Utilizing Geographic Information System for Prediction of Soil Erosion in Sono Sragen Catchment Area

Problem statement: Embung is a small dam created by manmade lakes and used to catch water during rainy season for an irrigation source. In Sragen area, Embung Sono is one of around twenty embung that used to catch rain water for irr igation. Soil erosions in catchment area are potent ial to reduce water capacity of the embung. Although ma ny embung have been constructed in Sragen areas, the numbers of soil erosion in catchment ar ea of embung are not well considered. Approach: Soil erosion in the Sono catchment area was analyze d using Geographic Information System (GIS) and Universal Soil Loss Equation (USLE) method. Results: The result showed that total number of soil erosion in the Sono catchment area is 63.50 ton/hayear and categorized in moderate classified. Value of Sediment Delivery Ratio (SDR) for catchment was 0.34. Empirical equation proposed by Renfro, Vanoni and USDA were also used to analyze the SDR v alue. Conclusion: It was found that USDA is the best method that can be used to predict soil er osion in Sragen area. It means the crop management and environmental conservation in Sono catchment ne ed to be improved.


INTRODUCTION
Water is very important to human being and living things. Human civilization will not reach like now, without developing water resources consistently. Therefore developing and managing water resources are base of human civilization. In rainy season, the excessive rain water can cause flood. On the other hand, in dry season, there is a little water that will cause dryness. To solve dryness in dry season, the cheaper strategy is water harvesting. Water harvesting effort coincide with improve water saving in river, dam and lake; all can preserve water supplies for farming, domestic and industries. An effort to manage excessive rain water is to build embung (onfarm reservoir). Embung is one simple technology, relative cheap and achievable by farmer society.
The main problem in using embung technology is excessive erosion that will make sedimentation within embung. Soil erosion caused moving soil particle to other place or sediment transportation. Particle soil transportation in the channel is being another problem concerned with erosion. Therefore there are many problem can be found due to sedimentation in river and dam in all of the word (e.g., Abu Hammad, 2010 andSchönbrodt et al., 2010). The effect of soil erosion is very serious; it's not only in Indonesia, but in the other country such as in west China, leading to heavy sediment load in many rivers (Suo, 2004) and also in Southern Neka Basin in Iran (Solaimani et al., 2009). In their studies, Narioka et al (2001) concluded that the erosion degree in the area ranges from very slight to very severe, depending on land status, while Lee and Lee (2010) explained that the result of simulation depending on model selection and measurement accuracy. Erosion occurred slightly in the forestland, but it was severe in the dry lands with slope more than 15%. To achieve sustainable farming in the slope lands, effective conservation measures combined with soil surface protection and appropriate cropping pattern should be establish in the tropical volcanic highland.
Scale of erosion vary in many places and depends on many factors. Result analysis showed that erosion in Buyan Lake Bali watershed dominated by erosion <2 ton h −1 year −1 in area 720 h or 37.091% of watershed area. In Candikuning, Bali, erosion prediction is between 2.82-871.52 ton ha −1 year −1 . Erosion varied from very slight to severe, depends on land condition. Erosion was found to occur slightly in the forestland but was more severe in the dry and shrub lands especially in areas with slope more than 15% (Narioka et al., 2001).
Embung is a solution for solve the irrigation problem during rainy season. On the other hand soil erosions in watershed are potential to reduce water capacity of the embung. Many previous studies (e.g., Suo, 2004;Narioka et al., 2001) showed that significant effect of soil erosion is depending on many parameters (e.g., soil type, slope area and land use area). Many local governments in Indonesia such as Sragen, Surakarta, Ungaran in Central Jawa have been developing many embung in order to irrigate the farmer area. In contrast, there is view study about effect of erosion to capacity of embung storage.
In addition, GIS technologies could provide a powerful tool to model the soil erosion for their spatial analysis and prediction. Many previous (e.g., Lufafa et al., 2003;Hasmadi et al., 2010 andMukhlisin et al., 2010) studies explained that by using GIS the collection, manipulation and analysis of the environmental data on soil erosion can be done much more efficiently. In this study we proposed a method to analyze the potential of soil erosion soil erosion in the Sono catchment Sragen using Geographic Information System (GIS) combining with Universal Soil Loss Equation (USLE) method. Result of this analysis then used to calculate the value of Sediment Delivery Ratio (SDR) and compared with empirical equation proposed by Renfro, Vanoni and USDA.

