## 2 Study Area  
### Location and Climate  
The study area is located at the outlet of the CSB (08\({}^{\circ}\)38\({}^{\prime}\)-08\({}^{\circ}\)47\({}^{\prime}\) W, 38\({}^{\circ}\)05\({}^{\prime}\)-38\({}^{\circ}\)07\({}^{\prime}\) N) in the southwest coast of Portugal (Figure 1a). The CSB covers an area of 31.5 km\({}^{2}\), has a mean elevation of 230 m a.s.l. (outlet is 2 m a.s.l. on the West and peak elevation is 290 m a.s.l. on the East), and its surface water and groundwater components flow westwards to the SAL [40]. The CSB is the main tributary to the SAL.  
Climate is warm-summer Mediterranean according to the Koppen classification, which means temperate dry summers and rainy winters [11; 41]. Weather data compiled from Sines and Monte Velho meteorological stations (Figure 1a) show that precipitation (P) occurs in three distinctive phases: (1) a predominant rainy phase from October to January with average monthly P from 70 to 90 mm, which represents around 60% of annual P; (2) a moderately rainy phase from February to May with  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Acronym** & **Definition** \\ \hline Aquifer H & Holocene alluvial aquifer \\ CSB & Cascalheira Stream Basin \\ CVMAE & Normalized MAE \\ CVRMSE & Normalized RMSE \\ CVSTD & Normalized STD \\ EC & Electrical conductivity \\ ER & Electrical resistivity \\ ERT & Electrical resistivity tomography \\ GDE & Groundwater-dependent ecosystem \\ GEC & Groundwater electrical conductivity \\ GER & Groundwater electrical resistivity \\ lnNSE & Logarithmic form of NSE \\ MAE & Mean error \\ MASW & Multichannel analysis of surface waves \\ MRE & Mean relative error \\ NAO & North Atlantic Oscillation \\ NSE & Nash–Sutcliffe efficiency coefficient \\ PBIAS & Percent bias \\ \(R^{2}\) & Coefficient of determination \\ RD & Relative difference \\ RMSE & Root-mean-square error \\ RSR & RMSE relative to STD \\ SAL & Santo Andre Lagoon \\ STD & Standard deviation of the measured data \\ SWQM & SAL water quality monitoring \\ VS & Shear-wave velocity \\ WFD & European Water Framework Directive \\ \hline \hline \end{tabular}
\end{table}
Table 1: Description of acronyms used.  
average monthly P from 30 to 50 mm, which means around 30% of annual P; and (3) a dry phase from June to September with average monthly P below 10 mm and occasional rainfall events exceeding 30 mm [40]. Average annual P is around 505 mm with a coefficient of variation of 0.35 over the period 1970-2016. Average monthly temperature (T) varies from 10.6 \({}^{\circ}\)C in January to 19.5 \({}^{\circ}\)C in July. Average minimum and maximum monthly T vary in the 5.8-15.4 \({}^{\circ}\)C and 14.1-24.9 \({}^{\circ}\)C ranges in January and July, respectively [42]. Average annual T is around 15.1 \({}^{\circ}\)C with a coefficient of variation of 0.05 over the period 1973-2007 [40]. Average annual actual evapotranspiration (E) is around 410 mm, thus the E-to-P ratio is about 0.2 [42].  
Figure 1: (**a**) Location of the study area in southwest Portugal, showing the Santo Andre Lagoon (SAL) and its contributing basin, the Casalheira Stream Basin (CSB), and the Sines (SI) and Monte Velho (MV) meteorological stations. (**b**) Geological map (scale 1:50,000) of the CSB according to [43] and direct field observations. (**c**) Land-use units of the CSB according to [42], aerial photographs, and direct field observations. (**d**) Holocene alluvial aquifer (aquifer H) contributing to the SAL, showing geophysical surveys performed along the main groundwater flow path at sites 1 (MASW1, ERT1), 2 (ERT2), and 3 (MASW3, ERT3), aquifer monitoring points (handmade open wells W1 to W6), geotechnical soundings S1 to S3 after [40], stream water EC measurements within the SAL Water Quality Monitoring (SWQM) Programme, and geographical features and sites cited in the text.