THE WATER OF THE ISLAND
This first step in a water saving project is to understand, describe and respect the water resources of the island. Its purpose is to help you describe the situation on your own island. Here, we use examples from the eight islands in our study to translate theory into understandable language.
Water is a very simple chemical compound – H2O - with a wide variety of special properties. It appears in various forms such as ice, water or vapour. One of the most important features of water is its extremely good solubility. Therefore, water can even break down minerals and rocks.
We – people, plants and animals – can’t do without it. An organism typically consists of 65-90 % water. We use it as a solvent and a carrier for nutrients, excretion products, oxygen and carbon dioxide. We use it in households and industry, on farms for animal husbandry and crop irrigation, but also for hydropower, shipping, fishing, recreation as well as a receiver of contaminants.
Global water resources
Fresh water is a very limited resource: the oceans surrounding our islands make up for 97.5 % of Earth's total water volume. This seawater is saline. Fresh water represents only 2.5 % of the total water on Earth.
Looking closer at these 2.5%, most of it is bound in glaciers and ice caps. Groundwater and surface water constitutes 1%. Much of the groundwater is located at large depths deep below the ground. It may be difficult to exploit, technically and economically, it can sometimes be saline, but it is our most important freshwater magazine. Surface water seems to be easier to use but is often under legal protection and may often be contaminated by agriculture, various industries and households.
Freshwater is a scarce and fragile resource we depend on, but too often take for granted.
The water cycle
All water on Earth is part of a cycle. We only have a given amount of water, it remains the same while moving continuously in a closed system. Little drops of water make the mighty oceans and the water we drink today may contain water molecules from a bath Cleopatra took 2.000 years ago.
As the sun shines, water evaporates from seas and lakes, and transpires from all organisms. Winds blow the water vapour up and away into the atmosphere, where the water molecules become so large that drops start falling as precipitation (rain, hail, snow, mist and fog). Most of the rain reaches the ground, infiltrating the soil or the bedrock via cavities and fractures, and watering plants.
The rainwater trickles and oozes down to the bedrock – we call it percolation. Eventually, the water reaches an area which, due to small fractures, slows down the percolation of water. The pores in the soil layer closest to the rock surface can then be filled up to 100 % with water. When all pores and fractures are filled with water, we call it groundwater, and the upper level that limits this groundwater-filled area is the groundwater surface.
The bedrock of an island can be sedimentary rocks that have both pores and fractures, or it can be rocks without pores but with smaller or larger fractures/cracks.
Limestone and sandstone are sediment rocks formed as deposits in rivers, lakes or ocean bays (such as calcareous clay or sand), and their pores result from their formation as sediment. After millions of years, sediments became rocks. Limestone has been chemically affected so that the lime has been dissolved and removed from the rock, with larger pores, sometimes even large cavities or caves. Limestone islands such as Inis Oírr can keep large amounts of water in their large pores, while bedrocks with only fractures like Houat have smaller groundwater magazines.
Inis Oírr (English: Inisheer) is the smallest, innermost of the three Aran islands on Ireland’s Western Coast. These islands are limestones interbedded with layers of shales which were laid down when the sea receded. Doctor Andrea Bartley explains what happened then: “The soil cover was removed by glacial activity exposing bare limestone. The bedrock surface has been weathered by the elements. This has resulted in the rock surface being fissured where it has been exposed. These fissures are generally shallow being less than 3 meters in depth. These fissures have been developed by rainwater. The rain that falls on the island runs down these fissures until it meets a more impermeable layer which usually takes the form of a thin shale band. The water then runs laterally along this stratum until it reaches the sea.”
The area of Inis Oírr is 8 km2. It has an average yearly rainfall of 1,153 mm. Yearly groundwater recharge can be calculated as rainfall from October to March x 0.8 = 461 mm.
Map: Rainfall on the Aran Islands, Ireland. Take special note of Inis Oirr.
Water from lakes and streams – surface water
The groundwater moves with the force of gravity, travelling deep and laterally, flowing from heights to the valleys where groundwater outflows are formed which we call springs that can form a stream, flow out to the sea around the island, or into a lake. A lake has an outlet, a river flowing to the sea. We call all stream water and lake water surface water.
On smaller islands, it is uncommon to find permanent flowing streams, and lakes are rare on islands in general. Islands made up of limestone usually have a rapid groundwater flow to the sea. If limestone rocks reach the coast, groundwater often flows out in springs in the coast cliffs. Sometimes these springs only exist during the rainy period (typically winter).
When the rains cease, the groundwater surface drops and the groundwater pressure in the soil or bedrock decreases, resulting in less water flowing out of the springs, and eventually no water flow at all. When new rain falls and the water magazine is refilled, new groundwater is formed, pressure increases, and water starts flowing from the spring again.
Places where groundwater flows out are usually below the surface of a lake, stream or even below the seabed on islands. This outflow is invisible but can be very large. The amount of groundwater flowing out to streams, lakes and seas equals the groundwater recharge or all rain minus vegetation transpiration.
In low-lying terrain, the groundwater surface may be at or very close to the ground surface, creating a wetland. In this part of the terrain no water moves downward, only laterally along the ground surface.
