Identication and Mapping Groundwater Potential Areas Using GIS and Remote Sensing in Wolaita Zone, Southern Region, Ethiopia

Recently water is becoming a vital natural resource that can be used for many things in human life i.e. hydropower generation, sanitation, drinking, irrigation, and transportation. But surface water is not enough to satisfy all demands of humans. So besides surface water, groundwater resource is by far huge in amount and not much exploited yet. Now a day exploitation of groundwater through hydrogeological surveying, identication, and delineation of better groundwater potential areas is implemented in many places of the world to satisfy the need for water. This research was done on the place where it is called the Wolaita zone located in the southern region of Ethiopia. The main objective of this research was to delineate and map the groundwater potential areas in terms of relative groundwater availability. Groundwater potential zone mapping was done using GIS and remote sensing integrated with the analytical hierarchical process to meet the objective. Seven thematic map layers (land use/cover, soil, slope, drainage density, lineament density, rainfall, and geology) were used as an input to the weighted overlay of layers for delineation of groundwater potential zones. As a result, the study area was classied into “poor”, “good” and “very good” groundwater potential zones. Accordingly, “poor” and “good” groundwater potential areas were occupying almost the same percentage from the total area which is 44.19% (198,0445ha) and 44.51% (199,460ha) respectively. But “very good” groundwater potential zones are covering 11.30% (50,652.2ha) of the study area. In this regard better groundwater potential areas were identied in the east and northern east direction of the study area. Wolaita zone was also clustered in 12 woredas and one town. From all twelve woredas, a High percentage of “very good” groundwater potential is found in Boloso sore, Duguna Fango, Boloso bombe and Damot sore woredas and relatively “poor” groundwater potential is situated in Kindo Koyisha, Kindo Didaye, Ofa, Humbo, and Sodo zuriya woredas and the remaining part of Wolaita zone is dominantly covered by “good” groundwater potential in Wolaita zone. Furtherly each woredas were also classied as “poor”, “good” and “very good” groundwater potential zones with their percentage of area coverage. Therefore identied groundwater potential areas can be used for digging of wells and bore holes, conducting further research, a benchmark for water resource management practice, or as an input for policymakers to make research-based decisions.


Introduction
Water is one of the vital natural resources which can be obtained from rain as surface runoff or from groundwater under the surface of the earth. It could be used for sanitation, drinking purpose, irrigation, hydropower, shing, recreation, transportation, and livestock production through the provision of different hydraulic structures according to the purposes that the water is intending to use [9]. Groundwater is a valuable resource that plays a fundamental role in maintaining terrestrial and aquatic ecosystems, as well as the well-being of human civilizations. However, population growth and the expansion of agricultural lands have mounted increasing pressure on groundwater resources, resulting in their overexploitation as well as a deterioration in water quality [15,24].
The conventional methods used to prepare groundwater potential zones are mainly based on ground surveys.
With the advent of remote sensing and Geographic Information System (GIS) technologies, the mapping of groundwater potential zones within each geological unit has become an easy procedure [13,18]. The AHP is most useful when nding decisions to complex problems with high stakes. It stands out from other decision-making techniques as it quanti es criteria and options that traditionally are di cult to measure with hard numbers. Rather than prescribing a "correct" decision, AHP helps decision makers nd one that best suits their values and their understanding of the problem (https://www.passagetechnology.com). Particularly in ground water modelling many researchers have been using the integration of GIS with AHP model for systematic approach.
In Ethiopia, most towns and villages get their domestic water supply from groundwater sources through developed springs, bore wells and shallow wells.
According to the International Atomic Energy Agency (IAEA) (2013), over 70 % of Ethiopia's water supply comes from groundwater and only 34 % of the population has access to an improved water supply. This shows that there is no proper utilization of the water resources and poor water management strategy to explore the available ground water resources and access it to the community, at least for half of it. Different factors would affect the ground water potential of a given place or country. Among them geology, land use/cover, slope, rain fall, soil types, lithology, drainage density, lineament density and geomorphology are the main factors which in uences the ground water potential of the place or region [1].
Remote sensing and GIS possess new possibilities for hydrogeological studies [14,21]. High-resolution satellite imageries are widely used in groundwater studies due to their high spectral and spatial resolution. In this research GIS and remote sensing integrated with Analytical hierarchical processing was used to analyze and interpret the data.
In Wolaita zone most households are using small bore holes around their living houses for sanitation, animal drinking and garden irrigation. This shows there is certain signals for ground water existence but not scienti cally supported and analyzed. Therefore this research was done to delineate ground water potential areas or zones for spotting out dominant water potential areas. This research will help researchers, policy makers and managers at different level to minimize cost of investigating hydrogeological survey, ground water assessment and used as a bench mark for further analysis. More over in exploitation of groundwater in that unsuccessful rate of well production encounter and requires huge amount of investment cost to utilize this precious resources. Thus, in order to ensure wise use of groundwater, a systematic evaluation of groundwater is required. by the Lake Abaya which separates it from Oromia Region. The administrative center of Wolaita is Sodo. Other major towns are Dimtu, Bedessa, Bekilo Segno, Areka, Gununo, Gesuba, Humbo-Tebela, Bitena and Boditi.
Wolaita has 358 kilometers of all-weather roads and 425 kilometers of dry-weather roads, for an average road density of 187 kilometers per 1000 square kilometers. The highest point in this Zone is Mount Damota (2738 meters). (https://en.wikipedia.org/wiki/Wolayita_Zone).

