Hydraulic Conductivity Map Research Articles

Geospatial based groundwater resources susceptibility and contamination risk mapping for Yeşilköy aquifer, Cyprus

In rural areas where the goals of sustainable agriculture are intertwined with the availability and supply of groundwater resources, it becomes imperative to perceive and search the status of groundwater resource’s susceptibility to contamination. A well-known overlay & index method, the DRASTIC model that assesses the state of susceptibility to contamination using vulnerability index map has been applied to the Yeşilköy aquifer. The seven different hydro-geologic parameters; aquifer media, vadose zone, and soil media, surface topography, recharge capacity, water table depth, and the hydraulic conductivity maps were overlaid to find the vulnerability of the aquifer. Results depicted that the aquifer is categorized into two vulnerability zones; low and moderate. The available susceptibility of the aquifer due to ongoing anthropogenic activities like agriculture was also studied by implementing the Weighted Arithmetic Water Quality Index (WQI) method. The aquifer’s overall water quality was “poor water quality”. Hence, there was no significant relationship between WQI and the vulnerability of the aquifer. A modification of the DRASTIC to enhance the relationship with WQI is proposed by utilizing the Analytic Hierarchy Process (AHP), as one of the successful multi-criteria decision-making models. The modification of the DRASTIC model with AHP revealed that the performance of the DRASTIC model was improved and the correlation between WQI and the DRASTIC vulnerability index was significantly enhanced. The results also show that the effective contributing parameters for groundwater vulnerability in the Yeşilköy aquifer were the aquifer media and the vadose zone. On the other hand, the water table’s depth and topography were almost independent of the aquifer’s vulnerability.

Building inexpensive topsoil saturated hydraulic conductivity maps for land planning based on machine learning and geostatistics

Saturated hydraulic conductivity (Ks) mapping is an application tool in many fields of science, technology, and land management. Hydrologically, Ks is the main factor controlling aquifer recharge, but it is one of the more difficult hydraulic properties of the soil to predict. This work develops a novel methodology for topsoil Ks mapping at the catchment scale, based on auxiliary data that are quick to determine and low-cost. It includes a double-scale sampling of the Ks to account for small-scale variability in the spatial geostatistical interpolation. A supervised selection of variables through correlation analysis and hierarchical clustering of variables precedes the development of site-specific pedotransfer functions (PTFs) with machine learning techniques. The approach was applied to a heavily anthropized area subject to flooding on the island of Mallorca, Balearic Islands, Spain. The variable selection process filtered out four predictor variables from the initial pool of fourteen predictors. An artificial neural network (ANN) with one hidden layer and six input variables (latitude, longitude, silt, clay, medium sand, and land use) provided the best Ks prediction model. Latitude and longitude coordinates and land use are surrogates for other physical and environmental factors. This ANN was found to be much more accurate than other classical PTFs. The ANN was used to estimate new Ks values from a subsequent sampling of model covariates, which allowed doubling the input information for spatial interpolation using ordinary kriging (OK). The consistency of the resulting Ks map was evaluated by direct comparison with a geomorphological base map. Overall, the spatial distribution of Ks was consistent with the lithological variability as well as with other superimposed anthropic factors.

Improving our ability to model crystalline aquifers using field data combined with a regionalized approach for estimating the hydraulic conductivity field

Modelling of heterogeneous aquifers, such as crystalline aquifers, is often difficult and, flow and transport predictions are always uncertain, suffering of our imperfect knowledge of the spatial distribution of aquifer parameters. This paper aims to test the robustness of a first-order hydraulic conductivity map estimated from both a detailed water-table map and hydraulic-conductivity statistics on a granitic watershed in Brittany (57 km2), compare it to local estimates and assess it through various numerical models. Map values range on four orders of magnitude (10-7 to 10-4 m/s, mean: 3.9x10-6 m/s) and their comparison to local estimates from various sources (pumping tests, MRS measurements, streamflow) gives satisfactory results.Four hydraulic conductivity fields were assessed through numerical modelling under steady-state condition. Model 1 used the regionalized hydraulic conductivity field directly, Model 2 used a uniform value (average of Model 1), Model 3 used a hydraulic conductivity field obtained by inverse modelling of the water table and Model 4 used two zones of uniform values based on the analysis of Model 1 and Model 3 fields. Model results were analysed based on their ability to reproduce the observed water-table levels and the groundwater flow directions. Modelled groundwater discharges to streams at sub-catchment scale were compared to spatial streamflow measurements performed during low water condition, which help validating models. The comparison between the fields obtained from Model 3 and that from the regionalized method (Model 1) shows that they are close in terms of mean values and spatial distribution. Model 1 reproduces rather well the water-table map and the groundwater flow directions. Model 2 shows the less good results. Model 4 has led to satisfactory results and shows that the hydraulic conductivity is higher (2.1x10-6 m/s) where the water table is located in the fractured zone, and lower (3.3x10-7 m/s) where it is located in the saprolites (highly weathered rock), which is expected for such aquifer system. Modelled groundwater discharges to streams are comparable in all models to streamflow measurements in most sub-catchments, but the models overestimate them in certain places, mainly because of sub-surface drains in a forest capturing part of the groundwater that can no longer return to streams (drains were not considered in the models).In addition, experiment on a second watershed (40 km2) shows how with much less field data the methodology can already provide interesting information on the hydraulic conductivity field (values and spatial distribution).Results are very encouraging and open up prospects for using quantitative and qualitative information from the mapping of hydraulic conductivity to constrain the spatialization of hydrodynamic parameters on models and thus our ability to model such complex aquifers.

