Introduction to Hydrology of Volcanic Islands - Science Direct

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ScienceDirect IERI Procedia 9 (2014) 135 – 140

2014 International Conference on Environment Systems Science and Engineering

Introduction to Hydrology of Volcanic Islands Juan C. Santamartaa*, Rafael J. Lario-Basconesb, Jesica Rodríguez-Martínc , Luis E. Hernández-Gutiérrezd, Roberto Poncelae b

a Área de Ingeniería Agroforestal, Universidad de La Laguna, Ctra. Geneto, 2, La Laguna, 38200 Tenerife (Canary Islands), Spain Servicio de Minas - Dir. Gral. de Industria y Energía, Gobierno de Canarias. Avda de Anaga 35. Ed Usos Múltiples I, SC de Tenerife 38071, Canary Islands, Spain c Urb. Jardines de Guajara, 1,La Laguna 38296, Tenerife, Canary Islands, Spain d Consejería de Obras Públicas, Transportes y Política Territorial. Gobierno de Canarias, Spain e Eurogeólogo-Hidrogeológo. Moreiba, 2,38111, Tenerife, Canary Islands, Spain



Abstract Among the volcanic regions in Spain, the most important are the Canary Islands. A remarkable amount of geological, ecological, botanical and hydrological singularities are concentrated in the archipelago. From the point of view of water resources, the Islands are a worldwide example of their use and management. This knowledge, developed trough years of research and experience in the Canary Islands, can be transferred to other oceanic island systems. In this paper, we start introducing the formation processes and geological oddities that condition water uses in the volcanic islands, with a special spot for the underground resources. Afterwards, waterworks more characteristic of islands and the effect of “fog rainfall” are described. Finally, we aim to the transfer of technology and success cases on water resources between global volcanic islands is proposed. © 2014 Authors. Published by Elsevier © 2014The Published by Elsevier B.V. B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer review under responsibility of Information Engineering Research Institute Selection and peer review under responsibility of Information Engineering Research Institute

Keywords: Water resources;Oceanic islands;Volcanism;Hydrology;Water exploitation;Groundwater

* Corresponding author name. Tel.: +3-492-231-8550 E-mail address: [email protected]

2212-6678 © 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer review under responsibility of Information Engineering Research Institute doi:10.1016/j.ieri.2014.09.053

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1. Introduction Islands are isolated systems, so they have had to evolve as different environmental units with respect to the continental land in several fields (Santamarta, 2009a); on one hand, vegetal diversity -including endemic oneis found on most of the islands; this effect is greater the farther away from the continents is the island, all resulting from a combination of evolution and geographic isolation. The other aspect to consider is the origin of volcanic islands, produced by a volcanic eruption located on the seabed. The islands have a short life cycle, compared to the continental land. From the beginning, they involve following several stages: x Formation x Growth x Dismantling large gravitational landslide and erosion x Disappearance under the sea They also share common environmental, economic and social characteristics such as: x Unique forest ecosystems, very sensitive to small habitat perturbations x Uniqueness botany x High presence of the primary sector x Tourism dependency x High-energy dependency, but able to integrate renewable energy x High population density, an illustrative example is Hawaii; which has the highest population density in the United States of America x Soil likeness between islands. These volcanic soils share more similarities than differences 2. Singularities of volcanic islands All volcanic islands have common origins. The most accepted theory about their formation - especially in the case of the Canary and Hawaiian archipelagos - is based on what is known as “hotspot”; according to this theory, offshore volcanic eruptions are generated at a location (the “hotspot”) on the seabed where the magma from the Terrestrial mantle rises; this point is mobile, due mainly to mantle convective forces and to relative plate translation; in this way, a hot spot can form in the oceanic plate a series of islands, an archipelago, in which the age of the materials increases with its distance to the hot spot. There are several types of volcanoes (and therefore, volcanic islands), depending on lava acidity and eruption characteristics, but in the Canary Islands almost every type of volcanic material can be found (Hernández-Gutiérrez, 2011) and for this reason this archipelago is very important for the scientific community. 2.1 Origin, formation and geology of the volcanic islands The formation process of a volcanic island can be described very briefly as follows: x Local anomaly in the seabed. x Lava plume ascent. x Underwater eruptions x Constitution of the base complex - plutonic rocks, submarine lavas, sediments - forming a submarine volcanic building x Sub aerial volcanic series – island emerged areas - island buildings rise from the seabed but they are only partly visible above sea level (less than 10 %) x Once at surface level, a series of eruptions occurs, usually referred to as Series I, Series II, Series III, depending on their age

