Integrated soil-water management for coastal regions1
H.S.Sen2
Former Director, Central Research Institute for Jute & Allied Fibres (ICAR), Barrackpore
24 Parganas (N), West Bengal PIN 700 120
Introduction
Coastal areas in India and elsewhere are by and large heavily populated. Nearly 40 % of cities larger than 500,000 population in India are located in the coast. Overall about 50-70 % of the global population live within 100 km of the coastline covering only about 4 % of earth’s land (Poyya and Balachandran, 2008), thereby drawing heavily on coastal and marine habitats for food, building sites, transportation, recreational areas, and waste disposal. According to another estimate (Wikipedia, 2009), coastal areas (within 200 km from the sea) share less than 15 % of the earth surface area, and this predicts that three-fourths of the world population are expected to reside in the coastal areas by 2025. It is important that coastal ecosystems have an economic value beyond their aesthetic benefit supporting human lives and livelihoods. By one estimate (Poyya and Balachandran, 2008), the combined global value of goods and services from coastal ecosystems is about US$ 12-14 trillion annually--a figure larger than the United States' Gross Domestic Product worked out in 2004. Notwithstanding these facts, the areas face a number of challenging areas worth consideration for planned development, and these include a series of constraints threatening productivity in agriculture and allied areas, the very base for the livelihood of the poverty-stricken inhabitants, human and climate induced factors threatening the ecological sustenance, and catastrophic effects of the weather disaster, which further aggravates due to climate change phenomenon. If the strength in this ecosystem rests with bountiful natural resources the weakness also lies in the same and its unplanned and arbitrary use, deserving therefore appropriate and scientific exploitation, in the first place, of the land and water preferably in an integrated fashion.
Definition and delineation of coastal areas
As per the 1991 notification, the Coastal Regulation Zone (CRZ) in India extends upto 500 km from the high tides and includes the land between high tides and low tide lines. The ecosystem thus includes saline, brackish (mixed saline and fresh) and fresh waters, as well as coastlines and the adjacent lands, the latter being the landward extension or the coastal plain. The different components, like estuaries, macrophyte communities, mangroves, coral reefs, salt marshes and the remaining continental shelves normally present in the coastal ecosystem, are in dynamic equilibrium and believed to be highly vulnerable to any change in the system. Velayutham et al. (1998) for the first time made a scientific attempt to characterize soil resources and their potentials of the coastal plains belonging to different Agro-ecological Sub Regions (AESR) in India showing a total of 10.78 million hectare area including the islands. Delineations made by them need to be further refined with due consideration to the prevalence of biota or their remnants, an all important parameter, in the ecosystem. _____________________________________________________________________________________
1Lead paper presented during Brainstorming session on “Sustaining agricultural productivity through integrated soil management”, held at NAAS, New Delhi, 29 Sept, 2011
2 Present address: 2/74 Naktala, Kolkata 700 047, WB; Email: hssen.india@gmail.com, hssen2000@hotmail.com; Mobile: 09874189762
Technological constraints
Different factors limiting agricultural productivity as well as ecological sustenance of the coastal plains are listed as (1) Excess accumulation of soluble salts and alkalinity in soil, (2) Pre-dominance of acid sulphate soils, (3) Toxicity and deficiency of nutrients in soils, (4) Drainage congestion particularly in lowlying areas, (5) Inadequate availability of good quality water suitable for irrigation, (6) Intrusion of seawater into coastal aquifers, (6) Shallow depth to underground water table generally rich in salts, (7) Periodic inundation of soil surface by the tidal water vis-à-vis climatic disaster and their influence on soil properties, (8) Heavy soil texture and poor infiltrability of soil, (9) Eutrophication, hypoxia and nutrient imbalance, (10) Erosion and sedimentation of soil, and (11) High population density, etc.
A number of location specific studies have been conducted for the management of coastal stressed soils for higher agricultural productivity through drainage & desalinization of water congested areas particularly in the lowlying belts through adoption of appropriate sub-surface drainage or through suitable land shaping technique, leaching of excess soluble salts in saline soils, reclamation of acid sulphate and sodic soils, and remedial measures of the nutritional disorders, etc. Methods developed for each will be sustainable in the long run if due attention is paid to integrated practices of land and water resources and through it or otherwise the necessary measures to conserve ecology at the same time.
