Competition for water resources of the Rio Guayas, Ecuador

Transcription

1 Optimal Allocation of Water Resources (Proceedings of the Exeter Symposium, July 1982). IAHS Publ. no Competition for water resources of the Rio Guayas, Ecuador INTRODUCTION P, J. WAITE* Cremer S Warner, 140 Buckingham Palace Road, London SW1W 9SQ, UK ABSTRACT Increasing demands on limited water resources have led to increased salt water intrusion hazards in many estuaries. A particular example from Ecuador is examined in this paper. Guayaquil obtains its water supply from the Rio Daule, one of the tributaries of the Rio Guayas. The river also supplies water for irrigation and other uses. Increasing use of the limited fresh water has resulted in such low residual fresh-water flow that salt water has intruded upstream to the water supply intake during the dry season. A major new development is the Daule-Peripa dam, for hydropower, irrigation and river control. It will be necessary to cater for all users of Rio Daule water both during construction and after completion of the dam. It is difficult to measure the low flows in the estuary and data are only available for part of the basin. This paper describes the analysis of flow and rainfall records to discover the nature of changes in the flow and in particular to evaluate the risk to the city water supply due to high salinity levels in the dry season. The Guayas basin, in Ecuador, highlights many of the water resources problems which are affecting tropical regions as competition for fresh-water supplies grows with increasing development, both urban and agricultural. It is difficult to assess the total water resources of such basins because river flow records are often available only for short periods and cover only part of the basin. Coastal plain estuaries frequently have long tidal sections e.g. the Gambia with over 500 km (Sanmuganathan & Abernethy, 1975). The residual fresh-water flow, in the reaches close to the sea, is hard to estimate particularly where many tributaries join in the tidal section. Dry season flow rates are particularly difficult to measure since they may be only a small percentage of the average flow (1% or less is quite usual). The reliability of flow measurements is difficult to maintain in rivers with variable flows. Frequent recalibration of gauging stations is required, particularly at or near minimum flows but is not practicable in many places so considerable changes may occur between calibrations, e.g. Waite (1982). However, stage-discharge * Formerly : Senior Scientific Officer, Hydraulics Research Station, Walling ford, Oxon, UK. 79

2 80 P.J.Waite curves are regularly checked and amended in the Guayas basin. There has been increasing modification of the natural flows of many rivers by control structures and abstractions. These changes affect the statistics of flows particularly when considering seasonal, monthly or shorter period means. This paper describes the declining residual dry season flow in the Rio Daule and the availability of fresh water for Guayaquil (population over 1.2 million). In the years the dry season flows were so low that the abstraction of water ceased for periods of several days because salt water had reached the intake, thereby closing the city water supply. Location Guayaquil is situated near the confluence of the Rios Babahoyo and Daule, which become the Rio Guayas estuary, Fig.l. A third river, the Rio Vinces, divides into several distributaries downstream of Vinces town, see Fig.2. Flow from the Vinces to the Daule is via the Pula. The intake for Guayaquil's water supply is at La Toma just downstream of the confluence of the Rio Pula with the Rio Daule. Approximately half of the fresh-water flow at La Toma in the dry season comes from the Rio Pula. La Toma is well within the tidal reach of the river and the current reverses there for most of the year. FIG.l Guayas basin showing rainfall and flow gauging stations.

3 Competition for water resources, Ecuador 81 Gauging stations 1979 abstractions-total 10-8m Vs Additional future demandtotal 26m Vs ilechugat La Capilla < Vernaza Junquillo R. San Pabio Babahoyo Guayaquil FIG.2 Schematic plan of Daule, Vinces, and Babahoyo rivers showing abstractions. Climate The Guayas basin, described in Waite (1976) and (1981), has a total area of 32 OOO km" and a tropical savannah climate with mean monthly temperatures varying between 24 and 27 C. Seasons are well defined. Rainfall is concentrated in the period January-April when total 3 1 flows to the estuary may reach 5000 m s or more. The dry season months are June-November inclusive when the flow of the Rios Babahoyo and Daule may be less than 20 m s each. Rainfall increases towards the north where the maximum contribution to the flow of the Rios Daule and Vinces occurs. At Pichilingue, just north of the centre of the basin, the mean annual rainfall increases to approximately 2000 mm.

