The Cunene Province (Southern Angola) is facing recurrent and pluriannual droughts. Surface water supply could be reinforced using the groundwater resources of the multilayered aquifer systems (MAS) hosted in the siliciclastic sediments of the Kalahari Group. The MAS were first identified in the early 2000s in Northern Namibia and recently in the Cunene Province, by studies of the PLANAGEO project based on modern processing and reinterpretation of legacy data from the 1960s and 1970s (electrical resistivity data and deep boreholes). This article presents the results of a time domain electromagnetic (TDEM) survey conducted in the Cunene Province to: (i) contribute to the design of the hydrogeological conceptual model of the transboundary MAS, namely their geometry and extension; (ii) validate the reprocessing of the legacy data; and (iii) guide the future location of boreholes. Results depict the geometry of the sedimentary basin and the characterization of the MAS, with particular emphasis on the intermediate and deep aquifers. The borehole siting, based on the interpretation of the new TDEM data and the legacy data (clay markers in borehole logs), was successful, with a good agreement between estimated and observed horizons of the deep aquifers. However, the presence of clayey layers, a clay-rich matrix in the detrital deposits and saline/brackish groundwater led to uncertainties in the interpretation of the electrical transects. As such, recommendations are made to improve future data collection and mapping of the MAS.

The new, two-way coupled, distributed and transient MARMITES-MODFLOW (MM-MF) model, coupling land surface and soil zone domains with groundwater, is presented. It implements model-based partitioning and sourcing of subsurface evapotranspiration ( ET ss ) as part of spatio-temporal water balance (WB). The partitioning of ET ss involves its separation into evaporation ( E ) and transpiration ( T ), while the sourcing of E and T involves separation of each of the two into soil zone ( E soil and T soil ) and groundwater ( E g and T g ) components. The objective of that development was to understand the system dynamics of a catchment with shallow water table, through spatio-temporal quantification of water fluxes and evaluation of their importance in water balances, focusing on the E g and T g components of ET ss . While the E g is computed using formulation from published study, the T g is obtained through a novel phenomenological function, based on soil moisture availability and transpiration demand driven by climatic conditions. The MM-MF model was applied in the small La Mata catchment (~4.8 km 2 , Salamanca Province, Spain), characterized by semi-arid climate, granitic bedrock, shallow water table and sparse oak woodland. The main catchment characteristics were obtained using remote sensing, non-invasive hydrogeophysics and classical field data acquisition. The MM-MF model was calibrated in transient, using daily data of five hydrological years, between 1 st October 2008 and 30 th September 2013. The WB confirmed dependence of groundwater exfiltration on gross recharge. These two water fluxes, together with infiltration and E soil , constituted the largest subsurface water fluxes. The E g was higher than the T g , which is explained by low tree coverage (~7%). Considering seasonal variability, E g and T g were larger in dry seasons than in wet season, when solar radiation was the largest and soil moisture the most depleted. A relevant observation with respect to tree transpiration was that during dry seasons, the decline of T soil , associated with the decline of soil moisture, was compensated by increase of T g , despite continuously declining water table. However, in dry seasons, T was far below the atmospheric evaporative demand, indicating that the groundwater uptake by the tree species of this study constituted a survival strategy and not a mechanism for continued plant growth. The presented MM-MF model allowed to analyze catchment water dynamics and water balance in detail, accounting separately for impacts of evaporation and transpiration processes on groundwater resources. With its unique capability of partitioning and sourcing of ET ss , the MM-MF model is particularly suitable for mapping groundwater dependent ecosystems, but also for analyzing impacts of climate and land cover changes on groundwater resources.

The Kalahari-Ohangwena transboundary aquifer system, recently identified in Northern Namibia, comprises 3 major aquifers with very different characteristics. The shallowest is discontinuous and with limited reserves, but it has local importance in water supply for the population, since it is easy to reach, and often presents good hydrochemical quality. An intermediate deeper aquifer is characterized by high salinity while the deepest aquifer, also mostly saline, can present zones with fresh water. However, the latter is located at considerable depths and is shaped by the bottom of the basin basement. There hasn't been a systematic hydrogeological data acquisition for decades in this area of Angola, but legacy electrical resistivity data reprocessing from geophysical surveys conducted >50 years ago in the Cunene Province allowed the construction of a quasi-3D geoelectrical model for the Angolan side of KOH aquifer system in the Cuvelai-Etosha basin. This model is based on 482 vertical electrical soundings carried out in 1966-67, using the Schlumberger array, that contribute to confirming the presence of the Kalahari-Ohangwena aquifer system in Angola. The obtained quasi-3D model highlights the geoelectrical features of hard bedrock and is validated with other hydrogeological and geophysical information. The quasi-3D electrical resistivity data is interpreted using selected boreholes and two time-domain electro-magnetics transects carried out in Namibia, in the 2000s. Although both geophysical data acquisitions were >40 years apart, the results show a very good correlation between the deeper aquifer and the aquitard separating the intermediate aquifer from the deeper aquifer either with the results from Namibia or the borehole data. This is a direct result of the lack of alteration in the hydraulic conditions over these decades, without significant anthropogenic activity and negligible extraction from deep wells. Based on this analysis, the original dataset was considered a reliable source and this quasi-3D model was validated. Furthermore, the model can be considered in the future as an important tool for groundwater resources management, as well as a good starting point for further hydrogeological research in the province of Cunene.

