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Global scale evaluation of coastal fresh groundwater resources
Ocean & Coastal Management 52 (2009) 197–206
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Global scale evaluation of coastal fresh groundwater resources Priyantha Ranjan a, *, So Kazama b,1, Masaki Sawamoto c, 2, Ahmad Sana d, 3 a
Department of Civil Engineering, Curtin University of Technology, GPO Box U1987, Perth WA6845, Australia Graduate School of Environmental Studies, Tohoku University, 6-6-06 Aramaki Aza Aoba, Sendai 980-8579, Japan c Alpha Hydraulic Engineering Consultants Co., Ltd., Mita-Hillcrest, 4-15-35, Mita, Minato, Tokyo 108-0073, Japan d Department of Civil and Architectural Engineering, Sultan Qaboos University, PO Box 33, Al-Khod 123, Oman b
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 1 October 2008
This paper presents a simplified approach to assess the effects of global warming on global coastal groundwater resources over the next century based on the smallest but necessary number of elements such as rainfall, temperature, hydraulic conductivity of the aquifers, and population changes regarding the consumption of groundwater. The positive aspect in this approach is that there is availability of the above elements in the majority of the planet. Methodology includes a sharp interface concept model and simplified estimation of groundwater recharge using limited climate data. The evaluation shows that the future climate changes would decrease fresh groundwater resources in Central American, South American, South African and Australian regions whereas most of the areas in Asia, except South-East Asia. Combinations of fresh groundwater loss and global population are considered to state the vulnerability of future fresh groundwater supply. Vulnerability assessment shows that South Asia, Central America, North Africa and the Sahara, South Africa and the Middle East countries are highly vulnerable whereas, Northern Europe, Western part of South America, New Zealand and Japan are less vulnerable with respect to future fresh groundwater supply. Further, this paper highlights the necessity Integrated Coastal Management (ICM) practices in these vulnerable coastal regions to ensure the sustainable development in coastal regions. Crown Copyright Ó 2008 Published by Elsevier Ltd. All rights reserved.
1. Introduction Since groundwater systems in coastal areas are in contact with saline water, the major problem is the intrusion of saltwater into the aquifer. Already at this moment, many coastal aquifers in the world, especially shallow ones, experience an intensive saltwater intrusion caused by both natural as well as man-induced processes [24]. Salinization of these groundwater systems can lead to a severe deterioration of the quality of existing fresh groundwater resources and can create a reduction of the amount of freshwater resources available in coastal aquifers. Therefore, use of coastal aquifers as operational reservoirs in water resources systems requires development of tools that facilitate the prediction of the behavior of coastal aquifers under different conditions. Climate change due to increasing concentration of greenhouse gas is likely to affect groundwater recharge and thus affect the * Corresponding author. Tel.: þ61 8 9266 3530 (office). E-mail addresses: [email protected] (P. Ranjan), kazama@kaigan. civil.tohoku.ac.jp (S. Kazama), [email protected] (M. Sawamoto), [email protected] (A. Sana). 1 Tel.: þ81 22 217 7458; fax: þ81 217 7458. 2 Tel./fax: þ81 3 5445 6543. 3 Tel.: þ968 513 333x2524; fax: þ968 513 416.
saltwater intrusion through changes in precipitation and temperature [1,40]. Any reduction in groundwater flow toward the sea will cause intrusion of saltwater into the aquifer as the saltwater– freshwater interface moves inland. Coastal aquifers within the zone of influence of mean sea level are threatened by accelerated rise in global sea level, because the salinization of coastal aquifers accelerates due to sea level rise [46,21,20]. Taking hydro-geological changes in coastal aquifers due to sea level rise, Melloul and Collin [26] presented a simple hydro-geological conceptual model relating seawater intrusion to the impacts of sea level rise and discussed the uncertainty in accurately forecasting future sea level rise. In addition, other anthropogenic activities such as land use change have an impact on salinity intrusion in coastal aquifer systems through reduced groundwater recharge. Also, the increased demand for fresh groundwater resources due to future population growth will lead to higher vulnerability of fresh groundwater supply due to higher demand in the future [11]. Availability of freshwater has been recognized as a global issue [6]. Coastal groundwater plays an important role in global freshwater resources. Quantification of coastal groundwater resources not only in individual aquifers but also at the global scale is required to support the sustainable use of fresh groundwater for coastal communities. Evaluation of the integrated effects of climate
0964-5691/$ – see front matter Crown Copyright Ó 2008 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ocecoaman.2008.09.006
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change on coastal groundwater resources in global scale becomes rather difficult. Even though global scale evaluations are very important in order to evaluate the effects of climate change on coastal groundwater resources, global scale assessments remain very difficult because of the lack of data. Also available methodologies are laborious, time-consuming and costly. A large number of more or less empirical methods have been developed by numerous scientists and specialists to evaluate freshwater resources in different climatic conditions [50,27–29,15]. Testing the accuracy of these methods in global scale is laborious, time-consuming and costly, because of the necessity of calibration for many variables. To overcome these restrictions when assessing future global coastal groundwater simulations, global evaluations of fresh groundwater resources should be carried out using methodologies which need only few hydrological and hydro-geological data. To accommodate users with limited available data, a method has been proposed by Ranjan et al. [36] which evaluates coastal fresh groundwater resources based on a sharp interface model and a simplified estimation of groundwater recharge using limited hydrological data. Using the developed methodology, they evaluated the impacts of climate change on coastal fresh groundwater resources for several water resources stressed regions [37]. The proposed method can give acceptable first order estimates, depending on the location, availability and the quality of the measured data, even though the results have certain uncertainty. The main objectives of this study are to use the introduced simple approach to provide a global scale evaluation of coastal groundwater resources considering the integrated effect of global warming (i.e. climate change and sea level rise) and anthropogenic stress such as land use change. 2. Methods and materials This study follows the developed method by Ranjan et al. [36] to evaluate fresh groundwater resources in coastal aquifers. It integrates two approaches; (i) Estimate the salinity intrusion in coastal groundwater systems using the sharp interface concept estimation, (ii) Estimate the groundwater recharge based on precipitation and temperature as measured climate data. Fig. 1 summarizes the integration of these two methodologies and required data. Precipitation and temperature are the main hydrological inputs. Hydraulic conductivity, land use type, soil type and sea level data are the other main inputs.
Precipitation and Temperature Land use Map Estimation of Groundwater Recharge
Kc Soil Data Map
CN
Groundwater Recharge
Sharp Interface Model
Sea Level Rise
Hydraulic Conductivity (K)
Loss of Fresh Groundwater Resources
Fig. 1. Steps of the estimation of global fresh groundwater resources.
characterize fully such systems and processes. Therefore this study follows the sharp interface model developed by Ranjan et al. [36] to estimate the salinity intrusion to coastal aquifers and hence to evaluate the loss of fresh groundwater resources in the aquifer. Sharp interface models couple the freshwater and saltwater flow based on the continuity of flux and pressure. In this approach, together with the Dupuit approximation for each flow domain, the equation of continuity may be integrated over vertical direction and come up with following system of differential equations [2].
" " # # f f v v f i vh f i vh K h h K h h þ þ qf vx fx vy fy vx vy # " vhf vhs vhf vhf þ aq ¼ Sf q ð1 þ dÞ d vt vt vt vt
(1)
2.1. Modeling of salinity intrusion To simulate the dynamics of freshwater–saltwater interface, it is necessary to examine the coupled freshwater and saltwater flow domains. The first concept regarding freshwater–saltwater interfaces, now widely cited as the Ghyben–Herzberg principle, is based on the hydrostatic equilibrium between freshwater and saltwater. After introducing the Ghyben–Herzberg principle, many models have been developed to study the problem of saltwater intrusion. They range from relatively simple analytical solutions to complex numerical models [49,51,32,2]. These studies are classically used in two different approaches [38]. In the first approach, freshwater and saltwater are assumed completely immiscible and a sharp interface exists between these two phases. In the other approach, freshwater and saltwater are assumed to be in a dynamic equilibrium resulting from the flow and dispersion mechanisms within the aquifer. Since, the main objective of this study is to evaluate the long-term overall behavior of the coastal groundwater systems due to the effect of global warming, the sharp interface model is the ideal way to
vhs vhs v v þ þ qs Ksx hi zb Ksy hi zb vx vy vx vy # " vhs vhs vhf ¼ Ss þ q ð1 þ dÞ d vt vt vt
(2)
The location of the interface elevation (hi) is given by
hi ¼
rs rf hs hf rs rf rs rf
(3)
where rf and rs are specific weight in fresh and saltwater, respectively, hf and hs are the piezometric heads of freshwater and saltwater regions, qf and qs are flow rate in fresh and saltwater, respectively. Kf and Ks represent the hydraulic conductivity in fresh and saltwater regions. Storage coefficients in fresh and saltwater regions are given by Sf and Ss, respectively. q is the porosity of the aquifer media. a ¼ 1 for unconfined aquifer and a ¼ 0 for confined aquifer.
