distinct
generating processes responsible for precipitation in the two rainier seasons appear to
respond in differing fashions to ENSO, while the overall response at any gauging
point is related to the up-stream hypsometric curve. The response of portions of a
basin below approximately 1000 m a.s.l. show the typical Caribbean pattern, while
areas at higher elevations are similar to the Pacific. Flows along the length of the
river reflect varying mixtures of combined effects.
The practical significance of the exact extent and nature of this spill-over of the
Pacific pattern can be gauged by considering the marked differences in probability
distributions of annual flows, particularly during warm years, along the Pacific
coast. The fact that almost 50% of Costa Rica's national power is derived from two
reservoirs, the Arenal and Cachi (Fig. 1), both of which are located in the Caribbean
Interannual variability of annual streamflow and the Southern Oscillation in Costa Rica
423
watershed, but which garner water from above 500 and 1000 m a.s.L, respectively,
should also be considered. Both reservoirs experienced sufficiently low levels early
in the 1990s to require national power rationing. Plans are also being considered for
a very large hydroelectric scheme on the Terraba River itself, which would
experience synchronous fluctuations in levels with the other major projects, even
though these lie in the Caribbean watershed. From the perspective of maintaining a
balanced water supply, this study suggests that candidate rivers in the Caribbean
watershed with high proportions of their basin areas below 1000 m might better be
sought.
Acknowledgements Thanks are due to G. Poveda, O. Mesa and D. Schultz for their
comments concerning the larger regional setting.
REFERENCES
Barros, A.-P. & Lettenmaier, D. P. (1994) Dynamic modeling of orographically induced precipitation. Reviews
Geophys. 32, 265-284.
Cavazos, T. & Hastenrath, S. L. (1990) Convection and rainfall over Mexico and their modulation by the Southern
Oscillation. Int. J. Climatol. 10, 377-386.
Department of Commerce (1996) Near real-time analyses ocean/atmosphere. Climate Diagnostic Bulletin. Climate
Prediction Center, National Oceanic and Atmospheric Administration, National Weather Service, National Centers
for Environmental Protection, Washington, DC.
Douglas, A. V. & Englehart, P. J. (1981) On a statistical relationship between autumn rainfall in central equatorial
Pacific and subsequent winter precipitation in Florida. Mon. Weath. Rev. 109, 2377-2382.
Estoque, M. A., Luque, J., Chandek-Monteza, M. & Garcia, J. (1985) Effects of El Nino on Panama rainfall. Geofis.
Internac. 24, 355-381.
Fernandez, W., Chacon, R. E. & Melgarejo, J. W. (1996) On the rainfall distribution with altitude over Costa Rica.
Revista Geofisica 44, 57-72.
Fernandez, W. & Ramirez, P. (1991) El Nino, la oscilacion del sur y sus effectos en Costa Rica: una revision (The
effects of El Nino-Southern Oscillation in Costa Rica, in Spanish). Tecnologia en Marcha 11, 3-11.
Filliben, J. J. (1975) The probability plot correlation coefficient test for normality. Technometrics 17, 111-117.
Hastenrath, S. L. (1967) Rainfall distribution and regime in Central America. Archiv fur Météorologie, Geophysik und
Bioklimatologie, Série B 15(3), 201-241.
Hastenrath, S. L. (1988) Climate and Circulation of the Tropics. Reidel, Dordrecht, The Netherlands.
Hastenrath, S. L. (1990) Diagnostics and prediction of anomalous river discharge in northern South America. /. Climate
3, 1080-1096.
Herrera, J. L. (1994) Analisis pluviometrico de las temporadas de illuvia 1991-1993 y su relacion con el fenomeno "El
Nino-Oscilacion Surefia" en Guatemala (A pluviométrie analysis of the 1991-1993 rainy season and its relation to
the El Nino-Southern Oscillation phenomenon, in Spanish). Internal Report, Secciôn Agrometeorologia,
INSIVUMEH (Institution for Vulcanological and Meteorological Investigation), Guatemala.
Klaus, D. (1973) Las invasiones de aire frio en los tropicos a sotavento de las Montanas Rocallosas (Invasions of cold air
in the tropics on the leeward side of the Rocallosas Mountains, in Spanish). Geofis. Internac. 13, 99-143.
Lilliefors, H. W. (1967) On the Kolmogorov-Smirnov test for normality with mean and variance unknown. /. Am.
Statist. Ass. 68, 399-402.
Portig, W. (1965) Central American rainfall. The Geographical Review 55, 68-90.
Poveda, G. & Mesa, O. (1997) Feedbacks between hydrological processes in tropical South America and large scale
oceanic-atmospheric phenomena. J. Climate 10, 2690-2702.
Pulwarty, R. S. & Diaz, H. F. (1993) A study of the seasonal cycle and its perturbation by ENSO in the tropical
Americas. IV Int.. Conf. on Southern Hemisphere Meteorology and Oceanography, 262-263. American
Meteorological Society.
Reding, P. J. (1992) The Central American cold surge: an observational analysis of the deep southward penetration of
North American cold fronts. Unpublished Master's Thesis, Texas A&M University.
Rogers, J. C. (1988) Precipitation variability over the Caribbean and tropical Americas associated with the Southern
Oscillation. /. Climate 1, 172-82.
Ropelewski, C. F. & Halpert, M. S. (1987) Global and regional scale precipitation patterns associated with the El
Nino/Southern Oscillation. Mon. Weath. Rev. 115, 1606-1626.
424
Richard K. George et al.
Schultz, D. M., Bracken, W. E. & Bosart, L. F. (1998) Planetary- and synoptic-scale signatures associated with Central
American cold surges. Mon. Weath. Rev. 126, 5-27.
Vargas, A. B. & Trejos, V. F. S. (1994) Changes in the general circulation and its influences on precipitation trends in
Central America: Costa Rica. Ambio 23, 87-90.
Velasco, I. & Frisch, M. (1987) Mesoscale convective complexes in the Americas. /. Geophys. Res. 92(D8), 95919613.
Waylen, P. R. & Caviedes, C. N. (1990) Annual and seasonal fluctuations of precipitation and streamflow in the
Aconcagua river basin, Chile. /. Hydrol. 120, 79-102.
Waylen, P. R., Quesada, M. E. & Caviedes, C. N. (1996a) Temporal and spatial variability of annual precipitation in
Costa Rica and the Southern Oscillation. Int. J. Climatol. 16, 173-193.
Waylen, P. R., Caviedes C. N. & Quesada M. E. (1996b) Interannual variability of monthly precipitation in Costa Rica.
/. Climate 9, 2606-2613.
Yarnal, B. & Diaz, H. F. (1986) Relationship between extremities of the Southern Oscillation and the winter climate of
the Anglo-American Pacific coast. J. Climatol. 6, 197-219.
Received 21 March 1997; accepted 9 November 1997