MATERIALS AND METHODS
Study area: Embung Kedung Sono is one of around twenty embung in Sragen, Central Jawa that have been used to catch rain water for irrigation ( Fig. 1 and 2). Sono catchment covers area in Pengkok, Celep, Karangpelem, Mojodoyong, Jenggrik and Kuto village and located between 110° 59' 0.44" E and 111° 2' 54" E as well as between 7° 32, 25" S and 7°29, 5" S. The area, length and slope average of Sono catchment are 1.112 h; 9.360 m and 2.2%, respectively. The elevation of the catchment is between 324 m and 112.5 m asl . Sono catchment area has been used mostly for wet rice crop. Geologycal stucture of Sono watershed is Lawu mountain deposit: volcanic sandstone, volcanic siltclaystone, volcanic breccia and lava.  Figure 2 shows Sono catchment area located in Sragen, Central Jawa. Embung Kedung Sono, Pengkok, Sragen located in 7° 29' 6" S and 110° 59' 10" E. Embung Kedung Sono can be used to irrigate rice field around Pengkok village. With availability of these embung the production of rice crop and the people economic grade can be increased. (USLE) is the most widely used of all soil erosion models. USLE was designed to predict soil loss from surface soil with respect to land use area (e.g., soil type, vegetation and management). USLE equation was designed to predict long-term average annual soil loss (Risse et al., 1993).

USLE: Universal Soil Loss Equation
Simplification of the complex reality in a catchment is required to analyze utilizing the model. Decisions need to be made on a suitable level of complexity or simplicity depending on the objective (Morgan, 2005). Soil erosion in a catchment area will reduce soil productivity and causes a loss of reservoir capacity. The well-known Universal Soil Loss Equation (USLE) represents a standardized approach (Zhou and Wu, 2008).
An USLE method is commonly used to predict erosion. The USLE is simple and very good to be implemented to area which has main factor erosion is rain water and run off (Schönbrodt et al., 2010). (1) E = Mean annual soil loss R = Rainfall erosivity factor K = Soil erodibility factor L = Slope length factor S = Slope steepness factor C = Crop management factor P = Erosion-control practice factor Sediment Delivery Ratio (SDR): Sediment prediction can be done by calculating SDR. SDR calculation is important for realistic prediction of total sedimentation base on erosion calculation in watershed. Accurate prediction of sediment delivery ratio is an important and effective approach to predict sediment yield which is usually not measured. Presently available prediction models are not generally applicable to a particular watershed (Ouyang and Bartholic, 1997).

Drainage area approach:
The relationships between SDR and other factors have been established as curves. Watersheds with large drainage area and the fields with a long distance to the streams have a low sediment delivery ratio.
As reported on (Ouyang and Bartholic, 1997 ( 2) where, A = drainage area in km 2 Valona used the data from 300 watersheds throughout the world to develop a model by the power function. This model is considered a more generalized one to estimate SDR: where, A, drainage area in square miles. The USDA SCS developed a SDR model based on the data from the Blackland Prairie, Texas. A power function is derived from the graphed data points: where, A, drainage area in square miles.
Slope, gradient and relief-length ratio: SDR is affected by the topographic features of the watershed. A watershed with short and steep slopes will deliver more sediment to a channel than a watershed with a long and flat landscape. Ouyang and Bartholic (1997) explained that Williams and Berndt's used slope of the main stream channel to predict sediment delivery ratio. The model is written as: where, SLP, % slope of main stream channel.
Manner's studies suggested that SDR was better correlated with relief and maximum length of a watershed expressed as Relief-Length ratio (R/L) than with other factors. Renfro 1975 modified the model (R 2 = 0.97) as follows: Log (SDR) = 2.94259 + 0.82362 log (R/L) where, R, relief of a watershed, defined as the difference in elevation between the average elevation of the watershed divide and the watershed outlet. L = maximum length of a watershed, measured approximately parallel to mainstream drainage.

RESULTS
Ruster Method was applied to be used to analysis the potensial erosion in the catchment area. For each factores in the USLE method such as the rainfall erosivity (R), soil erodibility (K), crop management C), length of slope (L), steepness of the slope (S) and the erosion control practice factors were modeled in builder model with sieze for each sel 20×20 m.
The rainfall erosivity was anayzed using 14 years rainfall data from two raingauge stations. For soil erodibility was analyzed based on 15 samples which collected from the entire catchment area (Fig. 3). Figure  4 shows LS factor thas was analyzed from topography map with scale 1:25,000. While Fig. 5 and 6 show the map fo crop management and erosion control practice, respectively that were analyzed absed observed land survey.  Figure 7 shows the model that was used to calculate erosion in catchment area using USLE's Eq. 1. From the figure can be seen that professeive erossion occured on some part of the catchment, particularly on step slope.
USLE_Result from the modeling is in image condition and there is not show the amount of soil erosion. Using zonal statistic function in Arc Gis, mean annual of soil erosion can be calculated as follows:

CONCLUSION
In this study can be analyzed that total number of soil erosion in the Sono catchment area is 63.50 ton h −1 year. Regarding Indonesian Forestry ministry's soil lost classification Sono catchment can be categorized to moderate erosion. It means the crop management and environmental conservation in Sono catchment need to be improved. In addition, the value of Sediment Delivery Ratio (SDR) for the catchment is 0.34. Empirical equation proposed by Renfro, Vanoni and USDA were also used to analyze the SDR value. It was found that USDA is the best method that can be used to predict soil erosion in the Sragen area.