Impact of Climate Change on Groundwater Resources C. P. Kumar Scientist ‘F’ National Institute of Hydrology Roorkee – 247667 (India)
Presentation overviewGroundwater in Hydrologic CycleWhat is Climate Change?Hydrological Impact of Climate ChangeImpact of Climate Change on GroundwaterClimate Change Scenario for Groundwater inIndiaStatus of Research StudiesMethodology to Assess the Impact of ClimateChange on Groundwater ResourcesConcluding Remarks
Types of Terrestrial Water Surface Water Soil Moisture Ground water
Pores Full of Combination of Air and Water Unsaturated Zone / Zone of Aeration / Vadose (Soil Water) Zone of Saturation (Ground water)Pores Full Completely with Water
Groundwater Important source of clean waterMore abundant than Surface Water Baseflow Linked to SW systems Sustains flows in streams
Why include groundwater in climate change studies?Although groundwater accounts for smallpercentage of Earth’s total water,groundwater comprises approximately thirtypercent of the Earth’s freshwater.Groundwater is the primary source of waterfor over 1.5 billion people worldwide.Depletion of groundwater may be the mostsubstantial threat to irrigated agriculture,exceeding even the buildup of salts in soils.(Alley, et al., 2002)
Groundwater• An important component of water resource systems.• Extracted from aquifers through pumping wells and supplied for domestic use, industry and agriculture.• With increased withdrawal of groundwater, the quality of groundwater has been continuously deteriorating.• Water can be injected into aquifers for storage and/or quality control purposes.
Groundwater contamination by:• Hazardous industrial wastes• Leachate from landfills• Agricultural activities such as the use of fertilizers and pesticides Management of a groundwater system, means making such decisions as:• The total volume that may be withdrawn annually from the aquifer.• The location of pumping and artificial recharge wells, and their rates.• Decisions related to groundwater quality.
What is Climate Change?IPCC usage: •Any change in climate over time, whether due to natural variability or from human activity.Alternate: •Change of climate, attributed directly or indirectly to human activity, that •Alters composition of global atmosphere and •Is in addition to natural climate variability observed over comparable time periods
GLOBAL CIRCULATION MODELSFormulated to simulate climate sensitivity to increased concentrations of greenhouse gases such as carbon dioxide, methane and nitrous oxide.
Global Climate Models (GCMs)• Divide the globe into large size grids• Physical equations• Lots of computing• Predict the climatological variables 16
Global Climate Modelstranslated to local impactsFive step process outlined by Glieck & Frederick (1999)• Look at several Global Climate Models (GCMs) and look for consensus & ranges• Downscale to level needed (statistical and dynamical methods)• Apply impact ranges to hydrologic modeling• Develop systems simulation models• Assessment of the results (historic and GCMs) at representative time frames
Overview of the Climate Change Problem Source: IPCC Synthesis Report 2001
Hydrological Impact of Climate Change According to the Technical Paper VI (2008) of Intergovernmental Panel onClimate Change (IPCC), the best-estimate in global surface temperature from1906 to 2005 is a warming of 0.74°C (likely range 0.56 to 0.92°C), with a morerapid warming trend over the past 50 years. Temperature increases also affect the hydrologic cycle by directly increasingevaporation of available surface water and vegetation transpiration. Consequently, these changes can influence precipitation amounts, timings andintensity rates, and indirectly impact the flux and storage of water in surface andsubsurface reservoirs (i.e., lakes, soil moisture, groundwater). In addition, there may be other associated impacts, such as sea water intrusion,water quality deterioration, potable water shortage, etc.
While climate change affects surface water resources directly through changes inthe major long-term climate variables such as air temperature, precipitation, andevapotranspiration, the relationship between the changing climate variables andgroundwater is more complicated and poorly understood. The greater variability in rainfall could mean more frequent and prolonged periodsof high or low groundwater levels, and saline intrusion in coastal aquifers due to sealevel rise and resource reduction. Groundwater resources are related to climate change through the directinteraction with surface water resources, such as lakes and rivers, and indirectlythrough the recharge process. The direct effect of climate change on groundwater resources depends upon thechange in the volume and distribution of groundwater recharge.
Therefore, quantifying the impact of climate change on groundwater resourcesrequires not only reliable forecasting of changes in the major climatic variables, butalso accurate estimation of groundwater recharge. A number of Global Climate Models (GCM) are available for understandingclimate and projecting climate change. There is a need to downscale outputs of GCM on a basin scale and couple themwith relevant hydrological models considering all components of the hydrologicalcycle. Output of these coupled models such as quantification of the groundwaterrecharge will help in taking appropriate adaptation strategies due to the impact ofclimate change.