4 82 P.J.Waite USE OF WATER The 1979 average abstraction rate at La Toma was 2.8 m s" By the end of the century, Guayaquil's total demand may reach 12 ir/s -1. All future water supplies are to be taken from La Toma. There are upstream users taking water from the rivers for irrigation and local water supplies in competition with Guayaquil. Current abstraction by these users is estimated to be 8 m 3 s -1 (Waite, 1981), downstream of the lowest gauging stations compared with a river flow of 20 m s -. Clearly the residual flow in the Daule after all^abstractions for irrigation and water supply can 3 1 It has been calculated that residual flows fall below ÎO m s (Sanmuganathan, 1979; Sanmuganathan & should be above 20 m s Abernethy, 1979) to prevent salt water affecting La Toma. Fig. 3 illustrates the calculated effects of low flows on salinity. November 1979 Flows in this region give low salinities at La Torna Flows in this region give high salinities at La Toma Key Calculated points interpolated salinity contours FIG.3 Maximum salinity at La Toma after 140 days of low flows. As shown in Fig.2, future abstraction for irrigation is likely to increase, up to 20 m s~ according to some estimates (Waite, 1981). At times this would leave no residual flow. However, after completion of the Daule-Peripa dam (Fig.l), control of the Rio Daule should be possible. But irrigation will increase and water transfer to arid regions further west is possible. When allocating water in the reservoir, the operators will have to consider the requirements for hydroelectric power, irrigation of new schemes, irrigation from run of the river users, extra basin transfer, water supply and salinity control. Notice will also have to be taken of any increased use of Rio Vinces water, as reductions of flow there will affect the Rio Pula and hence the flow for La Toma and salinity control.

5 Competition for water resources, Ecuador 83 PREVIOUS WORK The river basin authority (CEDEGE) and the National Hydrometeorological Service (INAMHI) have observed flow and rainfall at numerous stations. There are several gauging stations which have 16 years continuous data, gauging most of the flows upstream of the dry season tidal limits. However, there is only one rainfall station with complete records over the same period There are incomplete records from other stations (see Fig.l). In Waite (1976) a method of assessing the total flow into the Rio Guayas estuary appropriate for the dry season was proposed. Sufficient data were collected to enable a simple model of salinity (Sanmuganathan, 1975) to be adapted for use on the branched Guayas estuary (Sanmuganathan, 1979). This model has been used to predict that the residual fresh-water flow required to keep salinity below 0.5 g 1 (TDS) at the La Toma intake is 20 m s, independent of Babahoyo flow (see Fig.3). At this stage of the investigation insufficient data existed on the irrigation abstractions and it was thought that they were insignificant (Waite, 1976). The increasing problem of salt water reaching the La Toma intake has led to a reassessment of the abstractions for irrigation described above. There has been no systematic collection of data on abstractions upstream of Vinces town on the Rio Vinces and upstream of La Capilla on the Daule. Therefore the influence of the upstream abstractions on the statistics of the flow records at the Vinces and La Capilla gauging stations was unknown before this study. METHOD OF STUDY The study reported in this paper attempts to discover whether the period of low dry season flows during was a statistically plausible occurrence or whether there is some unnatural interference, or even a long term trend which is reducing available water at La Toma. The rainfall from one station and the flows at three gauging stations are analysed. Analysis of flow records The eight gauging stations with complete records have been used to calculate the flows of the Rios Babahoyo and Daule following the method in Waite (1976). No allowances have been made for irrigation abstractions. A further gauging station, Laurel on the Rio Pula, has six complete years of records and this has been extended by correlation with the Vinces flow records. Minimum mean monthly flow The minimum flows at the three chosen gauging stations, (La Capilla, Pula en Laurel, Vinces en Vinces) usually occur in November. Because the flows at La Toma are arrived at by estimation more reliability has been placed on the analysis of flow records from the gauging stations further upstream. In order that minor variations will not affect the analysis of the records the average dry season flows have been examined for each gauging station,

