Impacts of climate change on groundwater levels in coastal aquifers

Abstract

Groundwater in coastal aquifers is affected by many environmental factors including rainfall, evapotranspiration, sea level, coastal erosion, aquifer recharge and land use. The key factors that affect groundwater levels in two target sites have been evaluated up to the year 2100: (1) a coastal sand dune system in North West England and (2) confIned aquifers at Samut Prakan near Bangkok in Thailand. A water balance model was constructed and tested against observed well levels. The model comprised a daily simulation of rainfall, soil moisture defIcit and recharge based on a potential evapotranspiration sub model using the Penman-Montieth equation and crop/soil descriptors. Groundwater flow is simulated with the Darcy's law assumption for unconfmed flow. Residual water is assumed to contribute to groundwater storage and this storage is converted into modelled water table levels based on dune porosity functions. The model was calibrated by parameter estimation against observed data for 1972-2007 in North West England but limited data availability restricted the calibration in Thailand. In the North West England study, UKCIP02 "Medium-High" climate change scenarios were used in the model on a daily basis between 2008-2100. The modelled groundwater levels were found to be highly dependent not only on the amount of rainfall but when it falls and the size of each rain storm event. A Monte Carlo modelling approach enabled the model to be run with 1,000 sequences per simulation of rainfall patterns along with variations in estimates of sea-level rise, climate change (changes in precipitation and temperature), land use change, and erosion scenarios. This approach was able to deal with uncertainties in these factors effectively and it is strongly linked with the climate inputs such as rainfall and evapotranspiration which are expected to change in the future. The most important parameters that affected the modelled groundwater levels at both sites were found temperature, actual evapotranspiration, tree coverage and coastal erosion (which will become more signifIcant as erosion progresses). The models were less sensitive to uncertainties in system properties such as permeability. Sea-level rise and crop variety had the smallest impact on coastal groundwater recharge. The impact of sea-level rise was relatively unimportant over the short time period studied (100 years). The relative importance of these factors will probably change further into the future. to be rainfall, followed by change in Findings from the North West England study are that on average, hotter drier summers will cause a reduction in total evapotranspiration due to soil moisture defIcit and that wetter winters will cause an increased range of amplitude of water table levels. The overall trend is that groundwater levels may fall by about 1m in the next 100 years, causing the fragile ecosystems in the dune floors to dry out completely. The model also showed that for areas planted with pine trees, increased interception losses may cause the water table to fall by up to 1.5m, with prolonged periods of low water table levels. This will have the effect of drying out the inland slack floors and reducing the biodiversity of these areas. In Thailand, the expected change in shallow groundwater levels is that the water table will fall as less recharge arrives from rainfall and decreased river flows in the dry season. Runoff could decrease more than 50 per cent in certain areas of the lower basin. This will lead to water shortage in the central plain, especially during the dry season.

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