Impact of Climate Change on Groundwater It is important to consider the potential impacts of climate change on groundwatersystems. Although the most noticeable impacts of climate change could be fluctuations insurface water levels and quality, the greatest concern of water managers andgovernment is the potential decrease and quality of groundwater supplies, as it isthe main available potable water supply source for human consumption andirrigation of agriculture produce worldwide. Because groundwater aquifers are recharged mainly by precipitation or throughinteraction with surface water bodies, the direct influence of climate change onprecipitation and surface water ultimately affects groundwater systems. As part of the hydrologic cycle, it can be anticipated that groundwater systems willbe affected by changes in recharge (which encompasses changes in precipitationand evapotranspiration), potentially by changes in the nature of the interactionsbetween the groundwater and surface water systems, and changes in use related toirrigation.
(a) Soil Moisture The amount of water stored in the soil is fundamentally important to agricultureand has an influence on the rate of actual evaporation, groundwater recharge,and generation of runoff. Soil moisture contents are directly simulated by global climate models, albeitover a very coarse spatial resolution, and outputs from these models give anindication of possible directions of change. The local effects of climate change on soil moisture, however, will vary not onlywith the degree of climate change but also with soil characteristics. The water-holding capacity of soil will affect possible changes in soil moisture deficits; thelower the capacity, the greater the sensitivity to climate change. For example,sand has lower field capacity than clay. Climate change may also affect soil characteristics, perhaps through changesin cracking, which in turn may affect soil moisture storage properties.
(b) Groundwater Recharge Groundwater is the major source of water across much of the world, particularlyin rural areas in arid and semi-arid regions, but there has been very littleresearch on the potential effects of climate change. Aquifers generally are replenished by effective rainfall, rivers, and lakes. Thiswater may reach the aquifer rapidly, through macro-pores or fissures, or moreslowly by infiltrating through soils and permeable rocks overlying the aquifer. A change in the amount of effective rainfall will alter recharge, but so will achange in the duration of the recharge season. Increased winter rainfall, asprojected under most scenarios for mid-latitudes, generally is likely to result inincreased groundwater recharge. However, higher evaporation may mean that soil deficits persist for longer andcommence earlier, offsetting an increase in total effective rainfall.
Various types of aquifers will be recharged differently. The main types areunconfined and confined aquifers. An unconfined aquifer is recharged directly by local rainfall, rivers, and lakes,and the rate of recharge will be influenced by the permeability of overlyingrocks and soils. Unconfined aquifers are sensitive to local climate change, abstraction, andseawater intrusion. However, quantification of recharge is complicated by thecharacteristics of the aquifers themselves as well as overlying rocks and soils. A confined aquifer, on the other hand, is characterized by an overlying bedthat is impermeable, and local rainfall does not influence the aquifer. It isnormally recharged from lakes, rivers, and rainfall that may occur at distancesranging from a few kilometers to thousands of kilometers.
Several approaches can be used to estimate recharge based on surface water,unsaturated zone and groundwater data. Among these approaches, numericalmodelling is the only tool that can predict recharge. Modelling is also extremely useful for identifying the relative importance ofdifferent controls on recharge, provided that the model realistically accounts for allthe processes involved. However, the accuracy of recharge estimates depends largely on the availability ofhigh quality hydrogeologic and climatic data. The medium through which recharge takes place often is poorly known and veryheterogeneous, again challenging recharge modelling. Determining the potential impact of climate change on groundwater resources, inparticular, is difficult due to the complexity of the recharge process, and thevariation of recharge within and between different climatic zones. In general, there is a need to intensify research on modeling techniques, aquifercharacteristics, recharge rates, and seawater intrusion, as well as monitoring ofgroundwater abstractions.
(c) Coastal Aquifers Coastal aquifers are important sources of freshwater. However, salinityintrusion can be a major problem in these zones. Changes in climatic variablescan significantly alter groundwater recharge rates for major aquifer systems andthus affect the availability of fresh groundwater. Sea-level rise will cause saline intrusion into coastal aquifers, with the amountof intrusion depending on local groundwater gradients. For many small island states, seawater intrusion into freshwater aquifers hasbeen observed as a result of overpumping of aquifers. Any sea-level rise wouldworsen the situation.
