PRESENTATION ON HYDROGEOLOGY, CLIMATE AND WATER SUPPLY OF TONGA. Akapei. Vailea Kingdom of Tonga

Transcription

1 PRESENTATION ON HYDROGEOLOGY, CLIMATE AND WATER SUPPLY OF TONGA Akapei Vailea Kingdom of Tonga

2 INTRODUCTION The Kingdom of Tonga (the Friendly Islands) was united into a Polynesian kingdom in Constitutional monarchy in 1875 and a British protectorate in Tonga acquired its independence in 1970 and became a member of Commonwealth of Nations. It remains the only monarchy in the Pacific.

3 GEOGRAPHY LOCATION - The Kingdom of Tonga is a Polynesian country which lies in the South West Pacific between Fiji, Western Samoa and New Zealand.

4 Location Map of the Kingdom of Tonga Tonga comprises some 176 islands of which 35 are inhabited. Tonga consists of four clusters of islands extended over the north- south axis: Tongatapu and Eua in the south, Ha'apai in the middle Vava'u in the north Niuafo'ou and Niua Toputapu in the far north.

5 Nuku'alofa, the capital is situated in Tongatapu,, the largest island (King Palace).

6 POPULATION The last census in the Kingdom of Tonga was in The census revealed that the total population of the Kingdom of Tonga was 97,784. Of that total, 66,979 were in the Tongatapu Division 4,934 in the Eua Division 8,138 in Ha apai Division 15,715 in Vava u Division 2,018 in the Niuas Division

7 LANDS USE The pursuit of agriculture in Tonga has followed traditional patterns of subsistence cropping and fallowing for centuries. The mainstay of agriculture for export is yam, cassava, kumala,, taro, vanilla, squash pumpkin, water melons and western vegetables.

8 GEOLOGY The islands are recognized as either volcanic or uplifted coral limestone on a deep pile of sediments of volcanic origin. The volcano s form a linear chain on the western side and the coral islands to the east. Volcanic islands are recently formed and several are currently dormant or slightly active. The limestone islands have been uplifted in blocks in response to crustal movements. During this uplift, which occurred in stages, coral terraces and accumulation of volcanic debris formed the base of the islands seen today.

9 Tonga's archipelago lies along the boundary of the Pacific and Indian-Australian tectonic plates. It comprises both volcanic and uplifted coral islands and reefs, which cap the peaks of two parallel submarine ridges stretching south of Fiji. Tongatapu and Eua are limestone capped islands which with low islands form the Tongatapu group. The south of the Vava'u group is generally composed of high volcanic and elevated limestone islands with reef communities or fringing reefs. Ha apai group has high volcanic and low limestone islands. The Niuas are high volcanic islands surrounded by fringing and barrier reefs.

10 CLIMATE The Kingdom of Tonga lies within the trade wind zone of the South Pacific. A belt of high pressure spanning the South Pacific is centered on the latitudes 25S to 30S. In northern Tonga easterly and south easterly winds have their highest frequencies during the period May to October, When the trade wind are strongest. Mean monthly wind speeds vary from four to eleven knots. Mean wind speeds from May to October in northern Tonga are stronger than at other times of the year. In southern Tonga the mean speeds are stronger during November to April tropical cyclone season.

11 The humidity of the air may reach 75% at midday. By contrast, during the Dry season, the amount of rain received is less than 250mm and the humidity is lowered to 67% at midday. Rainfall is highly variable from year to year with abnormally low or high rainfall persisting for more than three months is extremely rare. Two predominant causes of rainfall variation in Tonga are ENSO and Tropical Cyclones. ENSO causes prolong drought whereas Tropical cyclones can result in unusually wet years. The mean annual rainfall for the major island groups, Tonga was calculated starting from year 1947 to Nuku alofa, Tongatapu received 1753mm, 1689mm (Lifuka( Lifuka, Haapai), and 2185mm (Neiafu( Neiafu, Vavau)

12 CLIMATIC PARAMETERS RAINFALL The area of convergence between the equatorial easterly trades and the sub-tropical south-easterlies. Tonga s annual rainfall can be defined by two seasons, the Wet and Dry seasons. A wet season (also the cyclone season) is noticeable from November to April and a dry season runs from May to October. Wet season contributes to about two thirds of the total annual precipitation. The wettest months are particularly January, February and March which may receive up to 250mm of rain.

13 Vava u Group

14 Mean Annual Rainfall (mm) in major island groups, Tonga ( ) Mean Annual Rainfall (mm)in major island groups, Tonga ( ) Mean Annual Rainfall (mm) Nukualofa, Tongatapu Pangai, Haapai Neiafu, Vavau Major island groups, Tonga

15 Water Supply and Means of augmentations The water supplies for most of the villages and outer islands are sourced from groundwater. Groundwater is normally pumped from drilled wells and some old dug wells. In some cases, supplementary rainwater is harvested from roof catchments. On the island of Eua Eua,, small streams flowing from caves are used for the primary water supply. On the island of Lifuka,, where the groundwater resource (freshwater lens) is limited, water supply improvements have been implemented by constructing infiltration galleries to replace earlier boreholes.

