Forum Geografi, 35(2), 2021; DOI: 10.23917/forgeo.v35i2.13943

Erosion Analysis in the Mrica Reservoir Catchment Area in Indonesia using the Soil Erosion Status Method

Beny Harjadi^*

Watershed Management Technology Center (WMTC). Jl. Ahmad Yani, Pabelan PO BOX 295, Solo 57102 Indonesia

 

*) Corresponding Author (e-mail: adbsolo@yahoo.com)

Abstract

The reservoir catchment area (RCA) of Mrica in Banjarnegara district is a powerplant in Central Java with a capacity of 184.5 MW. Mrica Dam, is also called the dam of Great Commander General Sudirman, has seen a gradual decrease in its functions due to sedimentation from massive erosion, especially in the upland regions. RCA Mrica, with the upland area in Wonosobo district, has an area of ​​93,546.4 hectares, consisting of six sub-watersheds, Lumajang, Serayu upstream, Begaluh, Serayu, Tulis and Merawu. In 2017, sedimentation in the dam reached 238,236,588.20 m3/year, resulting from an erosion rate of 524,948.33 tons/year. Considering this serious erosion problem in the Mrica RCA, this study aims to estimate the distribution of the erosion level, categorised as slight, moderate and severe, using the SES (soil erosion status) formula. SES was calculated by mapping the level of each influential parameter: aspect, drainage, land cover, slope and soil texture. The calculation used SRTM (Shuttle Radar Topography Mission) satellite imagery and 2017 Landsat TM7 images. The results show slight erosion (<50 tons/ha/year) in 1,468.7 ha (1.6%); moderate erosion (50-100 tons/ha/yr) in 56,258.8 ha (60.1%); and severe erosion (> 100 tons/ha/year) in 35,818.9 ha (38.29%). Sampling in the field took into account the slope class of the nine classes and was repeated three times, so the number of samples taken in the field was 27. From field visits to the 27 location points, there was conformity in the results of the sensing analysis is much more than 85%.  The results of the erosion calculation using the SES method showed severe erosion of 27.9% (26,137 ha); moderate erosion of 70.2% (65,679 ha); and slight erosion of 1.8% (1,731 ha).  Further erosion calculation using the SES method needs to be compared with calculation using other methods.

1. Introduction

Mrica reservoir, located in Banjarnegara district, is the location of an electricity power station serving several regions in Central Java under PT. Indonesia Power management. The reservoir also functions as a tourist attraction and for irrigation of the downstream area. Considering the importance of the reservoir functions, the area upstream of the reservoir catchment area (RCA)of Mrica needs to be managed properly to avoid land degradation. The Mrica Dam only has a capacity of 101,990,000 m³, so it can be said that it is in good condition, but the observation in the field shows that it is not able to accommodate the volume of sediment transportation that occurs annually.

According to Sunandar et al. (2013), the reservoir capacity was 101,990,000 m³ and the sediment that entered the reservoir was 238,236,588 m³/year, with an overflow sediment of 136,246,588 m³/year. The severe sedimentation in Mrica has shortened the reservoir’s lifetime, which was previously estimated to be up to 100 years (1988-2088) (Sunandar et al., 2013).

Mrica reservoir is a dam for the hydropower electric company Perusahaan Listrik Tenaga Air (PLTA), commonly known as PLTA Panglima Besar Sudirman. The reservoir was built by damming the Serayu River and inundating 32 villages in seven sub-districts. It began to be inundated in April 1988. The reservoir collects rainfall from the area upstream of Serayu River and is located in Bawang sub-district, Banjarnegara, Central Java Province. The construction of the reservoir was allocated to PLTA, with a 180.93 MW capacity, and is used for the irrigation of an area of 6,550 ha, for the fisheries of the karamba system (bamboo cage), and as a tourism location (Wulandari, 2007). It is located at an elevation +231 m above sea level (a.s.l.), with an inundated area of 8.85 km². The reservoir can collect 140 million m³ of water, with a flow of 11 m³/second. Based on the results of a sedimentation analysis, it was estimated that reservoir age for sediment accumulation ranged from 38.5 to 46 years old (Hanafi, 2015). However, land use changes that occur in the reservoir catchment area will change reservoir capacity and affect the hydrology.