Studies on the Quality and Vulnerability of Groundwater to Contamination

To effectively and properly protect groundwater, it is crucial being able to identify areas where groundwater may be most vulnerable to contamination and translate this information into vulnerability maps that can be used by potential end-users, such as land and water-resources managers to prevent or minimize harmful impacts on groundwater quality. The focus of the study is to evaluate the quality of groundwater around the Niger Delta Basin Development Authority in Nigeria and assess the aquifer characteristics and its vulnerability to contamination About hundred (100) boreholes spread to cover the study area were sampled. The water samples were analyzed using standard procedures for assessing drinking water qualities in order to understand the existing condition of groundwater quality within the study area. In addition, existing borehole logs were acquired and lithological mapping of the study area was done to acquire relevant data that was needed to generate thematic maps such as; depth to groundwater map, aquifer recharge map, aquifer media map, soil media map, hydraulic conductivity map and the vadose zone map. The topographic map of the study area was generated from the digital elevation model (DEM). To generate the groundwater contamination map, pairwise overlay analysis using geographic information systems (GIS) was employed.Based on the final DRASTIC map, it was observed that most of the locations from where water samples were collected fell within the light brown colouration with computed DRASTIC index of between (140-160) indicating high rate of vulnerability of the aquifer to contamination. Keywords: DRASTIC, Groundwater quality, Lithological mapping, Overlay analysis, Aquifer media. DOI : 10.7176/IKM/9-11-05 Publication date: December 31 st 2019

Assessing groundwater accessibility in the Kharga Basin, Egypt: A remote sensing approach

We used multi-map analysis of remote sensing and ancillary data to identify potentially accessible sites for groundwater resources in the Kharga Basin in the Western Desert of Egypt. This basin is dominated by Cretaceous sandstone formations and extends within the Nubian Sandstone Aquifer. It is dissected by N-S and E-W trending faults, possibly acting as conduits for upward migration of groundwater. Analysis of paleo-drainage using Digital Elevation Model (DEM) generated from the Shuttle Radar Topography Mission (SRTM) data shows that the Kharga was a closed basin that might have been the site of a paleo-lake. Lake water recharged the Nubian Sandstone Aquifer during the wetter Holocene time. We generated the following layers for the multi-map analysis: (1) Fracture density map from the interpretation of Landsat Operational Land Imager (OLI), SRTM DEM, and RADARSAT data. (2) Thermal Inertia (TI) map (for moisture content imaging) from the Moderate Resolution Imaging Spectroradiometer (MODIS) data. (3) Hydraulic conductivity map from mapping lithological units using the Landsat OLI and previously published data. (4) Aquifer thickness map from previously published data. We quantitatively ranked the Kharga Basin by considering that regions of high fracture density, high TI, thicker aquifer, and high hydraulic conductivity have higher potential for groundwater accessibility. Our analysis shows that part of the southern Kharga Basin is suitable for groundwater extraction. This region is where N-S and E-W trending faults intersect, has relatively high TI and it is underlain by thick aquifer. However, the suitability of this region for groundwater use will be reduced significantly when considering the changes in land suitability and economic depth to groundwater extraction in the next 50 years.

