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The Philippines is one of the most vulnerable countries in the world to the potential impacts of climate change. To fully understand these potential impacts, especially on future hydrological regimes and water resources (2010-2050), 24 river basins located in the major agricultural provinces throughout the Philippines were assessed. Calibrated using existing historical interpolated climate data, the STREAM model was used to assess future river flows derived from three global climate models (BCM2, CNCM3 and MPEH5) under two plausible scenarios (A1B and A2) and then compared with baseline scenarios (20th century). Results predict a general increase in water availability for most parts of the country. For the A1B scenario, CNCM3 and MPEH5 models predict an overall increase in river flows and river flow variability for most basins, with higher flow magnitudes and flow variability, while an increase in peak flow return periods is predicted for the middle and southern parts of the country during the wet season. However, in the north, the prognosis is for an increase in peak flow return periods for both wet and dry seasons. These findings suggest a general increase in water availability for agriculture, however, there is also the increased threat of flooding and enhanced soil erosion throughout the country.

Flood magnitude and frequency estimation are essential for the design of structural and nature-based flood risk management interventions and water resources planning. However, the global geography of hydrological observations is uneven, with many regions, especially in the Global South, having spatially and temporally sparse data that limit the choice of statistical methods for flood estimation. To address this data scarcity, we pool all available annual maximum flood data for the Philippines to estimate flood magnitudes at the national scale. Available river discharge data were collected from publications covering 842 sites, with data spanning from 1908 to 2018. Of these, 466 sites met criteria for reliable estimation of the annual maximum flood. Using the index flood approach, a range of controls was assessed at both national and regional scales using modern land cover and rainfall data sets, as well as geospatial catchment characteristics. Predictive equations for 2 to 100 year recurrence interval floods using only catchment area as a predictor have R2≤0.59. Adding a rainfall variable, the median annual maximum 1 d rainfall, increases R2 to between 0.56 for Q100 and 0.66 for Q2. Very few other topographic or land use variables were significant when added to multiple regression equations. Relatively low R2 values in flood predictions are typical of studies from tropical regions. Although the Philippines exhibits regional climate variability, residuals from national predictive equations show limited spatial structure, and region-specific equations do not significantly outperform the national equations. The predictive equations are suitable for use as design equations in ungauged catchments for the Philippines, but statistical uncertainties must be reported. Our approach demonstrates how combining individually short historical records, after careful screening and exclusion of unreliable data, can generate large data sets that can produce consistent results. Extension of continuous flood records by continuous and rated monitoring is required to reduce uncertainties. However, the national-scale consistency in our results suggests that extrapolation from a small number of carefully selected catchments could provide nationally reliable predictive equations with reduced uncertainties.

Fresh groundwater lenses on karstic oceanic islands form a vital resource sustaining local populations. However, this resource is susceptible to saltwater intrusion through human drivers (over-abstraction) and natural processes (variable precipitation and storm surges). There is a paucity of means to assess the risks that freshwater lenses are exposed to. This is partly driven by a poor understanding of the root causes of saltwater intrusion, which leads to potentially inappropriate freshwater management strategies. Thus, effective management of these freshwater lenses requires a baseline understanding of the processes that drive saltwater intrusion and the degradation of freshwater lenses, and the temporal and spatial variability of these processes. Dynamics of such freshwater lenses involve an interplay between physical, chemical, and socio-economic processes; therefore, finding a solution necessitates an interdisciplinary approach and a range of data collection strategies. This approach was formalized in a Freshwater Lens Assessment Protocol (FLAP). Results from the research developed and tested on Bantayan Island in the Philippines reveals a sufficient freshwater lens to support the current and projected population; however, local officials are operating abstraction wells from the wrong locations on the island. Such locations are utilized due to ease of access to existing infrastructure and government boundaries, but do not consider technical factors that influence saltwater intrusion. FLAP is an appropriate, cost-effective, interdisciplinary tool that uses a pragmatic approach to data collection, interpretation, and integration into an observational model. Continuous adjustments are possible through ongoing monitoring of the model, offering opportunities to evaluate the efficacy of resource management strategies.