Integrated hydrological modelling (IHM) can reliably characterize surface-water/groundwater interactions in complex hydrological systems such as hard-rock systems (HRS), located in water-limited environments (WLE). Such HRS-WLE conditions are represented by Sardon catchment (~80 km 2 ) in Spain, where the MODFLOW 6 modelling environment was tested, applying the following improvements as compared to previous works in that catchment: a new conceptual model, driving forces redefined based on remote sensing data, an unstructured Voronoi grid, and, most importantly, a novel cascade-routing and reinfiltration (CRR) concept. In the standard MODFLOW 6, rejected infiltration and groundwater exfiltration have always been considered as sinks (evaporation). However, in reality, that water can not only evaporate but also reinfiltrate back to the subsurface or move as runoff towards drainage water bodies. The CRR improves surface–unsaturated-zone interactions and also surface-water/groundwater interactions. The standard and new capacities of MODFLOW 6 are presented in the transient model of the Sardon catchment, calibrated using 7 years of daily groundwater heads and streamflows. The results showed: the large spatio-temporal variability of the groundwater fluxes, the substantial role of groundwater exfiltration, the low catchment storage, the fast reaction of the water table and streams to rainfall, and the mosaic character of the net recharge. These characteristics are typical for HRS-WLEs with a shallow water table. MODFLOW 6 has many improvements compared to previous MODFLOW versions, so with the proposed CRR concept (still can be improved), the single-environment MODFLOW 6 has modelling capacity comparable with multienvironment IHMs, while being more flexible and more efficient.

Studies on evapotranspiration partitioning under eddy covariance (EC) towers rarely address the separate effects of transpiration and evaporation on groundwater resources. Such partitioning is important to accurately assess groundwater resources, especially in arid and semi-arid areas.The main objective of this study was to partition (evaluate separately) the evaporation and transpiration components of evapotranspiration, originated either from saturated or unsaturated zone, and estimate their contributions in a semi-arid area characterized by relatively shallow groundwater Table (010 m deep).Evapotranspiration, tree transpiration and subsurface evaporation were estimated with EC tower, using sap flow methods and HYDRUS1D model, respectively. To set up the HYDRUS1D model, soil material properties, soil moisture, soil temperature, soil matric potential and water table depth were measured in the area. The tree transpiration was sourced into groundwater and unsaturated zone components (similar to 0.017 mm d(-1) for both) and accounted for only similar to 6% of the evapotranspiration measured by the EC tower (similar to 0.565 mm d(-1)), due to the low canopy coverage in the study area (7%). The subsurface evaporation fluxes were also sourced into groundwater and unsaturated zone components using the SOURCE package, and their relative relevance in total evapotranspiration was assessed.Subsurface evaporation was the main flux year-round (similar to 0.526 mm d(-1)). During late autumn, winter and early spring time, the unsaturated zone evaporation was dominant, while in dry summer the relevance of groundwater evaporation increased, reaching one third of evapotranspiration, although errors in the water balance closure point still at its possible underestimation. The results show that, in arid and semi-arid areas with sparse vegetation, the often neglected groundwater evaporation is a relevant contribution to evapotranspiration, and that water vapor flow should be taken into account in the calculation of extinction depth. (C) 2017 The Authors. Published by Elsevier B.V.

Geoelectrical and electromagnetic (time and frequency domains) hydrogeophysical methods were applied and jointly interpreted together with auxiliary information such as regional piezometric maps, borehole lithological logs and offshore data. The objective was to retrieve the structure and geometry of the Albufeira-Ribeira de Quarteira coastal aquifer system (Algarve, Portugal) and to upgrade the current hydrogeological conceptual model. The results allowed for the detection of the freshwater–saltwater interface along the coastline and identification of the water-bearing layers and aquitards and their hydraulic relationships. An explanation for the location of the inter- and subtidal fresh groundwater discharge is also presented and a new modeling unit is proposed for groundwater flow modeling. Limitations of the used hydrogeophysical methods are indicated and recommendations are made for follow-up studies.

