KEY WORDS: HYPER SALINITY; RESIDENCE TIME; SALT BUDGET; WATER EXCHANGE
Author: H.B. JAY ASIRI / J.K. RAJAP AKSHA
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The Mundel Lake is an extremely shallow lagoon on the west coast of Sri Lanka. It is connected to the Puttalam Lagoon through 15 km long Dutch Canal. Salinity measurements and daily sea level data were obtained fortnightly from January 1993 to March 1994 and they were used to quantify the salt and water budget along with precipitation, evaporation and freshwater runoff. Extreme fluctuations of salinity and sea level are striking features of the system. Salinity of the Mundel Lake and Dutch Canal varied from 5- 46.5 and 6- 61 ppt respectively while the sea level ranged from -0.25 to + 1.2 m.
Tidal variations were not seen in the lagoon due to it’s long narrow canal system. Salt budget showed that the deposition of salt on the lagoon bottom during periods of decreasing water level. During increasing water level, salt is dissolved again. Flow of water through the Dutch Canal between the Puttalam Lagoon and Mundel Lake is driven by the changes in sea level. These changes are mainly due to seasonal changes of net freshwater supply and, to a lesser degree, to seasonal changes in sea surface height. As the flow rates are small due to the long and narrow canal, the residence times ranges between two months and several months in the Mundel Lake, except during season of high freshwater supply. As the water exchange is weak, the Mundel lake becomes hypersaline with strong fluctuations in salinity. This implies a stress to all lagoon dwelling aquatic organisms and also to aquaculture practices in the area
The areas around Puttalum Lagoon and Mundel Lake have been found to be suitable· for prawn culture. A great number of prawn farms have already been established connecting the two water bodies (Corea et al., 1995). Prawn farms require salinities between 10 and 35 ppt and all the farms depend on the Dutch Canal and the lake as a discharge point for water and associated effluents. Because of intermittently low fresh-water supply and high evaporation very high salinities might appear in the area (Arulananthan et al., 1995 and J ayasiri et al., 1995).
The prawn farms therefore require freshwater to avoid hypersalinity. During low waters, vast areas are suffering from evaporite salt deposits. The salinity of the Mundel Lake depends on the flushing through the Dutch canal and on local evaporation, precipitation and runoff. During periods of limited exchange of water, the Mundel Lake is likely to become strongly hypersaline. This intern may cause intrusion of salt water into farm lands thereby affecting the flora and fauna. Increasing population in the area experience strong difficulties in obtaining drinking water of high quality, among other difficulties as a result of salt-water intrusion in to the wells. Rainy seasons on the other hand will cause flooding in the area as there is no proper drainage outlet. The Dutch Canal is long and· narrow and presently there is a sand bar at Udappuwa.
Aim of this study is to estimate the · exchange of water and salt between Mundel Lagoon and Indian Ocean. The observed salinities and sea levels within the lagoon are related to the net freshwater balance. Moore and Slinn (1984) studied the annual hydrological cycle (long term) of the Caimanero-Huizachelagoon system on the Pacific coast ofMexico. They studied seasonal changes of meteorological parameters such as precipitation and evaporation and fresh-water runoff. When the water level was decreasing during dry season, salt water entered, some of which was stored on the lagoon bed as evaporite deposits like Mundel Lake/Dutch Canal system. Robinson et al., 1983 studied the mechanisms which control sea level fluctuations of the Fleet, a long system of channels and lakes (12.5 km) in Southern England where the tidal wave almost completely 4isappears by the time it reaches the end of the system. There is no tide in the Mundel Lake too due to long, narrow canal system (Jayasiri et, al1998).
The Study Area
Mundel Lake is an extremely shallow coastal lagoon on the west coast of Sri Lanka. The surface area of the lake is approx. 30 km2 at local mean sea level, which is above the oceanic. The mean depth is about 0.75 m. The lagoon is nearly 12 km long and the width varies from 1-3 km. The Lake is connected to the Puttalum Lagoon through Dutch Canal in the Northern end and to Chilaw Lagoon in the south. The length of the Dutch Canal from Puttalum Lagoon to Mundel Lake is 16 km and its width is about 10 m but at high water large areas may be flooded. The salinity of the Mundel Lake is low during rainy season, but high during the dry season (Guruge et al., 1987).