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x Recent volcanic sites, which may include eruptions in the historical age As it can be seen, the geology of the islands is formed by the continuous succession of diverse geologic materials and structures, including pyroclastic deposits - volcanic ashes -. Note that some islands have volcanic processes active; in Europe, we can highlight Sicily, Iceland and recently the island of El Hierro, in the Canary archipelago. 2.2 Surface and underground hydrology In order to study the water resources in the volcanic islands is necessary to understand geological processes and structures involved in the hydrological cycle. The initial permeability of young volcanic terrains affects dramatically the behavior and typology of islands’ water resources. This issue conditions the existence of islands, considered as young ones, with greatly reduced runoff compared to older islands. An illustrative example is the island of El Hierro with an age of a million years and no large ravines in it – ravines are the usual rainwater evacuation routes on the rough islands -, so most of the rainfall infiltrates into the ground. As the islands grow older, materials usually become more impermeable; with age and due to erosive action, the islands lower their altitude and length, but have a larger number of more evolved soil orders than younger islands (Ibáñez and Effland, 2011). On the other hand, we must highlight the heterogeneity of the different volcanic materials, unlike the more homogeneous sedimentary formations. From a hydrogeological point of view, the most relevant structures are (Santamarta, 2013): x Base complex or island’s basement x Geological dikes x Paleosoils or “Almagres”: x Large landslides x Ash deposits Base complex or island’s basement: sometimes visible on the surface and with a higher impermeability; for this reason, the areas with emerged base complex are strategic locations for dams and reservoirs construction, if permitted by environmental and geological conditions; an example of such facilities can be found in the island of La Gomera, a small island with 18 large dams. Geological dikes: perhaps they are the most important structure, they are, basically, magma that has cooled very quickly inside previous terrain so that form walls in it with a great range of thickness (1-10 m). Families starting from the basal complex to the upper layers of the Island’s dorsal group dikes. Seen from above, they look like some kind of cells with different sizes. They are fundamental in order to understand the island aquifers behavior as they rise their level, because of their impermeability (unless they are altered or cracked); as a result of this effect, the water level in a volcanic island can reach 1,500 m above sea level, but, of course, in most cases, there are not a unique underground water level, as multiple highly impermeable “cells” lead to a complex system of local aquifers. Paleosoils or “almagres”: they are formed by a lava cast from an active eruption that steps over a previously consolidated soil; lava heats the soil forming a characteristic reddish colored (ochre) soil layer with and a thickness of about 10 cm; these layers are highly impermeable, in fact, they are similar to a cooked clay, so they play a main role in the hydrogeological system and, in some cases, can contribute to a surface spring formation, being the lower bed of a perched aquifer. Large landslides: they change completely the behavior and internal structure of an existing aquifer, establishing an impermeable layer formed by the avalanche deposits produced by the slide itself. This layer is called “mortalon” in the Canary Islands. Ash deposits: depending on their thickness, they can generate impermeable layers and, like the “almagres” are linked to perched aquifers.

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The islands’ water resources have a double origin: on one hand, rainfall, conditioned by the island location and altitude and on the other hand, the moisture of the trade winds that is captured by the forest through the so-called “hided”, “horizontal” or “fog” rainfall, which can represent as far as from 1.5 to 3 times the amount of conventional rainfall (Santamarta and Seijas, 2009b). Except very specific islands, for example Hawaii, volcanic islands don’t use to have rivers, but only small seasonal streams, especially after heavy rainfall. This means that only a small percentage of the surface water resources are used, mainly in agriculture (Santamarta and Rodriguez-Martin, 2012c). In some cases, these waters are mixed with a lower quality groundwater, -highly salted- to improve the final water characteristics. It may seem that volcanic islands with large rainfall rates, as for example, the island of Terceira in the Azores with an average of 1300 mm per year or Jeju Island in South Korea with a rainfall of 1975 mm per year, may have rivers, permanent throughout the year, but they don’t due to the high permeability of volcanic lands (Jong-Ho et al., 2005).