Soil health & carbon sequestration
The importance of improved soil quality in the coastal plains through higher SOC level of the soils is established. IRRI characterized lowland rice soils (excluding deepwater rice) in Asia in respect of soil quality (Haefele and Hijmans, 2009), which includes large areas under coastal plains. They grouped soil qualities into four categories. These were: Good, Poor, Very Poor and Problem soils. ‘Good’ and ‘Poor soils’ represent those with different degrees of weathering but without major constraints; ‘Very Poor’ represents soils with multiple chemical constraints (acidity, deficiency of phosphorous, or toxicities of iron and aluminum); while ‘Problem soils’ represent those with the most frequently cited soil problems, including acid sulphate, peat, saline, and alkaline soils, which partly cause low fertility, and partly soil chemical toxicity.
Modeling C sequestration so far indicated that coastal marsh ecosystems tend to sequester C continuously with increasing storage capacity as marsh age progresses and its area increases. Thus, C sequestration in coastal marsh ecosystems under positive accretionary balance acts as a negative feedback mechanism to global warming. Choi and Wang (2004) were of the opinion that dynamics of carbon cycling in coastal wetlands and its response to sea level change associated with global warming is still poorly understood. However, they also observed during their study at Florida that salt marshes in this area have been and continue to be a sink for atmospheric carbon dioxide. Because of higher rates of C sequestration and lower CH4 emissions, coastal wetlands could be more valuable C sinks per unit area than other ecosystems in a warmer world. Brigham et al. (2006) stated that the estuarine wetlands sequester carbon at a rate about 10-fold higher on an area basis than any other wetland ecosystem due to high sedimentation rates, high soil carbon content, and constant burial due to sea level rise.
In India, possibly, the first ever study made by Bhattacharyya et al. (2000) more than a decade back showed SOC pool in two soil strata under different physiographic regions including coastal areas. The data based on soil analyses covering 43 soil series showed the SOC data varied from 2.4 Pg to 10.9 Pg from 30 cm to 150 cm soil depth. It will be prudent to concentrate on elaborate studies in future on monitoring SOC pool in different soil strata in coastal areas over a long period of time, and relate them with sea level rise, extent and nature of land submergence with water, seawater quality, extent and nature of vegetative cover, relevant soil and climatic parameters, nature and amount of agricultural, industrial and city effluents discharged into the sea, and any other anthropogenic factors of the locality likely to influence SOC, etc. It should also be possible to create a databank on SOC and related factors of the long time past using radiocarbon dating.
Integrated soil & water management
If the water table, rich in salts, is present at a very shallow depth (generally not exceeding a depth of 2 m below the soil surface), it contributes salts to the root zone during the dry season through upward capillary rise in response to evapotranspiration demand of soil moisture. The net salt loading in the root zone will be positive (salinity will build up) or negative (desalinization will take place) depending upon the relative rate of recharge of salts by upward rise to rate of downward flux of salts through leaching. The relative salt loading will thus be treated generally as positive during dry season, and negative (waterlogging on the soil surface) during wet season due to high rainfall, and the process will be repeated in each year in a seasonally cyclic mode.
Working models have been developed, even for small holdings dominating the coastal ecosystem, based on the hydrological processes, and the same validated for different agro-climatic regions in India for (i) computation of soil water balance, (ii) optimal design of water storage in the ‘On-farm reservoir (OFR)’ by converting 20 % of the watershed, (iii) design of surface drainage in deep waterlogged areas to reduce water congestion in 75 % of the area, and (iv) design of a simple linear programme to propose optimal land allocation under various constraints of land, water or other critical inputs to arrive at a contingency plan for maximization of profit. It has also been reported to use remote sensing and GIS in mapping lowland lands and performance assessment of irrigation/ drainage systems (Ambast and Sen, 2006).
