3.1

Factors influencing groundwater potential

To evaluate groundwater potential zones, five parameters: lineament, slope, rainfall, slope, and drainage were selected as the influential factors. Groundwater resource and occurrence is believed to be influenced and largely depended on those factors. The comprehensive research methods of the groundwater potential evaluation is shown in Figure 2.

Figure 2.

Flow chart of the methods for estimate the groundwater potential of the study area

Lineaments occur as straight, curvilinear, parallel or en-echelon features. Lineaments may represent fracture systems, discontinuity planes, faults, and shear zone in rocks. Lineaments can be identified on satellite images ⁴⁶. Lineaments were identified from Digital Elevation Model (DEM). DEM of study area were provided as DEMNAS published by Indonesian Geospatial Agency. DEMNAS has spatial resolution of 0.27 arc second. Lineaments layer usually is converted into measurable quantity such as density ^{36 37 38 39}. However, in this study lineaments were conventionally assigned and classified following their capacity to promote groundwater occurrence. This approach was performed in Groundwater Potentiality Index ⁴⁷. This approach for lineaments is suitable for hard rock geology condition where groundwater occurrence is mainly governed by fractures.

Slope is principal factor of the superficial water flow since it govern the effect of gravity on the water movement ⁴⁷. Slope map was generated from DEMNAS data using tools in ArcGIS 10.4. Then, slope map was presented in degree units. Rainfall data in study area was provided by BPS-Statistics of Banjarnegara. Rainfall is the main source of groundwater recharge ^{29 48}. It determines the amount of water which would percolate into the groundwater system ⁵⁰.

The lithology influences both the permeability of the aquifer rocks and the distribution of the fracture pattern ⁴⁷. Lithology map in this study was based on geological map in the scale of 1:50,000. The maps were published by Geological Survey Center of Indonesia. Those geological map were generated from remote sensing interpretation using several basic data including IFSAR, RADARSAT2, TERRASAR X, SRTM 30 m and 90 m; LANDSAT V and ETM +7; ASTER, ALOS (AVNIR), regional geology map and topographic map.

Basic data of drainage was available as hydrography layer which was provided by Indonesian Geospatial Agency. Drainage was processed as density. The drainage density is the ratio of the sum of the lengths of streams to the area of the grid ^{29 49 50}. Drainage density was calculated using grid size of 8.5 km² through the equation 1. This analysis was performed based on analysis which was conducted by Mohammadi-Behzad (2018) ³⁹. Where, ∑Di is the total length of all streams i (km) and A is the area of the grid (km²) ³⁹. The values obtained for each grid were plotted at the center of the grid, then drainage density map is produced for the area by kriging interpolation technique ³⁹.

3.2

Analytic Hierarchy Process

First step of AHP method is to assign the level of importance of each factors based on Saaty’s scale values. Consequently, all factors are compared in a pairwise comparison matrix. The weight which was assigned to different thematic layers were normalized using Saaty’s AHP techniques. To control and test the consistency and judgement of the assigned weights, Consistency Ratio (CR) is calculated. First step to calculate CR is to compute maximum eigenvalue (λ_max). Then, calculate the Consistency Index (CI) using equation 2, where n is number of factors. CR is resulted by dividing CI by RI (Ratio Index). The value of RI is given based on Saaty’s 1-9 scale. If the value of CR is less than 0.1, the judgement of weights is acceptable and consistent.

3.3

Overlay Analysis

All five thematic layer maps were integrated using ArcGIS 10.4 as a summation of overall groundwater influencing factors to produce the groundwater potential map (GPM) of study area. The following formula was used to estimate the groundwater potential map ^{18 51 52}.

where GPM is groundwater potential map, MC1–MC5 is the main criteria (1–5 thematic layer map), w is the weight of the thematic map, SC1–SC5 is the sub-criteria of each thematic layer map and r is the sub-criteria class ranking.

4.1

Weights and Classes of Layers

The weights for each factors were decided based on the local field experience and expert opinions. The comparison of importance level of all five thematic layers are shown in a pairwise comparison matrix (Table 1). Normalized weight is presented in Table 2. Based on calculation, Consistency Ratio (CR) of this research is 0.0095 which mean that the judgement of the pairwise comparison matrix is consistent. Hence, the assigned weight for lineament, slope, rainfall, lithology, and drainage are 0.3892, 0.2141, 0.1987, 0.1213, and 0.0767 respectively. Ranks was assigned to different class of the individual themes are presented in Table 3. The thematic maps for all layers are presented in Figure 3.

Table 1.

Pairwise comparison matrix for AHP processing

Table 2.

Normalized weights for thematic layers

Table 3.

Assigned rank for various classes of all thematic layers

Figure 3.

All five thematic maps and its classes

221 lineaments were identified from DEM data. Then, all lineaments were processed using buffer tools with the total width of 250 meters. Buffering of 250 meters width was conducted based on background fracturing zone according to fault zone model by Braathen & Gabrielsen (2000) ⁵³. Each lineament buffer zone was given a rank of 1-5 based on interpretation of its capacity to promote groundwater occurrence. Major long faults were given the highest rank of 5. Interconnected local faults and frequents faults were attributed with rank of 4. Local faults and frequent fractures were given the moderate rank. While, fractures and short lineaments were attributed with rank of 2 as they were believed as low groundwater storage potential. Lastly, area of no lineament were given the lowest rank of 1.