Land use changes and intensive cropping patterns sometimes generate benefits; however, they also put the area at risk due to intensive erosion, environmental damage and flash floods. The occurrence of massive erosion will lead to reservoir silting, decrease the volume capacity of the irrigation canals and disrupt the power plant system. Another negative impact of erosion is that it can also degrade the quality and quantity of natural resource conditions.

Erosion that occurs in the upstream area will have an impact downstream, such as on reservoir sustainability, human activity and the environment. The ongoing erosion process will also lead to changes in soil structure aggregate and fertility level, in turn affecting the productivity of agricultural land. An increase in erosion will increase sedimentation and affect electricity supply and irrigation water capacity.  Erosion calculations using the MUSLE (Modification of the Universal Soil Loss Equation) method in Mrica showed 16,775,896 tons/year or 11,183,931 m³/year, or a reduction in soil thickness of 16 mm/year (Hanafi, 2015). With the USLE (Universal Soil Loss Equation) method, the calculation of erosion is 524,948.33 tons/ha/year, with the sedimentation of 238236588.20 m³/year (Sunandar et al., 2013).

The process of erosion in the upstream area and the accompanying sedimentation in the downstream area will adversely affect the reservoir’s lifetime. The process of erosion, therefore, needs to be studied to identify the distribution of the erosion level and to provide countermeasures. With the development of remote sensing technology, qualitative erosion can be calculated with satellite image analysis and radar imagery. Both imagery types can be calculated and analysed to obtain erosion distribution from slight to severe levels. The calculation results can help in the planning process before prevention steps and countermeasures are taken so that continuous erosion can be avoided. Satellite and radar imagery will reduce the time needed for surveys and field checks. On the other hand, if there is no satellite imagery, a census survey of the whole RCA/reservoir watershed area needs to be conducted.

Intensive land management, unsuitable land use in terms of land capability and unmatched site species will lead to land becoming infertile more quickly. To study land degradation levels in hilly and mountainous areas, erosion needs to be calculated. The objective of this research is to calculate and map the distribution of qualitative erosion classes using the SES method and satellite images of RCA Mrica. The SES method not only calculates the total erosion accumulated into the sediment in the reservoir but also calculates each unit of land in the field, so it is more detailed than the previous erosion calculation method. The advantage of calculating erosion using the SES method is that it can be done quickly and throughout the catchment area, while its weakness is that it only produces a qualitative calculation of erosion, but does not determine the amount of erosion that has occurred.

2. Research Method

The RCA Mrica research location is located at the latitude and longitude coordinates 7^o09’-7^o30’ S and 109^o36’-109^o65’ E. RCA Mrica is between Wonosobo and Temanggung regencies and is surrounded by four other regencies: to the west is Purbalingga regency; to the east Temanggung regency; to the north Pekalongan regency; and the south of Kebumen regency. The research site includes areas that often experience surface and landslide erosion, meaning the land easily experiences a degradation in soil fertility.  This is due to the existing land use is not following the land use capability (LUC) class.

The research method was based on qualitative calculations by conducting field surveys and also analysis of satellite image data. The field surveys involved collecting biophysical data from river conditions, slopes, slope direction, soil texture and land cover. To complete the erosion analysis, the primary data were supported by secondary data from the field. The field surveys were also intended to validate the results of the qualitative erosion analysis calculations using the soil erosion status (SES) method. According to Trisakti (2014), the qualitative erosion calculation method is very suitable for long-term planning, while quantitative erosion calculation for producing definite erosion figures is more for short-term planning.

