Quantifying the impact of evapotranspiration at the aquifer scale via groundwater modelling and MODIS Data

none4noIn shallow alluvial aquifers characterized by coarse sediments, the evapotranspiration rates from groundwater are often not accounted for due to their low capillarity. Nevertheless, this assumption can lead to errors in the hydrogeological balance estimation. To quantify such impacts, a numerical flow model using MODFLOW was set up for the Tronto river alluvial aquifer (Italy). Different estimates of evapotranspiration rates were retrieved from the online Moderate Resolution Imaging Spectroradiometer (MODIS) database and used as input values. The numerical model was calibrated against piezometric heads collected in two snapshots (mid-January 2007 and mid-June 2007) in monitoring wells distributed along the whole alluvial aquifer. The model performance was excellent, with all the statistical parameters indicating very good agreement between calculated and observed heads. The model validation was performed using baseflow data of the Tronto river compared with the calculated aquifer-river exchanges in both of the simulated periods. Then, a series of numerical scenarios indicated that, although the model performance did not vary appreciably regardless of whether it included evapotranspiration from groundwater, the aquifer-river exchanges were influenced significantly. This study showed that evapotranspiration from shallow groundwater accounts for up to 21% of the hydrogeological balance at the aquifer scale and that baseflow observations are pivotal in quantifying the evapotranspiration impact.openColombani N.; Gaiolini M.; Busico G.; Postacchini M.Colombani, N.; Gaiolini, M.; Busico, G.; Postacchini, M

Modelling Shallow Groundwater Evaporation Rates from a Large Tank Experiment

A large tank (1.4 m x 4.0 m x 1.3 m) filled with medium-coarse sand was employed to measure evaporation rates from shallow groundwater at controlled laboratory conditions, to determine drivers and mechanisms. To monitor the groundwater level drawdown 12 piezometers were installed in a semi regular grid and equipped with high precision water level, temperature, and electrical conductivity (EC) probes. In each piezometer, 6 micro sampling ports were installed every 10 cm to capture vertical salinity gradients. Moreover, the soil water content, temperature and EC were measured in the unsaturated zone using TDR probes placed at 5, 20 and 40 cm depth. The monitoring started in February 2020 and lasted for 4 months until the groundwater drawdown became residual. To model the groundwater heads, temperature, and salinity variations SEAWAT 4.0 was employed. The calibrated model was then used to obtain the unknown parameters, such as: maximum evaporation rates (1.5-4.4 mm/d), extinction depth (0.90 m), mineral dissolution (5.0e-9 g/d) and evaporation concentration (0.35 g/L). Despite the drawdown was uniformly distributed, the increase of groundwater salinity was rather uneven, while the temperature increase mimicked the atmospheric temperature increase. The initial groundwater salinity and the small changes in the evaporation rate controlled the evapoconcentration process in groundwater, while the effective porosity was the most sensitive parameter. This study demonstrates that shallow groundwater evaporation from sandy soils can produce homogeneous water table drawdown but appreciable differences in the distribution of groundwater salinity

Groundwater-surface water interaction revealed by meteorological trends and groundwater fluctuations on stream water level|Interazione acque sotterranee-acque superficiali rivelate dalle tendenze meteorologiche e dalle fluttuazioni delle acque sotterranee sul livello di un corso d’acqua

The importance of considering groundwater (GW) and surface water (SW) as a single resource of two interconnected components has rapidly increased during the last decades. To investigate GW-SW interaction in an aquifer system exploited by several pumping wells, an integrated continuous monitoring of the hydrological conditions was carried out. The sub-catchment (14 km2), located in the Aspio basin near Ancona (Central Italy), is drained by a small stream named Betelico, and it is characterised by the presence of an unconfined alluvial aquifer and a semi-confined limestone aquifer. The aim of this study is to evaluate the drivers of stream drying up occurred during the last couple of years. This has been achieved by applying a trend analysis on rainfall, air temperatures, piezometric and stream level, and well pumping rates. Precipitation trends were analysed over a 30-years period through the calculation of the Standard Precipitation Index (SPI) and through heavy rainfall events frequency plots, while the correlation between piezometric stream levels and pumping rate was analysed during the last six years. The groundwater level was compared with the stream baseflow level, highlighting the interconnection between GW-SW over the years. The analysis on the water surplus (WS) trend, together with the rainfall events characterisation, supports the hypothesis of the decrease in recharge rate as the main driver of the stream drying up. This case study stresses the importance of studying GW-SW interactions in a continuously changing climatic context characterised by a decreasing precipitation trend, coupling both the advantages of a robust method like trend analysis on time series and the field continuous monitoring

Modelling Shallow Groundwater Evaporation Rates from a Large Tank Experiment

A large tank (1.4 m x 4.0 m x 1.3 m) filled with medium-coarse sand was employed to measure evaporation rates from shallow groundwater at controlled laboratory conditions, to determine drivers and mechanisms. To monitor the groundwater level drawdown 12 piezometers were installed in a semi regular grid and equipped with high precision water level, temperature, and electrical conductivity (EC) probes. In each piezometer, 6 micro sampling ports were installed every 10 cm to capture vertical salinity gradients. Moreover, the soil water content, temperature and EC were measured in the unsaturated zone using TDR probes placed at 5, 20 and 40 cm depth. The monitoring started in February 2020 and lasted for 4 months until the groundwater drawdown became residual. To model the groundwater heads, temperature, and salinity variations SEAWAT 4.0 was employed. The calibrated model was then used to obtain the unknown parameters, such as: maximum evaporation rates (1.5-4.4 mm/d), extinction depth (0.90 m), mineral dissolution (5.0e-9 g/d) and evaporation concentration (0.35 g/L). Despite the drawdown was uniformly distributed, the increase of groundwater salinity was rather uneven, while the temperature increase mimicked the atmospheric temperature increase. The initial groundwater salinity and the small changes in the evaporation rate controlled the evapoconcentration process in groundwater, while the effective porosity was the most sensitive parameter. This study demonstrates that shallow groundwater evaporation from sandy soils can produce homogeneous water table drawdown but appreciable differences in the distribution of groundwater salinity