3.1. Quantification 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 infiltrate 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.
Table 1
Land use/cover of Wolaita zone
FID
|
Id
|
grid code
|
LULC_Name
|
Area (Ha)
|
percentage of area
|
0
|
1
|
4
|
crop Land
|
2989.54
|
66.26
|
1
|
2
|
7
|
Bare Land
|
2.97701
|
0.07
|
2
|
3
|
3
|
Grass Land
|
730.246
|
16.19
|
3
|
5
|
1
|
Forest Land
|
229.641
|
5.09
|
4
|
7
|
10
|
water bodies
|
14.1136
|
0.31
|
5
|
10
|
2
|
Shrub Land
|
471.391
|
10.45
|
6
|
12
|
5
|
wet Land
|
53.4903
|
1.19
|
7
|
165
|
8
|
Urban Land
|
20.2626
|
0.45
|
ii. Geology
Results from Table 2 indicated that the tertiary intrusive rocks (Ti) is dominant and covered 75.41% of the total area of Wolaita zone. Quaternary volcanic flow rocks (Qv), Alluvium (Q) and water (H2O) are covering 22.23%, 1.34% and 1.02% of the total area respectively.
Table 2
FID
|
Perimeter
|
Geology symbol
|
Geology Type
|
Area (Ha)
|
percentage of area
|
0
|
29.87222
|
Q
|
Alluvium (mostly Holocene), Quaternary no marine and marine
|
6,028.50
|
1.336189782
|
1
|
150.24724
|
Ti
|
Tertiary intrusive rocks
|
340,237
|
75.41199351
|
2
|
0.26414
|
H2O
|
water
|
4,583.44
|
1.015898763
|
3
|
58.47867
|
Qv
|
Quaternary volcanic flow rocks
|
100,322
|
22.23591794
|
iii. Soil
The infiltration and depercolation capacity of the direct rainfall and the surface flow 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 infiltration capacity of the soil.
Table 3
soil types in Wolaita zone
FID
|
Perimeter
|
Lands unit
|
FAO class
|
Dominant Soil
|
Major soil
|
Area (Ha)
|
percentage of total area
|
0
|
13.3516
|
Rgv
|
Lithosols/Eutric Cambisols
|
I-Be,s,l
|
Lithosols
|
101,919
|
22.59015741
|
1
|
4.51105
|
Vt1
|
Vitric Andosols
|
Tv,so
|
Vitric Andosols
|
12,234.2
|
2.711687749
|
2
|
3.428
|
Vs2
|
Dystric Nithosols
|
Nd'-I
|
Dystric Cambisols
|
87,200.3
|
19.32778484
|
3
|
0.802197
|
Vp1
|
Pellic Vertisols
|
Vp
|
Pellic Vertisols
|
31,346.2
|
6.947827118
|
4
|
1.07143
|
Vj1
|
Eutric Nithosols/Pellic Vertisols
|
Ne'-Vp
|
Eutric Nithosols
|
24,355.9
|
5.398440082
|
5
|
0.436424
|
Vh1
|
Chromic Luvisols
|
Lc,s'
|
Chromic Luvisols
|
38,942.6
|
8.631555096
|
6
|
3.08954
|
Rt1v
|
Chromic Vertisols/Eutric Nithosols
|
Vc'-Ne
|
Chromic Vertisols
|
108,241
|
23.99141699
|
7
|
0.975981
|
Rh2v
|
Orthic Acrisols/Dystric nithosols
|
Ao,s-Nd
|
Orthic Acrisols
|
45,419.6
|
10.06717014
|
8
|
2.15748
|
Lake
|
Lake
|
lake
|
water
|
500.135
|
0.110853996
|
9
|
0.619333
|
Af1
|
Eutric Fluvisols
|
Je
|
Eutric Fluvisols
|
1,006.58
|
0.223106591
|
As indicated in Table 3, almost 46.58% of the study area is covered by chromic vertisols and Lithosols. The remaining 53.42% of the area is covered by the other eight types of soils like Dystric cambisols (19.33%), Orthic acrisols (10.07%), chromic luvisols (8.63%), pellic vertisols (6.95%), Eutric Nithosols (5.398%), vitric andosols (2.71%), Eutric Fluvisols (0.22%) and water (0.11%) respectively.
iv. Drainage density and Lineament density
Drainage and lineament characteristics of a watershed provide important clues about the hydrogeology of the area [6, 7]. Around 33.17% of the study area is 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 influences the flow 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 flat surface would be existed. Therefore whenever the slope becomes gentler or flat 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 flow 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 significant influence on the groundwater flux 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.
Table 4
slope characteristics of Wolaita zone
FID
|
grid code
|
Area in Ha
|
Slope in degree
|
steepness
|
Percentage of total area
|
0
|
1
|
20,395.9
|
0–1
|
Very low
|
4.520
|
1
|
2
|
108,723
|
1–3
|
Low
|
24.099
|
2
|
3
|
91,360.1
|
3–5
|
Medium
|
20.251
|
3
|
4
|
52,541.8
|
5–7
|
High
|
11.646
|
4
|
5
|
178,126
|
> 7
|
Very High
|
39.483
|
Rainfall is the main actor and influential variable in hydrological cycle of surface and ground water hydrology. The higher or intense rainfall would cause surface water flow 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).
3.2. Determination of percentage of influence for thematic layers
Each and every parameters had their degree of influence on the ground water recharging capacity. Some might have major influence on the others while some might have minor influence 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 influence for each parameter. The higher the weight of the parameter the higher the influence on groundwater recharge potential and low influence connotes low groundwater recharge potential as discussed in previous studies. The percentage influence 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].
