Bataan’s Flashy Discharges

The west side of Manila Bay is characterized by a mountainous topography, with Mt. Natib, Mt. Mariveles, and the Zambales mountains forming the drainage divide of Bataan River Basin (bit.ly/BataanDrainageDivide). This results in a radial drainage pattern of numerous short streams that yield relatively low flows. In a hydrologic simulation of recent years (2017-2019), the average daily flow per river outlet is 0.6 cubic meters per second (cms) during the dry season and around 3 cms during the wet season. For context, 3 cms is less than 1% of Pampanga River’s average daily flow for the same simulation period. There are a total of 21 identified river outlets in Bataan River Basin that drain to Manila Bay, not including smaller streams along the shores that have much lower discharges. Nineteen of these twenty-one outlets have average discharges of less than 1.5 cms only, comprising just around 39% of the total flow of the whole basin. The average seasonal flow for Bataan River Basin is around 12 cms during the dry season and around 63 cms during the wet season.

Aside from the radial drainage pattern, other factors also have effects on the river discharges. Topography and weather both affect the resulting flows of a basin (see more about hydrologic inputs here: bit.ly/HydrologicInputs). Almost one-third of the 886-square kilometer basin have slopes considered as steep hills or cliff-like to mountainous (slopes equal to 30% and above). Steep areas can result to higher peak discharges because of higher flow velocities and lower infiltration rates, which may explain the flashy nature of the hydrographs generated. Streams in these areas are prone to be intermittent also, meaning, they may not have flowing water during some dry periods, and they have lower average flow. Some examples of intermittent streams in Bataan are the Apali, Marilao, and Santa Cruz Rivers. Drainage area is the topographical characteristic that can impact flows the most, it is worth noting that the 21 subbasins included in the hydrologic model only have an average size of 42 sq. km and so explains the relatively low flows. This is around 0.2% only of the whole area that drains to Manila Bay. Majority of the soils in the basin is classified as sandy clay loam with low infiltration rates and about 68% has either agriculture or forest land covers. Bataan River Basin receives an average monthly rainfall of around 368 mm during the wet season (May to Oct) and around 42 mm during the dry season (Nov to Apr). Temperature ranges from 24°C to 32°C. These averages were obtained from a 3-year period (2017-2019). These weather conditions are almost the same with that in Manila, except for the wet season where the monthly average rainfall in Bataan is around 86.5% of the monthly average in Manila.

In order to gauge the accuracy or performance of the hydrologic model, the flows were calibrated with observed river discharges (see bit.ly/ModelCalibration) to know more about calibration of models). River discharges are often derived from rating curves, which relate the water level with the corresponding discharge for a specific cross-section of a stream. In the case of Bataan where no discharge data are available as of the moment, the observed flows from basins with similar drainage channel patterns and topographic features were used. For Almacen River, the largest river (by discharge volume) in Bataan River Basin, the calibration resulted to a nearness coefficient (Nash-Sutcliffe parameter) of 0.18 which means that the simulated discharges are a better approximation of the real flows than the average of the observed discharges. A value of 1.0 means the simulated values are exactly equal with the observed flows. Also, the hydrologic model results fared better with using observed rainfall compared to using global or gridded weather datasets. Improvement in hydrologic model simulation can further be achieved by using observed discharges in Bataan, and by utilizing more weather stations to capture the spatial variability in climate in the basin, if available.

Almacen River is found at the northernmost part of Bataan River Basin. Its river system originates from both Mt. Natib and the Zambales mountains and has a drainage area of around 405 sq. km. which is almost half of the whole basin already. It has an average annual flow of 17.3 cms and a dependable flow of 0.52 cms based on an 8-year simulation. Dependable flow is the flow rate or discharge that is equaled or exceeded 80% of the time. This is often used as basis for flow diversion (hydropower, water supply, etc.) requirements, where 10% of which is allocated as environmental discharge or the required minimum flow that must be retained in rivers to sustain aquatic flora and fauna. Based on the results of the long-term hydrologic simulations, the rivers that yield the second and third largest flows are the Talisay and Abucay Rivers with 5.8 cms and 1.5 cms average annual flows, respectively. Overall, the contribution of Bataan River Basin to the freshwater influx to Manila Bay is around 6.8%.