On an island, it is usually easier to see groundwater flowing to the coasts, especially when the coast consists of mountains. From the mountain side, groundwater flows from fractures or cavities. The outflow occurs mostly during the rainy periods. Part of the year the outflow is small or simply does not exist. Do remember that most part of the outflow is invisible below sea level. Even groundwater formed in the middle of the island is transported to the sea by gravity even though it can take a long time.
Tilos is a solitary limestone rock rising 600 meters above sea level in the Aegean Sea, which is about 500 meters deep around the island. It is one of the islands of the Dodecanese, located 22 nautical miles (2-3 hours) NW of Rhodes and 222 nautical miles (15 hours) from Piraeus. The area of the island is 61.5 km2.
Walking on the island’s beautiful stone-paved tracks, passing settlements, dovecots, chapels, springs, stone bridges, small rushing watercourses, watermills, threshing circles and olive presses among holly oak, mastic and oleander trees, you can understand how the hydrological conditions of Tilos are a result of its climate, the karst limestone relief, the steep slopes, the fragmentation into many small or larger drainage basins, and the absence of vegetation on the mountain sides.
If it hasn’t rained for a long time, the plants’ roots have used up all the water in the upper parts of the soil and the soil runs dry. When the first rain falls on the surface of dehydrated soil, it will be bound to the surface of the soil particles as a very thin film (hygroscopic or bound water). The force with which water molecules adhere to the particles is so strong that the plants’ roots cannot absorb this water. It will eventually be lost through evaporation to the atmosphere if the particles are close to the ground surface.
When more rain falls and water is infiltrated, the water percolates to the cavities between the soil particles. It is the capillary tension that holds the water in the pores. It can be described as several molecules of water held together by the electrical forces of the molecules (oxygen and hydrogen ions). The smaller the pores, the easier the capillary tension can hold enough water in the pore to fill it, partially or completely. This is especially true for soils with clay and silt. If the pores are larger – as they are in sand or gravel – the pores can only partly retain capillary water, while the rest is filled with air and other gases from the plants’ roots.
Plants use capillary water in the pores. Through the roots’ sucking ability, the water is raised to the plant's parts above the ground surface wherefrom the transpiration of the plant occurs.
What we call green water is the part of the rainwater that is retained in the soil. It is the “water reservoir” of the plants and it provides at least 70 % of the freshwater used for cultivating food in the world through rain-fed agriculture. When the green water reservoir cannot hold more water, the water starts percolating to the area below the plants’ roots.
The challenge of fresh and saline groundwater
As fresh groundwater is lighter than saline water from the sea, an island’s fresh groundwater is a floating cushion above the deeper situated saline water. In between the fresh and saline water, a mixed zone of brackish groundwater is formed. The level of this brackish zone depends of groundwater recharge and discharge, defined by the transportation speed of the groundwater that, by the force of gravity, moves out to the sea.
Sketch: Sustainable well usage
For fresh groundwater to be used as potable water, recharge and discharge must be in balance. When pumping fresh groundwater with a tube well, we make the discharge greater than the recharge. The groundwater level will sink, the freshwater cushion will become lighter and the brackish zone will move upwards. Caused by the physical difference of fresh and saline water the up-movement is greater than groundwater level depression.
We risk getting brackish or saline groundwater in our tube well. Summertime, with huge needs of potable water, when rain is rare and no groundwater recharge exists, is an extremely dangerous time to get the discharge of fresh groundwater unbalanced.
The best source of potable water is always fresh groundwater – both due to its quality and its costs.
Houat (Enez Houad in Breton) is a French island in the Atlantic Ocean, off the south coast of Brittany. It is a massive of gneiss and granite in a mild slope, 5 km long and 1.5 km at its widest point. There are sand dunes in the northeast and the southeast, the rest of the coast is from 14 to 29 meters steep, cut by ravines with sandy beaches. Inside the islands, the terrain is mostly flat (25 to 28 meters).
The surface of Houat is 2.9 km2. Since the average rainfall is 910 mm per year, that means that a total of 750,000 m3 rainwater falls on the island per year. Groundwater recharge is about 480 mm in most parts of the island, the visible runoff from the mountain sides into the sea is about 220 mm and the rest of the groundwater runoff is submarine. The thin soil cover on a large part of the island's surface, as well as the bedrock's thin cracks, hampers infiltration in the bedrock.
Groundwater is the source of Houat's overall water used for consumption, although very deep wells sometimes cause saltwater penetration. The bedrock has cracks of various sizes and amounts, allowing for a wide variety of possibilities for outlets across the island.
What about climate change?
Islands will face climate changes over the next decades and beyond, until the end of this century. Scientists make scenarios of rain and temperatures across different geographical areas, and it seems both extreme rain (very heavy rain in a short time causing floods) and extreme dry periods will become more common. Displacements of the rain periods throughout the year may occur but with significant variations over the years. Temperature and rain changes will affect the vegetation periods, and longer vegetation periods mean greater transpiration from plants and less groundwater recharge with declining groundwater levels.