Sea Level Rise: A Global Concern • Mean sea level has risen globally by 25 cm (1 - 2.5 mm/yr) on average over the last century (IPCC, 2001). • Global warming is also occurring, causing temperatures to gradually increase worldwide. • Global warming is exacerbating sea level rise, due to the increase in glacial melt and thermal expansion of the water which results from temperature change. Based on IPCC estimates, sea level could rise by another 50 cm (5 mm/yr) by 2100. • Increased sea levels will vastly affect coastal regions. • Increased sea levels will lead to increased frequency of severe floods.
Source: Intergovernmental Panel on Climate Change (2001) Future sea level rise = 1.990 - 2.100 meters Even if greenhouse gas concentrations are stabilised, sea level will continue to rise for hundreds of years. After 500 years, sea level rise from the thermal expansion of oceans may have reached only half its eventual level, glacier retreat will continue and ice sheets will continue to react to climate change. Thermal expansion and land ice changes were calculated using a simple climate model calibrated separately for each of seven air/ocean global climate models (AOGCMs). Light shading shows range of all models (in the next slide) -
A link between rising sea level and changes in the water balance is suggestedby a general description of the hydraulics of groundwater discharge at the coast. The shape of the water table and the depth to the freshwater/saline interfaceare controlled by the difference in density between freshwater and salt water, therate of freshwater discharge and the hydraulic properties of the aquifer. To assess the impacts of potential climate change on fresh groundwaterresources, we should focus on changes in groundwater recharge and impact ofsea level rise on the loss of fresh groundwater resources in water resourcesstressed coastal aquifers.
Climate Change Scenario for Groundwater in India Impact of climate change on the groundwater regime is expected to be severe. Due to rampant drawing of the subsurface water, the water table in many regionsof the country has dropped significantly in the recent years resulting in threat togroundwater sustainability. The most optimistic assumption suggests that an average drop in groundwaterlevel by one metre would increase India’s total carbon emissions by over 1%,because withdrawal of the same amount of water from deeper depths will increasefuel consumption. Climate change is likely to affect groundwater due to changes in precipitation andevapotranspiration. Rising sea levels may lead to increased saline intrusion into coastal and islandaquifers, while increased frequency and severity of floods may affect groundwaterquality in alluvial aquifers. Sea-level rise leads to intrusion of saline water into the fresh groundwater incoastal aquifers and thus adversely affects groundwater resources.
For two small and flat coral islands at the coast of India, the thickness offreshwater lens was computed to decrease from 25 m to 10 m and from 36 m to 28m, respectively, for a sea level rise of only 0.1 m (Mall et al., 2006). Agricultural demand, particularly for irrigation water, which is a major share oftotal water demand of the country, is considered more sensitive to climate change.A change in field-level climate may alter the need and timing of irrigation. Increaseddryness may lead to increased demand, but demand could be reduced if soilmoisture content rises at critical times of the year. It is projected that most irrigated areas in India would require more water around2025 and global net irrigation requirements would increase relative to the situationwithout climate change by 3.5–5% by 2025 and 6–8% by 2075 (Mall et al., 2006). In India, roughly 52% of irrigation consumption across the country is extractedfrom groundwater; therefore, it can be an alarming situation with decline ingroundwater and increase in irrigation requirements due to climate change (Mall etal., 2006). In a number of studies, it is projected that increasing temperature and decline inrainfall may reduce net recharge and affect groundwater levels. However, littlework has been done on hydrological impacts of possible climate change for Indianregions/basins.
Status of Research Studies There have been many studies relating the effect of climate changes on surface waterbodies. However, very little research exists on the potential effects of climate change ongroundwater. Available studies show that groundwater recharge and discharge conditions are reflection ofthe precipitation regime, climatic variables, landscape characteristics and human impacts suchas agricultural drainage and flow regulation. Hence, predicting the behavior of recharge and discharge conditions under future climaticand other changes is of great importance for integrated water management. Previous studies have typically coupled climate change scenarios with hydrological models,and have generally investigated the impact of climate change on water resources in differentareas. The scientific understanding of an aquifer’s response to climate change has been studied inseveral locations within the past decade. These studies link atmospheric models tounsaturated soil models, which, in some cases, were further linked into a groundwater model. The groundwater models used were calibrated to current groundwater conditions andstressed under different predicted climate change scenarios. Some of the recent studies on impact of climate change on groundwater resources arementioned here.
