PDF full version: Participatory Watershed Land use Management (PWLM) Guidebook (2021/6/21)

Introduction

Many weather-related disasters, such as floods and landslides, have occurred in Asia in recent years. One of the main causes of these disasters is the large-scale land transformation that has accompanied industrialization and urbanization. In addition, there is a possibility of climate change effects. Climate change is expected to change flow regimes in running water and, consequently, to affect habitats and species that are sensitive to floods (IPCC 2014).

Against this background, the Institute for Global Environmental Strategies (IGES) conducted a pilot project focusing on the effects of land-use changes on flood risk in order to help improve the local governments' plans for land use and action on climate change. The study area is the Silang-Santa Rosa Subwatershed in the Philippines, which is located about 40 km south of Manila, the national capital. The subwatershed is adjacent to Lake Laguna, the largest lake in the country (Fig. 1). This page provides a brief explanation of the project.

Methodology

This project employed a framework called participatory watershed land-use management (PWLM), which consists of the following four steps (Endo et al., 2017): (i) scenario analysis, (ii) risk assessment, (iii) development of Climate Change Adaptation (CCA) measures, and (iv) land-use plan improvement (Fig. 2). The process is circular and repetitive to allow the results and findings from each step to inform previous steps and improve analyses and decisions.

The first step, scenario analysis, attempts to understand the problems that local governments face in addressing natural disasters and other impacts of climate change, and assess the government's development and land-use plans (Fig. 3) for the next decade (to 2025). The second step, risk assessment, involves estimating the total population at risk of flooding (e.g., because of a typhoon's intensive rain). In the third step, the development of CCA measures is aimed at devising possible actions for adaptation in consultation with local governments. The fourth step, land-use plan improvement, intends to support local governments to strengthen their land use and related development plans through dialogue on the issues and recommendations generated from the previous three steps.

Fig. 2 Participatory watershed land-use management (PWLM) approach

 

Fig. 3 Land-use change in Santa Rosa from 2014 (left) to 2025 (right)

 

In order to estimate the number of people at risk for flooding, during the second step, runoff simulation was implemented using a basin model1 with the precipitation and discharge data from the 2012 typhoon Ofel (international name: Son-Tinh). This extreme rainfall event data was prepared by the Philippine Atmospheric Geophysical and Astronomical Services Administration. The population change projection (Fig. 4) during the years 2014 and 2025 for the smallest administrative unit (barangay), roughly equivalent to a village/ward/city district in residential areas, was used. Then the population information and the flooded area were overlaid.

1.　These hydrologic and hydraulic modeling processes were implemented using the Hydrologic Engineering Center's Hydrologic Modeling System (HEC-HMS) and River Analysis System 　　(HEC-RAS)

Fig. 4 Population change in Santa Rosa from 2014 (left) to 2025 (right)

 

Result

The flood-affected area for 2025 was projected to increase by 21.68% in comparison with that for 2014 owing to land-use change (Fig. 5). It is possible that the affected population will increase from 200,000 people (in 2014) to 280,000 people (by 2025) mainly because of population growth in the area and changes in land use.

 

Adaptation measures

Based on existing literature, IGES created a list of potential measures for mitigating and adapting to flood risk (step three), as shown in Table 1. The list was shared with government officials to identify priority measures based on local needs.

Local governments have created a list of priority measures based on Table 1 by considering the necessity and feasibility of each measure. The highest priority measures were (i) land-use development controls to be enforced in areas highly susceptible to flooding, which would prevent settlement construction and maintain as much vegetation as possible; (ii) strengthening building codes in high-risk areas to mandate such measures as the construction of floodwalls and the introduction of elevated flooring to protect buildings and infrastructures against flooding; (iii) a relocation plan for informal settlers who reside in flood-prone areas.

 

Table 1 List of possible measures for CCA, considered during focus group discussion sessions (Endo et al., 2017)