16 Water Supply in urban area The Tonga Water Board (TWB) operates the water supply systems for the four main urban centers. Groundwater extraction by the TWB has increased by 40% over the last 5 years. Usage averages around 100L/capita/day The rural water supplies are operated by Village Water Committees with support and training provided by the Ministry of Health

17 Groundwater in Rural Area and Outer islands Most of the islands in Tonga fall into the category very small island with and area not greater than 100 km square or a width greater than 3 km very small size, the water resources on these islands are primarily ily dependent on rainwater and supplemented by underground water. Furness and Helu (1993) report on visit to all inhabited island of Tonga and make preliminary assessments of the potential for groundwater development and recommendation for the improvement of water resources. Most of these recommendations were to improve the rainwater collection system such as the guttering and also additional rainwater tank. The outer small islands of Tonga have received much less attention in terms of hydrogeological investigation and climate data. location map of the Tonga islands with a zooming out map of Lifuka island.

18 LIFUKA ISLAND, HA APAI GROUP

19 Outlined of Tonga Water Resources GROUNDWATER Groundwater in the islands of Tonga is found in the form of freshwater lenses, which occurring under the island overlying seawater The interface between the freshwater zone and seawater is not sharp but rather is in the form of a transition zone. The upper surface of a freshwater lens is the groundwater table and the lower surface is defined by the maximum salinity limit adopted for potable water (Alam & Falkland, 1997).

20 Water quality test for Tongatapu 3500 Tongatapu Village wells (Houma 166, Vaini 27 and Tatakamotonga 19) 3000 Houma Vaini Takamtong Conductivity (us/cm) Date

21 Water quality test for Vava u Vava'u Village Wells (Ta'anea (7) & Pangaimotu (14) Villages) /03/90 26/09/90 19/06/91 21/08/93 4/02/94 3/05/94 19/08/94 23/03/95 22/06/95 0 Well 14 well 7 8/12/95 23/03/96 15/10/96 6/03/97 5/06/97 26/09/97 8/02/98 8/06/98 3/10/98 Date Conductivity (us/cm)

22 Water quality test for Ha apai Ha'apai wells (Lotofoa (34) & Koulo (39) Village) Well 39 Welll 34 3/08/85 05/03/ /01/91 28/01/94 18/08/94 13/06/95 14/03/96 15/03/97 3/10/97 14/06/98 Date Conductivity (us/cm)

23 Typical freshwater lens on a coral island atoll situation.

24 Surface water Surface water resources are only occurring on some of the high volcanic and mixed geology islands in the form of springs and lakes. Crater lakes exist on the islands of Niuafo'ou and Tofua. The former lake has been used in dry periods as a source of potable water. Surface water is collected from cave systems on the island of 'Eua' and used for potable water supply.

25 Rainwater Rainwater systems are another freshwater resource for the islands and they represent an important source of potable water on many of the islands. On some of the smaller islands in the Ha'apai and Vava'u groups they are the only source of freshwater. Rainwater is the supplementary source to groundwater, which is collected by rooftop and stored by different types of tanks such as galvanize ion, reinforced concrete and fiberglass. There have been numerous programs involving the installation of water catchments tanks for the rural area and outer islands (Furness & Helu,, 1993). In addition to private installation of water collection tanks, foreign aid and various NGO groups have organized construction and installation of rainwater tanks in rural area and outer islands.

26 Recharge Analysis A computer program called WATBAL9F based on a water balance simulation to estimate the recharge to groundwater using monthly rainfall data from the Tonga Metrological Services (TMS) and potential evaporation data estimates The period of analysis was from 1947 to 2001 (55 years). It allows for interception losses, and a linear relation between evapotranspiration ratio (EA/ET) and soil moisture content (Field Capacity Wilting Point). It was undertaken for the main islands of Tonga namely Tongatapu, Ha apai, Vava u, Niuatoputapu and Niuafo ou.

27 2001 Annual Recharge for Lifuka Lifuka, Ha'apai Annual Recharge From 1947 to Recharge Statistics for 55 years period Mean = 524mm Max = 1296mm Min = -36mm Year Annual Recharge (mm)

28 Annual Rainfall and Recharge Annual Rainfall and Recharge Lifuka, Ha'apai Annual Raifall and Recharge from 1947 to 2001 Rainfall Recharge Statistics for 55 years period Average Annual Rainfall=1749mm Annual Recharge Mean = 524mm Max = 1296mm Min = -36mm Year Annual Rainfall and Recharge (mm)

29 Summary and Conclusion The water resources of rural area and outer islands of Tonga are vulnerable to the impacts of climate and sea level changes. Impacts of climate and sea level changes are outlined below. Salinity data from selected wells has shown an impact of the decreasing rainfall on the quality of groundwater lens. Lifuka in the Ha apai group has the lowest percentage of recharge (27%) for the 55 years period ( ) of analysis in comparison to other islands of Tonga.

30 RECOMMENDATIONS Need to assess and monitor water resources of rural area and all outer islands along with their performance under current stresses (e.g. droughts, groundwater pumping) and possible additional stresses caused by climate change scenarios. Proper management of the existing water resources is essentially needed. The monitoring data for the management of water resources include rainfall, evaporation, groundwater levels and salinity, and groundwater pumping. The continuation of present sea-level monitoring programs for small islands is encourage to enable further data to be collected and analysed. This should be combined with data monitoring and analysis of climate and groundwater systems in order to assess relative impacts of changes in recharge, pumping and sea-level changes.

31 Malo Aupito Vinaka vaka levu Thank you very much