Inverse modelling for freshwater lens in small islands: Kish Island, Persian Gulf

Abstract

A number of challenges including instability, nonconvergence, nonuniqueness, nonoptimality, and lack of a general guideline for inverse modelling have limited the application of automatic calibration by generic inversion codes in solving the saltwater intrusion problem in real‐world cases. A systematic parameter selection procedure for the selection of a small number of independent parameters is applied to a real case of saltwater intrusion in a small island aquifer system in the semiarid region of the Persian Gulf. The methodology aims at reducing parameter nonuniqueness and uncertainty and the time spent on inverse modelling computations. Subsequent to the automatic calibration of the numerical model, uncertainty is analysed by constrained nonlinear optimization of the inverse model. The results define the percentage of uncertainty in the parameter estimation that will maintain the model inside a user‐defined neighbourhood of the best possible calibrated model. Sensitivity maps of both pressure and concentration for the small island aquifer system are also developed. These sensitivity maps indicate higher sensitivity of pressure to model parameters compared with concentration. These sensitivity maps serve as a benchmark for correlation analysis and also assist in the selection of observations points of pressure and concentration in the calibration process. Copyright © 2012 John Wiley & Sons, Ltd.

Sea‐level rise impact on fresh groundwater lenses in two‐layer small islands

“…Simulations using K B / K U < 1 show that FGL thickness is controlled mostly by the properties of the upper layer and is much less influenced by the base‐layer hydraulic properties. Examples of where K B / K U < 1 occur in real systems include the sandy islands of Manukan (Praveena et al ., ) and Kish (Ataie‐Ashtiani et al ., ; Ataie‐Ashtiani et al ., ). Lower K B / K U ratios produce greater FGL thicknesses.…”

“…Kiribati and Tuvalu, Majuru, Marshall, Maldives, Cocos and Cook Islands) and sandy islands (e.g. Manukan, Shelter and Kish Islands) are examples of islands that are classified as having geological layering (Falkland, ; Bricker, ; Bailey et al ., , , ; White and Falkland, ; Ataie‐Ashtiani et al ., ).…”

“…tourism) and climate variability, including climate change impacts of modified recharge rates, sea‐level rise (SLR) and enhanced oceanic events such as storm surges (e.g. Bricker, ; Terry and Falkland, ; Terry and Chui, ; Ataie‐Ashtiani et al ., , ). Consequently, small islands are often considered highly vulnerable to salinization from seawater intrusion.…”

“…For example, Ataie‐Ashtiani et al . (, ) studied the FGL of Kish Island in the Persian Gulf and identified a dual‐aquifer system with hydraulic conductivity values of the upper layer (1 to 100 m/day) greater than those of the base layer (0.01 to 10 m/day).…”

Polynomial chaos expansions for uncertainty propagation and moment independent sensitivity analysis of seawater intrusion simulations

“…2. A review of literature illustrates that SA in groundwater and SWI modeling studies has traditionally relied on local methods (examples in SWI modeling include Sanz and Voss (2006) and Ataie-Ashtiani et al (2013)). This widespread use of local SA method is due to: (1) the high computational cost of global SA methods, (2) the simplicity of local methods, and (3) the capabilities of generic codes such as PEST (Doherty, 2005) and UCODE (Poeter and Hill, 1999) in the estimation of local sensitivity indices.…”