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From Eqs. (1) and (2), it is possible to derive a numerical model using implicit finite difference techniques. To solve the two simultaneous linear algebraic difference equations, the strongly implicit procedure – SIP [39] is used as a suitable numerical technique. Empirical evidence suggests that for the cases of flow in heterogeneous or anisotropic media, strongly implicit procedure is much faster than the other methods. Also the strongly implicit method does not depend upon the complexity of the problem [9,10]. A sensitivity analysis is carried out using the developed model to investigate the effect of hydro-geologic factors mainly, specific storage, porosity and hydraulic conductivity on the dynamics of freshwater–saltwater flow systems. It concluded that hydraulic conductivity is the main hydro-geological parameter which affects to change the freshwater–saltwater interface [33]. To test the model in different climatic regions, the model is successfully applied and verified with observed seawater intrusion data in three different climatic regions, Sendai coastal plain in Japan, Southern coastal aquifer in Sri Lanka and Al Batinah coastal plain in Oman [34,36]. Comparison between observed and simulated salinity profiles concluded that the developed model is able to simulate the realistic changes of the salinity profiles and the model can be used to successfully simulate the salinity profile in different hydrological and climatic areas. 2.2. Simple approach to estimate groundwater recharge Groundwater recharge in watershed areas is the main source of fresh groundwater resources in coastal zones. It is imperative to understand the process of the salinity interface and predict the shape of interface profile due to changes in groundwater recharge and related activities such as climate changes, and changes in land use pattern and land management practices [25,11,54,23]. Estimation of groundwater recharge is very sensitive to measurement errors due to the involvement of large number of observation parameters especially in the estimation of evapotranspiration. If groundwater recharge can be represented as a function of limited climate parameters such as precipitation and mean temperature, it leads to reduce accumulated uncertainty in the estimations. Hence, a methodology to estimate long-term average of actual evapotranspiration, surface runoff and groundwater recharge has been developed [36]. This methodology can be used in areas with limited meteorological data and also in ungauged basins. Also this kind of simple method can be expected to compute the average condition of fresh groundwater resources in different regions and it is easy to compare the status of fresh groundwater resources in different regions. The developed methodology is based on water balance equation and assesses how changes in catchment conditions can alter the partitioning of rainfall into different components. The simple water balance for any catchment can be written as:
R ¼ P ET RO
(4)
Where R is the groundwater recharge, P is the precipitation, RO is the surface runoff and ET is the evapotranspiration. All variables have dimensions [L/T]. 2.2.1. Estimation of evapotranspiration with minimum data Since evapotranspiration is one of the major inputs in water balance equation, evapotranspiration has to be mainly evaluated to estimate groundwater recharge. Crop evapotranspiration is used to estimate the evapotranspiration, concerning the effects of land use pattern. Crop evapotranspiration is a simple representation of the physical and physiological factors governing the evapotranspiration process, taking vegetation parameters into account. Differences in the crop canopy and aerodynamic resistance relative to the hypothetical reference crop are accounted within the crop coefficient
199
(Kc). It represents an integration of the effects of four primary characteristics that distinguish the crop from each other. These characteristics are; crop height, Albedo (reflectance), canopy resistance and the evaporation from soil [53]. Crop evapotranspiration is estimated as a multiplication of reference crop evapotranspiration (ETo) and crop coefficient (Kc) as follows.
ETcrop ¼ ETo Kc
(5)
Several methods exist for the empirical estimation of reference evapotranspiration (ETo). Based on the data requirement, these methods are classified as temperature based methods, pan evaporation, radiation and combination methods. Combination methods such as the Penman equation require air temperature, relative humidity, wind speed and radiation information reflecting meteorological parameters influencing evapotranspiration. Most of those methods need large amount of parameters to estimate evapotranspiration. It accumulates measurement errors and the lack of data for the necessary parameters leads the final estimation of groundwater recharge erroneous. Considering data requirements of available methods to estimate evapotranspiration, temperature based methods are the only alternative for consumptive use operation. A widely used temperature based theoretical method to calculate reference crop evapotranspiration is SCS Blaney Criddle method [44]. The Blaney Criddle method is simple and temperature is the only climatic factor use in this method. SCS Blaney Criddle method gives the reference evapotranspiration in the following equation.
p ETo ¼ Kt T 100
(6)
Kt ¼ 0:0173 T 0:314
(7)
where T is the mean monthly temperature ( F) and p is percentage of daylight of the year occurring during a particular month. 2.2.2. Estimation of surface runoff The Soil Conservation Service Curve Number (SCS-CN) method is used to estimate surface runoff with the effect of land use and hydrologic soil classes [41]. Curve number method computes direct runoff through an empirical equation that requires the rainfall and a watershed coefficient as inputs. Watershed coefficient is called as curve number (CN), which represents the runoff potential of land cover and soil complex. Following equation shows the relationship between rainfall depth, P, and runoff depth RO;
RO ¼
ðP 0:2 SÞ2 ðP þ 0:8 SÞ
(8)
where P is precipitation (mm) and S is the potential maximum retention (mm) after runoff begins. The retention factor (S) is related to soil and land use condition of the watershed through curve number and S (in mm) is determined by;
S ¼
1000 10 25:4 CN
(9)
where CN is the curve number. The Soil Conservation Service [41] has developed tables of initial curve number (CN) values as a function of the watershed soil type and land use condition. 2.2.3. Data sources and global scale estimation of groundwater recharge Estimates of global warming are generally based on the application of general circulation models (GCMs), which attempt to
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predict the impact of increased atmospheric CO2 concentrations on weather variables. In this study we mainly utilized the data from Hadley Center Climate Simulations (HadCM3) with IPCC–SRES A2 scenario. Among the scenarios in the SRES storylines, scenario A2 has the largest population growth and the SRES A2 emission scenario assumes comparably strong greenhouse gas increases [22,43]. HadCM3 climate data are available at a spatial resolution of 2.5 latitude by 3.75 longitude, which is equivalent to a surface resolution of about 417 km 278 km at the equator. Since the coarse areas have been poorly resolved, the results of this study only show a regional average condition of future fresh groundwater resources. Average monthly precipitation and mean monthly temperature from HadCM3 GCM model are used as main climate variables. Crop coefficients for the estimation of evapotranspiration and curve number for the estimation of surface runoff are obtained based on land use patterns and hydrological soil data. The crop coefficient values referred by FAO are used for the study according to the main land use pattern of the coastal grids [7]. The land use pattern is assigned based on land use classification by Food and Agriculture Organization [12]. Based on these land use classes, four major land use scenarios are defined for the evaluation; forests, grass lands, agricultural lands and bare lands. The hydrological soil data are assigned using the one-degree soil textural map developed by FAO [13]. Each soil texture of the FAO map is assigned to one of the four soil types as defined in SCS curve number method based on the soil texture and infiltration rate of the soil type. In the estimation of groundwater recharge, the components of the water balance are estimated treating each cell as a separate catchment. The crop coefficient (Kc) is assigned using the dominant land use pattern in the grid cell and the SCS curve number (CN) is assigned using dominant land use and hydrologic soil condition in each grid cell. Land use and soil properties within each cell are assumed to be homogeneous and constant for the simulation period (100 years). Mean monthly groundwater recharge is estimated for each grid cell and averaged annual groundwater recharge is used as the input for the sharp interface model. Since the simulation of groundwater resources is carried out only in coastal grids, the grid cells in the coastal regions are taken into account. To select the representative grids for coastal regions, the most seaward grid cells of the continents are considered. The estimated average annual groundwater recharges in the coastal grid cells are shown in Fig. 2. It shows average groundwater recharge in mm/day for the decade 2000–2010, 2040–2050, and 2090–2100. 3. Evaluation of loss of fresh groundwater resources
hydro-geological factor which affects the movement of the freshwater–saltwater interface and groundwater recharge is the main external factor affecting the dynamics of the interface [36]. Groundwater recharge can be considered as the main variable factor since hydraulic conductivity is a constant property throughout the simulation period. Sea level changes are assigned as the main seaward influencing factor [35]. 3.3. Global hydraulic conductivity data Information on global distribution of hydraulic conductivity is difficult to obtain. The hydraulic conductivity map used here is created using the integration of several regional and global datasets of hydro-geological materials. Data sources for hydraulic conductivity of the coastal aquifers are obtained from; 1. Classification of global groundwater regions by The International Groundwater Resources Assessment Centre [19], 2. World-wide Hydro-geological Mapping and Assessment Program (WHYMAP), 3. Soil hydraulic conductivity data (FIFE), 4. WISE/ISRIC database – World Inventory of Soil Emission Potentials/International Soil Reference and Information Centre. The final integrated map of the distribution of hydraulic conductivity is then scaled up to 2.5 3.75 resolution. Even though aquifer properties are very heterogeneous in spatial scale, each grid is assumed as homogeneous aquifer. 3.4. Sea level rise data Mean sea level rise is assigned based on two data sources. 1. Permanent Service for Mean Sea Level (PSMSL) data, 2. GFDL R30 (2002) GCM climate model. GFDL R30 GCM model data calculates sea level change under the SRES A2 IPCC scenario whereas PSMSL provides the trend of sea level change based on historical observation of sea level changes. Here, the sea level change is expressed as the rate of change (mm/ year). Data clearly show that the sea level rises are higher in the northern hemisphere and lower in the southern hemisphere. 4. Results and discussions
The concept of a freshwater–saltwater interface can be used to estimate fresh groundwater resources in coastal aquifers. Increases in groundwater recharge shift the freshwater–saltwater interface seaward (Fig. 3, interface 1), and decreases in recharge shift it landward (Fig. 3, interface 2). This interface movement changes the available fresh groundwater resources in the aquifer. When the aquifer is totally filled with freshwater (Fig. 3, interface 1), the freshwater loss is zero. The landward movement of the salinity interface leads to reduce the freshwater amounts in the aquifer. When the salinity interface coincides with the piezometric head (Fig. 3, interface 3) the whole aquifer fills with saltwater and the freshwater loss is 100%.