Characterisation of hydromorphological attributes is crucial for effective river management. Such information is often overlooked in tropical regions such as the Philippines where river management strategies mainly focus on issues around water quality and quantity. We address this knowledge gap using the River Styles Framework as a template to identify the diversity of river morphodynamics. We identify eight distinct River Styles (river types) in the Bislak catchment (586 km2) in the Philippines, showing considerable geomorphic diversity within a relatively small catchment area. Three River Styles in a Confined valley setting occupy 57% of the catchment area, another three in a partly confined valley setting occupy 37%, and two in the remaining 6% are found in a laterally unconfined valley setting. Five characteristic downstream patterns of River Styles were identified across the catchment. We observe that variation in channel slope for a given catchment area (i.e., total stream power) is insufficient to differentiate between river types. Hence, topographic analyses should be complemented with broader framed, catchment-specific approaches to river characterisation. The outputs and understandings from the geomorphic analysis of rivers undertaken in this study can support river management applications by explicitly incorporating understandings of river diversity and dynamics. This has the potential to reshape how river management is undertaken, to shift from reactive, engineering-based approaches that dominate in the Philippines, to more sustainable, ecosystem-based approaches to management.

Custado, M.J.G. and David, C.P.C., 2021. Assessing the spatial and temporal relationship between coastal runoff and chlorophyll-a in the Philippines using gridded datasets. Journal of Coastal Research, 37(4), 726–736. Coconut Creek (Florida), ISSN 0749-0208. Publicly available gridded datasets were used in examining the relationship between long-term temporal patterns in runoff rates and chlorophyll-a within the coastal areas of the Philippines. Temporal plots generated from remotely sensed chlorophyll-a data and runoff from the Global Runoff Reconstruction dataset illustrated variabilities in seasonal patterns across coastal segments and climate types. The linear correlation analysis performed on the datasets revealed a characteristic distribution of negative coefficients within the internal seas of the archipelago and positive coefficients in open coasts and embayments. Riverine-influenced sites with positive correlation coefficients (r = 0.33–0.62, p < 0.00001) were mostly associated with delivery of excess nutrients from land-use activities. The distribution of these coefficients was demonstrated to be affected by the physical nature of the coast. Moreover, seasonal changes in total suspended solids adjacent to a river discharge site were discussed, demonstrating how high sediment load can dampen productivity because of limited light penetration in marine waters. The analysis of publicly available datasets in this study provided assessments that can be used to complement coastal management efforts, particularly in the context of increasing anthropogenic stress and climate change.

Karst islands such as those found in the Philippine Archipelago present challenges for local stakeholders to manage their water resources sustainably. Anthropogenic climate change, an increasing population and changes in land use and industry have all combined to altering the water balance on these islands. The freshwater lens is susceptible to saltwater intrusion through human drivers (such as over-abstraction) and natural processes (including variable precipitation and storm surges). The dynamics of such freshwater lenses involve an interplay between physical, chemical and socio-economical processes; therefore, finding a solution necessitates an interdisciplinary approach and a range of data collection strategies. This approach was formalized in a Freshwater Lens Assessment Protocol (FLAP) and expanded through the application of a Vulnerability Indicator Assessment (VIA). The VIA provides an accessible framework for data integration and an improved knowledge base, from which it is possible to make better decisions and establish effective management programs to protect and maintain this vital resource. The FLAP approach and VIA analysis were developed and tested on Bantayan Island (Cebu Province) in the Philippines. The results of the analysis reveal areas of saltwater intrusion vulnerability located along coastal boundary margins, mapped structural discontinuities and zones of upconing due to over-pumping. We infer that the VIA forms an appropriate, cost-effective interdisciplinary tool that synthesizes data sets through hazard characterization, integrates stakeholder knowledge pertaining to water resource management and operational policy, assesses the risk and assigns a risk designation for groundwater vulnerability to saltwater intrusion.

Even in relatively wet tropical regions, seasonal fluctuations in the water cycle affect the consistent and reliable supply of water for urban, industrial, and agricultural uses. Historic streamflow monitoring datasets are crucial in assessing our ability to model and subsequently plan for future hydrologic changes. In this technical note, we evaluate a new observation-based global product of monthly runoff (GRUN; Ghiggi et al., 2019) for 55 small tropical catchments in the Philippines with at least 10 years of data, extending back to 1946 in some cases. Since GRUN did not use discharge data from the Philippines to train or calibrate their models, the data presented in this study, 11 915 monthly data points, provide an independent evaluation of this product. We demonstrate across all observations a significant but weak correlation (r2=0.372) between the GRUN-predicted values and observed river discharge, as well as somewhat skillful prediction (volumetric efficiency = 0.363 and log(Nash–Sutcliffe efficiency) = 0.453). GRUN performs best among catchments located in climate types III (no pronounced maximum rainfall with short dry season) and IV (evenly distributed rainfall, no dry season). There was a weak negative correlation between volumetric efficiency and catchment area, and there was a positive correlation between volumetric efficiency and mean observed runoff. Further, analysis for individual rivers demonstrates systematic biases (over- and underestimation) of baseflow during the dry season and underprediction of peak flow during some wet months for most catchments. To correct for underprediction during wet months, we applied a log-transform bias correction which greatly improves the nationwide root mean square error between GRUN and the observations by an order of magnitude (2.648 mm d−1 vs. 0.292 mm d−1). This technical note demonstrates the importance of performing such corrections when determining the proportional contribution of smaller catchments or tropical islands such as the Philippines to global tabulations of discharge. These results also demonstrate the potential use of GRUN and future data products of this nature after consideration and correction of systematic biases to (1) assess trends in regional-scale runoff over the past century, (2) validate hydrologic models for unmonitored catchments in the Philippines, and (3) assess the impact of hydrometeorological phenomena to seasonal water supply in this wet but drought-prone archipelago.

