Laguna Lake’s hydrodynamics

In one of our previous posts, we have talked about the largest lake in the Philippines, the Laguna Lake or Laguna de Bay (http://bit.ly/PMLDB2_LDB). This shallow lake’s water level fluctuates by around 2 meters annually and is maintained by the influx of freshwater from the rivers of the Lake’s watershed, the tidal fluctuations at its outlet (Manila Bay), precipitation, and evaporation.

The Lake’s hydrodynamics is influenced by the combined effects of wind stress, river discharges, and the Lake’s interaction with Manila Bay through the Pasig River. These forces across the Lake vary spatially and more importantly, seasonally. The Southwest and Northeast Monsoons and the prevailing wind conditions in Laguna Lake have a strong impact on the Lake’s circulation. River discharges come from the surrounding subbasins and induce currents in Laguna Lake especially during the wet season when the river discharges are high. The Lake’s interaction with Manila Bay through the Pasig River is linked to the exchange of saltwater and freshwater between the two water bodies. The implications of these forces are further discussed in the next images.

We have learned from our previous post (bit.ly/PRHydrodynamics) about how Manila Bay and Laguna Lake interact through water level dynamics, and how this interaction facilitates the exchange of both water quantity and quality. The interaction of Laguna Lake with Manila Bay, and the river discharges from the surrounding subbasins have a combined effect on the hydrodynamics of the Lake that varies with season.

Shown in the figure is a comparison between the water levels in Laguna Lake and in Manila Bay in 2018. The Lake’s water level fluctuates up to around 2 meters within a year. The seasonal rise and drop in the lake levels is caused by the accumulation of precipitation and discharges received by the lake from surrounding subbasins during the wet season and the evaporation accompanied with low inflows during the dry season. In 2018, the total discharges from Laguna Lake tributaries peaked at around 2,000 cubic meters per second during the wet season, following an almost 5-month long low flow season.

For most parts of the year, Laguna Lake’s water level is higher than the sea level in Manila Bay. During the dry season (Feb-Jun), Manila Bay’s high tides become higher than the lake’s water level (roughly below 0.6 meters from mean sea level), allowing for Pasig River’s backflow and for saltwater intrusion in the Lake. It should be noted however that the saltwater intrusion in the Lake does not occur on a yearly basis but is dependent on Laguna Lake’s hydrology.

Laguna Lake’s connection to Pasig River is through the Napindan Channel, located at the northwest portion of the Lake. During low flows or dry season, Pasig River produces currents into and out of the Lake at the northwest portion, depending on whether it is high tide or low tide at Manila Bay. In comparison, the effect of discharge elsewhere in the Lake is weaker. Meanwhile, during the wet season when high flow rates are expected, stronger currents can be found near the river outlets. Another result of the increased flows is the increase in the lake water level. As the Lake drains through the Pasig River, an outward current is created in the northwest. Movement in the South and West Bays of the Lake are also directed to the northwest, towards Pasig River.

The Philippines’ wind dynamics is generally governed by the Southwest Monsoon (Habagat) from June to October and the Northeast Monsoon (Amihan) from November to May, although there can be variations in the timing from year to year. In addition, inland surface wind conditions can vary depending on the local topography which can interfere with and deflect winds.

Laguna Lake’s hydrodynamics is greatly influenced by wind conditions. The prevailing wind conditions over Laguna Lake in May 2018 (southeasterly winds), September 2018 (Habagat, southwesterly winds), and December 2018 (Amihan, northeasterly winds) were obtained from the Weather Research and Forecasting (WRF) model (https://www.mmm.ucar.edu/weather-research-and-forecasting…). Wind velocities can reach up to 12 m/s (around 40 kph) especially during Habagat and Amihan seasons, resulting in strong currents in Laguna Lake. The figures show the effects of wind stresses on Laguna Lake’s near-surface and near-bottom currents for different prevailing wind conditions.

During the dry season, current velocities are usually lower than 0.3 m/s. In September or during Habagat, strong currents can have velocities ranging from 0.4 to 0.5 m/s. Current velocities can also reach 0.4 to 0.5 m/s during Amihan but the gyres’ directions are different as shown in the figures.

The difference in near-surface and near-bottom currents are related to the stratification of water in the lake, as discussed in the next section.

Laguna Lake can be stratified due to temperature difference along the vertical, which is caused by solar heating at the surface. During the dry season, the stratification can be enhanced by intrusion of saltwater, which is denser than the Lake’s freshwater. Simulation results show that temperatures at the top and bottom layers can have a difference of as much as 2 degrees Celsius during daytime. The density can also vary by around 0.7 kg/m3.

The temperature and density difference along the water column decreases as the day turns to night. Late afternoon breeze breaks thermal stratification during the day, and convective cooling at night maintains isothermal condition until dawn of the next day (Herrera, E.C., & Nadaoka, K., 2021).

As shown here, near-surface circulation follows southeasterly wind forces while near-bottom current patterns move more to the south as the Lake receives saltwater backflows from Pasig River during this time of the year.

The resulting layer hydrodynamics of the Lake has important implications in its primary production (Herrera, E., Nadaoka, K., Blanco, A., and Hernandez, E., 2014).