Irrigation water resources
In spite of the coastal ecosystem presenting a delicate equilibrium among the different components there is however no firm strategy, as of now, for exploitation of water resources for irrigation and other purposes for long term solution in any sector. Planning should be such as not to disturb the ecosystem in the long run. It is suggested that location-specific programme on water allocation under different sources should be drawn up for each region, based on soil, climate, water, and crop parameters, as well as their spatial variations, as per appropriate strategies to be worked out, with minimal dependence on abstraction of water from the underground aquifer, but with increasing dependence on other means, like use of surface water sources by recycling of rainwater stored and fresh water available using innovative seawater desalination technology, and conjunctive use of marginally saline water available, with overall target to increase water productivity and cropping intensity phasewise, and conserve the ecosystem at the same time. Future vision on water budgeting in the coastal areas in India, which should be the crux towards planning for integrated and efficient land and water management practices, along with conceptualizing a model specifically designed for water budgeting for the coastal watersheds including all its components, have been worked out recently to ensure long term sustenance by Sen and co-workers (not yet published).
Seawater intrusion into inland areas may be minimized, if not to be eliminated altogether, through structural measures or with the help of ‘Optimization Models’, the latter however yet to be validated through testing under wide variety of situations in order to suggest optimal location of pumping with reference to the coast, rate & frequency of pumping of underground water, etc.
Ecology
Forest resources
The present status of forest areas in the East and West coastal belts constitute only about 18.7 and 29.0 percent, respectively of the total geographical area of the country. The forest coverage in the A&N Islands, however, is as high as nearly 88 percent of its total land area. Mangroves growing under natural conditions along the coastal shoreline occupy nearly 0.4 million hectare (6460 km2) in the country comprising about 7 percent of the world’s mangroves. Reefs are not abundant along the Indian coast occupying only 7 percent to a coastal length of 420 km. Seagrasses, on other hand, intermingle with both mangrove and reef communities at their respective seaward and landward boundaries. In the Indian coastline the seagrasses were found to be efficient in cleansing the water contaminated by oil spills and effluent discharge to the extent of 20-100 percent in the Gulf of Mannar and Palk Bay (Sen et al., 2000).
In spite of the fact that mangroves have a very useful role to maintain the level of CO2 and other toxic gases in the atmosphere they also remove toxic materials and excess nutrients from estuarine waters. In addition, sediment and other inert suspended materials are mechanically and chemically removed from the water and deposited in the marsh or swamp, reducing the sedimentation of navigation channels and shellfish beds. The vegetation also slows the surge of floodwaters and may help to reduce the severity of flooding. Vegetation serves to stabilize estuarine shorelines and prevent erosion; for example, mangrove trees not only preserve shorelines, but actually can extend the land’s edge by trapping sediments and building seaward. Thus, mangroves support unique coastal ecosystems especially on their intricate root systems. In areas where roots are permanently submerged, they further play a role of hosting a wide variety of organisms, including algae, barnacles, oysters, sponges, and bryozoans, which all require a hard substratum for anchoring while they filter feed. Shrimps and mud lobsters use the muddy bottom as their home. Mangroves thus form a vital food web link among various aquatic and wild animal species. It is irony that, globally, about 50 percent, and in some countries, as high as 85 percent, of mangrove forests have been lost in the last 50 years mostly due to human interventions. An estimated 35% of mangroves have been removed due to shrimp and fish aquaculture, deforestation, and freshwater diversion. Hydrology of the rivers in the coastal ecosystem or any other human intervention including soil and water management practices in the coastal plain that might threaten the natural forest environment should be highly unwarranted.
Sedimentation and Erosion
The dynamics of alluvial landscapes and natural sedimentation patterns that determine the nutrient and energy flows in coastal areas are increasingly being modified by human activities, in particular those that affect water flows (dams, increased water extraction, deviation of rivers) and erosion, and especially due to deforestation. This prevents or slows down vertical accretion, thus aggravating salt water intrusion and impairing drainage conditions in riverine, delta or estuarine areas. It reduces or blocks sediment supply to the coast itself, which may give rise to the retreat of the coastline through wave erosion. Beach erosion is a growing problem and affects tourism revenue, especially in island nations. In the Caribbean, as much as 70 percent of beaches studied over a ten-year period were eroded.
