Meycauyan River and Bulacan River Basin’s Hydrology

The Meycauayan River is one of the major rivers that drain to Manila Bay. It tansports almost all of the waters of Bulacan River Basin to Manila Bay through its downstream-most reach. Delineation of the Bulacan River Basin showed that all rivers within the basin eventually drain to the outlet of Meycauayan River, making the drainage system of the basin a classic example of a dendritic drainage pattern. Downstream areas of basins with dendritic drainage patterns are at risk of flooding since flows accumulate as tributaries converge. Discharge hydrographs at the outlet of Meycauayan River may also have sharp peaks (high flows discharging on a short amount of time) due to the basin’s round shape. Since flows are coming simultaneously from different directions, round-shaped basins have higher peaks than elongated basins, given that they have the same drainage area.

Though almost the whole Bulacan River Basin drains through the Meycauayan River, the discharge values derived from the simulations are relatively lower than the other major rivers discharging to Manila Bay. This is due to the basin size of Bulacan River Basin which is only around 621.4 sq. km., about 6% of the area of Pampanga River Basin.

A 10m-IFSAR DEM, NAMRIA land-cover map, FAO soil map, and a Thiessen-weighted weather dataset were used as inputs to the hydrologic model of the Bulacan River Basin. You may go to bit.ly/HydrologicModelling for some refresher on hydrologic models.

The main result of a hydrologic model is a time-series set of discharges. From this, we can characterize the discharges from a river basin or flows from a particular river. From a seven-year hydrologic model simulation, it was found that the annual average flow of Meycauayan River is around 22.3 cubic meters per second (cms). For context, this is just less than 10% of the annual average flow of the Pampanga River.

After initial simulation, the model was calibrated by adjusting some watershed parameters related to curve number and soil properties, among others. For Meycauayan River, the R-squared value upon comparing the observed and simulated discharges is 0.68, while the Nash-Sutcliffe Efficiency (NSE) value reached up to 0.52. NSE values greater than 0.50 are deemed satisfactory.

The mean seasonal flows for the calibrated model were found to be 5.6 cms for the dry season (Nov – Apr), and 31.2 cms for the wet season (May – Oct). Collectively, the seasonal variation of river discharges to Manila Bay plays a huge role in the hydrodynamics model of the bay.

Aside from flows, the water balance of the model can be checked to make sense of the model inputs and the results. The water balance describes what happens to the water as it enters the watershed. For the Bulacan River Basin model, around 63% becomes surface runoff, 21% goes out of the system through evapotranspiration, 7% seeps into the ground and moves as subsurface or groundwater flow before contributing to the river discharges, and 9% percolates deep beneath the ground surface.

The high surface runoff can be highly attributed to the land cover and soil-type distribution within the basin. Sixty-five percent of the soil is classified under hydrologic soil group D (all soil types in the image except for Gleyic Cambisols, which fall under soil group C). These are clayey soils that have the highest runoff potential among the hydrologic soil groups. This soil type is coupled with the 3 major land covers found in the basin: agricultural, urban, non-forested wetlands (stated in decreasing order in terms of area covered); all of these have high runoff potentials as well.

The land cover of a basin also affects the discharge output of a hydrologic model. To see its effects on the flows of Meycauayan River, two sets of a three-year model simulation were conducted: one representing the land cover in 2004, and another in 2015. Both land cover maps used were official datasets from NAMRIA. Other parameters and inputs were maintained to see if there was a noticeable change in the flows for a land cover difference in 11 years. Results showed that the annual average flows of Meycauayan River increased by 13% in the latter year. Subsequently, the increase in the mean seasonal flows for the dry and wet seasons were 14.5% and 12.6%, respectively.

Notable changes in land cover between the two datasets were a 10.7% decrease in agricultural land, a 62.8% increase in urban land, and a 78.2% decrease in lands covered in brushes, most of which were converted to agricultural land. All these changes increase surface runoff, leading to increased flows in the simulation with 2015 land-cover. Overall, this implies that the changes in land cover of the watershed not only affects the water quality of the flows but also its quantity.