As explained in Fig. 1, each coastal grid cell is simulated using the sharp interface model considering 2 km width of the coastal aquifer. Assuming all groundwater flows to the coastal border in each cell, estimated groundwater recharge is fed as the main flow input to each coastal grid cell. Each cell boundary is considered as no flow boundaries. Hydro-geological parameters and rate of sea level change are kept constant throughout the simulation period. Steady condition of freshwater–saltwater interface with respective to groundwater recharge in year 2000 is considered as the initial conditions and consecutive groundwater flow is set as the main influencing factor. Output of the sharp interface model gives the loss of fresh groundwater resources as a result of salinity intrusion in terms of percentage loss of fresh groundwater resources. The situations of loss of fresh groundwater resources in the 2010s, 2050s and 2100s are portrayed on maps (Fig. 4a–c) which show the situation of average global coastal fresh groundwater resources in each decade.
3.2. Inputs for sharp interface model
4.1. Coastal fresh groundwater resources over the next century
A sensitivity analysis of the developed sharp interface model showed that saturated hydraulic conductivity is the main
To estimate the change in loss of fresh groundwater resources over the next century, the difference of the loss of fresh
3.1. Fresh groundwater loss due to salinity intrusion
P. Ranjan et al. / Ocean & Coastal Management 52 (2009) 197–206
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Fig. 2. Average groundwater recharge (a) in the decade 2000–2010; (b) in the decade 2040–2050; and (c) in the decade 2090–2100.
groundwater resources between 2010s and 2100s is considered. The difference shows a wide range across the world. Here, we grouped the reduction in fresh groundwater resources into four main groups for the purpose of the discussion of different coastal regions. The classification is shown in Table 1 and the classified areas are shown in Fig. 5.
Fig. 3. Schematic representation of loss of fresh groundwater resources due to salinity intrusion in coastal aquifers.
Fig. 5 clearly shows the areas which can expect reductions in the availability of fresh groundwater resources and areas which can expect increases in the availability of fresh groundwater resources. Central America, North part of South America, South African and Australian regions show a higher reduction in fresh groundwater resources in the future. Most of the areas in Asian continent except South-East Asia show medium reduction. The upper latitudinal areas give a sign of increase in availability of fresh groundwater resources. Coastal areas in Africa, specifically the Sahara region, show higher loss in fresh groundwater resources than any other regions in the world. The future situation of coastal groundwater in Australia suggests a reduction of fresh groundwater resources all along the Australian coast. European areas indicate a slight increase in loss of fresh groundwater resources in Northern and Western Europe, with mixed results for other parts. Fresh groundwater resources in North American coastal regions show that the Central part of America experiences a higher reduction in groundwater recharge and related reduction in fresh groundwater resources. Also the Southern and Eastern parts of South America show a reduction in groundwater recharge while the projected increases in groundwater recharge at the North-Western boarder of the South American continent could be accompanied by an increase in the fresh groundwater resources in coastal regions.
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Fig. 4. Loss of fresh groundwater resources (%); (a) in the decade 2000–2010; (b) in the decade 2040–2050; and (c) in the decade 2090–2100.
4.2. Population growth and demand for fresh groundwater Evaluations of this kind of trend of changes in coastal fresh groundwater resources are very important to assess the impact of human activities in coastal areas. The reduction in fresh groundwater resources could be detrimental to various water-dependent socioeconomic activities. Fresh groundwater resources are affected Table 1 Classification of the change of fresh groundwater resources. Change in loss of fresh groundwater resources
Classification of fresh groundwater resources
0% Less than 0% 0–5% Higher than 5%
No change Increased Low reduction Higher reduction
Fig. 5. Change in fresh groundwater resources over the next century.
P. Ranjan et al. / Ocean & Coastal Management 52 (2009) 197–206
for local people in the areas, as a result of increases in demand resulting from population growth and water use patterns. Projections of future water use show substantial differences reflecting different population growth, water use and water use efficiency [42]. More potable freshwater will be needed annually to satisfy domestic needs, as well as the requirements of industry and public facilities. The level of demand will depend on the rates of economic and population growth and the expected improvement of people’s living standards. Therefore global population projections have to be considered as a potential indicator to assess the changes in global fresh groundwater resources. To provide world population projections as geographic coordinate data, Girded Population of the World (GPW) map has been developed by CIESIN [5]. In the GPW data set, the distribution of the human population has been converted from national and subnational units to a series of geo-referenced quadrilateral grids. These geo-spatial data have been utilized for several global scale evaluations [31,14]. SRES emission scenarios provide different predictions for future global and regional population. Storyline A2, in contrast, assumes a world of high population growth (15 billion global population by 2100 [21]). Therefore, we used GPW dada under the SRES A2 scenario for our assessment. We assumed a uniform population over the cells. Considering the global distribution of population density predicted under the SRES A2 scenario, in year 2000 and year 2100, the population growth over the next century is estimated as shown in Fig. 6. It highlights that there will be a higher population growth over Indian sub-continent, China, South-East Asia, Central America and the Mediterranean regions.
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(less than 70%) and a lower population density (less than 500 person/km2) show low vulnerability. The combination of high loss of fresh groundwater resources and low population density as well as smaller amount of fresh groundwater loss and higher population shows a moderate vulnerability. 4.4. Vulnerable coastal regions According to global coastal fresh groundwater loss and global population, the distribution of vulnerable coastal areas is shown in Fig. 8. It shows that areas such as North Africa and Sahara, Middle East, Central America (Mexican), South Africa, South Asia (particularly South India and Bangladesh region), China and South Australia are projected to have the highest vulnerability regarding future fresh groundwater supply. Even though the population density is less, the availability of fresh groundwater resource is scarce in African and Middle Eastern regions. It leads to a categorization of these regions as highly vulnerable. The higher availability of fresh groundwater and lower population density in North Europe, South America, New Zealand and Japan, creates less vulnerability with respect to fresh groundwater supply over the next century. North America, Northern Australia and the Mediterranean regions show a moderate vulnerability of future fresh groundwater supply. Even in the same continent, we can see regional differences in vulnerability of fresh groundwater supply. Fig. 9 shows the distribution of some regions/countries in three classified vulnerability groups.
4.3. Vulnerability of coastal areas to fresh groundwater supply
4.5. Fresh groundwater demand over the next century in highly vulnerable regions
Loss of fresh groundwater resources as well as population growth varies substantially from region to region. Reduction in fresh groundwater resources and increase in population threaten the supply of fresh groundwater in coastal areas. The vulnerability of fresh groundwater supply can be defined considering these two aspects. Fig. 7 shows a graph of fresh groundwater loss versus population density which explains the possible combinations to identify the vulnerable coastal areas for future fresh groundwater supply. We define three vulnerable regions to classify each coastal aquifer according to the availability of fresh groundwater resources and the population density. High loss of fresh groundwater resources (more than 90%) and high population density (higher than 1500 person/km2) lead to the highest vulnerability of future fresh groundwater supply. Areas having less fresh groundwater loss
The ever increasing water demand due to population growth and local economic development should be considered as a main factor for sustainable development of coastal regions. Many coastal regions face mounting challenges of providing their increasing population with adequate fresh groundwater. The future trend of population growth in the regions which are identified as high vulnerable regions represents the expected condition of fresh groundwater demand in vulnerable coastal regions. The average per capita groundwater use for identified vulnerable coastal regions; Middle East, North Africa, South Africa, Central America and South Asia is obtained from the study of fresh groundwater resources by World Resources Institute [52] (Table 2). According to these groundwater use data, the South Asia and Central American regions show higher groundwater use than other vulnerable areas.
Population density (person/km2)
2500
2000
Highly Vulnerable areas
1500 Moderately Vulnerable areas
1000
500 Less Vulnerable areas 0 60
70
80
90
Fresh groundwater loss (%) Fig. 6. Population growth over the next century.
Fig. 7. Define the vulnerability of coastal fresh groundwater supply.
100
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P. Ranjan et al. / Ocean & Coastal Management 52 (2009) 197–206 Table 2 Per capita annual groundwater use (Source: Ref. [52]).
Fig. 8. Distribution of the vulnerability of coastal fresh groundwater.
In the Central American area the groundwater use is also higher due to higher living standards. Since South Asia has favorable hydro-geological conditions, even though various constraints limit the availability of fresh groundwater, groundwater is the main water resources in most of the South Asian coastal areas. Furthermore, coastal populations are widely reported to be growing more rapidly than the global mean, due to net coastward migration, and urbanization. Coastal population change can be considered as twice the national population change to reflect a net coastal migration [45,27]. Coastal population is expected to be grown twice the national mean population in each country [28]. Therefore, rapid change of coastal population should be concerned as an extreme scenario. Taking these facts into account, we considered two population growth scenarios for the estimation of freshwater demand in coastal regions as follows; (i) Coastal population growth is as same as the global mean population growth, (ii) Coastal population growth is twice the national population growth.
Population density (person/km2).
