Assessment of groundwater potential zones using an integration of Remote Sensing, GIS and 2D Electrical Resistivity imaging in the Echway watershed, Baro River Basin, Southwest Ethiopia

The objective of this study is to delineate and identify the groundwater potential zone of degraded land in the Echway watershed, southwest Ethiopia, employing a combined approach that includes 2D electrical resistivity, the analytical hierarchy process (AHP), geographic information systems (GIS) and remote sensing (RS). Using a geographic information system (GIS) and remote sensing data, groundwater potential zones were identified by taking into account the soil types, drainage density, geology, lineament density, Land use and land cover, rainfall and slope. Using the AHP method, calculate the weight of thematic layers and rank of subclasses based on the effects of various thematic layers on groundwater potential. A significant weight was provided on the geology of the research region, total annual rainfall, and lineament density. Due to significant effects on groundwater potential zones, only three of the seven theme levels were assigned significant weight in this analysis. The weighted overlay analysis was used to construct the diagram of the zones with groundwater potential. 2D electrical resistivity was utilized to find the groundwater aquifer, and four major electrical layers were discovered: clayey silt, sand, moderately weathered/fractured rock, and basement parent rock. The groundwater potential zones in the study area have been classified into five categories: very low (22.97%), low (13.43%), moderate (32.50%), high (25.12%), and very high (5.98%) covered, respectively. The groundwater potential zone image was created by combining GIS, remote sensing, AHP, and an electrical survey


Introduction e 1282 (2023) 012012
IOP Publishing doi:10.1088/1757-899X/1282/1/012012 2 analytical hierarchy process (AHP) use to construct similarity datasets for multi-criteria decision analysis by applying weights to thematic strata and grouping relevant subclasses [6,7].Integrating remote sensing, GIS, and AHP method is required for practical groundwater resource evaluation, observation, planning, prediction, and testing.In recent years, geophysical 2D resistivity surveys used in various applications, including environmental monitoring evaluation, mineral and oil exploration, civil engineering projects and hydrologic study [8].The primary advantage of the 2D electrical resistivity concept over a 1D image, this method generates more detailed data on resistivity changes along a profile line in all vertical and lateral dimensions [9,10,11].The two-dimensional electrical resistivity survey is called electrical resistivity tomography.It evaluated subsurface layers, porous and permeable zones, fractures/cracks, and potential water areas.Ethiopia is one of the countries experiencing a reduction in groundwater quantity and quality due to land degradation.According to current data, the average rate of deforestation in Ethiopia, owing to agricultural growth, is roughly 1,410 km 2 [12].The Gambella region's unique forest cover, vegetation, and rich biodiversity will be extinct by 2133 if current deforestation rates continue.It will have a significant impact on the LU/LC ecosystems all across the Gambella region.Groundwater potential in the Gambela Region has steadily declined due to fast land use and land cover changes, increased population, and agricultural activities in the Echway watershed.As a result of the gambela region's groundwater shortage in recent years as a result of soil degradation, identifying groundwater potential zones is essential to this region.The Echway catchment is one of the most densely inhabited places in Ethiopia's Gambela region, and as a result, there is a considerable demand for residential water supplies.The water treatment plant is inadequate to provide clean water to all areas of the study area, particularly Gambela municipality and the neighboring region inside this watershed, pushing inhabitants to rely on unsafe dug wells and adjacent rivers, which are unsuitable for household use.In order to identify groundwater potential zones, a combination of remote sensing, geographic information systems, AHP, and 2D electrical resistivity mapping was used.There hasn't been any additional research on this method of defining zones of groundwater potential in the Echway watershed.