ABSTRACTMagnetic resonance sounding (MRS) provides quantitative hydrogeological information on hydrostratigraphy and hydraulic parameters of subsurface (e.g., flow and storage property of aquifers) that can be integrated in distributed hydrologic models. The hydraulic parameters are typically obtained by pumping tests. In this study, we propose an MRS integration method based on optimizing MRS estimates of aquifer hydraulic parameters through hydrologic model calibration.The proposed MRS integration method was applied in the 73 km2 Carrizal Catchment in Spain, characterized by a shallow unconfined aquifer with an unknown aquifer bottom. 12 MRS survey results were inverted with Samovar 11.3, schematized and integrated in the transient, distributed, coupled, hydrologic, MARMITES‐MODFLOW model. As the aquifer bottom was unknown, the aquifer was schematized into one unconfined layer of uniform thickness. For that layer, MRS estimators of specific yield and transmissivity/hydraulic conductivity were calculated as weighted averages of the inverted MRS layers. The MRS integration with hydrologic model was carried out by introducing multipliers of specific yield and transmissivity/hydraulic conductivity that were optimized during transient model calibration using 11 time‐series piezometric observation points. The optimized multipliers were 1.0 for specific yield and 3.5*10‐9 for hydraulic conductivity. These multipliers were used, and can be used in future MRS investigations in the Carrizal Catchment (and/or adjacent area with similar hydrogeological conditions), to convert MRS survey results into aquifer hydraulic parameters.The proposed method of MRS data integration in the hydrologic model of Carrizal Catchment not only allowed us to calibrate the model but also to confirm the functional capability of MRS in quantitative groundwater assessment. Most importantly however, it demonstrated that if pumping tests are not available, the use of MRS integrated in distributed coupled hydrological models, or even in standalone groundwater models, provides a valuable aquifer parameterization alternative.

Hard rock aquifers are highly heterogeneous and hydrogeologically complex. To contribute to the design of hydrogeological conceptual models of hard rock aquifers, we propose a multi-techniques methodology based on a downward approach that combines remote sensing (RS), non-invasive hydrogeophysics and hydrogeological field data acquisition. The proposed methodology is particularly suitable for data scarce areas. It was applied in the pilot research area of Sardon catchment (80 km(2)) located west of Salamanca (Spain). The area was selected because of hard-rock hydrogeology, semi-arid climate and scarcity of groundwater resources. The proposed methodology consisted of three main steps. First, we detected the main hydrogeological features at the catchment scale by processing: (i) a high resolution digital terrain model to map lineaments and to outline fault zones; and (ii) high-resolution, multispectral satellite QuickBird and WorldView-2 images to map the out-cropping granite. Second, we characterized at the local scale the hydrogeological features identified at step one with: i) ground penetrating radar (GPR) to assess groundwater table depth complementing the available monitoring network data; ii) 2D electric resistivity tomography (ERT) and frequency domain electromagnetic (FDEM) to retrieve the hydrostratigraphy along selected survey transects; iii) magnetic resonance soundings (MRS) to retrieve the hydrostratigraphy and aquifer parameters at the selected survey sites. In the third step, we drilled 5 boreholes (25 to 48 m deep) and performed slu g tests to verify the hydrogeophysical interpretation and to calibrate the MRS parameters. Finally, we compiled and integrated all acquired data to define the geometry and parameters of the Sardon aquifer at the catchment scale.In line with a general conceptual model of hard rock aquifers, we identified two main hydrostratigraphic layers: a saprolite layer and a fissured layer. Both layers were intersected and drained by fault zones that control the hydrogeology of the catchment. The spatial discontinuities of the saprolite layer were well defined by RS techniques while subsurface geometry and aquifer parameters by hydrogeophysics. The GPR method was able to detect shallow water table at depth between 1 and 3 m b.g.s. The hydrostratigraphy and parameterization of the fissured layer remained uncertain because ERT and FDEM geophysical methods were quantitatively not conclusive while MRS detectability was restricted by low volumetric water content. The proposed multi-technique methodology integrating cost efficient RS, hydrogeophysics and hydrogeological field investigations allowed us to characterize geometrically and parametrically the Sardon hard rock aquifer system, facilitating the design of hydrogeological conceptual model of the area. (C) 2014 Elsevier B.V. All rights reserved.