Integrated remote sensing and GIS approach for delineation of groundwater potential zones using aquifer parameters in Devak and Rui watershed of Jammu and Kashmir, India

Remote sensing and geographic information system (GIS) have become one of the leading tools in the field of hydrogeological science, which helps in assessing, monitoring and conserving groundwater resources. In this paper, integrated remote sensing, GIS and fieldwork techniques were combined to delineate the groundwater potential zones map of Devak and Rui watersheds of Jammu and Kashmir. Remote sensing data were used to prepare hydrogeomorphology, drainage density and land use/land cover maps. The advanced space borne thermal emission and reflection (ASTER) digital elevation model (DEM) data was used for preparing the slope map. Pumping test data were used to prepare discharge, specific capacity, static water level, transmissivity and hydraulic conductivity maps. Well inventory data were collected to prepare the pre-monsoon water table, post-monsoon water table, static water level and fluctuation maps. All the thematic maps pertaining to chosen parameters were converted to raster data. The raster maps of these parameters were assigned an appropriate theme weight and class score. All thematic layers were integrated using the weighted index overlay analysis (WIOA) method in the GIS environment to determine groundwater potential map. The ground water prospects zones were depicted in five categories excellent, good, moderate, low and runoff zone. The results revealed that excellent zone covers 13.5 %, good zone 22.7 %, moderate zone 15.8 %, low zone 18.5 %, and runoff zone covers 29.5 % of the total study area. The result depicts the groundwater potential zones found to be helpful in better planning and management of groundwater resources in the study area.

Distance-based kriging relying on proxy simulations for inverse conditioning

Let us consider a large set of candidate parameter fields, such as hydraulic conductivity maps, on which we can run an accurate forward flow and transport simulation. We address the issue of rapidly identifying a subset of candidates whose response best match a reference response curve. In order to keep the number of calls to the accurate flow simulator computationally tractable, a recent distance-based approach relying on fast proxy simulations is revisited, and turned into a non-stationary kriging method where the covariance kernel is obtained by combining a classical kernel with the proxy. Once the accurate simulator has been run for an initial subset of parameter fields and a kriging metamodel has been inferred, the predictive distributions of misfits for the remaining parameter fields can be used as a guide to select candidate parameter fields in a sequential way. The proposed algorithm, Proxy-based Kriging for Sequential Inversion (ProKSI), relies on a variant of the Expected Improvement, a popular criterion for kriging-based global optimization. A statistical benchmark of ProKSI’s performances illustrates the efficiency and the robustness of the approach when using different kinds of proxies.

Two-scale modeling of solute transport in an experimental stratigraphy

Summary A high-resolution non-stationary hydraulic conductivity map is generated based on an experimental stratigraphy. A heterogenous model is created, incorporating the complete conductivity variation. A hydrostratigraphic model (HSM) is also created which divides the space into discrete lithofacies units. For each unit, an equivalent conductivity is estimated using numerical up-scaling. Under a lateral hydraulic gradient, steady-state, incompressible groundwater flow experiments are conducted in both models. Within each flow field, conservative pulse-input line-source tracer is simulated. In the heterogeneous model, the tracer exhibits both scale-dependency in the observed longitudinal macrodispersivity and persistent long tailing associated with anomalous, non-Fickian dispersion. In comparison, HSM-predicted, global mean relative error of hydraulic head is 1.5%, that of groundwater flux is 0.77%. Using (small) hydrodynamic dispersivities, the HSM closely predicts the evolution of the tracer moments. A certain degree of tailing is also predicted, as this model has captured the largest scale, between-unit velocity variations. However, detailed plume shape is not captured, nor are the arrival and tailing of the breakthrough curves. Using macrodispersivity (both unit-specific and time-dependent), the breakthrough prediction has improved, especially the solute arrival time. Both macrodispersion models also capture the development of breakthrough asymmetry as well as power-law tailing. However, the development of a steep front and multiple peak concentrations are not captured. Similar observations are also found for a continuous-source injection. Overall, for the chosen boundary condition, the advection–dispersion equation can be used by the lithofacies model to capture certain key aspects of the bulk flow and transport behaviors, although displacement mapping reveals that heterogeneity-induced dispersion is correlated both in time and space, a likely result of the correlated velocity field.

Representative hydraulic conductivity of hydrogeologic units: Insights from an experimental stratigraphy

A critical issue facing groundwater flow models is the estimation of representative hydraulic conductivity assigned to the model units. In this study, an experiment-based, high-resolution hydraulic conductivity map offers a test case to evaluate this parameter. Various hydrogeological units are distinguished, each is of irregular shape with distinct heterogeneity pattern created by physical sedimentation. Extending a previous study which used numerical upscaling to compute equivalent conductivities for these units (at two upscaling scales) [Zhang, Y., Gable, C.W., Person, M., 2006. Equivalent hydraulic conductivity of an experimental stratigraphy – implications for basin-scale flow simulations. Water Resources Research 42, W05404. doi:10.1029/2005WR004720], this study compares them with local statistics and effective conductivities predicted by a stochastic theory. Results suggest that for a system with moderate ln K variance (4.07) and low topographic slope (∼1°), the arithmetic mean ( K A ) provides a good estimate for the maximum principal component ( K max ) of the equivalent conductivity. The minimum principal component ( K min ) lies between the harmonic and geometric means: its closeness to the geometric mean is affected by heterogeneity pattern and upscaling scale. Using K max (alternatively, the arithmetic mean), geometric mean, and ln( K ) variance, the stochastic theory predicts a K min that is consistent with the up-scaled value. Similarly, knowing K min , K max predicted by theory is also consistent with the up-scaled value. For most deposits (some with variance greater than 1), a low-variance version of the theory is more accurate than a high-variance version. However, the increase of topographic slope (to ∼4°) and total ln K variance (to 16) result in increased deviation of K max from K A . High variance also results in significantly larger anisotropy ratio, possibly due to the dominance of preferential flow. Finally, for select units, equivalent conductivity exhibits scale effect. Field scale representative elementary volume thus does not exist and upscaling the full unit is necessary to obtain the representative conductivity.

