연구보고서
- 저자
- 신용희 박사
- 작성일
- 2018.04.24
- 조회
- 476
- 요약
- 목차
All nations share the fundamental responsibility to develop plans for securing water resources and their equitable distribution among the population, so as to sustainably maintain livelihoods as well as promote economic growth. The risk of extreme events caused by global warming, population growth, and urbanization are increasing at a faster rate than ever before. Over the past few decades, extreme drought and flood due to El Niño-Southern-Oscillation (ENSO) events have both increased and intensified in the South Pacific region. Therefore, securing water resources and managing them sustainably are of the utmost importance to Pacific Island Countries, including the Kingdom of Tonga.
Background of the joint project
Given that fresh water supply systems are lacking and adequate manmade water storage infrastructure is insufficient in Tonga, identifying and protecting available water resources, namely, groundwater, in a sustainable manner are priorities for Tonga. To confront this urgent issue, a joint project by the APEC Climate Center (APCC), the Ministry of Lands, Survey and Natural Resources (MLSNR), and the Ministry of Agriculture, Forestry, Food, and Fishery (MAFFF) was launched in 2017. The project focused on development of a smart groundwater management system utilizing climate information in response to water shortages in Tonga. The aim of the project is to help stakeholders and decision-makers in Tonga’s water management and agriculture sector to secure and manage water resources sustainably by providing optimal alternatives to the current system. Both abovementioned ministries have actively participated and collaborated with the APCC in the project activities. Notably, the MLSNR has collected water datasets and assisted in the digitization of these datasets as well as those from previously conducted climate and geophysical surveys. The APCC analyzed the overall aspects of water resource management in Tonga, including rainfall climatology, climate change impact, and water recharge for Tongatapu Island, main island of the Kingdom of Tonga and the location of its capital Nukuʻalofa. In collaboration with teams from Dong-A University and a private company called HydroNET, the APCC installed a real-time groundwater observation system and developed a web-based model for analyzing and predicting the use and recharge of groundwater resources.
Prevailing conditions in Tonga with regard to water resources
The climate of Tonga consists of two seasons: a wet season, from November through April, and a dry season from May through October. About 60% to 70% of the annual rainfall occurs during the wet season. Average annual rainfall in Nuku'alofa for the period from 1945 to 2016 is 1,838 mm. Increasing population and climate change are exacerbating the vulnerability of limited freshwater and groundwater resources in Tonga. A steadily rising demand for groundwater and possible groundwater contamination are directly related to the rising population on Tongatapu Island.
Possible effects of ENSO and climate change on water resources in Tonga
Recent changes in the precipitation regime due to climate change and sea level rise have contributed to increased salinity of groundwater resources. The problem is worsened by the fact that Tonga belongs to a climatic zone directly affected by the ENSO. If a strong El Niño event should strike, there will be less than normal rainfall, and drought is very likely to occur. A composite analysis of precipitation as per the Global Precipitation Climatology Project (GPCP) shows that average monthly precipitation would decrease by about 90 to 180 mm during the El Niño wet season (December to February). The large-scale atmospheric environment is closely linked with the local hydro-climate in Tonga, wherein precipitation and Southern Oscillation Index (SOI) are positively correlated. If a strong El Niño event occurs, the extent of rainfall received by Tonga is likely to decrease. Therefore, continuous monitoring of El Niño is required to prepare for possible drought events in Tonga.
Climate change scenarios predict that the uncertainty about groundwater recharge is likely to increase in the future. Thus, it is crucial to analyze water balance in the project area according to events associated with climate change. The WATBAL model was applied to estimate the groundwater recharge rate for Tongatapu island. The annual total groundwater recharge was estimated to be 141,584,597 per year at a recharge rate of 30%.
Estimating soil water content in the project area
Since agriculture is the sector most vulnerable to rainfall variations in Tonga, we focused on the smart use of groundwater resources based on climate forecast information. The computer simulation model Soil-Water-Atmosphere-Plant (SWAP) was applied to estimate soil moisture content for ungauged areas, the intention being to build a smart water management system for the agriculture sector in Tonga. Soil moisture sensors (soil water content: SWC; soil water potential: SWP) were installed in crop-growing areas in selected agricultural fields to observe how SWC content affects crop growth. These sensors were installed at the Vaini Research Station and the Fua'amotu Farm of the MAFFF to monitor soil moisture in taro and vanilla cultivation areas, respectively. Statistical correlation between SWC and SWP in each cultivation area during low rainfall periods was analyzed. The parameters were found to be linearly and significantly correlated (R2≥ 0.9).
A soil moisture content model for crop-growing areas was then constructed using information on soil physical characteristics, weather, and crop growth. Soil samples were obtained using a spade and analyzed via hydrometric analyses. The results served as inputs to the ROSETTA model, which was used to estimate hydraulic parameters. Based on the physical characteristics of Vaini and Lapaha clay soils, which are representative of those on Tongatapu Island, and the information on the growth of taro, cassava, and yam (the main crops grown on the island), we devised a soil moisture content model for the project area. Overall, the estimation results from the soil moisture content model can be combined with groundwater modeling information for different crop growth stages to support an effective water management plan for the agricultural sector in Tonga.
Contribution of the project toward sustainable water resource use in Tonga
In conclusion, we found that both supply and demand of groundwater resources are significantly biased depending on the locations in question in the project area, and both the quantity and quality of these resources are at high risk because of contamination and depletion of surface water as well as seawater intrusion. Moreover, groundwater conditions in the project area are changing rapidly due to climate change and growing demands for water. Therefore, establishing a systematic and continuous monitoring and management system for groundwater is necessary for ensuring water sustainability in Tonga. The results of this project should guide the government of Tonga toward providing optimal solutions and establishing a strategic plan toward securing sustainable water resources in response to future water demand, climate change, and other extreme events. In doing so, this project will help enhance the quality of life and water security of local communities.