Materials and Methods
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Data collection and analysis
The presence, mobility and distribution of groundwater resources was established by combining GIS, remote sensing, AHP and 2D apparent resistivity techniques.The remote sensing and geographic information system data were collected from several sources, including the Ethiopian Ministry ofWater and Irrigation and the Ethiopian Geological Survey.In addition to assessing potential zones, these data were utilized to build seven different thematic maps, which included geology, soil, rainfall, lineaments density, drainage density, land use/cover and slope.The data from the Ethiopia Geological Survey was utilized to create the geology thematic map for this study using the ArcGIS clipping program.The Climatic Research Department provided rainfall data for the years 2011 through 2020.Data on land usage and land cover were collected from the Food and Agriculture Organization.Subsurface Soil information was acquired from the Ethiopian Ministries of Water and Energy, cropped and analysed with ArcGIS platform.The Schulte Radar Landscape Survey and a digital terrain map with a resolution of 30m were used to construct the slope, lineament density and drainage network.The ArcGIS clip tool extracted and manipulated the study's LULC.AHP is a well multi-stage decision-making methodology, and it is another essential strategy for stakeholders to manage complex choices in a simple manner by applying pair-wise correlation and assessing many elements based on its influence, particularly in aquifer delineation [14,15,16,17].The AHP approach was used in this research to distribute average weights per each parameter in the groundwater aquifer estimation.Using the AHP method, calculate the weight of thematic layers and rank of subclasses based on the effects of various thematic layers on groundwater potential.Significant weight is provided on the geology of the research region, total annual rainfall and lineament density.Only three thematic factors were given substantial weight in this study out of seven different thematic layers due to major effect characteristics on groundwater potential zones.The weighted sum software package in the ArcGIS 10.8 program was used to construct the aquifer areas map.Hydrogeological field surveys were conducted to visualize the research region's topographical plot and geological and hydrogeological formation.2D electrical resistivity geophysical investigation was conducted in four different profile lines in different directions of the study region to determine the fracture length, level of weathering and crystalline basement rocks.Following the field study, the site inspection data and electrical resistivity values were analyzed, reviewed and processed with the Res2DInv tool.This survey was carried out in order to generate the 2D inverted model resistivity profile demonstrating underground geo-electrical layering of the region required for water-bearing structure characterization.The AMEB 400 resistivity device is 2D electrical resistivity equipment that was employed in this field investigation.Depending on the model system, the number of electrodes ranges from 25 to 100.This study utilised forty-two electrodes within 400m of each other, with an electrode spacing of 10m.The Wenner array arrangement was used to conduct the 2D resistivity study.When compared to other array configurations, the Wenner array had the greatest signal intensity and a significant depth of research in the survey [18].The RES2DINV program was used to process and invert the acquired data.The RES2DINV inverts observed and estimated apparent resistivity values to produce an inverted design resistivity line corresponding to Earth's soil characteristics' true resistivity.

3.
Groundwater storage factors vary according to location based on hydrogeological, meteorological and anthropocentric activities.The groundwater regulation factors land use/land contain, topsoil, lithology, drainage network, lineament, slope and rainfall were discovered to be crucial in determining groundwater recharge in this investigation.A 2D resistivity survey was done in the research region to determine the magnitude of weathering, fracture extent and foundation rock lithology to validate the groundwater carrying aquifer.

Geology and lineament
The geological formation in the research region was classified as Precambrian and Pleistocene Holocene based on the geological time scale.The study area's Precambrian rocks consist primarily of metamorphic rocks such as syntectonic granitoid, post tectonic granitoid and lower complex basement rock.Such strata are poor prospectively and release little water.Only metamorphic rock faults and fractures may generate considerable volumes of water and function as porous material [19].The Pleistocene Holocene strata comprise quaternary deposits of sand, silt, gravel and clay (Figure 2).These deposits were commonly categorised as water potential formations because of their considerable permeability and porosity [19,5].Lineament is the vertical characteristic that reflects subsurface structures such as a crack, fracture, join and fault.It facilitates substrate additional permeability and porosity [20,21].The spatial statistical unit density and categorisation program was used to determine lineament density from a lineament image.In the study field, lineament frequency was already divided into five categories: very low, low, moderate, high and very high (Figure 2).The substantial lineament density enhances infiltration and recharges efficiency due to the high secondary porous structure that represents a location with extensive groundwater occurrence.Poor lineament density influences recharge velocity and infiltration, suggesting a zone with limited groundwater capacity

Results and Discussions
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Slope and drainage density
The slope is one of the most critical components controlling groundwater storage and flow.Infiltration amount decreases when the terrain is steeper, and infiltration ratios are raised when the topography is plain.The amount of precipitation that percolates and reaches the aquifer highly depends on the slope's nature [22].Most of the study area comprised plain to moderate slopes, with steep slopes representing only a small portion of the region.The catchment's slope was classified into five categories: very low, low, moderate, high, and very high (Figure 3).Drainages are surface and subsurface structures that control water flow above and below the earth [3,23].On the other hand, drainage density is the sum of the distances including all tributaries within a specific area.The research region's drainage density was divided into five groups: very high, high, moderate, low, and very low (Figure 3).More drainage density structure shows that the formation rocks are well resistant to erosion and weathering owing to very high surface water runoff and limited groundwater infiltration capacity, but low drainage density gives little surface runoff and considerable groundwater potential.