Climate
The climate is stable, with mean temperature variation between 24 and 30°C during the day and 16 to 20°C at night, all year round. The year is divided into two seasons: the wet season (balguwa) from June to October, and the dry season (boniya) from October to June, broken in February by a short period of so-called "little rains" (baddessa). The average rainfall for the entire region is 1350 millimeters per year. The dry season is characterized by a strong wind which blows from the east; the sky is absolutely blue and rarely crossed by small white clouds.
During the heavy seasons, heavy precipitation and violent storms which, at the end of the season can last a full evening or night are common events. Fog can be seen in the valleys almost every morning of the rainy season; it then evaporates in the rst hours of the sun. In both seasons either hail which destroys crops or tornadoes, which knock down trees, are possible events. Groundwater inevitably occurs in geological formations that require knowledge of how these earth materials formed and the changes they have gone to understanding the distribution of geologic materials of varying hydraulic conductivity and porosity [11,19]. The geologic map of Wolaita zone (Fig. 2) is prepared by downloading geological map from the site: https://energy.usgs.gov in vector format and clipped by the shape of the study area. Accordingly there are four types of geological formations observed with in the study area i.e. H 2 O, Q, Q v and Ti.

Soil
Soil properties in uence the relationship between runoff and in ltration rates which in turn control the degree of permeability, the principal factor in hydrogeology that determines the groundwater potential [3]. The soil types ( Fig. 3) with in the study area are extracted from FAO soil data base.

Land use/cover
Impacts of Land use/cover change on subsurface components of the hydrologic cycle are less well recognized, particularly ground water recharge [4]. The in ltration capacity of the soil is dependent on the land use/cover existed on the place and it in turn has a direct in uence on the ground water recharge and its potential to be used by the community as an end users. Satellite image (land sat 8) was downloaded from the online internet source (https://earthexplorer.usgs.gov/) and supervised classi cation was done using ERDAS imagine 2014. Finally the following (Fig. 4) land use/cover map was prepared from the extracted satellite image in 2020.

Drainage Density
Drainage density, a fundamental concept in hydrologic analysis, is de ned as the length of drainage per unit area. The term was rst introduced by Horton (1932) and is determined by dividing the total length of streams within a drainage basin by the drainage area. A high drainage density re ects a highly dissected drainage basin with a relatively rapid hydrologic response to rainfall events, while a low drainage density means a poorly drained basin with a slow hydrologic response [16]. The drainage density map (Fig. 5) was prepared using GIS software from DEM (having 30m resolution) data of the study area.

Slope
Topographic setting relates to the local and regional relief situation and gives an idea about the general direction of groundwater ow and its in uence on groundwater recharge and discharge. Steeper the slope, greater will be the runoff and thus lesser is the groundwater recharge [20]. Digital Elevation model (DEM) is derived using contour information from the topographical map for estimation of slope in degree. The study area (Fig. 6) was classi ed in to ve slope classes as 0-1 (very low), 1-3 (low), 3-5 (medium), 5-7 (high) and > 7 (very high) depending on the steepness of the topographic feature of Wolaita zone. The steep slope causes less penetration due to fast surface runoff while, level and mild slope areas encourages minimum runoff, accordingly allowing additional time to in ltrate rainwater and helps large groundwater rejuvenate. The level and moderate slopes are measured as great subsurface water revive.

