Quasi‐Saturated Layer: Implications for Estimating Recharge and Groundwater Modeling

Abstract

This study presents an extension of the concept of ''quasi-saturation'' to a quasi-saturated layer, defined as the uppermost dynamic portion of the saturated zone subject to water table fluctuations. Entrapped air here may cause substantial reductions in the hydraulic conductivity (K ) and fillable pore water. Air entrapment is caused by a rising water table, usually as a result of groundwater recharge. The most significant effects of entrapped air are recharge overestimation based on methods that use specific yield (S y ), such as the water table fluctuation method (WTF), and reductions in K values. These effects impact estimation of fluid flow velocities and contaminant migration rates in groundwater. In order to quantify actual groundwater recharge rates and the effects of entrapped air, numerical simulations with the FEFLOW (Version 7.0) groundwater flow model were carried out using a quasi-saturated layer for a pilot area in Rio Claro, Brazil. The calculated recharge rate represented 16% of the average precipitation over an 8-year period, approximately half of estimates using the WTF method. Air entrapment amounted to a fillable porosity of 0.07, significant lower that the value of 0.17 obtained experimentally for S y . Numerical results showed that the entrapped air volume in the quasi-saturated layer can be very significant (0.58 of the air fraction) and hence can significantly affect estimates of groundwater recharge and groundwater flow rates near the water table. Article impact statement: Quasi-saturation due to air entrapment near the water table has major effects on estimated recharge rates and modeling groundwater.

Episodic and Continuous Recharge Estimation from High‐Resolution Well Records

“…Each of the approaches to the WTF recharge estimation method used here has limitations, as with any attempt to quantify complex hydrologic processes occurring in heterogeneous unsaturated and saturated media. Both approaches require correction for the approximation in using specific yield in lieu of a more physically realistic incorporation of air entrapment and fillable porosity (Goncalves et al 2019). Some of this limitation is addressed by the "overshoot" correction of the EMR approach (Nimmo et al 2015).…”

“…Tracer methods focus on longer term recharge processes (e.g., Muller et al 2016) and require ready access for sampling. Unsaturated zone processes can be important for recharge estimation, especially in very shallow water table conditions (Goncalves et al 2019;Crosbie et al 2005) but estimating unsaturated zone parameters is challenging. Some researchers (Chang et al 2018;Liang et al 2017;Park 2012;Park and Parker 2008;Sophocleous 1991) have investigated both saturated zone and unsaturated zone processes, and a new recharge estimation approach is based on GIS analysis of seasonal water table patterns (Gilmore et al 2019).…”

“…Goncalves et al 2019), and using a conventional S Y value from the literature may overestimate resulting recharge because fillable porosity can be much less than S Y . Park (2012) developed a complex method that incorporates a more realistic fillable porosity parameter, but most WTF analyses simply assign an estimated S Y value, and that was the approach used for this study.…”

Stability and Time-Delay Effect of Rainfall-induced Landslide Considering Air Entrapment

“…Test results pointed out that when the air overcomes the critical air pressure and breaks through the saturated soil layer and escapes the atmosphere, the pore air pressure of the unsaturated soil layer decreases. Unfortunately, Wang's method is mainly used in agricultural irrigation (Vereecken et al 2019;Gonçalves et al 2020). A laboratory test suitable for the Masa sandy loam soil from weathered granite rock and a TUAT light clay and soils from volcanic ash considering the influence of air entrapment on hydraulic conductivity was performed (Sakaguchi et al 2005).…”

Groundwater Recharge in the Cerrado Biome, Brazil—A Multi-Method Study at Experimental Watershed Scale

“…When using numerical methods, the solution of the saturated or unsaturated flow equations is obtained from iterative computational numerical processes such as finite differences or finite elements [7,37,[56][57][58][59]. The physical domain is simulated and discretized by means of regular or irregular cells, in which the flow is simulated and propagated three-dimensionally from Darcy equations or Richards' equation variations using input data, calibrated parameters, and initial/known conditions for the simulation (boundary conditions) [31].…”