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.

The Guarani Aquifer System (GAS) is a strategic transboundary aquifer system shared by Brazil, Argentina, Paraguay and Uruguay. This article presents a groundwater flow model to assess the GAS system in terms of regional flow patterns, water balance and overall recharge. Despite the continental dimension of GAS, groundwater recharge is restricted to narrow outcrop zones. An important part is discharged into local watersheds, whereas a minor amount reaches the confined part. A three-dimensional finite element groundwater-flow model of the entire GAS system was constructed to obtain a better understanding of the prevailing flow dynamics and more reliable estimates of groundwater recharge. Our results show that recharge rates effectively contributing to the regional GAS water balance are only approximately 0.6 km³/year (about 4.9 mm/year). These rates are much smaller than previous estimates, including of deep recharge approximations commonly used for water resources management. Higher recharge rates were also not compatible with known ⁸¹Kr groundwater age estimates, as well as with calculated residence times using a particle tracking algorithm.

The determination of soil moisture is a crucial issue for various purposes, including hydrological, climatological, and agricultural studies. Over the past few decades, several distinct remote sensing approaches have been developed. One recent development is the Optical TRApezoil Model (OPTRAM). This approach is similar to the traditional TOTRAM, but it replaces the LST index (thermal band) with the STR index, which is calculated using the SWIR band. Numerous studies have demonstrated the effectiveness of OPTRAM in predicting soil moisture. However, the capability of OPTRAM to estimate soil moisture in urbanized areas has not yet been fully recognized. To address this gap, we conducted tests in the Rio Claro municipality, where land use and occupation vary significantly. By utilizing Sentinel-2 multispectral images, we constructed the NDVI-STR space, estimated soil moisture, and compared it with field measurements. The values of R2, MAE, and RMSE for the OPTRAM-derived soil moisture at urbanized of Rio Claro were 0.92, 0.0196, and 0.1413, respectively. These results demonstrate a high level of representativeness for the soil moisture estimates Furthermore, the freely distributed Sentinel-2 satellite images has a spatial resolution that is well-suited to the dimensions of the target areas in the evaluated scene.