Mount Bulusan, the Philippines' fourth most active volcano, erupted in February 21, 2011, sending volcanic ash and pyroclastic materials to its surrounding rivers. The waters drained into the estuary of harmful algal blooms plagued Sorsogon Bay. We aim to determine the impact of the 2011 volcanic eruption and the preceding volcanic ash emissions to the dissolved silica concentration of rivers draining the flanks of Mt. Bulusan and its possible implications to the phytoplankton assemblage of the bay. Six river water sampling periods from August 2010 to October 2012 overlapped with Mt. Bulusan's active phase of volcanism. Our data shows that mean river silica from pre-eruption levels of~500 µM increased by more than 200% during and post-eruption. Highest Si concentration of 2270 µM was measured from Cadacan River in August 2011. Here, we argue that the sustained general increase of dissolved silica is due to the silica-containing materials from Mt. Bulusan's eruption and that their concentration in river waters is also a function of watershed lithology and precipitation. Increase in dissolved silica and other nutrients caused a shift to diatom domination and, possibly, termination of Pyrodinium bahamense var. compressum blooms. Silica load increase in embayments is a natural process that controls the dominance of algae. Our study also highlights the importance of Philippine rivers to the global ocean silica budget as a function of high precipitation, tectonics in general, and volcanism in particular.

Polychlorinated Biphenyls (PCBs) are a group of man-made organic chemicals known as chlorinated hydrocarbons. PCBs were banned in 1979 due to its toxicity and persistence in the environment. Variations of PCBs were determined in soils and plants. Degradation of PCB Aroclors and congener patterns were investigated from known contaminated site at a former Military Air Base. PCB concentrations ranged between 17 - 1,040 µg L-1 at 0 - 0.5 m depth, between 36 - 898 µg L-1 at 1.0 - 1.5 m depth, between 20 - 73 µg L-1 at 4 m depth and 21 µg L-1 at 8 m depth. There is also a negative correlation between the mean of the Toxicity Characteristic Leaching Procedure (TCLP) of PCBs versus the sampling depth. Linear regression analyses were used to determine the correlation of the factors such as depth, Octanol/Water Partition Coefficient (Kow) and solubility affected Soil/Water Distribution Coefficient (Kd). Effects were found only when the soil depth is at 0.3 - 0.5 m (at 95% confidence level). Kd is positively correlated with Kow while Kd has some level of negative correlation with solubility. No effect was found at 1.0 - 1.5 m, 4.0 m, and 8.0 m. Organic Carbon/Water Partition Coefficient (Koc) showed high results exceeding the log Koc criteria of 4.5; as a result, this is now a matter of concern on the potential adverse effects of the substance on terrestrial organisms within its vicinity.

Purpose - The purpose of this paper is to integrate the proactive role of communities and the use of flood modeling in the implementation of a flood early warning system. Design/methodology/approach - Manual rain gauges were installed in 20 houses of volunteers living within the Bicol River basin to monitor rainfall. Rain information is sent twice daily via SMS message to a receiving computer. The received data are used to run a basin model that was developed in HEC-HMS, which converts precipitation excess to overland flow and channel run-off. Findings - Different watershed models were developed for different rainfall events. Geomorphic analysis using 3 s SRTM Digital Elevation Model (DEM) processed in a GIS platform was also done to refine the overland flow. The derived hydrographs were used in the HEC-RAS hydraulic model which has as main output threshold values for the rain-flood relationship. Research limitations/implications - Although SRTM DEM that was used for the geomorphic analysis was sufficient for the purpose of the study, higher resolution DEMs can further improve the mapping of spatial extent of flood areas. Practical implications - The results are used for the forecast of flood and the timely issuance of flood bulletins. Originality/value - This study is the first to incorporate the involvement of the community in establishing a flood early warning system. The method can also be used as a prototype for other flood models in other parts of the country.