Figure 1.   Flow chart SES analysis in the catchment area of Mrica reservoir.

Satellite images were taken in October 2014 including Shuttle Radar Topography Mission (SRTM) images and Landsat imagery 8. The SRTM images were employed for analysis of the drainage density, slope direction, slope and soil texture, while the Landsat 8 images were used for the analysis of land cover and use class distribution. Figure 1 shows a flow chart of the satellite image analysis methods, starting from radiometric and geometry correction to the qualitative erosion calculation SES results. Based on the SRTM imagery, maps were made, including drainage maps, slope direction maps, slope maps and texture maps. For the Landsat 8 images, classification of land cover and land use was made to produce a land cover map.

All five maps were further analysed by slicing or impact classification of the erosion. This resulted in the erosion class; i.e., (a) slight <50 tons/ha/yr, (b) moderate = 50-100 tons/ha/year and (c) severe > 100 tons/ha/yr. The results from the classification analysis of each parameter then generated an erosion map for each parameter. These comprised (a) a map of erosion caused by drainage, (b) a map of erosion due to differences in slope direction, (c) an erosion map due to slope, (d) an erosion map due to differences in soil texture and (e) erosion map based on land cover. Sampling in the field was distributed throughout the RCA Mrica area, representing nine different slope classes, each of which was repeated three times, meaning there were nine slopes x 3 three replications, equal to 27 samples. Validation of the accuracy level of the remote sensing analysis was made by comparing the erosion conditions in the field and the level of erosion on the map, which was expected to have an accuracy of more than 80%.

From the five erosion class maps, the average erosion that had occurred was then calculated, resulting in the qualitative SES erosion.  Validation of the erosion calculation results was made by checking the location to compare the erosion class on the map with the actual conditions in the field. The provision for drainage maps referred to drainage speed, as shown in Table 1 (Harjadi, 2015).

No	Description	mm/hour	Class
1	Good	>125	3
2	Very good	65-125	3
3	Moderate	20-65	2
4	Slightly slow	5-20	2
5	Slow	1-5	1
6	Very slow	<1	1

Table 1. Class provisions for drainage parameters.

 

The aspect map, or the direction of the sunlight onto the slopes, was based on Table 2.

Table 2. Class provisions for aspect parameters.

No	Symbol	Description	Degree	Class
1	N	North	22.5	1
2	NE	North-east	67.5	1
3	E	East	112.5	2
4	SE	South-east	157.5	3
5	S	South	202.5	3
6	SW	South-west	247.5	3
7	W	West	292.5	2
8	NW	North-west	337.5	1

 

The slope Map, from flat class to slightly sloping up to precipitous, was based on Table 3.

 

Table 3. Class provisions for slope parameters.

Slope	%	Description	Class
A	0 - 4	Flat to slightly sloping	1
B	4 - 8	Gently sloping	1
C	8 - 15	Moderately sloping	1
D	15 - 25	Strongly sloping	2
E	25 - 35	Moderately steep	2
F	35 - 45	Steep	2
G	45 - 65	Very steep	3
H	65 - 85	Extremely steep	3
I	> 85	Precipitous	3

 

There were 12 classes for the texture map, from rough to fine texture, as shown in Table 4.

Table 4. Class provisions for texture parameters.

No	Symbol	Texture	Class
1	S	Sand	3
2	LS	Loamy Sand	3
3	L	Loam	1
4	SL	Sandy Loam	1
5	SiL	Silty Loam	1
6	Si	Silty	3
7	SCL	Sandy Clay Loam	2
8	SiCL	Silty Clay Loam	1
9	CL	Clay Loam	2
10	SC	Sandy Clay	1
11	SiC	Silty Clay	1
12	C	Clay	2

 

The land cover map consisted of eight types of cover, using the criteria shown in Table 5.

Table 5. Class terms for land cover parameters.

