Soil Erosion Rates at the Various Land Use Types of Ngrancah Sub Watershed, Indonesia*)

Submitted by admin on Tue, 05/11/2010 - 13:49.

Ilmu Hutan

The study area is covered by critical land and located at the upper area of Sermo dam. The area of the Ngrancah sub watershed is 2,128.9 hectares. In fulfilling the human’s requirement, there are some types of land use, includes: forest (0.72 %), mixed cropping (68.13 %), settlement (15.28 %), dry rice field (2.13 %), dry field (6.12 %) and dam area (7.63 %). The objective of this research is to study the erosion rate of each land use type.

The USLE Model was used for predicting the erosion rates, A = RKLSCP. The area was consisted of 77 land unit. The soil samples were taken from each land use. The research resulted that the average of erosion rates of the various land units were: at forested land (3.54 – 5.06 ton/ha/yr), dry rice field (2.74 – 9.21 ton/ha/yr), mixed cropping (66.72 – 224.41 ton/ha/yr), dry field (44.76 – 519.95 ton/ha/yr), and settlement (165.85 – 600.18 ton/ha/yr). The majority of the area has a moderate (68 %) and high (15 %) level of erosion rate. The Duncan Multiple Range test showed that the erosion rate of the settlement, dry field and mixed cropping were not significantly different. Besides, between mixed cropping, dry rice field and forest were also not significantly different. Based on this research result, it is suggested that the soil conservation technique is urgently required to control the land degradation.

Key words: soil, erosion, land use, USLE, Indonesia

*) This paper is part of the dissertation and has been accepted for presentation at the 2^nd International Conference on Human Habitat and Environment to be held on 15-16^th, June 2010 in Putra Nilai, Negeri Sembilan, Malaysia, held by Institute of Malay World and Civilization (ATMA) Universiti Kebangsaan Malaysia

1) Faculty of Forestry, Gadjah Mada University, Indonesia.

2) Faculty of Geography, Gadjah Mada University, Indonesia.

INTRODUCTION

The rapid increasing of the number of population in Indonesia, especially in Java, has a great impact to the use of natural resources, because of the increasing human needs such as food, clothing, and a place to live. All of these requirements can be fulfilled by the natural resources management. The need of land was increased when the economic crisis happened in Indonesia lately, which also has an impact to the land exploitation.

Land exploitation without considering its carrying capacity will result the land degradation such as: erosion, critical land, floods and drought, water pollution, and sedimentation. Thus, soil management should be managed appropriately and sustainability. One of the efforts is the maintenance of soil fertility and the application of land rehabilitation and soil conservation practices, including: 1) rehabilitation of critical land, 2) increasing soil productivity, and 3) flood control (Anonymous, 1998).

Ngrancah sub watershed is located in Kulon Progo, consists of 2,200 hectares. This area was chosen as a study area because:

a). At the outlet of this sub watershed laid the Sermo dam which functions as water supply, tourism, fish pound and as a form of water harvest.

b). the erosion rate and erosion hazard level of this area is considered high and very high.

c). the age of dam can be shorter than predicted the sedimentation rate is high.

d). the form of sub watershed is oval so it has a unique characteristic.

e). There is a variety of land use, slope, rainfall and soil type so the area is interested to study.

f). There is an obstacle in applying soil conservation technique to this area.

g). It is easy to get to the study area.

The previous research done by Suharno (1999) mentioned that 85 % of the land of study area has a bad and very bad quality. Moreover, he concluded that 77 % of land was classified as high and very high in erosion hazard level. Based on the explanation mentioned above, the erosion and critical rate of the study area should be solved urgently. The first step should be carried out is understanding and predicting the erosion rate of the study area. Identification and assessment of erosion problems could have an important influencing better land use and conservation practices (Kusumandari and Mitchell, 1997).

MATERIAL AND METHOD.

The USLE model is used for predicting the erosion rate, that is: A = RxKxLSxCP. Maps of soil, topography, and land use are required. Data of rainfall, soil characteristics, slopes, land use, and conservation practices were collected. The land unit map was made and field study includes soil observation, soil sampling, slopes and land use checking and the description of soil erosion forms were carried out. The soil samples then were analyzed in the laboratory.

