Abstract
Abstract. In coastal zones with saline groundwater, fresh groundwater lenses may form due to infiltration of rain water. The thickness of both the lens and the mixing zone, determines fresh water availability for plant growth. Due to recharge variation, the thickness of the lens and the mixing zone are not constant, which may adversely affect agricultural and natural vegetation if saline water reaches the root zone during the growing season. In this paper, we study the response of thin lenses and their mixing zone to variation of recharge. The recharge is varied using sinusoids with a range of amplitudes and frequencies. We vary lens characteristics by varying the Rayleigh number and Mass flux ratio of saline and fresh water, as these dominantly influence the thickness of thin lenses and their mixing zone. Numerical results show a linear relation between the normalised lens volume and the main lens and recharge characteristics, enabling an empirical approximation of the variation of lens thickness. Increase of the recharge amplitude causes increase and the increase of recharge frequency causes a decrease in the variation of lens thickness. The average lens thickness is not significantly influenced by these variations in recharge, contrary to the mixing zone thickness. The mixing zone thickness is compared to that of a Fickian mixing regime. A simple relation between the travelled distance of the centre of the mixing zone position due to variations in recharge and the mixing zone thickness is shown to be valid for both a sinusoidal recharge variation and actual records of daily recharge data. Starting from a step response function, convolution can be used to determine the effect of variable recharge in time. For a sinusoidal curve, we can determine delay of lens movement compared to the recharge curve as well as the lens amplitude, derived from the convolution integral. Together the proposed equations provide us with a first order approximation of lens characteristics using basic lens and recharge parameters without the use of numerical models. This enables the assessment of the vulnerability of any thin fresh water lens on saline, upward seeping groundwater to salinity stress in the root zone.
Highlights
Rain-fed areas may suffer from salinity in the root zone when salt groundwater is found at shallow depths
In such areas, infiltrating rain water is the only source of fresh water, forming and maintaining thin fresh water lenses floating on top of the saline groundwater
Infiltration of rainwater is limited by upward seepage of the saline groundwater when the soil surface is below sea level as found in deltaic areas like the Netherlands (De Louw et al, 2011; Maas, 2007; Oude Essink et al, 2010), and foreseen due to future relative sea level rise in, for example, the deltas of the Nile and Mississippi (Jelgersma, 1996)
Summary
Rain-fed areas may suffer from salinity in the root zone when salt groundwater is found at shallow depths. Saline water is often present at a shallow depth, due to a history of flooding (Vos and Zeiler, 2008; Post, 2004), marine transgressions and sea spray (Stuyfzand and Stuurman, 1994). In such areas, infiltrating rain water is the only source of fresh water, forming and maintaining thin fresh water lenses floating on top of the saline groundwater. The integrity of fresh water lenses is threatened by both the expected sea level rise (Day et al, 1995; Lebbe et al, 2008) and the drainage of soil for agricultural
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