Lineament Density
The lineament de nes as linear features in a landscape identi ed on satellite images and aerial photographs, most likely have a geological origin. Generally, lineaments are underlain by structural zone, fractured zone, a series of fault or fold-aligned hills zone of localized weathering and zone of increased permeability and porosity [23]. Lineament distribution for Wolaita zone (Fig. 7), prepared using image of Landsat 8 Thematic Mapper (TM). The lineament density map was prepared by using Arc GIS 10.5 software.

Rainfall
Rainfall or precipitation is one of the factor that contribute for the ground water recharging through in ltration of the water under the surface of the earth.
Literally the higher the amount of the rain on the given place the higher the depercolation and the ground water table will increase as well and vice versa. The rainfall map is extracted from eight rainfall stations with in the region. By using IDW interpolation technique the point source data of mean annual rainfall for each stations was interpolated for whole area of study (Wolaita zone).

Methods
Geographic information system (GIS) integrated with remote sensing data was used for spatial analysis and overlaying of input maps i.e. soil map, slope map, Drainage density map, lineament density map, Land use/cover map, geology map and rainfall map. The following ow chart (Fig. 9) shows the method of analyzing the input data to delineate the ground water potential zones or areas in the study area.

Result And Discussion
3.1. Quanti cation of parameters i. Land use/cover As described in Table 1a higher (66.26%) percentage of the study area is covered by crop land which allows surface water to in ltrate down to the ground. This in turn contribute the rise of ground water table of Wolaita zone. The remaining part of the area is covered by grass land (16.19%), shrub land (10.45%), Forest land (5.09%), wet land (1.19%), urban land (0.45%), water bodies (0.31%) and bare land (0.07%) respectively based on the extent of the area they have covered.  iii. Soil The in ltration and depercolation capacity of the direct rainfall and the surface ow is highly dependent on the soil type and characteristics of the study area (Wolaita zone). The pore space of the soil differ for different types of the soil as the higher the pore space allows the higher the in ltration capacity of the soil. covered by medium drainage density which is larger in extent as compared to the others and 8.74% of it is covered by very high drainage density which considered as low area coverage than the remaining ones (Fig. 10). On the basis of Fig. 11, the higher percentage (35.24%) of the total area is covered by low lineament density and the smallest area (1.44%) is covered by very high lineament density. It is evident that areas having higher lineament density have higher groundwater potential and vice versa [2,8]. Furthermore, in perennial and seasonal streams, the water contact time with the bed and banks is long that it provides good opportunity for percolation. Therefore, areas nearby drainage channels/water courses may have good groundwater potential [22].

V. slope and Rainfall
Slope is one of the factor which in uences the ow characteristics of a given watershed or area of interest. The higher the slope the steepest the land feature and the lower the slope the gentler or at surface would be existed. Therefore whenever the slope becomes gentler or at it will allow the water to enter to the subsurface of the earth and allows the ground water to be recharged. Otherwise while considering steep slope the surface ow of the water will not get enough time to enter to the subsurface of the land and the ground water recharge will be very low in effect. Within the study area, the effect of topographic gradient has signi cant in uence on the groundwater ux than the groundwater depth and hence the hydraulic gradient [5,10].
In the study area, almost 50% of the area is covered by high slope (5-7 degree) and very high slope (> 7 degree) which indicates half of the study area has a lower contribution for the ground water recharge ( Table 4). The remaining half of the study area would have a better contribution for ground water recharge. Rainfall is the main actor and in uential variable in hydrological cycle of surface and ground water hydrology. The higher or intense rainfall would cause surface water ow and recharging of ground water in a given watershed. In the northern part of the study area there is a high rainfall distribution and in the eastern part there is relatively moderate to high rainfall distribution. On the other hand, in the western part of Wolaita zone very low rain fall distribution was observed. Moderate rain fall distribution is situated in the north east, south east and North West direction of study area. Much of the study area is covered by low to moderate rainfall distribution (Fig. 8).

Determination of percentage of in uence for thematic layers
Each and every parameters had their degree of in uence on the ground water recharging capacity. Some might have major in uence on the others while some might have minor in uence on other determining parameters. According to Das S. et al., (2017) the total weight of each factor results from the sum of the measure of in uence for each parameter. The higher the weight of the parameter the higher the in uence on groundwater recharge potential and low in uence connotes low groundwater recharge potential as discussed in previous studies. The percentage in uence score was derived from the interrelationship among all the factors using Analytical hierarchical processing (Table 5). Analytical hierarchical processing (AHP) method is very helpful for multi-parameter assessment [17]. 3.3. Weightage of thematic overlays using AHP As described earlier analytical hierarchical process was used for weighted overlaying each thematic layers in order to decide which factor had a better in uence on others in determining ground water potential zones or areas of the study area. Scale was provided from 1 to 5 in order to assign factor of in uence (Table 6). Where 1 represents less importance and 5 represents most importance with respect to their in uence in recharging ground water. While determining factor of in uence of each parameters in AHP, the consistent ratio (CR) was 0.087 < 0.1 which was acceptable. From Table 6, above the highest in uencing factor is rainfall (44%) and the lowest in uencing factor was slope (3%).
A 7*7 matrix was produced for all parameters described as a factor for ground water potential assessment (Fig. 12). The normalized principal Eigen vector indicated that the percentage in uence of each parameters for the study area (Wolaita zone).