1.Materials and tools

In this research some maps will be collected:

a. Geomorfological Maps.

b. Land use map of Ngrancah subwatershed.

c. Soil map of Ngrancah subwatershed.

d. Land slopes map of Ngrancah subwatershed.

e. Rainfall map of Ngrancah subwatershed.

f. Rainfall data of Ngrancah subwatershed.

g. Soil samples.

Tools that are required for this research: :

a. Scanner will be used for scanning the maps so that can be transferred from regular map to digital map.

b. Computer for data analysis and On screen digitizing using ArcViews 3.3 Software.

c. Printer.

d. Global Positioning System (GPS) for soil sampling and field checking.

e. Stationaries.

f. Soil Ring sampler.

g. Camera.

h. Plastic bag.

i. Label.

2.Data gathering

a. Primary data.

The primary data was collected from the field such as land use, soil samples, soil description/characteristics, vegetation, and slopes. The soil samples will be taken based on the land unit. The vegetation data were sampled and gathered by making vegetation plots in the field.

b. Secondary data.

The secondary data needed for this research are maps that mentioned in point 1 above.

Collecting soil samples from the field

The soil samples were taken from the field, both disturbed and undisturbed soils. The number of the samples were examined purposively. The soil laboratory anlysis includes: soil texture, soil structure, organic matter content and soil permeability.

Erosion prediction

The Universal Soil Loss Equation (USLE) that was developed by Weischmeier dan Smith (1978), was used in this research, that is:

A = R K L S C P

A = Soil erosion rate (ton/ha/th)

R = Erosivity factor (MJ.cm/ha.jam.th)

K = Erodibility factor (ton.ha.jam/ha.MJ.cm)

L = Length slope factor

S = Slope factor

C = Crops factor

P = Conservation practices faktor.

- Erosivity factor (R)

Erosivity factor will be calculated using Harper equation:

- Erodibility factor (K)

K factor can be calculated using 2 methods: a) equation and b) nomograph. When the percentage of silt and very fine sand is more than 70% so the use of the equation to calculate K factor is more appropriate. If the silt and very fine sand content is ≤ 70%, the use of nomograph is more appropriate.

K = 2.173 M^1.14(10^-4) (12-a) + 3.25 (b-2) + 2.5 (c-3)

100

Equation:

M = (% silt +very fine sand) x (100 - % clay)

a = % soil organic (table 2)

b = Code of soil structure (table 3)

c = Code of soil permeability (table 4).

When the texture data available is only % of sand, silt and clay, the percentage of very fine sand can be calculate as 1/3 of sand content. Also, when the quantitaive data of soil texture is not available, the M value can be determined by using table 1 below.

Table 1. M value (Hammer in Kusumandari, 2010).

Soil texture (USDA)	M Value	Soil texture (USDA)	M
Heavy clay Medium clay Sandy clay Light clay Sandy clay loam Silty clay Clay loam Sand	210 750 1213 1685 2160 2830 2830 3035	Loamy sand Silty clay loam Sandy loam Loam Silty loam Silt Unknown	3245 3770 4005 4390 6330 8245 4000

Table 2. Soil organic content (Hammer in Kusumandari, 2010).

Class	Percentage (%)
0 Very low 1 low 2 moderate 3 high 4 very high	<1 1 - 2 2.1 - 3 3.1 - 5 >5 (peat soil)

Table 3. Code of soil structure (Hammer in Kusumandari, 2010).

Type of soil structure	Value
very fine granular fine granular medium, coarse granular blocky, platy, massif	1 2 3 4

Table 4. Code of soil permeability (Hammer in Kusumandari, 2010).

Class	Cm/jam	Value
Rapid moderate to rapid moderate moderate to slow slow very slow	>25.4 12.7 – 25.4 6.3 – 12.7 2.0 – 6.3 0.5 – 2.0 <0.5	1 2 3 4 5 6

- Slope and slope length factors (LS)

The LS factors can be determined based on the slopes map and the value can be calculated using table 5 below.