Fig. 1. Geological dikes across paleosols “almagres”

In the islands, ravines are the true watersheds; they are highly steeped in the highlands of the islands – as the rainfall largest concentration is found in the dorsal structure – but they are much smaller than in the inland areas; the terrain is steeper, mostly in young islands; finally, ravines get broader and less steep when reaching the coast area. It is necessary to take certain precautions in order to develop and implement technical projects linked to the surface runoff estimation, because the mathematical models and calculus methods used in continental terrains tend to overestimate the amount of runoff, according to some studies (Santamarta, 2009a). 2.3 Water resources exploitation systems Agriculture is the most water-demanding activity in the Canary Islands; this demand pattern, excepting singular cases, is commonly found in volcanic islands; another example of this behavior is Hawaii, where some unique varieties, farming can be found, for example: coffee, avocado, pineapple and sugar cane; the case of sugar is as very special one as it needs heavy watering. Tourism is also an important feature in water demand, which is estimated in up to 500 liters per capita per day, in some water management planning systems. Most of islands’ the water resources are obtained from underground in comparison compared to those obtained from surface (Custodio, 1978); groundwater are mainly extracted through wells, boreholes and in some regions such as Hawaii, the Canary Islands of Jeju in Korea South by galleries or “water mines” (Fig. 3); this exploitation method, by excavating horizontal tunnels (Fig. 2), it is possible only when aquifers are over levelled due to geological dikes; in the Canary Islands more than 2,000 water mines can be found, with longitudes up to 6,500 m. The aquifers overexploitation can provoke the following negative effects: • Upland areas, the extraction of salts and fluoride richer waters

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• Water table lowering • Coast areas, seawater intrusion Surface exploitation is mainly based on ravine-through streams; usually, these streams can be found only in rainfall seasons and they are rarely perennial. In this case, if permitted by regulatory, technical and geological conditions, conventional water dams are used to collect surface water in the ravine itself. If dam construction it’s not possible, some indirect methods can be implemented, i.e. pipe intakes located in the ravines, in order to transport the water to either a dam or to an artificial waterproofed reservoir; it is case is necessary that the collected water is as sediment free as possible in order to not decrease the reservoir capacities. There are many water exploitation systems developed from a mix forestry engineering, historical tradition and specific strategies used in semiarid land, usually of Arab origin, that allow to increase the water resources available for crops.

Fig. 2. Water mine in Canary Islands

3. Conclusions The origin, geology and the formation mechanisms of volcanic terrains, significant condition the exploitation, use of water resources in the island environment, with both positive and negative effects. The natural resources, and specifically, water, must be managed in a special way in the case of volcanic islands, paying attention to its unique peculiarities, this means that sometimes the strategies and methodologies used at continental land can’t be applied in a straight way in limited and heterogeneous spaces as the islands (Santamarta, 2012a). A perfect example of this fact is the “rainfall fog” which is not taken into consideration in inland modelization but is a main source of water in the volcanic islands. In fact, forests regulate this resource and for this reason they are a part of the hydrological cycle of the islands, not only from the demanding point of view but increasing the water intake in the cycle. Accordingly to this undeniable reason, a specific regulation should be developed in this area (or some exceptions added to the current regulation), as it is done in other fields (i.e. economy or transports). Inland regulation, especially that related to environmental protection and natural resources management, usually can’t be successfully applied to these singular cases.

Acknowledgements This work has been developed in the framework of the RECLAND Project. It has been funded by the European Union under the Lifelong Learning Programme, Erasmus Programme: Erasmus Multilateral Projects, 526746-LLP-1-2012-1-ES-ERASMUS-EMCR, MSc Programme in Climate Change and Restoration of Degraded Land.

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