No	Land Cover	Class
1	Forest	1
2	Rice field	1
3	Open land	2
4	Vegetable garden	2
5	Village	2
6	Bush	2
7	Agroforestry	2
8	Wasteland	3

3. Results and Discussion

3.1. Results

a. Drainage

Light erosion occurred in areas with slow and very slow drainage, while severe erosion took place in areas with good or very good drainage. According to Tingsanchali (2012), slow drainage due to closed channels can cause flooding.

Class	Criterion	Drainage	%	Ha
1	Slight	Slow, Very slow	1.6	1,468.7
2	Moderate	Slight slow, Moderate	60.1	56,258.8
3	Severe	Good, Very good	38.3	35,818.9
			100	93,546.4

Table 6. Erosion classes of each level of drainage.

 

The most widespread erosion in areas with moderate drainage accounted for 56,258 ha, with light erosion-affected areas with slow drainage at 1,468 ha (Table 6).  Prevention of erosion in areas with rapid drainage can be overcome using soil conservation constructions mechanically, vegetatively and biologically. The distribution of moderate erosion that dominates the area with medium drainage can be seen in Figure 2. The distribution of severe erosion of 38.3% or 35.818 ha is mostly spread over the upper (upstream) region and a small part of the central area.

Figure 2. Map of erosion class with different drainage.

b. Aspect

Slope directions affect the occurrence of erosion. The southward slope areas (SW = south west, S = south, and SE = south east) are the most prone to erosion, at a level of around 49.4%. According to Lee and Pradhan (2007), aspect is one of the factors which causes landslides, together with other factors such as slope, lithology and land cover.

Table 7. Erosion classes of each aspect level.

Class	Criterion	Aspect	%	Ha
1	Slight	NW,N,NE	24.6	23,003.0
2	Moderate	W,E	26.0	24,322.1
3	Severe	SW,S,SE	49.4	46,221.3
			100	93,546.4

 

Severe erosion caused by the south facing area is 46,222 ha and is dominant in the RCA Mrica area (Table 7).  As suggested, in the RCA Mrica for south facing land should be managed carefully, as this land is easily eroded. Figure 3 shows that the distribution of severe erosion due to the predominantly south-facing slope conditions is spread over 49%.

Figure 3. Map of erosion class with different aspects.

 

c. Slope

The steeper the slope, the greater the potential for erosion. The majority of the slopes (60.1%.) at RCA Mrica are moderate, comprising slope D (15-25%), E (25-35%) and F (35-45%). According to Trisakti (2014), the slope factor is one of the contributors to erosion speed and leads to the dredging of the RCA (watershed) below it.

Table 8. Erosion classes of each level of slope.

Class	Criterion	Slope	%	Ha
1	Slight	A,B,C	1.57	1,468.7
2	Moderate	D,E,F	60.14	56,258.8
3	Severe	G,H,I	38.29	35,818.9
			100	93,546.4

 

From the slope factors, a medium erosion of 56.258.8 ha, or an area of 60.1% which dominates the RCA Mrica area, can be seen (Table 8).  Figure 4 shows that in the RCA Mrica map, severe erosion is located in the upper (upstream) area, and light erosion is in the lower area, or near the river. Moderate erosion spread throughout the RCA Mrica area.

Figure 4. Map of erosion class with different slopes.

 

d. Texture

The softer the (clay) soil texture is, the more easily the land will be eroded, while the coarser texture (sand) will be resistant to erosion. Regarding the texture factor, most of the land in RCA Mrica is classified as moderate erosion, at around 66,034 ha (70.6%). According to the US Department of Agriculture (USDA) (2011), 12 texture classes affect soil sensitivity to erosion.

Table 9. Erosion classes for each level of texture.

