Preprocessing approaches in machine learning-based groundwater potential mapping: an application to the Koulikoro and Bamako regions, Mali

Abstract

Abstract. Groundwater is crucial for domestic supplies in the Sahel, where the strategic importance of aquifers can only be expected to increase in the coming years due to climate change. Groundwater potential mapping is gaining recognition as a valuable tool to underpin water management practices in the region, and hence, to improve water access. This paper presents a machine learning method to map groundwater potential and illustrates it through an application to two regions of Mali. A set of explanatory variables for the presence of groundwater is developed first. Several scaling methods (standardization, normalization, maximum absolute value and min-max scaling) are used to avoid the pitfalls associated with the reclassification of explanatory variables. A number of supervised learning classifiers is then trained and tested on a large borehole database (n = 3,345) in order to find meaningful correlations between the presence or absence of groundwater and the explanatory variables. This process identifies noisy, collinear and counterproductive variables and excludes them from the input dataset. Tree-based algorithms, including the AdaBoost, Gradient Boosting, Random Forest, Decision Tree and Extra Trees classifiers were found to outperform other algorithms on a consistent basis (accuracy > 0.85), whereas maximum absolute value and standardization proved the most efficient methods to scale explanatory variables. Borehole flow rate data is used to calibrate the results beyond standard machine learning metrics, thus adding robustness to the predictions. The southern part of the study area was identified as the better groundwater prospect, which is consistent with the geological and climatic setting. From a methodological standpoint, the outcomes lead to three major conclusions: (1) because there is no aprioristic way to know which algorithm will work better on a given dataset, we advocate the use of a large number of machine learning classifiers, out of which the best are subsequently picked for ensembling; (2) standard machine learning metrics may be of limited value when appraising map outcomes, and should be complemented with hydrogeological indicators whenever possible; and (3) the scaling of the variables helps to minimize bias arising from expert judgement and maintains robust predictive capabilities.

A Review of Geological and Climatic Variables in Groundwater Availability Prediction in Africa: Machine Learning Approaches

“…In some cases, authors found some vegetation related indices such as Normalized Difference Vegetation Index (NDVI) and Normalized Difference Water Index (NDWI) to be necessary for their groundwater level prediction studies (Gómez-Escalonilla, et al, 2022a;Gómez-Escalonilla, et al, 2022b). High NDVI values from natural vegetation may potentially suggest the likelihood of groundwater availability (Sahour et al, 2022) as well as the NDWI which can detect water content and the presence of water bodies or moisture in the landscape.…”

“…Land use and land cover integration is frequently employed in studies that map groundwater potential. This approach recognizes that land use changes, primarily driven by human activities, can significantly impact groundwater resources (Gómez-Escalonilla et al, 2022a).…”

“…Geomorphology can be valuable in identifying characteristics that have the potential to facilitate groundwater infiltration and storage (Gómez-Escalonilla et al, 2022a). The depth of the water table is a valuable factor in mapping water tables as it helps identify the primary zones where aquifers are recharged and discharged (Gómez-Escalonilla et al, 2022a).…”

“…The method has shown efficiency in various studies because of its capability to incorporate various environmental variables and factors that affect groundwater availability, such as topography, soil characteristics, and rainfall patterns. For instance, Sarkar et al (2022) employed a variety of input parameters, with some highlighting the importance of climatic variables (Gonzalez et al, 2021) and geological factors (Gómez-Escalonilla et al, 2022a).…”

A national-level study on groundwater potentiality mapping using a hybrid machine learning models under the scenario of climate change

“…Because each model has its limitations, none was previously considered to be the ideal one for GWP modeling (Mallick et al, 2021;Gómez-Escalonilla et al, 2022). This is why researchers tried to integrate ensemble machine learning approaches, statistical techniques, and machine learning techniques in GWP modeling (Cai et (Islam et al, 2021a;Talukdar et al, 2021a;Talukdar et al, 2020b).…”