Ground water potential zones/areas
The ground water potential zones were delineated by using GIS and RS integrated with Analytical hierarchical processing. The weightage for each parameters was assigned depending on the in uence factor they were obtained from AHP analysis (Table 6). Therefore the nal ground water potential zone map was produced by overlaying all thematic layers considered as a factor for ground water recharging. Depending on the nal output "poor" and "good" groundwater potential areas were occupying almost the same percentage from the total area which is 44.19% (198,0445ha) and 44.51% (199,460ha) respectively. But "very good" ground water potential zones are covering 11.30% (50652.2ha) of the study area. Considering Fig. 13, poor ground water recharging is existed in the west and southern west direction of the study area and good ground water potential is also found in the northern east, northern west and southern east direction of Wolaita zone. More importantly very good groundwater potential zone is dominantly situated in the east and northern direction of the study area.
Therefore, while digging of the ground to explore the ground water better to start from the north and east direction of the place since there is very good potential of ground water is available here. Wolaita zone is found in southern nation nationalities regional state of Ethiopia which was clustered in to twelve woredas naming Kindo Didaye, Ofa, Kindo Koyisha, Sodo zuriya, Humbo, Damot woide, Duguna Fango, Damot gale, Damot pulasa, Damot sore, Boloso bombe, and one main town called Sodo town. In these clustered woredas the ground water potential zones were analyzed with their percentage of area coverage (Table 7). High percentage of very good ground water potential is found in Boloso sore, Duguna Fango, Boloso bombe and Damot sore woredas as viewed in Fig. 14. Relatively poor ground water potential is situated in Kindo Koyisha, Kindo Didaye, Ofa, Humbo and Sodo zuriya woredas and the remaining part of Wolaita zone is covered by good ground water potential in Wolaita zone. Each woredas of Wolaita zone had classi ed with poor, good and very good with their area coverage and its percentage of area coverage as displayed on Table 7.

Conclusion
In recent years mapping and delineation of ground water potential becomes highly implemented due to the fact that the demand for water resources is also increasing. High potential of water resources is hugely relying on ground water resources than surface water especially in Ethiopia. Different parameters were analyzed and overlaid to quantify and delineate ground water potential of Wolaita zone particularly. Geology, land use/cover, rain fall, slope, soil, drainage density and lineament density are among the factors used for ground water potential analysis. GIS and remote sensing application integrated with Analytical hierarchical process (AHP) was used to analyze and process the data.
Therefore based on the output resulted from data analysis and interpretation, the study area ground water potential zones were classi ed under poor, Good and very good as per the ground water resource existence and availability. Poor and good ground water potentials are relatively equal in area coverage than very good ground water potential area. The study area was also clustered with twelve woredas having their own ground water potential zone and their area coverage was analyzed to prioritize better ground water potential areas.
This research output is very essential to easily identify the ground water potential zones in Wolaita zone and would be taken as a reference to evaluate and identify the ground water potential zone in other parts of Ethiopia. In addition it could be a bench mark for policy makers to draft water policies regarding with sustainable management of water resources.

Declarations
Author contribution statement Tesfaye Hailu Estifanos: data preparation, synthesizing input information, materials, analyzing the data, interpret and write up of the information, wrote the full paper Funding statement This work was not supported by any organization or institutions.

Data availability statement
Data will be made available on request.

Declaration of competing interest statement
The authors declare no con ict of interest. Figure 1 Location map of Wolaita zone Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.  soil map of Wolaita zone Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 4
Land use/cover map of Wolaita zone Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors. Figure 9 ow chart to map ground water potential areas in Wolaita zone Figure 10 Drainage density Vs area coverage of the study area Figure 11 Lineament density Vs area coverage in Wolaita zone Matrix in AHP for each parameters Figure 13 delineated ground water potential zone of Wolaita zone Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors. Figure 14 spatial distribution of ground water potential zone in each woreda of Wolaita zone Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.