Soil and Land Use and Land Covers
Groundwater potential zones can be strongly influenced by soil properties such as porosity, soil composition, texture and permeability [24].Soil types were classified into five category, eutric cambisols, eutric fluvisols, eutric lithosols, gleysols, and eutric vertisols.The soils mentioned above are categorized depending upon size distribution as coarse-grained, moderate-grained and small-grained.As a result, eutric gleysols and eutric vertisols are small-grained soils with low to shallow potential groundwater zones.Eutric cambisols are intermediate grain soils with reasonable groundwater potential.Eutric fluvisols and eutric lithosols are coarse-grained soils with high and very high likely groundwater (Figure 4).The surface water runoff, infiltration and circulation of groundwater are all strongly affected by LULC.Anthropogenic changes in land use/land cover can affect recharge percentages, therefore, may impact a detrimental effect on groundwater chemistry [25].
During precipitation, the roots of plants may absorb and retain rainwater, resulting in high infiltration and little surface runoff.However, urban areas increase surface runoff and decrease the percentage of groundwater recharge [5].This research region's five types of LULC include annual crop, open grass, dense forest, habitation, and water body.An annual yield is a small unit in the region, with open grass and lush vegetation occupying most of the landscape (Figure 4).As a result, groundwater occurrence in the Echway watershed, the water body, and the forests are highly valued, annual crops and open grass is reasonably valued, and settlement is low valued [26].The potential groundwater zone image for the research region was created by combining the scores of seven parameters and associated subdivision score using the weighted overlay approach.(Table 1) [28,29,30].

Groundwater potential zones mapping using AHP approach
The potential groundwater zone of this study was classified into five categories, namely, very low, low, moderate, high, and very high, with an area covered of 229.44km2 (22.97%), 134.08km2 (13.43%), 324.88km2 (32.50%), 25.52km2 (25.12%), and 59.76km2 (5.98%), respectively (see table 5.8).Due to the complicated basement rock and lower rainfall within study site, the aquifer diagram of the area reveals shallow water capability regions in the northwest to east.High and very high groundwater potential occupied the study area's west, central, and southeast parts.These areas have coincided with quaternary sediment, highly weathered post-tectonic lineament, water body, and a relatively high amount of rainfall.

Identification of groundwater
According to the RS, GIS, and AHP methods groundwater potential map, Compared to the northwest and southeast, the research region's middle has a greater water possibility region.Four resistivity traverses were carried out in the central portion of the research area.To determine subsurface potential zone using 2D Electrical Resistivity lithology, number of aquifers and the depth of groundwater bearing formation the inverted model's pseudo-traverse data were used [31,32,33].The figure's x-axis measures the length above the surface in meters, while the y-axis shows the depth of penetration of the two-dimensional inverted traverse in meters (Figure 7).The traverse resistivity value is evaluated using the research area's local geology and hydrogeology.

Traverse one (AA')
The first traverse line (AA') was run perpendicular to the Baro River for 400m, running south-northeast (Figure 8).Due to the compact sandy silt that was located on the surface top layer on the left side of the traverse are high resistance value and its resistivity values vary from 200 to 270Ωm.The second layer, which occurs at the end of the inverted model's right wing, was interpreted as wet clayey soil, with a low resistivity value range of 20 to 85Ωm and thickness of 10m.The third subsurface layer, referred to as the sand layer, denotes a water-bearing zone with resistivity values ranging from 95 to 150 Ωm.The thickness of this water-bearing saturated sand layer is thin on the right side of the model and progressively increases toward the left side of the traverse.The last layer was characterized as a slightly weathered rock with a resistivity of 160 to 245Ωm.

Traverse two (BB')
The second traverse line (BB') was conducted parallel to the Baro River in the southwest direction (Figure 9).This traverse's data points were 145m long with a maximum length of 400m.This traverse showed four distinct geological layers: clayey silt, sand, weathered/fractured layer and massive parent basement rock.The top layer of the traverse was made up of moist clay soils, with a thickness of 3 to 5m and a resistivity range of 10 to 30 Ωm.The second layer, which lies beneath the top layer of clay, was characterized as saturated sand and had a resistivity range of 38-85Ωm.The thickness of this layer is relatively thin in the middle to left part of the traverse because of the occurrence of hard granitoidbasement rock, but this hard granitoid rock is absent on the right side of the traverse.The thickness of this second sand layer ranges from 30 to 45m, and it was assumed to be a suitable groundwater-bearing zone.The third layer was identified to be slightly weathered granitoid parent rock with a resistivity of 100 to 200 Ωm and a thickness of 6 m.The last layer is a considerable depth of solid granite that occurs at resistivity levels of 300 Ωm and higher.

Integration of Remote Sensing, GIS, AHP and 2D Electrical Resistivity
The 2D resistivity traverse result was integrated with groundwater potential zone mapping to improve accuracy, identify shallow groundwater potential zones, and recommend the best location for the community to drill the water well [34,33].The area's geo-electrical layers were divided into clayey silt, sand, slightly weathered/fractured rock and massive basement rock.Sand layers occurrence of Pleistocene Holocene region represents high groundwater saturated zone and thickness vary from 12 to 70 m.The southern part of the study area traverses BB' and CC' is excellent groundwater carrying site with a shallow depth of 12m.Although clayey silt layers constitute the moderately water-bearing zone, they are not a suitable location for drilling the water well.Poor and very poor potential zones are indicated by slightly weathered and massive hard granitoid rocks.Based on the overall interpretation results, the middle portion of the second layer of sand deposits in the Pleistocene Holocene region is a good groundwater potential zone with shallow water depth (Figure 12).This location is ideal for drilling water well to supply the water needs of Ethiopia's Gambella town.