Predictions of future water demand are estimated using the per capita groundwater use and the expected population in each decade for two scenarios. Fig. 10 shows the expected conditions of fresh groundwater demand over the next century in highly vulnerable coastal regions considering average expected population growth and the extreme scenario assuming that coastal population growth is twice the national population growth. Fig. 10 indicates that for both scenarios, South Asian population will be grown extremely higher than other areas. Not only South Asian countries have the highest overall population density, but also the increasing water demands in the domestic, agricultural and industrial sectors will put additional stress on water resources. 2500 Bangladesh
2000
China Highly Vulnerable areas
South India
1500 Moderately Vulnerable areas
1000
Central America/Mexico
500 Less Vulnerable North America North Europe South America New Zealand
0 60
70
Mediterranean South Australia Middle East
North Australia
80
North Africa/Sahara
90
Fresh groundwater loss (%) Fig. 9. Vulnerability of fresh groundwater supply.
100
Region/Classification
Per capita groundwater use (m3/person/year)
South Asia Central America Middle East North Africa/Sahara South Africa
233 275 95 55 108
Considering the coastal population growth is compatible with global mean population (scenario 1), annual groundwater demand in South Asia to be estimated approximately as 700,000 MCM in the end of next century. For extreme population scenario (as coastal population will be twice the national population), annual groundwater demand in South Asia will be approximately 1,400,000 MCM in the end of next century. These estimations show that, to balance water supply with water demand, introducing an efficient water management practices by improving water use efficiency is likely to be the best approach for the Asian region. The coastal areas in North Africa and the Sahara region show a higher loss in fresh groundwater resources than in any other regions in the world. The demand for fresh groundwater resources shows relatively lower values in this region, which has low population density. Freshwater demand in North Africa which has less population growth will be 100,000 MCM for scenario 1 and around 200,000 MCM for scenario 2. In Africa, poverty is linked to the environment in complex ways, particularly in economies that are based on exploitation of natural resources like groundwater. Hence, poor communities do not have the capacity and technology to access the deep groundwater and extract it. Degradation of these resources reduces the productivity of poor persons who rely on natural resources most, and makes poor communities even more vulnerable to extreme events (e.g. droughts). 4.6. Contribution of the assessments of coastal fresh groundwater resources to Integrated Coastal Management (ICM) practices This assessment emphasizes that salinity intrusion in coastal groundwater systems affects not only loss of fresh groundwater resources, but also related variety of phenomena such as social and economical and environmental aspects. Global warming threatens to exacerbate these problems by accelerating the rise of sea level, changing the frequency and intensity of precipitation, increasing global temperature while causing other changes in atmospheric and oceanographic conditions. Also rapid growth of coastal population shows that increasingly ominous consequences loom ahead. All these problems highlight the necessity of Integrated Coastal Management (ICM) practices. In the literature there have been numerous studies involving Integrated Coastal Management concepts including global scale general guidelines [47,4,8] and regional scale case studies [48,16–18,30,3]. They explained that generally there are three different modeling approaches as methods and techniques for investigations related to ICM; conceptual modeling, process modeling and simulation modeling. The developed integrated salinity intrusion model is useful as a conceptual model to identify the major factors affecting salinity intrusion in coastal areas. It can be also used as a process modeling to identify the roles played by various processes and mechanisms such as seal level change, groundwater recharge, and other changes in catchment hydrological cycle which are in response to climate change and human activities such as land use change. Also this model is useful as a simulation model to take into account both applicability and simplicity of the model and limited data requirement allowing predictions of spatial and temporal variations in coastal systems.
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1600000
Anual Groundwater use (MCM)
1400000 1200000
205
Middle East North Africa South Asia central America South Africa Middle East-scenario 2 North Africa-scenario 2 South Asia-scenario 2 central America-scenario 2 South Africa-scenario 2
1000000 800000 600000 400000 200000 0 2000
2010
2020
2030
2040
2050
2060
2070
2080
2090
2100
Year Fig. 10. The expected fresh groundwater demand over the next century.
Results of this study will be useful for ICM authorities, decision makers, planners and policy makers and to come up with better sustainable coastal development proposals, minimizing adverse effects of natural and anthropogenic activities. Also the identified vulnerable regions in this study should be considered as priority areas for ICM practices to introduce necessary adaptation strategies. Further, the outcome of this study highlights the necessity of a continues process for monitoring, collecting and disseminating necessary data including salinity data, hydrological and hydrogeological data, climate data, population data, water demand and water use data which need for assessing impacts on resources, coastal issues, functional uses and coastal developments as one of the important factors. Therefore, ICM activities at both national and regional level should be implemented in these vulnerable areas. It can be done by introducing community based coastal zone management programs, which involves local residences and local expertise who have first hand experiences in their own areas.
5. Conclusions Global scale quantification of coastal fresh groundwater resources is required to support the sustainable development in coastal communities. In this paper, an effort has been made to evaluate global coastal fresh groundwater resource. In order to obtain a reliable estimate of coastal fresh groundwater availability, we followed the methodology developed by Ranjan et al. [36] based on the sharp interface concept and simplified estimation of groundwater recharge using limited climate data. The proposed methodology will be useful in areas where limited local hydrological data are available. The effect of future climate change on the global groundwater recharge is evaluated using HadCM3 GCM data considering the SRES A2 scenario. The estimated groundwater recharge is then used to estimate the loss of fresh groundwater resources in coastal aquifers due to salinity intrusion. The global scale evaluation shows that Central America, South America, South Africa and Australian regions show a reduction in fresh groundwater resources in the future. Most of the areas in the Asian continent, except South-East Asia will experience a medium reduction. Coastal areas in Africa show higher loss in fresh groundwater resources than any other regions in the world. Fresh groundwater resources in Central part of America and Southern
and Eastern parts of the South America experience a higher reduction in groundwater recharge and related reduction in fresh groundwater resources. To investigate the potential impacts of change in fresh groundwater resources on the water-dependent socioeconomic activities, the population growth is considered as a potential indicator. Combinations of fresh groundwater loss and population are considered to state the vulnerability of future fresh groundwater supply. It indicated that South Asia, especially South India and Bangladesh, Central America, North Africa and Sahara, South Africa and the Middle East countries are highly vulnerable whereas, North Europe, Western part of South America, New Zealand and Japan are less vulnerable to future fresh groundwater supply. South Asia which shows highest population growth will be more stressed in the future. The interesting point is that the present and future water demand show relatively lower values in African continent, which have low population density. This study further proposes that the ability of water resources managers to respond is not only depending on the level of climate change, but also population growth and changes in demands, technology and economic, social, and legislative conditions highlighting the importance of an Integrated Coastal Management practices in coastal areas at both national and regional levels. Beyond this, the study shows how the outcome of this study can be implemented for successful Integrated Coastal Management practices ensuring sustainable coastal developments. Acknowledgments This work was made possible through the support for field observation from Dr. Ahmed Sana, Sultan Qaboos University, Oman and Mr. Mahesh Dilroy, University of Ruhuna, Sri Lanka. The authors would like to acknowledge the generosity of these supports. References [1] Arnell NW. Climate change and global water resources: SRES emissions and socio-economic scenarios. Global Environmental Change 2004;14:31–52. [2] Bear J, Cheng AHD, Sorek S, Ouazar D, Herrera I. Seawater intrusion in coastal aquifers – concepts, methods and practices. Kluwer Academic Publishers; 1999.
