The expected population affected by flooding in 2025 will be reduced by 19 per cent to about 183,349 people: There will be approximately 43,061 fewer people in the flood-affected areas under the runoff-neutral development scenario. Additionally, the exposure of these people to floodwater damage could be reduced further by restricting development and/or reinforcing building standards in the flooded areas. (Endo et al., 2017) The systematic review of Hudon and Botzen (2019) showed that there are limited cost-benefit analysis of flood-zoning policies. However, the studies identified found that zoning policies tended to have a positive CBA outcome, meaning zoning policies are economically desirable, at least when both changes in financial and environmental impacts are considered. However, these results should be interpreted with some caution since many of the stud- ies did not explicitly account for the full range of potential costs. (Hudson and Botzen, 2019, p. 17) Financial benefits: • Reduced annual expected damage to physical property due to flooding ◦ Due to lower exposure (less valuable assets located in the area) ◦ Due to lower vulnerability (higher level of preparedness due to regulations) • Reduced annual expected flood losses due to business interruption caused by flooding ◦ Due to lower exposure (fewer businesses located in the area) ◦ Due to lower vulnerability (the businesses remaining are less likely to be heavily affected) • Reduced indirect economic impacts due to flooding • Fewer ripple effects into economic activity in other areas, that is, a factory must shut down because it cannot get input materials from a flooded factory Financial costs: • Suboptimal land-use ◦ Lost income due to sub-optimal land-use ◦ Cost of purchasing land that does not meet zoning requirements • Loss of employment ◦ Average wage • Indirect economic impacts ◦ Indirect business losses due to sub-optimal land-use • Administrative/enforcement costs ◦ Cost of producing and maintain flood risk maps accounting for changing flood conditions ◦ Costs of hiring staff and transaction costs of enforcing the zoning policy ◦ Start-up and assessment costs ◦ Deadweight welfare loss due to increases taxation to finance the zoning department • Cost of employing risk management methods and maintenance costs • Change in tax revenue due to changes in economic activity (Hudson and Botzen, 2019, p. 5)

Participatory watershed land-use management (PWLM) should be done to inform zoning policy. Applying readily available data and techniques—participatory GIS and hydrological modeling—in a simple step-by-step procedure (i.e PWLM) makes this approach practical and applicable to various locations and institutions where limited expertise and resources are available. The GIS and remote sensing analyses conducted under the pilot project do not necessarily require a large amount of data, and most of them are publicly available (e.g. satellite images, population data, rainfall projections). The information about flooding and land-use can be obtained locally from consultation with local stakeholders. If the support from local experts in the relevant field of science (e.g. University of the Philippines Los Baños in our pilot project) is secured, similar data collection and analyses can be fully conducted at the local level. Our approach can be best described as a “partial” PGIS approach in a sense that all technical aspects of GIS were undertaken by experts (i.e. our study team) and that stakeholders other than local government representatives (e.g. local residents, companies) were not included (Canevari-Luzardo et al., 2015). (Endo et al., 2017)

The acceptability of the zoning policy will depend on the distribution of benefits across stakeholders. (Hudson and Botzen, 2019).

Environmental benefits: Newly developed eco-system services due to changing land-use patterns (different land-use patterns can promote different eco-system services) ◦ New recreational uses ◦ New landscapes ◦ Increased bio-diversity Environmental cost: • Negative changes in the environment (e.g., habitat loss) (Hudson and Botzen, 2019, p. 5)