Validation for the probable locations
Applying data from shallow and open dug wells, the GWPZs have been validated [34,35].The wells data were obtained from the Gambela water and irrigation bureau.The wells data, including wells, yield data,static water level, and transmissivity.There are forty-four wells in the study area.Out of 44 wells, only 20 wells have yield and transmissivity.The model's validity was examined using these 20 wells.The pump well yields vary between 0.01-14l/s and transmissivity range from 1 to 100 m 2 /d above in the study area.These bore wells yield and transmissivity have been classification as very low 0.01-0.5l/sand 1-10m 2 /d, low 0.5-2l/s and 10-50m 2 /d moderate 2-5l/s and 50-100m 2 /d, High 5-10l/s and100m 2 /d, and very high 10-14l/s and 100 m 2 /d above (Figure 13).Five wells fall on very high potential zones, seven wells on high potential zones, four wells on moderate potential zones, two wells on low potential zones and two wells on shallow potential zones.According to the validation findings, 16 out of the 20 pump wells are located in aquifer zones with a medium to high capacity.This revealed that 80% of the wells' yield and transmissivity correspond to potential groundwater zones.The identification of the aquifer potential regions matched well inventory information that was already available.The overall borehole wells drilled in the study area are shallow wells, with a depth range of 45-65m, exceptional of 2 open dug wells with a depth of 10m.Regional trends of rock type and rainfall can be seen in the spatial variability of the several aquifer potential regions that were determined by the results Groundwater is a reliable source which can be successfully controlled and managed to satisfy the country's demands.The current work studied an integrated combination of RS, GIS, AHP and 2D electrical resistivity to assess and identify significant groundwater potential zones.Water potential zones are considered using seven distinct thematic maps, including geology, soil, rainfall, lineaments density, drainage density, land use/cover and slope.The study area's Precambrian rocks consist primarily of metamorphic rocks such as syntectonic granitoid, post tectonic granitoid and lower complex basement rock.The Pleistocene Holocene strata comprise quaternary sand, silt, gravel and clay deposits.The geology of the research region, annual total rainfall and lineament density are all given significant weight.This study's groundwater potential zone was divided into five groups: very low (22.97%),low (13.43%),moderate (32.50%), high (25.12%),and very high (5.98%)covered,respectively.Based on RS, GIS, and AHP groundwater potential map, the research's west, north, and centre portions have moderate, high and very high groundwater potential.Because of the region's Quaternary sediments, flat to gentle slope, high lineament density and good precipitation.

9.
The authors acknowledge the Geology Department, College of Natural and Computational Science, and Arba Minch University in Ethiopia for their contributions

Figure 2 .
Figure 2.Geology and Lineaments density map of the study area

Figure 3 .
Figure 3. Map of the research area's slope and drainage percentage

Figure 4 .
Figure 4.Soil and Land use & Land cover map of the study area 3.1.4RainfallRainfall influences subsurface recharging and plays a vital role in the water balance cycle.Continuous long-period precipitation much more reflects good recharge than low precipitation, which indicates poor groundwater recharge[27].The rain in the research region was divided into five categories: very low, low, moderate, high, and very high.The rainfall image shows significant precipitation in the centre of the research zone, however, the research region's southeast trend indicates that the amount of precipitation there is quite low (Figure5).

Figure 5 .
Figure 5.Rainfall map of the study area

Figure 6 .
Figure 6.Groundwater potential map of the study area

Figure 8 .
Figure 8.Pseudo section of resistivity traverse line 1 5.2 Traverse two (BB') The second traverse line (BB') was conducted parallel to the Baro River in the southwest direction

Figure 10 .
Figure 10.Pseudo section of resistivity traverse line 3

Figure 11 .
Figure 11.Pseudo section of resistivity traverse line 4

Figure 12 .
Figure 12.Integration of groundwater potential zone with 2D resistivity traverse line

13 Figure 13 .
Figure 13.Validated Groundwater potential map Fourresistivity traverses were carried out in the central portion of the research area to determine subsurface lithology, number of aquifers, and depth of groundwater bearing formation.The area's geo-electrical layers were divided into clayey silt, sand, slightly weathered/fractured rock and massive basement rock.Sand layers occurrence of Pleistocene Holocene region represents high groundwater saturated zone and thickness vary from 12 to 70 m.The southern part of the study area traverses BB' and CC' is excellent groundwater carrying location with a shallow depth of 12m.Poor and very poor potential zones are indicated by slightly weathered and massive hard granitoid rocks Conclusions .

Table 1 .
The weight of criteria and their subcategory ranking

Table 2 .
Groundwater potential zones classification