Eutrophication, Hypoxia, Dead Zones and Nutrient Cycle
The urban developments are taking up fertile agricultural land and leading to pollution of rivers, estuaries and seas by sewage as well as industrial and agricultural effluents. In turn, this is posing a threat to coastal ecosystems, their biological diversity, environmental regulatory functions and role in generating employment and food. Overuse of fertilizer can result in eutrophication, and in extreme cases, the creation of ‘dead zones’. Dead zones occur when excess nutrients—usually nitrogen and phosphorus—from agriculture or the burning of fossil fuels seep into the water system and fertilize blooms of algae along the coast. In dead zones, huge growth of algae reduces oxygen in the water to levels so low that nothing can live. There are now more than 400 known dead zones in coastal waters worldwide, compared to 305 in the 1990s, according to a study undertaken by the Virginia Institute of Marine Science. Those numbers were up from 162 in the 1980s, 87 in the 1970s, and 49 in the 1960s. In the 1910s, only four dead zones were identified (Minard, 2008). Hypoxia in the Northern Gulf of Mexico, commonly named as the 'Gulf Dead Zone', has doubled in size since researchers first mapped it in 1985, leading to very large depletions of marine life in the affected regions (Portier, 2003).
The World Resources Institute (2006) reported that driven by a massive increase in the use of fertilizer, the burning of fossil fuels, and a surge in land clearing and deforestation, the amount of nitrogen available for uptake at any given time has more than doubled since the 1940s. In other words, human activities now contribute more to the global supply of fixed nitrogen each year than natural processes do, with human-generated nitrogen totaling about 210 million metric tons per year, while natural processes contribute about 140 million metric tons. This influx of extra nitrogen has caused serious distortions of the natural nutrient cycle. In some parts of northern Europe, for example, forests are receiving 10 times the natural levels of nitrogen from airborne deposition, while coastal rivers in the Northeastern United States and Northern Europe are receiving as much as 20 times the natural amount from both agricultural and airborne sources (Coastal Wiki, 2008).
Climate Change and its impact
Destruction of habitats in coastal ecosystem is caused by natural disasters, such as cyclones, hurricanes, typhoons, volcanism, earthquakes and tsunamis, frequency of which is increasing at almost exponential rate with time (Sen, 2010), causing colossal losses worldwide. Each year an estimated 46 million people risk flooding from storm surges.
It has also been predicted (IPCC, 2007) that increase in sea surface temperature of about 1-3°C might result in more frequent coral bleaching events and widespread mortality unless there is thermal adaptation or acclimatization by corals.
The impacts on sea level rise are expected to be more local than global. The relative change of sea and land is the main factor. Many cities, for instance, even suffer land subsidence as a result of ground water withdrawal. This may be compounded with sea level rise, especially since rates of subsidence may exceed the rate of sea level rise between now and 2100. In India, potential impacts on 1 m sea level rise might lead to inundation of 5,763 km2 of land including Ganges-Brahmaputra delta facing flood risks from both large rivers and ocean storms. Apart from the inundation of low lying coastal areas, including parts of many major cities, more significant are the direct loss of land caused by the sea rising and the associated indirect factors, including erosion patterns and damage to coastal infrastructure, salinization of wells, sub-optimal functioning of the sewage system of coastal cities (with resulting health impacts), loss of littoral ecosystems and loss of biotic resources. Climate change thus affects wide array of sensitive sectors like agriculture, forestry and fishery and thereby the livelihoods of millions of coastal communities.
Lowlying coastal soils: The probable effects on soil characteristics of a gradual eustatic rise in sea level will vary from place to place depending on a number of local and external factors, and interactions between them (Bramner and Brinkman, 1990). In principle, a rising sea level would tend to erode and move back existing coastlines. However, the extent to which this actually happens will depend on the elevation, the resistance of local coastal materials, the degree to which they are defended by sediments provided by river flow or longshore drift, the strength of longshore currents and storm waves, and on human interventions which might prevent or accelerate erosion.
Sediment supply and deltaic aggradation: In major deltas, such as those of the Ganges-Brahmaputra and the major Chinese rivers, sediment supplies delivered to the estuary will generally be sufficient to offset the effects of a rising sea level, but on the other hand it would impair the drainage system as well.