Table 5. LS factor (Anonymous, 1998)

Slope (%)	LS factor
0 – 8 >8 – 15 >15 – 25 >25 – 45 >45	0.25 1.20 4.25 9.50 12.00

- CP faktors.

The CP factor can be determined by field checking of the land use and the soil conservation practices in the field and then the CP value can be found out using CP table.

To study the difference of erosion rate between the various type of land use, the statistical analysis was used.

RESULT AND DISCUSSION

Erosion is a process of the soil detachment by water or wind (Arsyad, 2006). Usually this process is followed by transportation and sedimentation. According to Rahim (2003), erosion can be defined as a process of soil detachment and transportation from one place to another by the movement of water, air and glacier. In tropical area, like Indonesia, the dominant energy is water. Erosion can be devided into two types: natural and human induced erosion. Natural erosion is an erosion as a result of soil formation, whereas human induced erosion is caused by the land cultivation without soil and water techniques application (Asdak, 2002).

Arsyad (2006) explained that erosion will cause the loss of topsoil which is usually more fertile and good for plants growth. The transported soil will be sedimented to the dam, lake, reservoir, irrigation channels, and on the agricultural area. So, the impact of erosion can be on site and off site.

Furthermore, Suripin (2004) emphasized that erosion research is very important as a standard to plan the land use, planting pattern and its intensity, land management and soil conservation technique. However, Arsyad (2006) said that the good erosion research with a clear objective in Indonesia is still less.

The erosion prediction of Ngrancah sub watershed was used the USLE model. This model is famous and mostly used in Indonesia (Suripin, 2004). USLE was reported to be the most widely used model for predicting rainfall erosion (Weischmeier and Smith, 1965, 1978 in Kinnel, 2004). The erosion rate prediction of Ngrancah subwatershed using the USLE method can be seen in Table 6.

Table 6. Result of Erosion Prediction of Ngrancah sub watershed.