6 84 P.J.Waite i.e. the mean of the September, October and November flows. The probability of the low flows in occurring as part of the population represented by the data up to 1976 has been calculated using the Students' t-test: 0.08 for La Capilla for Pula en Laurel O.Ol for Vinces en Vinces If 1980 is added to the sequence of unusual events for La Capilla and Pula en Laurel, the probabilities decrease to 0.06 and respectively. The conclusion is that these four years were either very unusual events or the result of some disturbance to the river system which had not occurred previously. Seasonal distribution of flow It has been shown that the recent dry season flows of the Daule have been exceptionally low. However, it is possible that the overall volume of flow in a year is remaining constant (at least stationary in the statistical sense) but a higher proportion of runoff is occurring within the wet season. In order to check this hypothesis the ratio of three months' dry season flow to the annual total flow and to three months' wet season flow, have been calculated for three gauging stations, La Capilla, Pula en Laurel and Vinces en Vinces, together with the estimated total flows at La Toma. The ratios of dry season flows to mean annual flows and to wet season mean flows exhibit declining trends. In particular the trend lines have been found for the total flow at La Toma, by linear regression; the gradients are and respectively. The mean annual flow at La Toma has no apparent trend over the period Linear regression on the mean flows there gives a gradient of O.OOl and a correlation coefficient of 0.05 indicating the sequence is random. Further confirmation of this comes from the exact analysis of the mean annual flows at La Capilla and Vinces. Following the method given in section 2.2 of Clarke (1973), it is possible to show that the sequences of mean annual flows are random. So the dry season flows have been decreasing relative to the annual mean flows (which are behaving as a random sequence) and more of the flow is being concentrated in the wet season. Analysis of rainfall records Available data Fig.l shows some of the rainfall stations in the Guayas basin which have been examined. Four stations were found with long term, almost complete records; these are in the north of the basin where the major contribution to flow occurs. Pichilingue is the best of the four having complete records for the period of interest and earlier ( ). The other stations have gaps in their records and so it was decided to use Pichilingue records alone. It has been shown (Waite, 1981) that it is representative of the other three stations. Analysis of Pichilingue record The exact test for the lag one serial correlation coefficient given, for example, in section of Clarke (1973), indicates that the sequence is random if the lag one serial coefficient, r, is within the interval ( , ).

7 Competition for water resources, Ecuador 85 In this case r = , indicating a random sequence. Performing linear regression on the 21-year record gives a gradient of O.OOl indicating no trend. The years have been identified as the driest consecutive three years in the record corresponding to the exceptionally low dry season flows in the Rios Daule and Vinces for those years. The Students' t-test has again been used to ascertain the probability that the sequence of rainfall could be from the population represented by the record from ; a value of Similarly for these three years to be in the population represented by the record from , has a probability of So the probability of the Pichilingue rainfall and the La Capilla flow being of the historical record is similar. It is a reasonable deduction that the low flows recently recorded in the dry season at La Capilla are only the result of low rainfall. However, the rainfall does not show a declining trend over the past 21 years. On the other hand, the sequences of low flows observed at Vinces en Vinces and Pula en Laurel for are much more extreme events (O.Ol and respectively). These cannot be explained in terms of rainfall alone. Analysis of rainfall-runoff relationship Correlation of flows with rainfall The average dry season flow (i.e. mean flow during September, October and November) for the three gauging stations has been plotted against total annual rainfall at Pichilingue (annual rainfall is calculated over the period December to November) and is shown in Fig.4. Also shown are the straight line fits for both the whole period and for For the shorter period the correlation of dry season flow with rainfall is reasonable (correlation coefficient approximately 0.8) but for the longer period the correlation coefficient for Vinces and Laurel drops to less than O.6. The behaviour of flows at Vinces and Laurel for the last five years of record have been demonstrated to exhibit a different behaviour from previous years; Waite (1981) gives further details. These results suggest some interference with flows at Vinces and Laurel which has led to even lower flows in the three years than the exceptionally low rainfall would be expected to generate. In earlier years all three gauging stations exhibited a natural variation with rainfall. Effect of timing of rainfall on low flows In order to attempt to produce a forecasting method for low flows at La Toma two hypotheses were tested: I The driest month's (November's) flow Q]_2» i s given by a linear combination of the previous 11 months' and current November's rainfall. II That the dry season average flow (average flow during September, October and November), is given by a linear combination of the previous nine months' rainfall. The correlation coefficients for comparison between observed and the calculated values were:

8 86 P.J.Waite î! ï all I 1! i B0Û ÛÛ ' " a > /' ' / / /' ' / / // ' / // // / / / - '/// y, / / H y '' (V n, / / v / a ' * / o / / / / / 3 / O XD / If / Q 1 oxr / g / / / /A x a /// /;* n / >/// "// *» /i 1 e /' //i i r o //I. X /' o ^ /7 / /' // /a* // / e l / / \ i x ' / / / ' / / / # ^ / ' Y # ^y,' / / # / / / ' / / / / / s / s / / / / <' / s / s x * Pula en Laurel o e Vinces en Vinces a a La Capilla Correlation Dashed lines for period Sclid lines for whole period i i. i Dry season mean flows (m 3 FIG.4 Correlation of average dry season flows with rainfall. s'j Hypothesis I II La Capilla Vinces Laurel The greatest contributions to November and dry season average flows come from January to April rainfall. Forecasting of low flows from rainfall From the above analysis it appears it might be possible to make a forecast of low flows (due to low rainfall) at the end of the dry season by performing calculations in May, providing rainfall up to April is known. Because the correlation coefficients were higher for I above, we look at the possibility of forecasting November flows, Q]_2/ with the formula: Hypothesis III: Q-, o = Z. c i r i where r^ is rainfall in i-th month (rj_ for December) and c^ are constants to be determined. But the correlation coefficients found for this hypothesis were only 0.90 for La Capilla, 0.65 for Vinces and 0.59 for Laurel. To

