Laguna Lake’s Hydrology

The watershed of Laguna de Bay is subdivided into 24 subbasins (see https://bit.ly/LLSubbasins for the list of subbasins). A continuous hydrologic model of all the subbasins showed that of these 24, the Pagsanjan Subbasin yields the highest discharge into the Lake. It has an annual average flow of around 13.6 cms, or which is about 16.4% of the total inflows into the lake. This supports what is historically reported where the Pagsanjan Subbasin drains the biggest contribution of inflows to the lake (https://llda.gov.ph/laguna-de-bay/).

During strong typhoons or continuous heavy storm events, the Marikina Subbasin discharges more freshwater into the lake when the Manggahan flood gates are opened, and more flows are diverted to Laguna Lake. For most days of the year, Marikina Subbasin naturally discharges to the Pasig River, and subsequently to Manila Bay. Thus, Pagsanjan Subbasin remains to be the largest freshwater contributor to the lake.

The Pagsanjan Subbasin yields the highest annual average discharge of 13.6 cms. Excluding the subbasin of Marikina, the Pagsanjan Subbasin also has the biggest catchment area among Laguna Lake’s subbasins (330 sq. km or about 11% of the whole Laguna Lake Watershed). It is followed by Sta. Maria Subbasin which drains an annual average flow of around 12.7 cms and has a catchment area of 244 sq. km. San Juan Subbasin comes in third which drains an annual average flow of 8.4 cms and has a catchment area of 196 sq. km. If every other hydrological aspect were to be constant, larger basin areas would yield higher discharges. However, aside from the catchment area, other topographical features (such as land cover and soil type), and meteorological features (rainfall, temperature, etc.) vary from one basin to another, affecting the amount of discharge that the watersheds drain to their outlets.

The Laguna de Bay is divided into 4 bays (see https://bit.ly/LDB_Bays). The hydrologic simulation results are in agreement with the LLDA report that about half of the inflows come from the east side (38.9 cms or about 45%) of the watershed. The South, West, and Central Bays, receive an annual average flow of 21 cms, 14 cms, and 12 cms, respectively.

The rainfall distribution within the Laguna Lake Watershed varies widely at different parts of the watershed and at different times within the year. The isohyets (blue lines) represent areas that are experiencing approximately the same total annual rainfall (values in millimeters) while the red bar graphs represent total monthly rainfall.
In general, it can be seen that higher rainfall values are experienced at higher places or mountainous regions. This is due to the orographic effect which is the increase in rainfall intensity as elevation increases (moist rising air cools and condenses upon reaching higher altitudes).
The temporal variability in the graphs show consistency with the climate map of the Philippines generated with data from 1951 to 2010 (http://bagong.pagasa.dost.gov.ph/info…/climate-philippines).
The subbasins in the western side of the watershed have pronounced peaks in the middle of the year (around July or August), making them highly representative of Type I Climate which has a wet season from May to October and dry season for the rest of the year. The subbasins in the center portion have less pronounced peaks but the maximum rainfall still occurs within May to October (Type III Climate). Some subbasins in the east experience a clear Type II Climate which is characterized by evident maximum rainfall at the end of the year. Capturing an accurate rainfall distribution is crucial to hydrologic modelling, thus the importance of spatial, continuous, and reliable weather datasets.
This rainfall distribution within the lake’s watershed also supports the model results. The two highest freshwater contributors to the lake, Pagsanjan and Sta. Maria, are at the eastern side of the watershed. Aside from having large basins, these two experience high total annual rainfalls as can be seen in the image.

Different land-cover types have different capacities to convert rainfall into surface runoff. Highly urbanized areas result to higher surface runoffs while agricultural lands, grasslands, and brushlands yield relatively lower surface runoff (theoretical trends observed in the model results as the percentage of rainfall converted to surface runoff were plotted against different land-cover concentrations in each of the basins). This implies that converting agricultural lands into urbanized areas can increase surface runoff and lead to steeper discharge hydrographs. Steeper hydrographs (higher flows occurring in less amount of time) increase the risk of flooding. Aggressive land conversion can also lessen groundwater recharge that can eventually lead to drying up of wells and land subsidence, among other things.

A land-cover change scenario run was done for the subbasin of Tanay. A hypothetical scenario where all the brushes (36.5 sqm of land area or 65% of the basin) were converted into urban land, resulted to an average increase of about 10% in the annual peak flows. Furthermore, the annual average values of surface runoff increased by 17%, while groundwater contribution to the streamflow and groundwater recharge to aquifers decreased by 9% and 10% respectively. This signifies a lower baseflow through most of the year but higher discharge during storm events.