	Land unit	Area (Ha)	R	K	LS	C	P	A	Erosion class
1	Ht-IV-LkLi	12.25	749.49	0.40	6.8	0.005	0.35	3.54	SR
2	Ht-V-LkLi	3.01	749.49	0.41	9.5	0.005	0.35	5.06	SR
3	KC-III-LcLi	35.74	930.45	0.40	3.1	0.18	0.35	74.74	S
4	KC-III-LcLi	51.02	930.45	0.24	3.1	0.19	0.35	45.42	R
5	KC-III-LcLi	148.27	893.59	0.42	3.1	0.18	0.4	82.41	S
6	KC-III-LcLi	16.51	749.49	0.34	3.1	0.18	0.35	51.15	R
7	KC-III-LkLi	25.02	930.45	0.35	3.1	0.17	0.35	61.22	S
8	KC-III-LkLi	255.30	919.97	0.39	3.1	0.19	0.4	85.50	S
9	KC-III-LkLi	7.82	749.49	0.45	3.1	0.19	0.35	68.58	S
10	KC-III-LkLi	6.63	749.49	0.18	3.1	0.18	0.35	27.68	R
11	KC-III-LkLi	9.78	749.49	0.21	3.1	0.18	0.35	30.76	R
12	KC-IV-LcLi	533.77	930.45	0.37	6.8	0.18	0.35	148.31	S
13	KC-IV-LcLi	45.89	749.49	0.39	6.8	0.19	0.35	134.38	S
14	KC-IV-LkLi	179.52	930.45	0.38	6.8	0.18	0.35	149.17	S
15	KC-IV-LkLi	20.77	868.35	0.33	6.8	0.20	0.35	135.26	S
16	KC-IV-LkLi	25.87	749.49	0.39	6.8	0.19	0.35	131.25	S
17	KC-IV-LkLi	4.74	749.49	0.38	6.8	0.18	0.35	126.33	S
18	KC-IV-LkLi	9.32	749.49	0.41	6.8	0.19	0.35	137.41	S
19	KC-V-LcLi	68.67	913.97	0.38	9.5	0.19	0.35	224.41	T
20	KC-V-LkLi	33.43	930.45	0.29	9.5	0.18	0.35	165.43	S
21	KC-V-LkLi	9.33	930.45	0.38	9.5	0.18	0.35	214.15	T
22	KC-V-LkLi	27.87	930.45	0.40	9.5	0.19	0.35	231.27	T
23	PmPk-III-LcLi	4.84	930.45	0.37	3.1	0.36	0.35	131.07	S
24	PmPk-III-LcLi	6.27	930.45	0.40	3.1	0.36	0.35	145.53	S
25	PmPk-III-LcLi	9.49	930.45	0.34	3.1	0.33	0.35	111.97	S
26	PmPk-III-LcLi	7.78	930.45	0.65	3.1	0.32	0.35	210.78	T
27	PmPk-III-LcLi	16.84	930.45	0.37	3.1	0.34	0.35	128.03	S
28	PmPk-III-LcLi	13.66	930.45	0.64	3.1	0.36	0.35	230.79	T
29	PmPk-III-LcLi	10.93	749.49	0.68	3.1	0.36	0.35	198.00	T
30	PmPk-III-LcLi	10.20	749.49	0.54	3.1	0.35	0.35	152.38	S
31	PmPk-III-LcLi	7.03	749.49	0.67	3.1	0.34	0.35	184.12	T
32	PmPk-III-LkLi	6.63	930.45	0.49	3.1	0.33	0.35	159.42	S
33	PmPk-III-LkLi	16.19	930.45	0.39	3.1	0.33	0.46	169.14	S
34	PmPk-III-LkLi	9.07	930.45	0.56	3.1	0.34	0.35	193.52	T
35	PmPk-III-LkLi	7.95	930.45	0.39	3.1	0.36	0.35	140.83	S
36	PmPk-III-LkLi	9.77	749.49	0.17	3.1	0.36	0.35	50.08	R
37	PmPk-III-LkLi	4.43	749.49	0.49	3.1	0.34	0.35	133.99	S
38	PmPk-IV-LcLi	5.79	930.45	0.34	6.8	0.32	0.46	312.38	ST
39	PmPk-IV-LcLi	4.30	930.45	0.32	6.8	0.36	0.35	252.60	ST
40	PmPk-IV-LcLi	6.88	930.45	0.60	6.8	0.34	0.35	455.11	ST
41	PmPk-IV-LcLi	15.06	930.45	0.32	6.8	0.32	0.46	296.15	ST
42	PmPk-IV-LcLi	4.00	765.8	0.43	6.8	0.36	0.35	277.13	ST
43	PmPk-IV-LcLi	6.10	749.49	0.46	6.8	0.35	0.35	288.01	ST
44	PmPk-IV-LcLi	6.17	930.45	0.30	6.8	0.33	0.35	221.18	ST
45	PmPk-IV-LkLi	7.14	930.45	0.39	6.8	0.34	0.35	296.35	ST
46	PmPk-IV-LkLi	34.76	749.49	0.38	6.8	0.36	0.46	318.35	ST
47	PmPk-V-LcLi	5.95	930.45	0.28	9.5	0.34	0.35	299.91	ST
48	PmPk-V-LcLi	23.51	930.45	0.30	9.5	0.33	0.35	304.36	ST
49	PmPk-V-LkLi	1.77	930.45	0.28	9.5	0.35	0.46	406.00	ST
50	Sw-III-LcLi	6.91	749.49	0.32	3.1	0.01	0.35	2.54	SR
51	Sw-III-LcLi	4.87	749.49	0.45	3.1	0.01	0.35	3.58	SR
52	Sw-III-LkLi	12.14	781.22	0.34	3.1	0.01	0.35	2.74	SR
53	Sw-IV-LcLi	3.89	749.49	0.42	6.8	0.01	0.35	7.08	SR
54	Sw-IV-LkLi	4.03	749.49	0.57	6.8	0.01	0.35	9.21	SR
55	Tg-III-LcLi	8.32	930.45	0.24	3.1	0.20	0.35	48.24	R
56	Tg-III-LcLi	12.29	825.26	0.19	3.1	0.21	0.35	34.27	R
57	Tg-III-LkLi	1.17	749.49	0.58	3.1	0.21	0.35	97.39	S
58	Tg-IV-LcLi	4.72	930.45	0.45	6.8	0.17	0.35	167.27	S
59	Tg-IV-LcLi	10.45	930.45	0.30	6.8	0.20	0.35	132.48	S
60	Tg-IV-LcLi	3.46	930.45	0.31	6.8	0.20	0.35	139.33	S
61	Tg-IV-LcLi	5.97	930.45	0.49	6.8	0.21	0.35	225.23	T
62	Tg-IV-LcLi	7.97	930.45	0.16	6.8	0.21	0.35	72.68	S
63	Tg-IV-LkLi	11.04	930.45	0.34	6.8	0.18	0.35	132.75	S
64	Tg-V-LcLi	26.48	930.45	0.44	9.5	0.20	0.39	298.21	ST
65	Tg-V-LkLi	6.32	930.45	0.44	9.5	0.19	0.35	263.15	ST
66	Tg-V-LkLi	5.99	930.45	0.76	9.5	0.19	0.38	489.13	ST
67	KC-IV-Lk-Li	3.32	749.49	0.39	9.5	0.19	0.35	182.95	T
68	PmPk-III-LkLi	3.69	930.45	0.28	3.1	0.36	0.46	131.50	S
69	PmPk-III-LkLi	3.47	930.45	0.41	3.1	0.32	0.35	131.95	S
70	PmPk-III-LkLi	3.81	930.45	0.47	3.1	0.35	0.46	219.65	T
71	PmPk-IV-LcLi	3.77	930.45	0.18	6.8	0.34	0.35	133.74	S
72	PmPk-IV-LkLi	2.83	749.49	0.41	6.8	0.34	0.35	251.04	ST
73	PmPk-V-LcLi	3.85	930.45	0.41	9.5	0.34	0.35	424.45	ST
74	Tg-III-LcLi	3.19	753.06	0.31	3.1	0.20	0.35	51.78	R
75	Tg-IV-LcLi	3.48	930.45	0.30	6.8	0.19	0.35	125.92	S
76	Tg-IV-LcLi	3.07	930.45	0.33	6.8	0.18	0.35	130.35	S
77	Tg-IV-LcLi	3.04	930.45	0.18	6.8	0.20	0.35	80.29	S
	Dam	162.34