9 Competition for water resources, Ecuador 87 improve on this, the antecedent flows for the months December to April, Q-j_, i = 1, 5, are also used in the hypothesis: 5 5 Hypothesis IV: Q 12 =._ a i r i + _ b iqi + câ a., b. and c are constants and d is the year date. The values of the coefficients are given in Waite (1981). An improvement is observed in correlation coefficients with 0.98 for La Capilla, 0.84 for Vinces and 0.89 for Laurel. So for La Capilla we may have accounted for 97% of the variation of November flows. An equation of the form of hypothesis IV has been put forward as a forecasting method to be used for year by year, short term planning. Deductions on water use Abstractions downstream from Vinces The 1979 level of abstraction when combined with low rainfall resulted in residual flows of the Daule and Pula too small to maintain fresh water at La Toma. The number of pumps in 1979 was estimated to be 50% greater than in 1974, and further increases may be expected. Also there are plans to take much more water from the Vinces distributaries below Vinces Town. If increases in abstraction at La Toma are implemented then it is estimated that there will be a total further loss to the residual flow at La Toma of about 20 m s, i.e. more than minimum residual flows of recent times. Therefore alternative sources of water will be required either for the new irrigation schemes planned or for the abstractions at La Toma, or both; especially at times of low flows. Abstractions upstream from Vinces Town From the analysis of flow records at Vinces, it was concluded that there has been some interference with Vinces flow in recent years because the variation of low flows could not be fully explained by the variation in rainfall. Although the extreme series of low flows at La Capilla, in the adjacent sub-basin, could be completely explained by the extremely low sequence of rainfall recorded at Pichilingue, the reasons for the unnaturally low flows at Vinces should be investigated. There may be extra abstractions or diversions for irrigation supply. Interference with natural flows should be minimized especially at the end of the dry season, possibly by prohibiting irrigation during spring tides. CONCLUSIONS Sparse flow and rainfall data may be analysed, for inconsistencies, by means of simple statistical techniques. A simple correlation of end of dry season flow with wet season rainfall and flows appears to be a straightforward means of predicting low flows, in order to take emergency action to secure downstream flow. Ratio of dry season to annual flows may be used to assess changes in the character of basins. Annual flows may remain statistically random but dry season flows show a declining trend. Techniques have been demonstrated to identify areas (e.g. upstream

10 88 P.J.Waite of Vinces) where the use of water should be investigated or controlled. ACKNOWLEDGEMENTS The data used in this study were provided by EMAP-G, CEDEGE and INAMMI, the cooperation of these organizations is gratefully acknowledged. This work was carried out in the Hydraulics Research Station's Overseas Development Unit which is headed by Mr C.L.Abernethy. This paper is published with the permission of the Director, Hydraulics Research. REFERENCES Clarke, R.T. (1973) Mathematical Models in Hydrology. Irrigation and Drainage Pap. no. 19, FAO, Rome. Sanmuganathan, K. (1975) One dimensional analysis of salinity intrusion. Hydraulics Research Station Report OD 2, Wallingford, Oxon, UK. Sanmuganathan, K. (1979) Prediction of salinity intrusion in the Rio Guayas, Ecuador. Hydraulics Research Station Report OD 21, Wallingford, Oxon, UK. Sanmuganathan, K. & Abernethy, C.L. (1975) A mathematical model to predict long term salinity intrusion in estuaries. Proc. International Water Resources Association (Delhi), vol. III. Sanmuganathan, K. & Abernethy, C.L. (1979) Long term prediction of salinity intrusion in branched estuaries. Proc. 18th Congress IAHR. Waite, P.J. (1976) Rio Guayas, Ecuador: Field data for estuary salinity study. Hydraulics Research Station, Report OD 8, Wallingford, Oxon, UK. Waite, P.J. (1981) Rio Guayas, Ecuador: Salt water intrusion study, 3, analysis of low flows. Hydraulics Research Station, Report OD 35, Wallingford, Oxon, UK. Waite, P.J. (1982) Analysis of low flows in the Nilwala Ganga, Sri Lanka. Poster paper in this symposium, the abstract of which is published in Hydrol. Sci. J. 27(2), p. 200.