Table 5
Analytic Hierarchy Process (AHP) according to satty (1980)
Importance
|
Equal
|
Weak
|
Moderate
|
Moderate plus
|
Strong
|
Strong plus
|
Very strong
|
Very, very strong
|
Extreme
|
scale
|
|
1
|
|
|
2
|
|
|
3
|
|
|
4
|
|
|
5
|
|
|
6
|
|
|
7
|
|
|
8
|
|
|
9
|
|
1/9
|
|
1/8
|
1/7
|
|
1/6
|
1/5
|
|
1/4
|
1/3
|
|
1/2
|
1
|
|
2
|
3
|
|
4
|
5
|
|
6
|
7
|
|
8
|
9
|
|
|
|
|
|
|
3.3. Weightage of thematic overlays using AHP
Table 6
weightage or factor of influence of each thematic layers and influencing parameters
no
|
Component
|
classifications
|
Default value
|
Adjusted scale
|
Percentage of influence (%)
|
1
|
Rain fall (mm/yr.)
|
1,534.52-1,540.37
|
1
|
1
|
44
|
1,540.37-1,544.58
|
2
|
2
|
1,544.58-1,549.72
|
3
|
3
|
1,549.72-1,556.03
|
4
|
4
|
1,556.03-1,564.33
|
5
|
5
|
2
|
Geology
|
Quaternary alluvium (Q)
|
1
|
4
|
16
|
Tertiary intrusive rock (Ti)
|
2
|
3
|
Water (H2O)
|
3
|
5
|
Quaternary volcanic flow rocks (Qv)
|
4
|
2
|
3
|
Land use/cover
|
Crop land
|
1
|
3
|
15
|
Bare land
|
2
|
1
|
Grass land
|
3
|
2
|
Forest land
|
4
|
3
|
Water bodies
|
5
|
5
|
Shrub land
|
6
|
3
|
Wet land
|
7
|
4
|
Urban land
|
8
|
1
|
4
|
soil
|
Lithosols
|
1
|
2
|
4
|
Vitric andosols
|
2
|
2
|
Dystric cambisols
|
3
|
2
|
Pellic vertisols
|
4
|
3
|
Eutric nithosols
|
5
|
2
|
Chromic luvisols
|
6
|
3
|
Chromic vertisols
|
7
|
3
|
Orthic acrisols
|
8
|
3
|
Lake
|
9
|
5
|
5
|
Drainage Density (Km/Km2)
|
0.0431–0.7175
|
1
|
1
|
8
|
0.7175–0.9642
|
2
|
2
|
0.9642–1.1780
|
3
|
3
|
1.1780–1.4412
|
4
|
4
|
1.4412–2.1402
|
5
|
5
|
6
|
Lineament Density
(Km/Km2)
|
0-0.2927
|
1
|
1
|
10
|
0.2927–0.5958
|
2
|
2
|
0.5958–0.9303
|
3
|
3
|
0.9303–1.5260
|
4
|
4
|
1.5260–2.6653
|
5
|
5
|
7
|
Slope (degree)
|
0–1
|
1
|
5
|
3
|
1–3
|
2
|
4
|
3–5
|
3
|
3
|
5–7
|
4
|
2
|
> 7
|
5
|
1
|
summation
|
100
|
As described earlier analytical hierarchical process was used for weighted overlaying each thematic layers in order to decide which factor had a better influence 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 influence (Table 6). Where 1 represents less importance and 5 represents most importance with respect to their influence in recharging ground water. While determining factor of influence of each parameters in AHP, the consistent ratio (CR) was 0.087 < 0.1 which was acceptable. From Table 6, above the highest influencing factor is rainfall (44%) and the lowest influencing 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 influence of each parameters for the study area (Wolaita zone).
3.4. 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 influence factor they were obtained from AHP analysis (Table 6). Therefore the final ground water potential zone map was produced by overlaying all thematic layers considered as a factor for ground water recharging. Depending on the final 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.
Table 7
Groundwater potential area coverage of each woreda found in Wolaita zone
zone
|
woreda
|
Ground water potential area coverage (ha)
|
Percentage of area (%)
|
poor
|
good
|
Very good
|
Poor
|
good
|
Very good
|
Wolaita
|
Kindo Didaye
|
37,102
|
365.902
|
0
|
99.02343
|
0.976575
|
0
|
Ofa
|
37,756.04
|
589.1281
|
0
|
98.46362
|
1.536382
|
0
|
Kindo Koyisha
|
31,826.68
|
20,513.69
|
5.8668
|
60.80032
|
39.18847
|
0.011208
|
Sodo zuriya
|
22,854.3
|
17,485.3
|
95.1474
|
56.52144
|
43.24325
|
0.235311
|
Humo
|
51,699.8
|
33,435.4
|
0
|
60.7267
|
39.2733
|
0
|
Damot woide
|
7,005.634
|
28,145.57
|
0
|
19.93
|
80.07
|
0
|
Duguna Fango
|
6.32471
|
30,630.4
|
9,025.19
|
0.015947
|
77.22875
|
22.75531
|
Damot gale
|
6,400.049
|
19,039.95
|
0
|
25.15743
|
74.84257
|
0
|
Damot pulasa
|
784.247
|
14,990.7
|
659.621
|
4.771936
|
91.21445
|
4.013619
|
Damot sore
|
340.3196
|
14298.76
|
3452.728
|
1.88107
|
79.03445
|
19.08448
|
Boloso sore
|
0
|
3,375.515
|
26,658.08
|
0
|
11.23913
|
88.76087
|
Boloso Bombe
|
281.1566
|
15,677.30
|
10,741.29
|
1.053031
|
58.71704
|
40.22993
|
Sodo town
|
1,853.703
|
707.9337
|
0
|
72.36401
|
27.63599
|
0
|
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 classified with poor, good and very good with their area coverage and its percentage of area coverage as displayed on Table 7.