Class	Criterion	Texture	%	Ha
1	Slight	SiC, SC, SiCl, L, SiL, SL	27.3	25,538.2
2	Moderate	CL, C, SCL	70.59	66,034.4
3	Severe	S, LS, Si	2.11	1,973.8
			100	93,546.4

 

Texture conditions in RCA Mrica are dominated by the moderate class, meaning they are not too subtle or too rugged and are included in the silt texture class (medium) (Table 9). There is very little heavy texture. only 1.973 ha (2.1%).  Figure 5 shows a map of the distribution of moderate-dominated erosion, that spreads evenly from the top (upstream) to the bottom (downstream) of the area.

Figure 5. Map of erosion class with different soil textures.

 

e. Land Cover

From the existing land cover in RCA Mrica, there are medium and light erosion levels. Table 10 shows that erosion is dominated by 51.47% (48,148 ha) of moderate class and by light erosion at 47.98% (44,883 ha). Land cover conditions like this are sufficient to prevent erosion. According to Yan et al. (2016), more open land cover change will increase land degradation.

Table 10. Erosion classes for each level of land cover.

Class	Criterion	Land Cover	%	Ha
1	Slight	Forest, Rice field	47.98	44,883.5
2	Moderate	Vegetable, Village, Bush, Agroforestry	51.47	48,148.3
3	Severe	Wasteland	0.55	514.5
			100	93,546.4

 

The erosion in the RCA Mrica area is predominantly at moderate and light levels, with slight erosion at a level of 0.55% (514 ha). Figure 6 shows the distribution of moderate (51%) and light (48%) erosions in the RCA Mrica area when calculated from the land cover factor. In areas with moderate erosion, reforestation is required to allow more land to be covered by vegetation, thus reducing the erosion rate.

Figure 6. Map of erosion class with different land cover.

 

The results of the calculation for each parameter can be seen in Table 11; the five erosion factors resulted in almost equal erosion levels, all dominated by moderate erosion. Furthermore, severe erosion conditions are influenced by the aspect factor, drainage and slope.

Table 11. Erosion classes of each factor.

Percentage (%)	Aspect	Drainage	Land Cover	Slope	Texture
1	24.6	1.6	48.0	1.6	27.3
2	26.0	60.1	51.5	60.1	70.59
3	49.4	38.3	0.6	38.3	2.11
Area (ha)	Aspect	Drainage	Land Cover	Slope	Texture
1	23,003	1,469	44,884	1,469	25,538
2	24,322	56,259	48,148	56,259	66,034
3	46,221	35,819	515	35,819	1,974

 

Figure 7 shows that the most predominant erosion is moderate; the exception for aspect factor and slope direction which are predominantly severe erosion. Soil texture and land cover factors make very little contribution to severe erosion.

Figure 7. Erosion class with different factors.

The results of the erosion calculation using the SES method were moderate erosion 70% (65,678 ha), severe erosion 28% (26,137 ha) and 2% light erosion (1,731 ha) (Table 12).

 

Table 12. Qualitative erosion with the SES method.

Class	Criterion	%	Ha
1	Slight	1.85	1,730.6
2	Moderate	70.21	65,678.9
3	Severe	27.94	26,136.9
		100	93,546.4

 

Figure 8 shows a map of erosion distribution in the RCA Mrica area, which is dominated by moderate levels that are evenly distributed in the upper, middle and lower regions.

Figure 8. Map of qualitative erosion at Mrica dam.

3.2. Discussion

From the five factors that influence erosion, aspect, drainage, land cover, slope and texture, average erosion can be calculated, meaning the results refer to qualitative erosion. According to Saiya, Dibyosaputro, and Santosa (2016), erosion calculations using USLE also consider the biophysical factors of soil, especially soil texture and slope.  Different types of land use will also lose soil or erosion will occur (Martínez-Valderrama et al., 2016).