Bouraoui et al. (1999) Presented a general approach to evaluate the effect ofpotential climate changes on groundwater resources. A general methodology is proposed in order to disaggregateoutputs of large-scale models and thus to make informationdirectly usable by hydrologic models. Two important hydrological variables: rainfall and potentialevapotranspiration are generated and then used by coupling witha physically based hydrological model to estimate the effects ofclimate changes on groundwater recharge and soil moisture inthe root zone.
Sherif and Singh (1999) Investigated the possible effect of climate change on sea waterintrusion in coastal aquifers. Using two coastal aquifers, one in Egypt and the other in India,this study investigated the effect of likely climate change on seawater intrusion. Under conditions of climate change, the sea water levels will risewhich will impose additional saline water heads at the sea side andtherefore more sea water intrusion is anticipated. A 50 cm rise in the Mediterranean sea level will cause additionalintrusion of 9.0 km in the Nile Delta aquifer. The same rise in water level in the Bay of Bengal will cause anadditional intrusion of 0.4 km.
Ghosh Bobba (2002) Analysed the effects of human activities and sea-level changes on thespatial and temporal behaviour of the coupled mechanism of salt-waterand freshwater flow through the Godavari Delta of India. The density driven salt-water intrusion process was simulated with theuse of SUTRA (Saturated-Unsaturated TRAnsport) model. The results indicate that a considerable advance in seawater intrusioncan be expected in the coastal aquifer if current rates of groundwaterexploitation continue and an important part of the freshwater from theriver is diverted for irrigation, industrial and domestic purposes.
Allen et al. (2004) Used the Grand Forks aquifer, located in south-central BritishColumbia, Canada as a case study area for modeling the sensitivity ofan aquifer to changes in recharge and river stage consistent withprojected climate-change scenarios for the region. Results suggested that variations in recharge to the aquifer under thedifferent climate-change scenarios, modeled under steady-stateconditions, have a much smaller impact on the groundwater systemthan changes in river-stage elevation of the Kettle and Granby Rivers,which flow through the valley.
Brouyere et al. (2004) Developed an integrated hydrological model (MOHISE) in order tostudy the impact of climate change on the hydrological cycle inrepresentative water basins in Belgium. This model considers most hydrological processes in a physicallyconsistent way, more particularly groundwater flows which are modelledusing a spatially distributed, finite-element approach. The groundwater model is described in detail and results arediscussed in terms of climate change impact on the evolution ofgroundwater levels and groundwater reserves. Most tested scenarios predicted a decrease in groundwater levels inrelation to variations in climatic conditions.
Holman (2006) Described an integrated approach to assess the regional impacts ofclimate and socio-economic change on groundwater recharge from EastAnglia, UK. Important sources of uncertainty and shortcomings in rechargeestimation were discussed in the light of the results. Changes to soil properties are occurring over a range of time scales,such that the soils of the future may not have the same infiltrationproperties as existing soils. The potential implications involved in assuming unchanging soilproperties were described.
Mall et al. (2006) Examined the potential for sustainable development of surfacewater and groundwater resources within the constraints imposed byclimate change and future research needs in India. He concluded that the Indian region is highly sensitive to climatechange. The National Environment Policy (2004) also advocated thatanthropogenic climate changes have severe adverse impacts onIndia’s precipitation patterns, ecosystems, agricultural potential,forests, water resources, coastal and marine resources. Large-scale planning would be clearly required for adaptationmeasures for climate change impacts, if catastrophic human misery isto be avoided.
Ranjan et al. (2006) Evaluated the impacts of climate change on fresh groundwaterresources specifically salinity intrusion in five selected waterresources stressed coastal aquifers. The annual fresh groundwater resources losses indicated anincreasing long-term trend in all stressed areas, except in thenorthern Africa/Sahara region. They also found that precipitation and temperature individuallydid not show good correlations with fresh groundwater loss. They also discussed the impacts of loss of fresh groundwaterresources on socio-economic activities, mainly population growthand per capita fresh groundwater resources.
Scibek and Allen (2006) Developed a methodology for linking climate models andgroundwater models to investigate future impacts of climate changeon groundwater resources. Climate change scenarios from the Canadian Global CoupledModel 1 (CGCM1) model runs were downscaled to local conditionsusing Statistical Downscaling Model (SDSM). The recharge model (HELP) simulated the direct recharge to theaquifer from infiltration of precipitation. MODFLOW was then used to simulate four climate scenarios in 1-year runs (1961–1999, 2010–2039, 2040–2069, and 2070-2099) andcompare groundwater levels to present. The predicted future climate for the Grand Forks area (Canada)from the downscaled CGCM1 model will result in more recharge tothe unconfined aquifer from spring to the summer season. However,the overall effect of recharge on the water balance is small becauseof dominant river-aquifer interactions and river water recharge.
