Introduction
The December 2004 Tsunami approached and swept over the Maldives in an East-West Direction. The low elevation of the islands makes them very vulnerable to any rise in sea-level, and thus especially to tsunamis. It has been estimated that one third of the population of the country was severely affected, 1,300 people injured, 83 confirmed dead and another 25 missing and feared dead, 39 islands severely damaged, 14 completely destroyed and people remaining had to be evacuated. Nearly 12,000 people were displaced from their own islands and another 8,500 have been temporarily relocated to other places on their own islands. Livelihoods of many have been disrupted if not destroyed and the impact on the National Economy is likely to be substantial given that it is based largely on tourism, fisheries and agriculture. Furthermore, disruption to services has been substantial. Stand-alone electricity supplies on many islands were destroyed as well as telecommunication facilities, and these are now being slowly restored. It is estimated that 15 percent of islands suffered disruption to water supplies and 25 percent of islands suffered major damage to their essential infrastructure facilities including jetties and harbors. In the first weeks after the disaster FAO embarked on preliminary damage assessments that were followed by more in-depth missions investigating short and long-term impacts of the disaster to agriculture and land and water resources. The combined findings resulted in a representative assessment that is summarized in this text. On basis of the assessments FAO initiated projects to ensure and monitor the recovery of agriculture in the affected areas. The logistics of providing any level of service to the dispersed population even in normal times is a daunting matter. Provision of essential infrastructure services and their maintenance is fraught with transport difficulties and associated high cost, and is unusually challenging at the best of times. The needs of the post-tsunami efforts face similar problems.
Overview
The Maldives is a large and spread group of widely dispersed islands in atolls spanning a distance of some 900 Kilometers more or less stretched from North to South. The population is about 300,000 at latest count. The country consists of a total of some 1,190 islands of which199 were inhabited islands prior to the Tsunami. Of these, only 28 have a land area greater than one square kilometer. The total land area (covering all islands) varies in time but on basis of recent satellite measurements is currently indicated to be in the order of 230,000 ha. Al this land has similar sandy type coral soils and it is estimated that only 10 % of the land area has worthwhile agricultural potential. The geographical shape of the island varies from elongated (mostly in North-South direction), horseshoe shaped, triangular to circular but in almost all cases the East-West cross-section does not extend beyond 1-2 km. The elevation above mean sea level is generally stated to be only some 1.0-1.5 m. Some islands have pronounced marshy depressions (these are locally referred as taro lands, as they are suitable for growing the taro crop). One third of the inhabited islands have a population of less than 500, whilst 70 percent have a population of less than 1,000. Around 100 islands are of agricultural significance. Almost all of the farming is small holder subsistence farming. A total of 32 non-inhabited islands are leased for private sector commercial farming but in most cases development has yet started.
Natural conditions
As the country is located in the Western part of the Indian Ocean , the climate is warm and humid for most of the year. Rainfall is quite widespread and has an increasing trend from North (average 1840mm) to South (3500mm). Two monsoon seasons dominate the climatic regime; these are the high rainfall SW monsoon and the somewhat drier NE monsoon which respectively prevail from May/June till September and from October/November till February. There is a very distinct dry season from January to April with the heavy rainy season expected to begin usually around May.
All soils are calcareous residual soils derived from weathered coral formations that form the bed-rock foundation of these islands. The weathered layer is usually not more than 50-70 cm deep and consists almost entirely of medium sized calcium-carbonate sand grains. The topsoil of some 15- 20 cm thickness is dark colored by organic matter from cleared natural vegetation. As the soils do not have silt and clay material that provides soils with adsorption capacities for water and nutrients, all holding capacity of these soils is vested in the organic matter of the topsoil and in the little capillary holding capacity of medium-sized sand. Natural fertility of the soils is very low and total water retention capacity of the root zone (at field capacity) is probably not more than 30-40 mm. The soils have a very high vertical permeability. The hydraulic conductivity (K-value)1 of the medium sized-sand and the coral bedrock is in the order of several meters per day. Rainwater infiltrates without producing run-off.
All islands show the typical geohydrological sea island feature of a fresh water lens floating on deeper saline groundwater. The fresh water lens is periodically replenished by the rainfall recharge and depleted by the local groundwater use for household, irrigation and industrial water, groundwater uptake by the vegetation and lateral outflow to the sea (most houses have roof systems to collect rain water for cooking and drinking). The ground water table is present at depths of only twenty to fifty centimeters below ground level in most locations. The depth of the fresh water lens is a few meters, probably a little deeper in the South than in the drier North.