Slope of study area was classified into 5 classes as 0°-10°, 10°-20°, 20°-30°, 30°-45°, and >45°. Groundwater potential occurred in gentle slope to plain region as water flow is slow and the time is enough available to improve the infiltration of water to the underlying fractured aquifer ⁴⁷. Therefore, lower degree of slope was given higher rank than higher degree of slope. Slope of 0°-10° was given the highest rank of 5. Whereas, slope of more than 45° was given the lowest rank of 1.

Rainfall data of study area was obtained from BPS-Statistics of Banjarnegara. There are several limitation of rainfall of study area as follows (1) the location of rainfall station were unknown, (2) rainfall data were attributed based on administrative area of the sub-district, (3) only one of 4 sub-district which has complete annual rainfall data from the year of 2007 until 2016. There are 4 sub-district in the study area, Pagedongan sub-district which is located in the easternmost of the study area, Mandiraja sub-district which is located in the westernmost of the study area, Bawang sub-district which in the mid east, and Purwanegara sub-district which is located in the mid west. Pagedongan sub-district has average of annual rainfall of 4706 mm/year. Average of annual rainfall of 3331 mm/year was recorded in the Bawang sub-district. While, Purwanegara sub-district has average of annual rainfall of 3508 mm/year. Lastly, Mandiraja sub-district has average of annual rainfall of 3514 mm/year. Rainfall was classified into 2 classes, 3000-4000 mm/year which is given rank of 4, and 4000-5000 mm/year which is attributed with rank of 5 as expected to have highest groundwater potential.

Melange Luk Ulo Complex, Serpentinite, Mafic and Ultramafic, and Brecciated Rocks which were consisted of metamorphic and igneous rocks were attributed as the lowest groundwater potential due to lower permeability. Claystone Totogan Formation and Tuff Waturanda Formation were given rank of 2. Greywacke as the member of Luk Ulo Complex was given moderate rank of 3. While, Sandstone Waturanda Formation was attributed with rank of 4. The highest rank was assigned to Quaternary Terrace Deposits and Alluvium.

Drainage density of the study area ranging from 1.65 km/km2 to 5.23 km/km². Therefore, drainage density of the study are was classified into 4 classes: 1.65-2.1, 2.1-3.15, 3.15-4.2, and 4.2-5.25 km/km² as shown in Figure 3. The highest drainage density appeared in the eastern part of the study area. Higher drainage density were given lower rank, while lower drainage density were given higher rank. Hence, 4.2-5.25 km/km² was given the lowest rank of 1. Whilst, 1.65-2.1 km/km² was given rank of 4.

4.2

Groundwater Potential Zones Map

The systematic AHP analysis on weighted factors generated a groundwater potential zones map using raster calculator tool in ArcGIS software by integrating all thematic maps. The index of groundwater potential is ranging from 1.79 to 4.72. The classification of groundwater potential zone is based on equal interval method. Hence, the interval of 1-1.8, 1.8-2.6, 2.6-3.4, 3.4-4.2, and 4.2-5.00 is assigned to very low, low, moderate, high, and very high. Groundwater potential zones map is presented in Figure 4.

Figure 4.

Groundwater potential zones map

The groundwater potential zone at a glance is highly reflects lineament layer map. Almost to none of the study area is classified as very low groundwater potential zone. This class only covered a pixel which equivalent to area of 1529 m². The study revealed that 43.02% (37.12 km²) of the study area exhibits poor groundwater potential zones (Table 4). Poor groundwater potential zone is the largest index in the study area. Poor groundwater potential zone is characterized by having no lineament zone, slope more than 20°, lithology Melange Luk Ulo Complex, Serpentinite, Mafic and Ultramafic, Brecciated Rocks, Claystone Totogan Formation, and Tuff Waturanda Formation; and higher drainage density. Those rock assemblages consist of metamorphic rocks (amphibolite, serpentinite, schist and phyllite), igneous rock (granite, porphyry, gabbro, and basalt), and sedimentary rocks such as tuff, claystone, and shale.

Table 4.

Classification of groundwater potential zone

17.98% (15.52 km²) of the study area was classified as having high to very high groundwater potential zones. While moderate groundwater potential zone covered 38.99% (33.65 km²) of the study area. The presence of high to very high groundwater potential zone may pertain to the presence of interconnected local faults, frequent faults, and major long faults; greywacke, alluvium and quaternary unconsolidated terrace deposit, higher rainfall, gentle slope below 20°, and lower drainage density.

4.3

Result Validation

The occurrence and discharge of springs and bore wells were used for validation of groundwater potential map. Firstly, there are 52 springs and 2 bore wells in the study area which were discovered during the observation in the dry season of 2018. However, there are only 9 springs and 1 bore wells where appropriate measurement of discharge were conducted. The discharge of springs and bore wells ranging from 0.12 l/s to 2 l/s. This range of springs discharge is classified as sixth magnitude springs based on classification of springs by discharge according to Meinzer ⁵³. While, bore well yield of 2 l/s.

Based solely on the occurrence of springs and bore wells, 1 spring and 1 bore well are located in very high GPZ, 9 springs and 1 bore well in high GPZ, 30 springs in moderate GPZ, and 12 springs in low GPZ. While, no spring in very low GPZ. 30 springs are located in lineament zone. Apparently, high to very high GPZ and occurrence of springs correspond to lineaments.

The correlation of AHP raster values to the corresponding discharge of springs and bore wells showed a positive coefficient of determination (R²) of 0.80 (Figure 5). It shows that the groundwater potential zones map which was generated by using integration of RS, GIS, and AHP technique in the research area have a promising result.

Figure 5.

Plot of the AHP raster value to the corresponding discharge of springs and bore wells