Tidal flooding: In coastal lowlands which are insufficiently defended by sediment supply or embankments, tidal flooding by saline water will tend to penetrate further inland than at present for the lack of adequate drainage in case of the former, extending the area of perennially or seasonally saline soils. Where Rhizophora mangrove or Phragmites vegetation invades the area, would over several decades lead to the formation of potential acid sulphate soils. Impedance of drainage from the land by a higher sea level and by the correspondingly higher levels of adjoining estuarine rivers and their levees will also extend the area of perennially or seasonally reduced soils and increase normal inundation depths and durations in river and estuary basins and on levee backslopes. In sites which become perennially wet, soil organic matter contents will tend to increase, resulting eventually in peat formation. On the other hand, where coastal erosion removes an existing barrier of mineral soils or mangrove forest, higher storm surges associated with a rising sea level could allow seawater to destroy existing coastal eustatic peat swamps, which might eventually be replaced by freshwater or saltwater lagoons.
Subsidence of land: The probable response of lowlying coastal areas to a rise in sea level can be estimated in more detail on the basis of the geological and historical evidence of changes that occurred during past periods when sea level was rising eustatically or in response to tectonic or isostatic movements, e.g. around the Southern North Sea; in the Nile delta; on the coastal plain of the Guyanas; in the Musi delta of Sumatera. Major shifting of the river course of the Ganga-Brahmaputra river system and a large number of their tributaries took place due to neo-tectonic movement in Bengal basin during 16th to 18th century, which eventually led to massive change of the hydro-geological properties in lower Ganges delta affecting livelihood in both India (related areas) and Bangladesh. Contemporary evidence is also available in areas where land levels have subsided as a result of recent abstraction of water, natural gas or oil from sediments underlying coastal lowlands. Further studies of such contemporary and palaeo-environments are needed together with location specific studies in order to better understand the change processes, identify appropriate responses and assess their technical, ecological and socio-economic implications.
Erosion: It is an important area influenced by the climate change with rising temperatures leading to rise in the sea’s water mass. In India the mainland consists of 43 % sandy beaches, 11 % rocky coast with cliffs, and 46 % mud flats and marshy coast (SAARC Disaster Management Centre, 2009). The damages caused by sea erosion in different coastal states in India alone show a staggering annual loss of Rs. 368.387 crores. Various preventive and mitigation measures have been suggested and being adopted, which are mainly of two types: (i) Structural measures and (ii) Non-structural/ soft measures.
Summary and Conclusion
Coastal ecosystem poses a delicate equilibrium between land and water masses amongst its different components but with high degree of vulnerability in spite of bountiful natural resources. The equilibrium is further under serious threat due to climate change and global warming. On the other hand, it is significant that coastal marshes tend to sequester carbon continuously with increasing storage capacity and with time, and thus regarded as a valuable C sink per unit area, particularly in the tropics, to negate adverse impacts due to global warming. Planning for effective and sustainable development warrants specific attention to maintain the equilibrium. This will require adoption of integrated approach to soil and water management, in the first place, and through it or otherwise, necessary measures to conserve the ecology. Piecemeal approaches to reclaim location specific problems or interference with the hydrology of the rivers per se for short term gains for increase of productivity or otherwise, disregarding completely the practices on integrated management of different intervention areas and thereby conserve the ecology in coastal plains, may offset the equilibrium, as experienced in different parts of the world, leading to such adverse impacts, such as seawater intrusion into inland areas, massive loss of mangroves, coral reefs, seagrasses and various other aquatic plant & animal species, sedimentation & erosion, tidal flooding, subsidence of land, etc. The influx of reckless application with fast increasing dose of nitrogen or other inputs resulting in nutrient imbalance through human activities in the adjoining inland and coastal areas are glaring examples leading to such phenomena as eutrophication and formation of dead end zones in the coastal water bodies. Policy approach for water budgeting of different water resources, preferably on watershed basis, with minimal or planned dependence on abstraction of the underground water should be an essential strategy to be drawn in order to ensure sustainable increase in crop water productivity as well as water productivity in other sectors all along the coast.
References
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