## 2 Study Area  
### Location and Climate  
The MDQ is located in northern Ecuador at 0\({}^{\circ}\)14\({}^{\prime}\) N to 0\({}^{\circ}\)35\({}^{\prime}\) S and 78\({}^{\circ}\)10\({}^{\prime}\) W to 78\({}^{\circ}\)56\({}^{\prime}\) W (Figure 1), covers a surface area of 4320 km\({}^{2}\), and includes three main geomorphological sectors (Figure 1a). The 40-km-wide northern elongated Inter-Andean Valley (IAV) has an elevation ranging from 2100 to 3500 m a.s.l. and lies between the Western Andean Cordillera (WAC) (peak elevation 4776 m a.s.l., at Guagua-Pichincha Volcano) and Eastern Andean Cordillera (EAC) (peak elevation 4873 m a.s.l., at Sinclolagua Volcano) (Figure 1b). The Guayllamba River flows north through the IAV and is the main surface watercourse (Figure 1b).  
The MDQ exhibits a neo-tropical high-mountain climate, determined by the El Nino-Southern Oscillation and the Humboldt Current, and a steep orograph [8,19,20]. Consequently, it has a marked distribution of biorones and ecosystems at different elevations including tropical mountain rainforests in the lowlands, wet alpine meadows (locally named _paramo_) in mid-slope areas, dry and cold scrublands in the highlands, and permanent snow covers at volcanoes' peaks [4,6,21].  
Precipitation (P) follows a decreasing gradient from east to west, controlled by incoming Atlantic cloud fronts and elevation [22,23] and exhibits a positive gradient from low-lying areas to around 3500 m a.s.l. and a negative gradient above that elevation [19,20]. Most P occurs in February-May. In contrast, the lowest amount is recorded in July-September [23]. Annual mean P is around 1100 mm, with a coefficient of variation of 0.21 measured over the period 2003-2019. Annual mean temperature (T) is around 7.5 \({}^{\circ}\)C, with daily minimums in June-September and maximums in February-April. The decreasing T gradient with elevation is around 0.6-0.7 \({}^{\circ}\)C per 100 m elevation [21,22]. Insolation increases from low-lying areas to summits due to cloudiness induced by the Foehn effect in valleys from incoming Atlantic cloud fronts [7]. Annual mean potential evapotranspiration is around 1000 mm.  
Figure 1: (**a**) Location of the study area. (**b**) The MDQ displayed using the 30 m-resolution Digital Elevation Model from Shuttle Radar Topography Mission ([[http://srtm.csi.cgiar.org/srtmdata/](http://srtm.csi.cgiar.org/srtmdata/)]([http://srtm.csi.cgiar.org/srtmdata/](http://srtm.csi.cgiar.org/srtmdata/)) (accessed on 11 February 2021), showing the  
location and typology of the compiled 23 geophysical prospecting surveys and additional geographical features cited in the text: (1) MDQ, (2) Quito city, (3) Quito-Machachi Hydrogeological Unit [24; 25; 26], (4) Cayambe-Ruminahui Hydrogeological Unit [24; 25; 26], and (5) the water transfer system used by EPMAPS to supply the MDQ [8]. ERT--Electrical Resistivity Tomography; VES--Vertical Electrical Sounding; REMI--Refraction Microthermor; L-MTS--Low-frequency Magnetotelluric Sounding; VLF-EM--Very low-frequency Electromagnetic Method. (c) Hydrogeological map (scale 1:250,000) of the MDQ, updated and improved from [24; 25; 26], showing regional piezometry [24; 25; 26], and the location of the hydrogeological cross-section A-A\({}^{\prime}\) and a synthetic stratigraphic column of the IAV sector [18; 24; 25; 26].