They found the salinities lower than 3 ppt during the rainy season but 40-50 ppt during the dry period. The lagoon is separated from the ocean through an about 1 00 m wide permanent sand bar at Udappuwa which obstruct the direct exchange of water with the ocean. During periods of heavy rain, however the sand bar has been actively broken to prevent flooding of the low land. The Battulu Oya river enters freshwater from south, particularly during the rainy season. The area is located in the arid zone of Sri Lanka. It experiences two monsoon periods, the south-west (May-Sep) and the north-east monsoon (Oct-Jan). The rainfall is markedly seasonal and 70% of the rains occur during the northeast monsoon. According to measurements at V anathavillu, near the Puttalam weather station, the average rainfall is 1200 mmyear.
The average maximum daily air temperatures are high varying between of 30.4 — 33.6 °C and the average minimum temperature during night varies from 19 — 25 °C. In general, the temperatures are lower during the north-east monsoon. The average pan evaporation is 1800 mmyear-1, indicating that evaporation dominates over precipitation
Materials & Methods
Salinity measurements were carried out fortnightly at 14 stations from February 1993 to July 1994. The salinities were determined with an Autosallaboratory salinometer. The accuracy of this instrument is 0.001 ppt. Some water samples were diluted with water before being analyzed since the salinities are our-of the range of these samples for this salinometer (>42 ppt). The freshwater supply was measured from June to October 1993 in Battulu Oya. Two current meter moorings, each with three pendulums (Cederlof et al., 1996) hanging in a rope were deployed at surface, middle and bottom of the Battulu Oya river.
This devise gives a direct estimate of velocity and the direction. Daily sea levels in the Mundel Lake were carried out manually using a tide pole from 20 Feb 1993 to 31 March 1994. The accuracy of the tide pele measurements is about ±2 em. Daily rainfall and evaporation were obtained from the V anathavillu from Jan 1993 to March 1994~ which is close to the study area. These parameters are shown as monthly averages. Half hourly sea level data from Colombo harbour were obtained for the year 1993.
Salinity The salinity shows strong fluctuations on the time scale of months. In Puttalum Basin, the salinity varied from 7-63 ppt, in Mundel Lake from 5-45.5 ppt, in Dutch Canal (North) 6-60 ppt, and in Dutch Canal (South) from 0-33 ppt. There. is a gradient from south to north, basically during the whole 1.3 year period and there is good correlation between the four different areas, when the variations at 2 week intervals were taken into account. The salinity in Puttalum Basin and Dutch Canal (N) exhibits maxima in August and September 1993 respectively and minima in February 1994 after a period of very heavy rains.
The variation is similar in Mundel Lake and Dutch Canal but the salinities are generally lower (10-20 ppt). The overall mean salinity in Mundel Lake is 25.0 ppt and in the Puttalum Basin 3-9 ppt. There are two maximum salinity levels and two minimum salinity levels per year which could be related to the rains. Only in the Puttalam Basin, from February-March a maximum value was not clearly indicated. It is also interesting to note that the salinity of the lake from January to February 1994 dropped by 15 ppt when compared with values during 1993 which indicates that a study over a one year period is not sufficient to indicate all the variations.
The sea level in Mundel Lake is influenced by evaporation (about 1 m3 s-1 ), precipitation (0-3 m3 s-1 ), river discharge (0-7 m3 s-1 ) and the external, oceanic sea level. From October to December the water level was very high due to heavy rainfall. The lowest daily sea level (-0.35 m) occurred during July-August due to minimum rainfall and absence of river discharge including relatively high evaporation. In December, a rapid increase occurred when the area was flooded. The maximum sea level during this period was 1.10 m.
The hypsographic curve shows that the area of the Mundel Lake is changing linearly as a function of the height of water.