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[28] Nicholls RJ, Lowe JA. Benefits of mitigation of climate change for coastal areas. Global Environmental Change 2004;14:229–44. [29] Nilsson C, Reidy CA, Dynesius M, Revenga C. Fragmentation and flow regulation of the world’s large river systems. Science 2005;308:405–8. [30] Noronha L. Coastal management policy: observations from an Indian case. Ocean and Coastal Management 2004;47:63–77. [31] Parry ML, Rosenzweig C, Iglesias A, Livermore M, Fischer G. Effects of climate change on global food production under SRES emissions and socio-economic scenarios. Global Environmental Change 2004;14:53–67. [32] Polo JF, Ramis FJR. Simulation of salt–fresh water interface model. Water Resources Research 1983;19(1):61–8. [33] Ranjan SP, Kazama S, Sawamoto M. Modeling of the dynamics of saltwater– freshwater, interface in coastal aquifers. In: Proceedings of the joint AOGS annual meeting & APHW second conference. Singapore; 2004. p. 373–380. [34] Ranjan SP, Kazama S, Sawamoto M. Effect of sea level rise on the loss of fresh groundwater resources: case studies of Western American coast and Bay of Bengal. Journal of Hydraulic Engineering, JSCE 2005;49:97–102. [35] Ranjan SP. Evaluation of the effects of global warming on coastal groundwater resources. Doctoral Dissertation. Japan: Tohoku University; 2005. [36] Ranjan SP, Kazama S, Sawamoto M. Effects of climate and land use changes on groundwater resources in coastal aquifers. Journal of Environmental Management 2006;80(1):25–35. [37] Ranjan SP, Kazama S, Sawamoto M. Climate change and fresh groundwater resource in coastal regions. Global Environmental Change 2006;16:388–99. [38] Reilly TE, Goodman AS. Quantitative analysis of fresh–salt water relationship in groundwater systems – a historical perspective. Journal of Hydrology 1985;80:125–49. [39] Remson I, Hornberger GM, Molz FI. Numerical methods in subsurface hydrology. New York: Wiley-Interscience; 1971. p. 389. [40] Revenga C, Brunner J, Henninger N, Kassem K, Payne N. Pilot analysis of global ecosystems: freshwater ecosystems. Washington, DC: World Resources Institute and Worldwatch Institute; 2000. [41] SCS. National engineering handbook. Hydrology, Section 4. Washington, DC: Soil Conservation Service, US Department of Agriculture; 1956. [42] Seckler D, Amarasinghe U, Molden D, de Silva R, Barker R. World water demand and supply, 1990 to 2025: scenarios and issues. Research Report 19. Sri Lanka: International Water Management Institute; 1998. [43] Semmler T, Jacob D. Modeling extreme precipitation events; a climate change simulation for Europe. Global and Planetary Change 2004;44:119–27. [44] Shuttleworth WJ. Evaporation. In: Maidment DR, editor. Handbook of hydrology. New York: McGraw-Hill; 1992. [45] Small C, Nicholls RJ. A global analysis of human settlement in coastal zones. Journal of Coastal Research 2003;19(3):584–99. [46] Titus JG. Greenhouse effect, sea level rise and land use. Land Use Policy 1990;7(2):138–53. [47] UNEP, Grigalunas TA, Opaluch J, Brown G. Regional Seas Reports and Studies No. 164: Environmental economics for integrated coastal area management: valuation methods and policy instruments. UNEP, http://www.unep.org/ regionalseas/sitemap/default.asp; 1995. [48] UNEP. Regional Seas Reports and Studies No. 161: Guidelines for integrated management of coastal and marine areas – with special reference to the Mediterranean Basin. UNEP; 1995. p. 80. [49] Van Der Veer P. Analytical solution for steady interface flow in a coastal aquifer involving a phreatic surface with precipitation. Journal of Hydrology 1977;34:1–11. [50] Vorosmarty CJ, Green P, Salisbury J, Lammers RB. Global water resources: vulnerability from climate change and population growth. Science 2000;289:284–8. [51] Wilson JL, Costa ASD. Finite element simulation of salt–fresh water interface with indirect toe tracking. Water Resources Research 1982;18(4):1069–81. [52] World Resources Institute. Water resources and freshwater ecosystems data base. World Resources Institute, http://earthtrends.wri.org; 2005. [53] Wright JL. New evapotranspiration crop coefficients. Journal of Irrigation and Drainage Division, ASCE 1982;108:57–74. [54] Zhang L, Dawes WR, Walker GR. Response of mean annual evapotranspiration to vegetation change at catchment scale. Water Resources Research 2001;37(3):701–8.
Contents lists available at ScienceDirect
Ocean & Coastal Management journal homepage: www.elsevier.com/locate/ocecoaman
Global scale evaluation of coastal fresh groundwater resources Priyantha Ranjan a, *, So Kazama b,1, Masaki Sawamoto c, 2, Ahmad Sana d, 3 a
Department of Civil Engineering, Curtin University of Technology, GPO Box U1987, Perth WA6845, Australia Graduate School of Environmental Studies, Tohoku University, 6-6-06 Aramaki Aza Aoba, Sendai 980-8579, Japan c Alpha Hydraulic Engineering Consultants Co., Ltd., Mita-Hillcrest, 4-15-35, Mita, Minato, Tokyo 108-0073, Japan d Department of Civil and Architectural Engineering, Sultan Qaboos University, PO Box 33, Al-Khod 123, Oman b
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 1 October 2008
This paper presents a simplified approach to assess the effects of global warming on global coastal groundwater resources over the next century based on the smallest but necessary number of elements such as rainfall, temperature, hydraulic conductivity of the aquifers, and population changes regarding the consumption of groundwater. The positive aspect in this approach is that there is availability of the above elements in the majority of the planet. Methodology includes a sharp interface concept model and simplified estimation of groundwater recharge using limited climate data. The evaluation shows that the future climate changes would decrease fresh groundwater resources in Central American, South American, South African and Australian regions whereas most of the areas in Asia, except South-East Asia. Combinations of fresh groundwater loss and global population are considered to state the vulnerability of future fresh groundwater supply. Vulnerability assessment shows that South Asia, Central America, North Africa and the Sahara, South Africa and the Middle East countries are highly vulnerable whereas, Northern Europe, Western part of South America, New Zealand and Japan are less vulnerable with respect to future fresh groundwater supply. Further, this paper highlights the necessity Integrated Coastal Management (ICM) practices in these vulnerable coastal regions to ensure the sustainable development in coastal regions. Crown Copyright Ó 2008 Published by Elsevier Ltd. All rights reserved.
1. Introduction Since groundwater systems in coastal areas are in contact with saline water, the major problem is the intrusion of saltwater into the aquifer. Already at this moment, many coastal aquifers in the world, especially shallow ones, experience an intensive saltwater intrusion caused by both natural as well as man-induced processes [24]. Salinization of these groundwater systems can lead to a severe deterioration of the quality of existing fresh groundwater resources and can create a reduction of the amount of freshwater resources available in coastal aquifers. Therefore, use of coastal aquifers as operational reservoirs in water resources systems requires development of tools that facilitate the prediction of the behavior of coastal aquifers under different conditions. Climate change due to increasing concentration of greenhouse gas is likely to affect groundwater recharge and thus affect the * Corresponding author. Tel.: þ61 8 9266 3530 (office). E-mail addresses: [email protected] (P. Ranjan), kazama@kaigan. civil.tohoku.ac.jp (S. Kazama), [email protected] (M. Sawamoto), [email protected] (A. Sana). 1 Tel.: þ81 22 217 7458; fax: þ81 217 7458. 2 Tel./fax: þ81 3 5445 6543. 3 Tel.: þ968 513 333x2524; fax: þ968 513 416.
saltwater intrusion through changes in precipitation and temperature [1,40]. Any reduction in groundwater flow toward the sea will cause intrusion of saltwater into the aquifer as the saltwater– freshwater interface moves inland. Coastal aquifers within the zone of influence of mean sea level are threatened by accelerated rise in global sea level, because the salinization of coastal aquifers accelerates due to sea level rise [46,21,20]. Taking hydro-geological changes in coastal aquifers due to sea level rise, Melloul and Collin [26] presented a simple hydro-geological conceptual model relating seawater intrusion to the impacts of sea level rise and discussed the uncertainty in accurately forecasting future sea level rise. In addition, other anthropogenic activities such as land use change have an impact on salinity intrusion in coastal aquifer systems through reduced groundwater recharge. Also, the increased demand for fresh groundwater resources due to future population growth will lead to higher vulnerability of fresh groundwater supply due to higher demand in the future [11]. Availability of freshwater has been recognized as a global issue [6]. Coastal groundwater plays an important role in global freshwater resources. Quantification of coastal groundwater resources not only in individual aquifers but also at the global scale is required to support the sustainable use of fresh groundwater for coastal communities. Evaluation of the integrated effects of climate
0964-5691/$ – see front matter Crown Copyright Ó 2008 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ocecoaman.2008.09.006
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change on coastal groundwater resources in global scale becomes rather difficult. Even though global scale evaluations are very important in order to evaluate the effects of climate change on coastal groundwater resources, global scale assessments remain very difficult because of the lack of data. Also available methodologies are laborious, time-consuming and costly. A large number of more or less empirical methods have been developed by numerous scientists and specialists to evaluate freshwater resources in different climatic conditions [50,27–29,15]. Testing the accuracy of these methods in global scale is laborious, time-consuming and costly, because of the necessity of calibration for many variables. To overcome these restrictions when assessing future global coastal groundwater simulations, global evaluations of fresh groundwater resources should be carried out using methodologies which need only few hydrological and hydro-geological data. To accommodate users with limited available data, a method has been proposed by Ranjan et al. [36] which evaluates coastal fresh groundwater resources based on a sharp interface model and a simplified estimation of groundwater recharge using limited hydrological data. Using the developed methodology, they evaluated the impacts of climate change on coastal fresh groundwater resources for several water resources stressed regions [37]. The proposed method can give acceptable first order estimates, depending on the location, availability and the quality of the measured data, even though the results have certain uncertainty. The main objectives of this study are to use the introduced simple approach to provide a global scale evaluation of coastal groundwater resources considering the integrated effect of global warming (i.e. climate change and sea level rise) and anthropogenic stress such as land use change. 2. Methods and materials This study follows the developed method by Ranjan et al. [36] to evaluate fresh groundwater resources in coastal aquifers. It integrates two approaches; (i) Estimate the salinity intrusion in coastal groundwater systems using the sharp interface concept estimation, (ii) Estimate the groundwater recharge based on precipitation and temperature as measured climate data. Fig. 1 summarizes the integration of these two methodologies and required data. Precipitation and temperature are the main hydrological inputs. Hydraulic conductivity, land use type, soil type and sea level data are the other main inputs.
Precipitation and Temperature Land use Map Estimation of Groundwater Recharge
Kc Soil Data Map
CN
Groundwater Recharge
Sharp Interface Model
Sea Level Rise
Hydraulic Conductivity (K)
Loss of Fresh Groundwater Resources
Fig. 1. Steps of the estimation of global fresh groundwater resources.
characterize fully such systems and processes. Therefore this study follows the sharp interface model developed by Ranjan et al. [36] to estimate the salinity intrusion to coastal aquifers and hence to evaluate the loss of fresh groundwater resources in the aquifer. Sharp interface models couple the freshwater and saltwater flow based on the continuity of flux and pressure. In this approach, together with the Dupuit approximation for each flow domain, the equation of continuity may be integrated over vertical direction and come up with following system of differential equations [2].