Equivalent hydraulic conductivity of an experimental stratigraphy: Implications for basin‐scale flow simulations

Critical issues facing basin‐scale groundwater flow models are the estimation of representative hydraulic conductivity for the model units and the impact of nonrepresentation of within‐unit conductivity heterogeneity on the model flow prediction. In this study, high‐resolution, fully heterogeneous basin‐scale hydraulic conductivity map is generated by scaling up an experimental stratigraphy created by physical sedimentation processes and by assuming increasing conductivity for increasing gray scale (proxy for sand content). A fully heterogeneous model is created, incorporating the complete conductivity variation. Two hydrogeologic framework models are also created, one of coarser stratigraphic division. A novel numerical up‐scaling method is developed to compute an equivalent conductivity for each irregularly shaped framework model unit by conducting basin‐scale flow experiments in the fully heterogeneous model. In each experiment, different boundary conditions are specified, subjecting the basin to various flow conditions. To evaluate the impact of using equivalent conductivity on the prediction of basin‐scale hydraulic head and groundwater flow, the flow experiments conducted in the fully heterogenous model are repeated in the framework models. Results indicate that for most deposits, the behavior of the equivalent conductivity with increasing ln(K) variance is consistent with the prediction of an analytic‐stochastic theory. The equivalent conductivity is also insensitive to the boundary condition and the number of flow experiments performed, indicating the possible emergence of an effective conductivity. Although all equivalent conductivities are full tensors, the off‐diagonal term is 2–3 orders of magnitude smaller than the diagonal terms. Ignoring the off‐diagonal term has minimal impact on the framework‐model‐predicted hydraulic head and groundwater flow paths, when compared to the impact of nonrepresentation of within‐unit conductivity heterogeneity. Under certain boundary conditions, significant head deviation can develop within framework model units that contain trended or strongly stratified deposits. However, the accuracy of head prediction is improved when the length of the no‐flow boundary is increased. In a topography‐driven system, progressive degradation is observed in the prediction of basin‐scale flow pattern, flow rate, and location of recharge/discharge, when the progressively up‐scaled framework models are used. In summary, the accuracy of the framework models is controlled by the level of stratigraphic division, conductivity heterogeneity, and boundary conditions.

Constructing a saturated hydraulic conductivity map of Spain using pedotransfer functions and spatial prediction

Currently, most of the research focused on general pedotransfer functions (PTFs) has been generated under humid climatic conditions. Nevertheless, this article proposes an estimation of pedotransfer functions for a great variety of climatic and physiographic conditions, with a predominance of soils developed under semi-arid conditions, such as those occurring in Spain. The results obtained, in spite of the variety of measuring methods of the variables used, show that it is possible to estimate saturated hydraulic conductivity values for soils from sand content data. These are compared with results obtained from the application of other authors' PTFs. This is an important support to the present attempts at establishing soil inference systems. The PTFs obtained have increased the number of profile points in networks with saturated hydraulic conductivity data, allowing the transformation of point data to a continuous saturated hydraulic conductivity surface. To reach this goal, three methods have been applied: that of Batjes, inverse distance weight and kriging interpolation, with the latter giving the best results. Finally, a saturated hydraulic conductivity map with a spatial resolution of 1×1 km for the Spanish part of the Iberian Peninsula has been generated. The resulting map showed a good spatial fit after being compared with lithological distribution, which confirms the applicability of the method for future hydrological applications.

On the theory and practice of non-linear least-squares

Nonlinear least squares has proven to be a very useful tool for the estimation of parameters in groundwater modeling. The reason for this success is the power of the approach: the only fundamental requirement is to posses one numerical simulator able to calculate the output ~(.,c) of the model (for example the computed pressure history) once a value x (for example the hydraulic conductivity map) has been assessed to the unknown parameter (hence cp denotes the parameter --* output or modelling operator). Given a value z of the experimental data (for example the measured pressure history), nothing can prevent you from forming the least-squares error: