Despite the importance of tropical ecosystems for climate regulation, biodiversity, water and nutrient cycles, only a few Critical Zone Observatories (CZOs) are located in the tropics. Among these, most are in humid climates, while very few data exist for semi-arid and sub-humid climates, due to the difficulty of estimating hydrogeochemical balances in catchments with ephemeral streams. We contribute to fill this gap by presenting a meteorological and hydro-geochemical dataset acquired at the Mule Hole catchment (4.1 km 2 ), a pristine dry deciduous forest located in a biosphere reserve in south India. The dataset consists of time series of variables related to (i) meteorology, including rainfall, air temperature, relative humidity, wind speed and direction, and global radiation, (ii) hydrology, including water level and discharge at the catchment outlet, (iii) hydrogeology, including manual (monthly) and/or automated (from 15 min to hourly) groundwater levels in nine piezometers and (iv) geochemistry, including suspended sediment content in the stream and chemical composition of rainfall (event based), groundwater (monthly sampling) and stream water (storm events, 15 min to hourly frequency with an automatic sampler). The time series extend from 2003 to 2019. Measurement errors are minimized by frequent calibration of sensors and quality checks, both in the field and in the laboratory. Despite these precautions, several data gaps exist, due to occasional access restriction to the site and instrument destruction by wildlife. Results show that large seasonal and interannual variations of climatic conditions were reflected in the large variations of stream flow and groundwater recharge, as well as in water chemical composition. Notably, they reveal a long-term evolution of groundwater storage, suggesting hydrogeological cycles on a decadal scale. This dataset, alone or in combination with other data, has already allowed to better understand water and element
While basaltic volcanic aquifers are well described in the literature, andesitic groundwater systems are less studied. Nevertheless, these aquifers supply a large population in all the subduction areas, where this volcanism mainly occurs. In Indonesia, the growing needs of the population induce an increase of the pressure on such aquifers. We present in this paper the case study of the Arjuno-Welirang, and compare it with a well-known hydrogeological system, the Bromo-Tengger. Based on geological, geomorphological, and water chemistry data, this study highlights the diversity of andesitic history inherent to this kind of volcanism, and its strong implications on the groundwater availability downstream. The aim of this paper is also to show that simple investigations can help building a preliminary conceptual model (PCM) of complex volcanic settings. Such PCM is needed to define further detailed hydrogeological investigations required to set-up aquifer exploitation, management and preservation rules.
<p class="western" lang="en-US">Space and time variability of water content in aquifers are fundamental issues to understand complex interactions taking part in the critical zone, such as land use and irrigated agricultural production. Fundamental parameters on aquifer behavior are commonly monitored through hydrogeological methods, such as piezometric levels and pumping tests in boreholes. Precisions on the water quality and residence time are provided by geochemical analyses of samples collected in surface streams and boreholes. Several studies showed how additional data can be obtained from non-invasive hydrogeophysical methods, that reveal structural heterogeneities of hydrogeological parameters filling the gaps between boreholes.</p> <p class="western" lang="en-US">We carried out a multimethod geophysical survey in the Berambadi experimental catchment (India) which is part of the M-TROPICS CZO (Multiscale TROPIcal CatchmentS Critical Zone Observatory). Two surveys including seismic, electrical, and electromagnetic methods have been repeated for contrasting piezometric levels (high in December 2019, low in May 2022) corresponding to contrasted water contents. We considered time-lapse imaging using electrical resistivity tomography (ERT) and audio-magneto-tellurics (AMT), which sensitivities apply at complementary scales. Changes in the electrical resistivity from ERT shallow cross-sections and deeper jointly inverted ERT-AMT vertical profiles are compared for the two seasons. Results are discussed in terms of water content and porosity of the regolith as well as uncertainties caused by inherent repeatability issues of time-lapse measurements. Final discussion concerns perspectives of combined time-lapse electrical and seismic velocity models to assess the impact of the spatial variability of regolith properties at the catchment scale.</p>
<p>Intense irrigation along with extensive use of fertilizers significantly effects the hydrological and biogeochemical cycles in shallow aquifers. Land use changes associated with human activities are known to be a major controlling factor of the terrestrial silicon cycle, altering silicon fluxes to surface and groundwater. In the present study we determined dissolved silicon concentration (DSi) and &#948;<sup>30</sup>Si of shallow groundwater samples collected from bore wells and piezometers of two watersheds in Southern India under contrasting land use: one intensely cultivated (Berambadi) and one forested (Mule Hole).</p><p>Intense groundwater irrigation in the Berambadi region leads to water table depletion, progressive salinization and occurrence of nitrate hotspots in groundwater. We collected groundwater samples during two periods, during the summer (dry) season in March and during the South-West monsoon season in August from both watersheds. DSi values ranged from &#160;410 &#181;M to 1487 &#181;M, with a lower value during August sampling indicating dilution effects caused by monsoon precipitation. Mule Hole and Berambadi aquifer recharge mostly occurs through surface water percolation or from lateral flow. Groundwater composition thus exhibits seasonal variation depending on precipitation which can be traced using water isotopes (&#948;<sup>18</sup>O and &#948;<sup>2</sup>H). The depleted values in Berambadi groundwater (average &#948;<sup>18</sup>O of -2.99 &#8240; and &#948;<sup>2</sup>H of -15.86 &#8240;) compared to forested watershed in Mule Hole indicate higher contribution from meteoric water likely due to quicker turnover resulting from continuous irrigation.</p><p>Silicon isotope fractionation in natural waters is majorly controlled by soil-water interaction consisting in dissolution of primary minerals and formation of secondary minerals and also from biogenic sources and uptake.&#160; Preliminary results show no significant differences in &#948;<sup>30</sup>Si signatures in groundwater from the two watersheds (1.1 &#177; 0.3 &#8240;) in dry season despite higher and more variable DSi concentration in cultivated watershed (1100 &#177; 260 &#181;M vs. 790 &#177; 120 &#181;M for the forest). Assuming similar discharge, higher DSi concentration in Berambadi during both seasons indicates increased export/mobilization of Si into aquifer when compared to forested landscape.</p><p>We will further refine our understanding of Si biogeochemistry in groundwater and the changes associated with land use by comparing the water and silicon isotopes with the germanium/silicon ratio and major element compositions in comparison with surface water data.</p>
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