Tsunami Impact Assessment
The tsunami waves generally approached the Maldives islands from the East. The tide in visited area was reported to be in the order of 0.50 - 0.75 m. The tsunami occurred at low tide but nevertheless the waves were high enough (reaching to 2-3 m + MSL at the sea side) to sweep over some (parts) of the islands. Three waves swept over the islands in a 2-3 minutes interval, lasting less than 6-7 minutes in total. Afterwards the flood water drained back to the sea which process, depending on local topography lasted between 0.5 hr to a full day with some of the enclosed depressions remaining inundated for several days.
Agricultural impacts
Agriculture on almost all islands is restricted to growing field crops and fruit trees. There are no livestock or fodder crops. Principal field crops are: water-melon, pumpkin, cucumber, cabbage, cassava, sweet potato, yam, chilies and taro. These crops are generally cultivated in cleared plots in the natural jungali vegetation (a mixture of flood and salt-resistant bushes and shrubs, intermixed with coconut trees. The plots vary in size from 0.5 to 1 acre with individual plots usually being separated by a strip of jungali. Usually, no facilities are installed apart from a shallow dug well (for manually watering crops) and an elementary shelter. Fruit trees are generally grown within the walled compound around the houses and include mango, papaya, bread fruit, stone apple, drumstick and various citrus varieties. Papaya and bananas are grown both in the compound and in the field plots. The agricultural impact of the tsunami is generally limited to the loss of field crops and the dying of fruit trees due to osmotic stress and toxicity of the flood water. Very little erosion of the agricultural fields or deposition of sediments was observed. Damage to infrastructure (irrigation/ drainage/road systems, fences, farm buildings, etc.) also appears to be negligible as little infrastructure had been constructed. All field crops in the flooded areas are lost beyond rescue. Almost all fruit trees have shed their leaves and many appeared to be dead, It was, however, observed in the beginning of March that a fair percentage of the fruit trees (estimated at some 30-50 %) sprouted again and formed new shoots. At the same time, most mango trees, however, showed no sign of life but some apparently dead bananas sprouted again after being cut back. Coconuts and all natural vegetation, apart from some limited leave shedding show no signs of suffering. Young trees (especially seedlings) have suffered more than mature trees. Local people generally confirmed that the sprouting was induced by rains. The best sprouting was generally observed in areas where strong rains had occurred quite soon after the tsunami but people also stated that trees had responded to rains which came as late as 6-7 weeks after the saline flooding. Some households, of their own initiative, started applying excess water to their fruit trees soon after the Tsunami and they have had success with the trees responding by showing new growth.
Impacts on the soils
Although most of the saline flood water eventually drained back to the sea and the period of flooding in some cases was quite short, enough sea water infiltrated or remained on the land to leave a considerable salt load in the soil. After the retreat of the saline flood water, most of this salt remained in the root zone, raising the soil salinity in this zone to an estimated level as high as EC e =50-60 dS/m (thresholds for most crops and fruit trees is EC e <4 dS/m)2. These high soil salinity values prevailed for some time after the flooding, until the arrival of the first rain. Post-tsunami rainfall has been varied and is in line with the decreasing trend in total rainfall in a South to North direction. Between December and early March, there had been more than 150 mm in the southern islands in a few intensive falls whilst in the northern islands visited there has been about just over 100 mm but only one event of which was over 40 mm and the others smaller. These rains have leached out most of the salts and reduced soil salinity in the root zone to safe levels. Due to topographic differences and variance in rainfall salinity is still a problem in some areas, especially in the North. This remaining salinity is confirmed by disappointing experiences of the few farmers who have started planting again (failed cucumber seedlings) and by the analysis results of the soil samples taken in affected fields. Unlike in other regions the tsunami flood has not caused any deterioration of soil structure, since there is no silt/clay fraction. Dispersion of organic matter was observed in some fields but this has not materially affected the soil structure.
Impacts on groundwater
The tsunami flooding has salinized the groundwater by three mechanisms:
1) The saline flood water has raised the watertable and as such salinized the upper groundwater. All salts leached from the soil profile have also ended up in this upper groundwater layer;
2) Many of the wells have been flooded and thereby locally salinized the surrounding groundwater;
3) the sea water not only flooded the land surface but also intruded laterally into the groundwater aquifer, thereby salinizing the fresh water lens over some distance inland (generally confirmed by the high salinity measured in the sea-side wells).
The first mechanism presumably led to a rather uniform and area-wide salination of the upper groundwater zone (where the land was flooded). The second mechanism locally added to this salination while the third mechanism salinized the fresh water lens from the sea side. The measured salinity values reflect this variation. Salinity levels in flooded wells were found to be mostly in the range of EC = 5-7 dS/m. (as compared to pre-tsunami salinity levels of EC = 0.5-0.6 dS/m). As the groundwater is generally only used for general household purposes and not for drinking/cooking water, most flooded village wells were operational quite soon. Water quality in the wells improved after a few days by draining and dilution from the fresh water lens.