" " # # f f v v f i vh f i vh K h h K h h þ þ qf vx fx vy fy vx vy # " vhf vhs vhf vhf þ aq ¼ Sf q ð1 þ dÞ d vt vt vt vt
(1)
2.1. Modeling of salinity intrusion To simulate the dynamics of freshwater–saltwater interface, it is necessary to examine the coupled freshwater and saltwater flow domains. The first concept regarding freshwater–saltwater interfaces, now widely cited as the Ghyben–Herzberg principle, is based on the hydrostatic equilibrium between freshwater and saltwater. After introducing the Ghyben–Herzberg principle, many models have been developed to study the problem of saltwater intrusion. They range from relatively simple analytical solutions to complex numerical models [49,51,32,2]. These studies are classically used in two different approaches [38]. In the first approach, freshwater and saltwater are assumed completely immiscible and a sharp interface exists between these two phases. In the other approach, freshwater and saltwater are assumed to be in a dynamic equilibrium resulting from the flow and dispersion mechanisms within the aquifer. Since, the main objective of this study is to evaluate the long-term overall behavior of the coastal groundwater systems due to the effect of global warming, the sharp interface model is the ideal way to
vhs vhs v v þ þ qs Ksx hi zb Ksy hi zb vx vy vx vy # " vhs vhs vhf ¼ Ss þ q ð1 þ dÞ d vt vt vt
(2)
The location of the interface elevation (hi) is given by
hi ¼
rs rf hs hf rs rf rs rf
(3)
where rf and rs are specific weight in fresh and saltwater, respectively, hf and hs are the piezometric heads of freshwater and saltwater regions, qf and qs are flow rate in fresh and saltwater, respectively. Kf and Ks represent the hydraulic conductivity in fresh and saltwater regions. Storage coefficients in fresh and saltwater regions are given by Sf and Ss, respectively. q is the porosity of the aquifer media. a ¼ 1 for unconfined aquifer and a ¼ 0 for confined aquifer.
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From Eqs. (1) and (2), it is possible to derive a numerical model using implicit finite difference techniques. To solve the two simultaneous linear algebraic difference equations, the strongly implicit procedure – SIP [39] is used as a suitable numerical technique. Empirical evidence suggests that for the cases of flow in heterogeneous or anisotropic media, strongly implicit procedure is much faster than the other methods. Also the strongly implicit method does not depend upon the complexity of the problem [9,10]. A sensitivity analysis is carried out using the developed model to investigate the effect of hydro-geologic factors mainly, specific storage, porosity and hydraulic conductivity on the dynamics of freshwater–saltwater flow systems. It concluded that hydraulic conductivity is the main hydro-geological parameter which affects to change the freshwater–saltwater interface [33]. To test the model in different climatic regions, the model is successfully applied and verified with observed seawater intrusion data in three different climatic regions, Sendai coastal plain in Japan, Southern coastal aquifer in Sri Lanka and Al Batinah coastal plain in Oman [34,36]. Comparison between observed and simulated salinity profiles concluded that the developed model is able to simulate the realistic changes of the salinity profiles and the model can be used to successfully simulate the salinity profile in different hydrological and climatic areas. 2.2. Simple approach to estimate groundwater recharge Groundwater recharge in watershed areas is the main source of fresh groundwater resources in coastal zones. It is imperative to understand the process of the salinity interface and predict the shape of interface profile due to changes in groundwater recharge and related activities such as climate changes, and changes in land use pattern and land management practices [25,11,54,23]. Estimation of groundwater recharge is very sensitive to measurement errors due to the involvement of large number of observation parameters especially in the estimation of evapotranspiration. If groundwater recharge can be represented as a function of limited climate parameters such as precipitation and mean temperature, it leads to reduce accumulated uncertainty in the estimations. Hence, a methodology to estimate long-term average of actual evapotranspiration, surface runoff and groundwater recharge has been developed [36]. This methodology can be used in areas with limited meteorological data and also in ungauged basins. Also this kind of simple method can be expected to compute the average condition of fresh groundwater resources in different regions and it is easy to compare the status of fresh groundwater resources in different regions. The developed methodology is based on water balance equation and assesses how changes in catchment conditions can alter the partitioning of rainfall into different components. The simple water balance for any catchment can be written as:
R ¼ P ET RO
(4)
Where R is the groundwater recharge, P is the precipitation, RO is the surface runoff and ET is the evapotranspiration. All variables have dimensions [L/T]. 2.2.1. Estimation of evapotranspiration with minimum data Since evapotranspiration is one of the major inputs in water balance equation, evapotranspiration has to be mainly evaluated to estimate groundwater recharge. Crop evapotranspiration is used to estimate the evapotranspiration, concerning the effects of land use pattern. Crop evapotranspiration is a simple representation of the physical and physiological factors governing the evapotranspiration process, taking vegetation parameters into account. Differences in the crop canopy and aerodynamic resistance relative to the hypothetical reference crop are accounted within the crop coefficient
199
(Kc). It represents an integration of the effects of four primary characteristics that distinguish the crop from each other. These characteristics are; crop height, Albedo (reflectance), canopy resistance and the evaporation from soil [53]. Crop evapotranspiration is estimated as a multiplication of reference crop evapotranspiration (ETo) and crop coefficient (Kc) as follows.
ETcrop ¼ ETo Kc
(5)
Several methods exist for the empirical estimation of reference evapotranspiration (ETo). Based on the data requirement, these methods are classified as temperature based methods, pan evaporation, radiation and combination methods. Combination methods such as the Penman equation require air temperature, relative humidity, wind speed and radiation information reflecting meteorological parameters influencing evapotranspiration. Most of those methods need large amount of parameters to estimate evapotranspiration. It accumulates measurement errors and the lack of data for the necessary parameters leads the final estimation of groundwater recharge erroneous. Considering data requirements of available methods to estimate evapotranspiration, temperature based methods are the only alternative for consumptive use operation. A widely used temperature based theoretical method to calculate reference crop evapotranspiration is SCS Blaney Criddle method [44]. The Blaney Criddle method is simple and temperature is the only climatic factor use in this method. SCS Blaney Criddle method gives the reference evapotranspiration in the following equation.
p ETo ¼ Kt T 100
(6)
Kt ¼ 0:0173 T 0:314
(7)
where T is the mean monthly temperature ( F) and p is percentage of daylight of the year occurring during a particular month. 2.2.2. Estimation of surface runoff The Soil Conservation Service Curve Number (SCS-CN) method is used to estimate surface runoff with the effect of land use and hydrologic soil classes [41]. Curve number method computes direct runoff through an empirical equation that requires the rainfall and a watershed coefficient as inputs. Watershed coefficient is called as curve number (CN), which represents the runoff potential of land cover and soil complex. Following equation shows the relationship between rainfall depth, P, and runoff depth RO;
RO ¼
ðP 0:2 SÞ2 ðP þ 0:8 SÞ
(8)
where P is precipitation (mm) and S is the potential maximum retention (mm) after runoff begins. The retention factor (S) is related to soil and land use condition of the watershed through curve number and S (in mm) is determined by;
S ¼
1000 10 25:4 CN
(9)
where CN is the curve number. The Soil Conservation Service [41] has developed tables of initial curve number (CN) values as a function of the watershed soil type and land use condition. 2.2.3. Data sources and global scale estimation of groundwater recharge Estimates of global warming are generally based on the application of general circulation models (GCMs), which attempt to
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predict the impact of increased atmospheric CO2 concentrations on weather variables. In this study we mainly utilized the data from Hadley Center Climate Simulations (HadCM3) with IPCC–SRES A2 scenario. Among the scenarios in the SRES storylines, scenario A2 has the largest population growth and the SRES A2 emission scenario assumes comparably strong greenhouse gas increases [22,43]. HadCM3 climate data are available at a spatial resolution of 2.5 latitude by 3.75 longitude, which is equivalent to a surface resolution of about 417 km 278 km at the equator. Since the coarse areas have been poorly resolved, the results of this study only show a regional average condition of future fresh groundwater resources. Average monthly precipitation and mean monthly temperature from HadCM3 GCM model are used as main climate variables. Crop coefficients for the estimation of evapotranspiration and curve number for the estimation of surface runoff are obtained based on land use patterns and hydrological soil data. The crop coefficient values referred by FAO are used for the study according to the main land use pattern of the coastal grids [7]. The land use pattern is assigned based on land use classification by Food and Agriculture Organization [12]. Based on these land use classes, four major land use scenarios are defined for the evaluation; forests, grass lands, agricultural lands and bare lands. The hydrological soil data are assigned using the one-degree soil textural map developed by FAO [13]. Each soil texture of the FAO map is assigned to one of the four soil types as defined in SCS curve number method based on the soil texture and infiltration rate of the soil type. In the estimation of groundwater recharge, the components of the water balance are estimated treating each cell as a separate catchment. The crop coefficient (Kc) is assigned using the dominant land use pattern in the grid cell and the SCS curve number (CN) is assigned using dominant land use and hydrologic soil condition in each grid cell. Land use and soil properties within each cell are assumed to be homogeneous and constant for the simulation period (100 years). Mean monthly groundwater recharge is estimated for each grid cell and averaged annual groundwater recharge is used as the input for the sharp interface model. Since the simulation of groundwater resources is carried out only in coastal grids, the grid cells in the coastal regions are taken into account. To select the representative grids for coastal regions, the most seaward grid cells of the continents are considered. The estimated average annual groundwater recharges in the coastal grid cells are shown in Fig. 2. It shows average groundwater recharge in mm/day for the decade 2000–2010, 2040–2050, and 2090–2100. 3. Evaluation of loss of fresh groundwater resources
hydro-geological factor which affects the movement of the freshwater–saltwater interface and groundwater recharge is the main external factor affecting the dynamics of the interface [36]. Groundwater recharge can be considered as the main variable factor since hydraulic conductivity is a constant property throughout the simulation period. Sea level changes are assigned as the main seaward influencing factor [35]. 3.3. Global hydraulic conductivity data Information on global distribution of hydraulic conductivity is difficult to obtain. The hydraulic conductivity map used here is created using the integration of several regional and global datasets of hydro-geological materials. Data sources for hydraulic conductivity of the coastal aquifers are obtained from; 1. Classification of global groundwater regions by The International Groundwater Resources Assessment Centre [19], 2. World-wide Hydro-geological Mapping and Assessment Program (WHYMAP), 3. Soil hydraulic conductivity data (FIFE), 4. WISE/ISRIC database – World Inventory of Soil Emission Potentials/International Soil Reference and Information Centre. The final integrated map of the distribution of hydraulic conductivity is then scaled up to 2.5 3.75 resolution. Even though aquifer properties are very heterogeneous in spatial scale, each grid is assumed as homogeneous aquifer. 3.4. Sea level rise data Mean sea level rise is assigned based on two data sources. 1. Permanent Service for Mean Sea Level (PSMSL) data, 2. GFDL R30 (2002) GCM climate model. GFDL R30 GCM model data calculates sea level change under the SRES A2 IPCC scenario whereas PSMSL provides the trend of sea level change based on historical observation of sea level changes. Here, the sea level change is expressed as the rate of change (mm/ year). Data clearly show that the sea level rises are higher in the northern hemisphere and lower in the southern hemisphere. 4. Results and discussions
The concept of a freshwater–saltwater interface can be used to estimate fresh groundwater resources in coastal aquifers. Increases in groundwater recharge shift the freshwater–saltwater interface seaward (Fig. 3, interface 1), and decreases in recharge shift it landward (Fig. 3, interface 2). This interface movement changes the available fresh groundwater resources in the aquifer. When the aquifer is totally filled with freshwater (Fig. 3, interface 1), the freshwater loss is zero. The landward movement of the salinity interface leads to reduce the freshwater amounts in the aquifer. When the salinity interface coincides with the piezometric head (Fig. 3, interface 3) the whole aquifer fills with saltwater and the freshwater loss is 100%.