Woldeamlak et al. (2007) Modeled the effects of climate change on the groundwater systems inthe Grote-Nete catchment, Belgium. Seasonal and annual water balance components includinggroundwater recharge were simulated using the WetSpass model, whilemean annual groundwater elevations and discharge were simulatedwith a steady-state MODFLOW groundwater model. Results show that average annual groundwater levels drop by 50 cm.
Hsu et al. (2007) Adopted a numerical modeling approach to investigate the responseof the groundwater system to climate variability to effectively managethe groundwater resources of the Pingtung Plain in southwesternTaiwan. A hydrogeological model (MODFLOW SURFACT) was constructedbased on the information from geology, hydrogeology, andgeochemistry. The modeling result shows decrease of available groundwater in thestress of climate change, and the enlargement of the low-groundwater-level area on the coast signals the deterioration of water quantity andquality in the future.
Toews (2007) Modeled the impacts of future predicted climate change ongroundwater recharge for the arid to semi-arid south Okanagan region,British Columbia. Climate change effects on recharge were investigated usingstochastically-generated climate from three GCMs. Spatial recharge was modelled using available soil and climate datawith the HELP 3.80D hydrology model. A transient MODFLOW groundwater model simulated rise of watertable in future time periods, which is largely driven by irrigationapplication increases.
Concluding Remarks on the Research Studies These studies are still at infancy and more data, interms of field information, are to be generated. This will also facilitate appropriate validation of thesimulation for the present scenarios. However, it is clear that the global warming threat isreal and the consequences of climate changephenomena are many and alarming.
Methodology to Assess the Impact of Climate Change onGroundwater ResourcesThe methodology consists of three main steps. To begin with, climate scenarios can be formulated for the future yearssuch as 2050 and 2100. Secondly, based on these scenarios and present situation, seasonal andannual recharge are simulated with the UnSat Suite (HELP module forrecharge) or WetSpass model. Finally, the annual recharge outputs from UnSat Suite or WetSpass modelare used to simulate groundwater system conditions using steady-stategroundwater model setups, such as MODFLOW, for the present conditionand for the future years.
The influence of climate changes on goundwater levels and salinity, due to: a. Sea level rise b. Changes in precipitation and temperatureMethodology1. Develop and calibrate a density-dependent numerical groundwater flow model that matches hydraulic head and concentration distributions in the aquifer.2. Estimate changes in sea level, temperature and precipitation downscaled from GCM outputs.3. Estimate changes in groundwater recharge.4. Apply sea level rise and changes in recharge to numerical groundwater model and make predictions for changes in groundwater levels and salinity distribution.
The main tasks that are involved in such a study are: Describe hydrogeology of the study area. Analyze climate data from weather stations and modelled GCM, and build future predicted climate change datasets with temperature, precipitation and solar radiation variables (downscaled to the study area). Define methodology for estimating changes to groundwater recharge under both current climate conditions and for the range of climate- change scenarios for the study area. Use of a computer code (such as UnSat Suite or WetSpass) to estimate groundwater recharge based on available precipitation and temperature records and anticipated changes to these parameters.
Quantify the spatially distributed recharge rates using the climate data andspatial soil survey data.Development and calibration of a three-dimensional regional-scalegroundwater flow model (such as Visual MODFLOW). Simulate groundwater levels using each recharge data set and evaluatethe changes in groundwater levels through time.Undertake sensitivity analysis of the groundwater flow model.
A typical flow chart for various aspects of such a study is given below. The figure shows the connection fromthe climate analysis, to recharge simulation, and finally to a groundwater model. Recharge is applied to athree-dimensional groundwater flow model, which is calibrated to historical water levels. Transientsimulations are undertaken to investigate the temporal response of the aquifer system to historic and futureclimate periods.
Concluding Remarks Although climate change has been widely recognized, research onthe impacts of climate change on the groundwater system is relativelylimited. The impact of future climatic change may be felt more severely indeveloping countries such as India, whose economy is largelydependent on agriculture and is already under stress due to currentpopulation increase and associated demands for energy, freshwaterand food. If the likely consequences of future changes of groundwaterrecharge, resulting from both climate and socio-economic change, areto be assessed, hydrogeologists must increasingly work withresearchers from other disciplines, such as socio-economists,agricultural modelers and soil scientists.