In the long run (1 year), spatial variations in the salt concentration of the groundwater will have evened due to ground water flow, salt diffusion, advection and dispersion, dilution3 by rain and other mixing processes while in the still longer run, all tsunami added salts may be expected to drain back to the sea. Although full completion of the latter process will take considerable time (several years), it is conceived that one good rainy season with several intense showers will flush out enough salts from the fresh water lens to be safe for agricultural purposes.
Processes related to the flow of soluble salts in the soil: diffusion is a chemical activity, the flow of ions (salt molecules) from areas of high concentration to low concentration; advection is the mechanical transport of soluble salts through groundwater movement and dispersion is the subsequent dilution, influenced by pore shapes and sizes. Dilution can also occur by simply adding fresh (rain)water, diminishing the overall salt-concentration.
The way forward
Practical advice tor farmers on coping mechanisms with salinity
After initial severe salinisation of agricultural lands, most field crops and fruit trees have died. In areas where the rains were on time, some of the trees survived. Post-tsunami rainfall has cleaned most of the affected area and it is expected that after the monsoon season, all salinity problems will be eradicated. Nevertheless, there are some relevant measures that farmers can take to ensure crop and fruit tree survival, such as:
- Farmers (in the North) should begin re-planting at the onset of the first heavy rains (>50-60 mm).
- Farmers should be advised not to irrigate field crops with well water of EC > 5 dS/m and to over-irrigate (daily with 50% more water than normal practice) when the well water is of marginal quality (EC ~ 4 dS/m).
- Farmers on northern islands and close to the sea should limit their abstractions from the wells in order to prevent intrusion of deeper salts.
- Water conservation is to be promoted through mulching and run-off reducing measures.
Short term interventions needed
- Provision of seeds and seedlings of fruit-trees to farmers on affected islands to quickly re-establish their livelihoods and help them return to normality. (Inputs currently being procured by FAO for immediate distribution)
- Provision of extension services to tree-owners at the Agricultural Centers, on how to enhance tree survival/recovery chances through watering/pruning.
Mid-term interventions needed
Present expertise in salinity management in the MFAMR is very limited and although the current tsunami related salinity problems may be expected to be essentially of a temporary nature, it is considered that the country needs to strengthen its capacity in this field of expertise. This need arises from the prevailing geohydrological regimes of these coral islands under which all irrigation water is drawn from a thin fresh groundwater lens floating on highly saline deeper groundwater. The water and salt balances of these fresh water lenses should be carefully managed and monitored in order to assure that the irrigation water salinity remains within safe limits. Expertise is also needed to monitor the capillary salination of the root-zone during the dry season. Under the Tsunami relief operations, FAO has already provided 15 salinometers (EC-meters) and training to MFAMR staff on how to use them.
The medium term activities needed related to salinity are:
- raising the awareness and establishing elementary knowledge on salinity hazards facing the country's agriculture amongst the staff of the MFAMR;
- establishment of a dedicated, suitably equipped soil salinity laboratory at the Hanimaadhoo Agricultural Centre;
- providing professional training to selected staff of the Hanimaadhoo Centre in salinity assessment and management;
- establishing a program for the monitoring and assessment of salinity conditions in the fresh water lens, the salinity of irrigation (well) water and of soil salinity in the root zone;
- establishing an exchange and networking relationship with other salinity centers in the region (a.o. Central Soil Salinity Research Station at Karnal , India ; Centre for Saline Agriculture in Dubai );
These activities could be part of a comprehensive project to strengthen integrated land and water resources management, and take into consideration soil fertility, other water quality aspects, etc.
Sources
The main sources for this brief were drawn from FAO-consultancy missions and technical reports by L. Smedema (soil specialist) and A. Somesan (water management specialist). Missions were carried out in collaboration with the Ministry of Fisheries, Agriculture and Marine Resources (MFAMR) and supported by the FAO-Maldives Representative and AG Tsunami Coordination at FAO-HQ.
Notes:
1 Hydraulic conductivity is a measure for the ease with which the soil pores permit water movement. It depends on the type of soil, porosity, and the configuration of the soil pores. For an explanation of this and other technical terms on salinity used in this text, check the FAO-field guide: 20 things to know about the impact of salt water on agricultural land in Aceh Province
2 EC is Electrical Conductivity, a measure of soil and water salinity; and is measured in deci-Siemens per meter (dS/m). ECe refers to the salinity of the saturated extract of the soil.
3 Processes related to the flow of soluble salts in the soil: diffusion is a chemical activity, the flow of ions (salt molecules) from areas of high concentration to low concentration; advection is the mechanical transport of soluble salts through groundwater movement and dispersion is the subsequent dilution, influenced by pore shapes and sizes. Dilution can also occur by simply adding fresh (rain)water, diminishing the overall salt-concentration.