Precise and non-invasive measurement of groundwater depth is essential to support management of groundwater resources. In that respect, GPR is a promising tool for high resolution, large scale characterization and monitoring of hydrological systems. We applied GPR in a semi-arid catchment (Sardon, Salamanca, Spain) in order to investigate the water table depth in weathered granites. We used a pulse radar with a single 200MHz bowtie antenna combined with a differential GPS and a survey wheel for accurate positioning. Measurements were performed following a series of transects crossing perpendicularly the bed of the Sardon streams, which were dry during the survey period (September 2009). In order to transform the GPR data from time to depth we estimated the soil dielectric constant using frequency domain reflectometry (FDR) or water level depth information from several observation wells. Electrical resistivity tomography (ERT) was applied along the GPR profiles and compared to the GPR results. GPR signals were also simulated using forward modeling (GprMax2D) of several hypothetic configurations of the subsurface. Those techniques helped us to better understand and interpret the GPR data. In general, the shallow water table was sparsely detected in the GPR profiles ranging from ∼1 to ∼3 meters the entire catchment. The results showed a good agreement of ERT and GPR profiles. The comparison of measured and simulated GPR data showed multiple reflections in presence of the saturated fractured granite.

Dry seasons in arid and semiarid areas are characterized by large potential evapotranspiration, which in shallow water table condition results in substantial evapotranspiration (ET). The ET consists of two different processes, plant transpiration and bare soil evaporation, each sourcing water either from saturated or unsaturated zone. The partitioning and sourcing of ET into these four flux components is important in hydrological modelling and water management. We attempted partitioning of dry season ET in the granitic Sardon catchment in Spain at the footprint of an eddy covariance tower. From that tower we obtained nearly continuous estimate of ET. The transpiration of the trees (Quercus pyrenaica and Quercus ilex species) occurring in the footprint of the tower was measured by sap flow sensors while the sourcing of that transpiration was identified by stable isotope analysis of groundwater, moisture of unsaturated zone and stem sap. The evaporation of the bare soil areas, in-between the tree canopies, and the partitioning of that evaporation into saturated and unsaturated zone sourcing components was modelled on the base of thermal, soil water potential and soil moisture profiles installed at the bare soil locations. The partitioning results of tree transpiration and bare soil evaporation were finally compared with the tower estimate of ET. Total ET for dry season was 0.6 mm/d. Evaporation was the most relevant term in the dry season water balance, representing 84% of total ET, while transpiration was low, representing 6 % of total ET. The direct evaporation from groundwater was relevant, representing 37% of total ET.

Topsoil thickness is a critical input in hydrological modeling because it controls, in conjunction with soil hydraulic properties, the partitioning of water fluxes between the atmosphere and the subsurface. To parameterize a distributed hydrological model that computes groundwater recharge, we developed a data-integration method to predict the clayey topsoil thickness (CTT) that we applied in a small catchment in Portugal (similar to 19 km(2)). The prediction method is based on the integration of: (i) invasive sampling used as a CTT reference dataset (61 invasive measurements); (ii) surface geophysics applied to complement the time-consuming invasive sampling; (iii) remote sensing (RS) image processing (high resolution Quick Bird image, aerial photographs and ASTER GDEM) used to derive soils classes and terrain parameters; (iv) geostatistical mixed linear model (MLM) applied to integrate the CTT variability at the catchment scale using geophysical and RS derived auxiliary variables. The selection of the appropriate statistical model derived from the MLM was based on the verification of model assumptions using diagnostic tools.We first converted 436 Geonics (TM) EM-31 field measurements of soil apparent electrical conductivity (ECa) into CTT. This was achieved by building MLM based calibration models that integrated 25 invasive CTT measurements paired with corresponding ECa, and RS-derived auxiliary variables. Next, we predicted the CTT at the catchment scale by applying the MLM approach and integrating the RS-derived auxiliary variables with: (i) the 436 CTT values derived from surface geophysical dataset; (ii) the 61 CTT values from the reference invasive dataset. The two maps had similar CTT patterns which depicted the spatial variability of the CTT over the geomorphologic catchment features. The prediction map derived from the geophysical dataset resulted in slightly lower CTT values than the reference map (median of 0.87 m against 1.11 m) and a comparable accuracy (RMSE of 0.76 m against 0.88 m). As these differences will be minimized during the calibration process of the hydrological model, the presented methodology is considered suitable for hydrological and environmental studies, in which catchments often need to be investigated over large areas. (C) 2011 Elsevier B.V. All rights reserved.

We present a coupled model approach to improve the water balance at the catchment scale. The model is composed of a land surface and unsaturated zone model (MARMITES) coupled with the groundwater model MODFLOW. We aim to quantify spatio;temporally the water fluxes of the unsaturated and saturated zones and to assess the impact of typically underestimated water fluxes, such as groundwater transpiration and groundwater evaporation, on groundwater resources.

Unaltered hard rocks are hydrogeologically characterized by low primary porosity and permeability. Productive aquifers can however be supported by fractured and/or weathered hard rocks. In this type of composite aquifers, the upper weathered layers have typically a storage function while the underlying fissured layers have a transmissive function. Such aquifer sequence is usually highly heterogeneous because the weathered and fractured zones are controlled by various factors such as mineralogy and texture of lithologies, regional and local tectonics, paleoclimate and interaction between these factors.