Battulu Oya enters the Mundel Lake from its southern end. All the water which discharge from the river can not be expected. to enter the lagoon. During dry seasons there is probably no significant freshwater supply. because there is a dam upstream to prevent salt water intrusion into upstream areas. During rainy season maximum discharge of 9m3 was indicated. Measurements were carried out form January to October, and during the other months, freshwater discharge was estimated from rainfall data. From the results it was possible to identify a sill or other obstructions in the Dutch Canal. This sill implies a complete cut-off of the canal flow at depths of .about 10 em below the mean sea level. This ··sill .. implies that no ocean water entered or exchanged from Day 200 up to Day 295.
Salt and Water Budget
Salt and water budget during different periods are described below. I. During Day 50-95, the sea level was slightly higher than the lagoon. There is basically no change of volume in the lagoon and the precipitation and evaporation were 0.35 m3 s1 and 0.95 m3 s1 respectively.
The river discharge was zero. Conservation of volume claims an inflow of 0.6m3s1 • During this period the salinity of the lagoon increased from 19 to 34 ppt due to a net inflow of saline water through the Dutch Canal (N). Conservation of salt indicated an inflow of 0.85 m3s1. As the average salinity of the Dutch canal water was 45 ppt, the transport of salt into the lagoon is 38 kgs.
The residence time for the lagoon waters in this case was 82 days. II. During the period from Day 100-140, the water level of Mundellake was higher than the sea level. The lagoon level however did not change during this period, as the precipitation and evaporation were almost equal. The river discharge which is about 1 m3 s1.
This means that the outflow was equal to the river discharge. As the freshwater discharged into the lagoon, the salinity decreases from 33 to 23 ppt. The conservation of salt also gave similar results ( 0.65 m3 s1 ). During this period the salt transport from Mundel to Puttalam was 28 kg s1 while the mean salinity of the lagoon was 28 ppt
Discussion & Conclusions
The seasonal variations in water and salt exchange result from interactions among precipitation, freshwater runoff, evaporation and sea level variations. Sea level variations determine the direction of the flow through the Dutch canal. Often the dominance of one or the other of these processes determines the quality of the water at a particular season. Due to dominance of different phenomena strong fluctuations of salinity and water level in the lagoon occur. ·. It indicated that the periods of inflow are short compared to the periods of out flow, but the salinity was relatively high. The net freshwater input dominates during the rainy season but ceases in between. Mean . salinity of the Mundel Lake was 25 ppt during 1993 — 1994 with variations between 5 — 46.5 ppt. Mean salinity of Puttalam Lagoon was normally higher than the oceanic mean (Arulananthan et al., 1995), but in the southern end it was even above 40 ppt. During the period of observation, the mean salinity of the Puttalam Basin was above 40 ppt, but it varied from 7- 63 ppt indicating very large fluctuations. Meshal (1987) showed that the Habita Lagoon of the Red sea is hypersaline through out the year and salinity varied from 51-113 ppt and he related the salinity in the lagoon to the mean water level of the Red sea, and to the precipitation/evaporation cycle. From 20 July- 8 Sept 1993, the sea level was higher than the water level of Mundel Lake. As there was no flow in to Mundel Lake, it was assumed that there is a sill or a barrier somewhere within the canal. During this period sea ‘level decreased by 4.60 mm day-1. The observed evaporation was 4.58 mm day~.
Seepage of seawater through the sand bar at Udappuwa may be a reason for salinity increase in the lagoon during drought periods. The entry of sea water occur probably only through the canal. In October due to heavy rains, water level in Mundel Lake increased but still lower than the oceanic sea level. During this period the flow in the canal was towards to Mundel Lake as indicated by increasing salinities by day 300. During November and thereafter a very large out flow which also resulted flooding in the area between Mundel Lake and Puttalam Lagoon was observed. Minimum salinity was observed during this period and in certain areas it became nearly freshwater. This situation however is probably not very common; the precipitation during 1994 strongly exceeded normal values