Erosion at RCA Mrica is dominated by moderate erosion, at 70.2%, or an area of 65,678.9 ha. Moderate and uniform erosion throughout the area is a serious problem that needs to be addressed immediately; for example, using soil and water conservation measures. Soil conservation can be performed mechanically as well as vegetatively; vegetative erosion control uses plants that can control erosion (Pasaribu, Rauf, & Slamet, 2018). Land use such as grasses and arid climatic conditions will increase the intensity of erosion (Jianga et al., 2018).

The conditions of erosion distribution in the RCA Mrica area are dominated by moderate erosion that is evenly distributed from the upper to the lower slopes, while the remaining instances include severe erosion and very little area that includes slight erosion. Severe erosion occurs in the upstream or mountainous areas and hills or on the RCA Mrica borders. The level of erosion that occurs is not only determined by the height of the location, but also by the K factor, soil erodibility, or the sensitivity of the soil to erosion (Sulistyo, 2015). Integration of soil erosion will be a consideration or have an impact on the occurrence of land degradation heterogeneity (Al Sayah et al., 2021).

Severe erosion is common in sloping to steep terrains on mountain ranges and hills with landslide and gully erosion. On the other hand, moderate erosion occurs in middle regions, such as alluvial-colluvial regions, with moderate slopes and is dominated by sheet and rill erosion.  Light erosion on the lower basin that is not too steep is dominated by surface erosion due to open area conditions. This is in accordance with the conditions in the field, with the validation results obtaining an accuracy of more than 85%. Qualitative erosion calculations using the SES method in the Tulis sub-watershed also had an accuracy of more than 80% (Harjadi & Susanti, 2019). The severity of erosion occurring in a certain location also indicates that land degradation has occurred on a massive scale (Tsymbarovich et al., 2020).

Furthermore, for light erosion, it is possible to conduct soil conservation measures mechanically, vegetatively and biologically, with limited use of chemical fertilizers, which tend to damage the land as they are unable to improve soil structure aggregation. The role of the government is also very important in helping to handle erosion so that it will not be a cause of flood disasters due to river shallowing (Hutauruk et al., 2020). In order to reduce the risk, mitigation must be made by understanding the signs of disaster due to mild or severe erosion (Shimizu et al., 2020).

4. Conclusion

Qualitative erosion analysis can be made using the SES (Soil Erosion Status) method. The defining parameters for such calculation are aspect, drainage, land cover, slope and soil texture. From the erosion calculation using the SES method, erosion at RCA Mrica was shown to comprise slight erosion 1.85% (1,730.6 ha), moderate erosion 70.21% (65,678.9 ha) and severe erosion 27.94% (26,136.9 ha). Light erosion is when the erosion value is <50 tons/ha/yr, with moderate erosion at 50-100 tons/ha/yr, and severe at > 100 tons/ha/yr.

 

In areas with severe erosion, integrated soil conservation using civil, technical, vegetative, chemical and biological methods needs to be undertaken. If such techniques are performed incorrectly, they can become a contributor to sedimentation in reservoirs and silting of rivers. Besides being a contributor to sedimentation, land that has experienced severe erosion will suffer rapid degradation and land can become marginal more quickly. Severe erosion occurs mostly in mountainous areas and hills, or at the top border of RCA Mrica.

The erosion conditions that dominate at RCA Mrica are moderate and uniform, so management of the upper land should be conducted carefully and appropriate land conservation rules followed. Suggestions for areas with severe erosion levels include civil and mechanical soil and water conservation measures; e.g., by repairing terraces, drainage and gully plugs or retaining dams. For medium erosion, soil and water conservation action with a combination of civil and vegetative measures, with an improvement of the grass, cropping patterns, and vegetation choices that are suitable for the condition of land should be taken.

Acknowledgements

The authors would like to express their gratitude and appreciation to all the parties involved in the project; i.e., the researchers and technicians at WMTC (Watershed Management Technology Center), and stakeholders for assistance in the field survey. Thanks also to the electrical company manager in Central Java responsible for the functioning of the Mrica dam i.e. PT. Indonesia Power in Banjarnegara.

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