As explained in Fig. 1, each coastal grid cell is simulated using the sharp interface model considering 2 km width of the coastal aquifer. Assuming all groundwater flows to the coastal border in each cell, estimated groundwater recharge is fed as the main flow input to each coastal grid cell. Each cell boundary is considered as no flow boundaries. Hydro-geological parameters and rate of sea level change are kept constant throughout the simulation period. Steady condition of freshwater–saltwater interface with respective to groundwater recharge in year 2000 is considered as the initial conditions and consecutive groundwater flow is set as the main influencing factor. Output of the sharp interface model gives the loss of fresh groundwater resources as a result of salinity intrusion in terms of percentage loss of fresh groundwater resources. The situations of loss of fresh groundwater resources in the 2010s, 2050s and 2100s are portrayed on maps (Fig. 4a–c) which show the situation of average global coastal fresh groundwater resources in each decade.
3.2. Inputs for sharp interface model
4.1. Coastal fresh groundwater resources over the next century
A sensitivity analysis of the developed sharp interface model showed that saturated hydraulic conductivity is the main
To estimate the change in loss of fresh groundwater resources over the next century, the difference of the loss of fresh
3.1. Fresh groundwater loss due to salinity intrusion
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Fig. 2. Average groundwater recharge (a) in the decade 2000–2010; (b) in the decade 2040–2050; and (c) in the decade 2090–2100.
groundwater resources between 2010s and 2100s is considered. The difference shows a wide range across the world. Here, we grouped the reduction in fresh groundwater resources into four main groups for the purpose of the discussion of different coastal regions. The classification is shown in Table 1 and the classified areas are shown in Fig. 5.
Fig. 3. Schematic representation of loss of fresh groundwater resources due to salinity intrusion in coastal aquifers.
Fig. 5 clearly shows the areas which can expect reductions in the availability of fresh groundwater resources and areas which can expect increases in the availability of fresh groundwater resources. Central America, North part of South America, South African and Australian regions show a higher reduction in fresh groundwater resources in the future. Most of the areas in Asian continent except South-East Asia show medium reduction. The upper latitudinal areas give a sign of increase in availability of fresh groundwater resources. Coastal areas in Africa, specifically the Sahara region, show higher loss in fresh groundwater resources than any other regions in the world. The future situation of coastal groundwater in Australia suggests a reduction of fresh groundwater resources all along the Australian coast. European areas indicate a slight increase in loss of fresh groundwater resources in Northern and Western Europe, with mixed results for other parts. Fresh groundwater resources in North American coastal regions show that the Central part of America experiences a higher reduction in groundwater recharge and related reduction in fresh groundwater resources. Also the Southern and Eastern parts of South America show a reduction in groundwater recharge while the projected increases in groundwater recharge at the North-Western boarder of the South American continent could be accompanied by an increase in the fresh groundwater resources in coastal regions.
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Fig. 4. Loss of fresh groundwater resources (%); (a) in the decade 2000–2010; (b) in the decade 2040–2050; and (c) in the decade 2090–2100.
4.2. Population growth and demand for fresh groundwater Evaluations of this kind of trend of changes in coastal fresh groundwater resources are very important to assess the impact of human activities in coastal areas. The reduction in fresh groundwater resources could be detrimental to various water-dependent socioeconomic activities. Fresh groundwater resources are affected Table 1 Classification of the change of fresh groundwater resources. Change in loss of fresh groundwater resources
Classification of fresh groundwater resources
0% Less than 0% 0–5% Higher than 5%
No change Increased Low reduction Higher reduction
Fig. 5. Change in fresh groundwater resources over the next century.
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for local people in the areas, as a result of increases in demand resulting from population growth and water use patterns. Projections of future water use show substantial differences reflecting different population growth, water use and water use efficiency [42]. More potable freshwater will be needed annually to satisfy domestic needs, as well as the requirements of industry and public facilities. The level of demand will depend on the rates of economic and population growth and the expected improvement of people’s living standards. Therefore global population projections have to be considered as a potential indicator to assess the changes in global fresh groundwater resources. To provide world population projections as geographic coordinate data, Girded Population of the World (GPW) map has been developed by CIESIN [5]. In the GPW data set, the distribution of the human population has been converted from national and subnational units to a series of geo-referenced quadrilateral grids. These geo-spatial data have been utilized for several global scale evaluations [31,14]. SRES emission scenarios provide different predictions for future global and regional population. Storyline A2, in contrast, assumes a world of high population growth (15 billion global population by 2100 [21]). Therefore, we used GPW dada under the SRES A2 scenario for our assessment. We assumed a uniform population over the cells. Considering the global distribution of population density predicted under the SRES A2 scenario, in year 2000 and year 2100, the population growth over the next century is estimated as shown in Fig. 6. It highlights that there will be a higher population growth over Indian sub-continent, China, South-East Asia, Central America and the Mediterranean regions.
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(less than 70%) and a lower population density (less than 500 person/km2) show low vulnerability. The combination of high loss of fresh groundwater resources and low population density as well as smaller amount of fresh groundwater loss and higher population shows a moderate vulnerability. 4.4. Vulnerable coastal regions According to global coastal fresh groundwater loss and global population, the distribution of vulnerable coastal areas is shown in Fig. 8. It shows that areas such as North Africa and Sahara, Middle East, Central America (Mexican), South Africa, South Asia (particularly South India and Bangladesh region), China and South Australia are projected to have the highest vulnerability regarding future fresh groundwater supply. Even though the population density is less, the availability of fresh groundwater resource is scarce in African and Middle Eastern regions. It leads to a categorization of these regions as highly vulnerable. The higher availability of fresh groundwater and lower population density in North Europe, South America, New Zealand and Japan, creates less vulnerability with respect to fresh groundwater supply over the next century. North America, Northern Australia and the Mediterranean regions show a moderate vulnerability of future fresh groundwater supply. Even in the same continent, we can see regional differences in vulnerability of fresh groundwater supply. Fig. 9 shows the distribution of some regions/countries in three classified vulnerability groups.
4.3. Vulnerability of coastal areas to fresh groundwater supply
4.5. Fresh groundwater demand over the next century in highly vulnerable regions
Loss of fresh groundwater resources as well as population growth varies substantially from region to region. Reduction in fresh groundwater resources and increase in population threaten the supply of fresh groundwater in coastal areas. The vulnerability of fresh groundwater supply can be defined considering these two aspects. Fig. 7 shows a graph of fresh groundwater loss versus population density which explains the possible combinations to identify the vulnerable coastal areas for future fresh groundwater supply. We define three vulnerable regions to classify each coastal aquifer according to the availability of fresh groundwater resources and the population density. High loss of fresh groundwater resources (more than 90%) and high population density (higher than 1500 person/km2) lead to the highest vulnerability of future fresh groundwater supply. Areas having less fresh groundwater loss
The ever increasing water demand due to population growth and local economic development should be considered as a main factor for sustainable development of coastal regions. Many coastal regions face mounting challenges of providing their increasing population with adequate fresh groundwater. The future trend of population growth in the regions which are identified as high vulnerable regions represents the expected condition of fresh groundwater demand in vulnerable coastal regions. The average per capita groundwater use for identified vulnerable coastal regions; Middle East, North Africa, South Africa, Central America and South Asia is obtained from the study of fresh groundwater resources by World Resources Institute [52] (Table 2). According to these groundwater use data, the South Asia and Central American regions show higher groundwater use than other vulnerable areas.