Source: Data Analysis, 2010

Note: SR=very low, R=low, S=moderate, T=high, ST=very high.

Table 6 shows that the erosion rates of forest and paddy field were classified as a very low level. The moderate class of erosion happened at mixed cropping, settlement, and dry field. Furthermore, the high level of erosion resulted from settlement area and small area of mixed cropping and dry field. For the very high of erosion level happened at the settlement area, especially at the IV and V classes of slopes and at the dry field with the highest slope (V) class. Based on the criteria launched by the Forestry Department, 1998, the erosion rates can be classified into 5 classes as seen on Graph 1.

Note:

Erosion class : 1 – 5

Dam : 6

Graph 1. Erosion Class.

Graph 1 shows the majority of the area (68%) and 15% has a moderate and high classes of erosion respectively. Only a small area (5.5% is classified as a low erosion rate, and 2.2% has a very low erosion rate, and 1.6% considered as a very high rate.In details, the erosion rate for each land use types an be explained bellow.

Graph 2. The erosion rates of each land use type.

The above graph shows the erosion rates from the lowest to the highest, respectively are: forest, dry paddy field, mixed cropping, dry field, and settlement. The graph also shows that the higher the slopes, the higher the erosion rates of each land uses. Forest area has the lowest erosion rate (3.54 to 5.06 ton/ha/yr) because the forest at this study area consists of teak, acacia and palm tree plantations. At this area, the human disturbance is relatively low because there is no logging activities. Besides, the mulch is also undisturbed. The lower ground cover, although they are not exist during the dry season, they grow well during the rainy season. This ground cover plays an important role in protecting the soil from the rainfall energy. It was mentioned by Dissmeyer and Foster (1981) that the major subfactor operating in the forest environment are these: 1) amount of bare soil or ground cover, 2) canopy, 3) soil reconsolidation, 4) high organic content, 5) fine roots, 6) residual binding effect, 7) on site storage, 8) steeps and contour tillage.

At the dry paddy field area, the erosion rates is also low ( 2.74 – 9.21 ton/ha/yr), resulted from the typical paddy field area which usually developed at the flat area. In this stduy the dry paddy field area is located at the land with the slopes class of III (>15 to 25%) and IV (>25 to 45 %), but they applied terraces for soil and water conservation services so that the erosion rate is low. The dry paddy field fully depends on the rainfall water, it does not have the irrigation system.

Mixed planting area has the erosion rate of higher than that of dry paddy field and forest areas, but lower than that of dry field. The mixed planting area is marked by the various type of trees so that it has a complete canopy strata, that are: dominant, codominant, intermediate, and suppressed. The complete canopy strata plays an important role in reducing the rainfall energy which fall to the canopy so that when the water comes to the lower stratum, the size of rainfall is much smaller. Mulch also influences in protecting the soil from the detachment of the rainfall. Based on this explanation, the erosion rate of mixed cropping area is relatively low.

The erosion rate at the dry field area was 44.76 to 519.95 ton/ha/yr which was higher than that of mixed cropping. This situation as a result of at the dry field, both trees and crops are planted together. Besides, at the dry paddy field area there were soil tillage so that the soil structure was disturbed. Although in some cases the soil the soil tillage has a positive impact such as increasing of soil infiltration, but the disturbance to the soil result in the easier of soil ispersion process and the tranportation through the erosion process. The planting of crops in the dry field area resulted in the less canopy cover compare to that of trees. As a consequence, the rainfall can hit the soil directly. Moreover, the soil tillage result in the thinner organic matter layer.

The erosion rate at the settlement area was 165.80 to 600.18 ton/ha/yr and was classified as the highest erosion rate compare to other land uses. This high erosion rate was influenced by the soil compaction process as a result of the frequent and intense human activities at this area. According to Yaalon (2007), humans are the current major cause of soil erosin. The compact soil has the lower infiltration rate and capacity. Thus, the rainfall tends to change to runoff which potentially impact to erosion. Rahim (2003) explained that when there runoff during the rainfall, the erosion process will be dominated by transportation and diispersion by the runoff. It was also mentioned that the erosion caused by runoff will give higher influence to the sedimentation process at the river, lake, dam and the sea. Runoff has a high energy for erosion and transporation of soil particles.

The Duncan analysis of erosion rate was shown in table 7.

Table 7. Statistical analysis using Duncan Multiple Range Test

Duncan grouping	Mean	N	Sampel
A	271.78	6	PmPk/settlement
A	180.57	6	Tg/dry field
AB	138.55	6	Kc/mixed cropping
B	5.52	4	Sw/Dry paddy field
B	4.30	2	Ht/Forest

Table 7 shows that the erosion rate of the settlement, dry field and mixed cropping were not significantly different as well as between mixed cropping, dry rice field and forest.

CONCLUSION AND RECOMENDATION

It can be concluded that the average of erosion rates of the various land uses were: at forested land (3.54 to 5.06 ton/ha/yr), dry rice field (2.74 to 9.21 ton/ha/yr), mixed cropping (66.72 to 224.41 ton/ha/yr), dry field (44.76 to 519.95 ton/ha/yr), and settlement (165.85 to 600.18 ton/ha/yr). The majority of the area has a moderate (68 %) and high (15 %) level of erosion rate. The Duncan Multiple Range test showed that the erosion rate of the settlement, dry field and mixed cropping were not significantly different. Besides, between mixed cropping, dry rice field and forest were also not significantly different.

Based on this research result, it is suggested that the soil conservation technique is urgently required to control the land degradation.

ACKNOWLEDGEMENT

I would like to thank and acknowledge Institute for Research and Community Servives (LPPM UGM) for providing the grant of Doctorate Research, year 2009.

REFERENCES

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