Population density (person/km2)
2500
2000
Highly Vulnerable areas
1500 Moderately Vulnerable areas
1000
500 Less Vulnerable areas 0 60
70
80
90
Fresh groundwater loss (%) Fig. 6. Population growth over the next century.
Fig. 7. Define the vulnerability of coastal fresh groundwater supply.
100
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P. Ranjan et al. / Ocean & Coastal Management 52 (2009) 197–206 Table 2 Per capita annual groundwater use (Source: Ref. [52]).
Fig. 8. Distribution of the vulnerability of coastal fresh groundwater.
In the Central American area the groundwater use is also higher due to higher living standards. Since South Asia has favorable hydro-geological conditions, even though various constraints limit the availability of fresh groundwater, groundwater is the main water resources in most of the South Asian coastal areas. Furthermore, coastal populations are widely reported to be growing more rapidly than the global mean, due to net coastward migration, and urbanization. Coastal population change can be considered as twice the national population change to reflect a net coastal migration [45,27]. Coastal population is expected to be grown twice the national mean population in each country [28]. Therefore, rapid change of coastal population should be concerned as an extreme scenario. Taking these facts into account, we considered two population growth scenarios for the estimation of freshwater demand in coastal regions as follows; (i) Coastal population growth is as same as the global mean population growth, (ii) Coastal population growth is twice the national population growth.
Population density (person/km2).
Predictions of future water demand are estimated using the per capita groundwater use and the expected population in each decade for two scenarios. Fig. 10 shows the expected conditions of fresh groundwater demand over the next century in highly vulnerable coastal regions considering average expected population growth and the extreme scenario assuming that coastal population growth is twice the national population growth. Fig. 10 indicates that for both scenarios, South Asian population will be grown extremely higher than other areas. Not only South Asian countries have the highest overall population density, but also the increasing water demands in the domestic, agricultural and industrial sectors will put additional stress on water resources. 2500 Bangladesh
2000
China Highly Vulnerable areas
South India
1500 Moderately Vulnerable areas
1000
Central America/Mexico
500 Less Vulnerable North America North Europe South America New Zealand
0 60
70
Mediterranean South Australia Middle East
North Australia
80
North Africa/Sahara
90
Fresh groundwater loss (%) Fig. 9. Vulnerability of fresh groundwater supply.
100
Region/Classification
Per capita groundwater use (m3/person/year)
South Asia Central America Middle East North Africa/Sahara South Africa
233 275 95 55 108
Considering the coastal population growth is compatible with global mean population (scenario 1), annual groundwater demand in South Asia to be estimated approximately as 700,000 MCM in the end of next century. For extreme population scenario (as coastal population will be twice the national population), annual groundwater demand in South Asia will be approximately 1,400,000 MCM in the end of next century. These estimations show that, to balance water supply with water demand, introducing an efficient water management practices by improving water use efficiency is likely to be the best approach for the Asian region. The coastal areas in North Africa and the Sahara region show a higher loss in fresh groundwater resources than in any other regions in the world. The demand for fresh groundwater resources shows relatively lower values in this region, which has low population density. Freshwater demand in North Africa which has less population growth will be 100,000 MCM for scenario 1 and around 200,000 MCM for scenario 2. In Africa, poverty is linked to the environment in complex ways, particularly in economies that are based on exploitation of natural resources like groundwater. Hence, poor communities do not have the capacity and technology to access the deep groundwater and extract it. Degradation of these resources reduces the productivity of poor persons who rely on natural resources most, and makes poor communities even more vulnerable to extreme events (e.g. droughts). 4.6. Contribution of the assessments of coastal fresh groundwater resources to Integrated Coastal Management (ICM) practices This assessment emphasizes that salinity intrusion in coastal groundwater systems affects not only loss of fresh groundwater resources, but also related variety of phenomena such as social and economical and environmental aspects. Global warming threatens to exacerbate these problems by accelerating the rise of sea level, changing the frequency and intensity of precipitation, increasing global temperature while causing other changes in atmospheric and oceanographic conditions. Also rapid growth of coastal population shows that increasingly ominous consequences loom ahead. All these problems highlight the necessity of Integrated Coastal Management (ICM) practices. In the literature there have been numerous studies involving Integrated Coastal Management concepts including global scale general guidelines [47,4,8] and regional scale case studies [48,16–18,30,3]. They explained that generally there are three different modeling approaches as methods and techniques for investigations related to ICM; conceptual modeling, process modeling and simulation modeling. The developed integrated salinity intrusion model is useful as a conceptual model to identify the major factors affecting salinity intrusion in coastal areas. It can be also used as a process modeling to identify the roles played by various processes and mechanisms such as seal level change, groundwater recharge, and other changes in catchment hydrological cycle which are in response to climate change and human activities such as land use change. Also this model is useful as a simulation model to take into account both applicability and simplicity of the model and limited data requirement allowing predictions of spatial and temporal variations in coastal systems.
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1600000
Anual Groundwater use (MCM)
1400000 1200000
205
Middle East North Africa South Asia central America South Africa Middle East-scenario 2 North Africa-scenario 2 South Asia-scenario 2 central America-scenario 2 South Africa-scenario 2
1000000 800000 600000 400000 200000 0 2000
2010
2020
2030
2040
2050
2060
2070
2080
2090
2100
Year Fig. 10. The expected fresh groundwater demand over the next century.
Results of this study will be useful for ICM authorities, decision makers, planners and policy makers and to come up with better sustainable coastal development proposals, minimizing adverse effects of natural and anthropogenic activities. Also the identified vulnerable regions in this study should be considered as priority areas for ICM practices to introduce necessary adaptation strategies. Further, the outcome of this study highlights the necessity of a continues process for monitoring, collecting and disseminating necessary data including salinity data, hydrological and hydrogeological data, climate data, population data, water demand and water use data which need for assessing impacts on resources, coastal issues, functional uses and coastal developments as one of the important factors. Therefore, ICM activities at both national and regional level should be implemented in these vulnerable areas. It can be done by introducing community based coastal zone management programs, which involves local residences and local expertise who have first hand experiences in their own areas.
5. Conclusions Global scale quantification of coastal fresh groundwater resources is required to support the sustainable development in coastal communities. In this paper, an effort has been made to evaluate global coastal fresh groundwater resource. In order to obtain a reliable estimate of coastal fresh groundwater availability, we followed the methodology developed by Ranjan et al. [36] based on the sharp interface concept and simplified estimation of groundwater recharge using limited climate data. The proposed methodology will be useful in areas where limited local hydrological data are available. The effect of future climate change on the global groundwater recharge is evaluated using HadCM3 GCM data considering the SRES A2 scenario. The estimated groundwater recharge is then used to estimate the loss of fresh groundwater resources in coastal aquifers due to salinity intrusion. The global scale evaluation shows that Central America, South America, South Africa and Australian regions show a reduction in fresh groundwater resources in the future. Most of the areas in the Asian continent, except South-East Asia will experience a medium reduction. Coastal areas in Africa show higher loss in fresh groundwater resources than any other regions in the world. Fresh groundwater resources in Central part of America and Southern
and Eastern parts of the South America experience a higher reduction in groundwater recharge and related reduction in fresh groundwater resources. To investigate the potential impacts of change in fresh groundwater resources on the water-dependent socioeconomic activities, the population growth is considered as a potential indicator. Combinations of fresh groundwater loss and population are considered to state the vulnerability of future fresh groundwater supply. It indicated that South Asia, especially South India and Bangladesh, Central America, North Africa and Sahara, South Africa and the Middle East countries are highly vulnerable whereas, North Europe, Western part of South America, New Zealand and Japan are less vulnerable to future fresh groundwater supply. South Asia which shows highest population growth will be more stressed in the future. The interesting point is that the present and future water demand show relatively lower values in African continent, which have low population density. This study further proposes that the ability of water resources managers to respond is not only depending on the level of climate change, but also population growth and changes in demands, technology and economic, social, and legislative conditions highlighting the importance of an Integrated Coastal Management practices in coastal areas at both national and regional levels. Beyond this, the study shows how the outcome of this study can be implemented for successful Integrated Coastal Management practices ensuring sustainable coastal developments. Acknowledgments This work was made possible through the support for field observation from Dr. Ahmed Sana, Sultan Qaboos University, Oman and Mr. Mahesh Dilroy, University of Ruhuna, Sri Lanka. The authors would like to acknowledge the generosity of these supports. References [1] Arnell NW. Climate change and global water resources: SRES emissions and socio-economic scenarios. Global Environmental Change 2004;14:31–52. [2] Bear J, Cheng AHD, Sorek S, Ouazar D, Herrera I. Seawater intrusion in coastal aquifers – concepts, methods and practices. Kluwer Academic Publishers; 1999.
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