Erosion hazard index analysis of several land uses in Watatu Village, Donggala Regency

Watatu Village is one of the villages located in Donggala Regency and as the capital city of South Banawa Sub-District which has significant potential in the agricultural sector, including cocoa plantations, oil palm trees, and paddy fields. This research aims as a source of information correlated to land-use policies and the erosion hazard index in Watatu Village, South Banawa Sub-District, Donggala Regency. The research was carried out by using the descriptive explorative method, and the observed variables were carried out through a survey. Soil sampling was determined purposively (purposive sampling) based on the category of land use and slope at the research location. Intact and incomplete soil samples were taken as many as 21 samples at seven land-use units. Sample analysis was performed at the Soil Science Laboratory on the Erosion Hazard Index, namely (1) soil texture, (2) soil structure, (3) organic matter content, and (4) permeability, and (5) bulk density. The results of the Erosion Hazard Index analysis showed that the high erosion hazard index was found in Land use unit 2 (mixed gardens), Land use unit 4 (cocoa gardens), Land use unit 5 (mixed gardens), and Land use unit 7 (Shrub). Thereby, conservation actions needed to preserve the environment to prevent or inhibit the rate of erosion and maintain the stability of land use.


Introduction
One of the primary natural resources on earth is the land. The land described as a land space and is a part of the land that has many functions for life [1]. Erosion is one type of land degradation. Erosion is one indication that land is damaged [2]. Furthermore, erosion is the process of erosion and transport of soil or parts of soil by natural media in the form of water. Erosion is an indicator of gaps in land use in an area [3]. However, it should be noted that erosion can occur naturally also by human activities that use the land for various purposes such as land use for agriculture, settlement, and others.
Direct land use may cause damage in an area if not accompanied by land damage prevention measures [4,5]. It will result in visible land degradation marked by high levels of erosion and sedimentation and low levels of rainwater infiltration [6,7].
This research aims as a source of information correlated to land-use policies and the erosion hazard index in Watatu Village, South Banawa Sub-District, Donggala Regency

Methods
This research was conducted in Watatu Village, Banawa Selatan District, Donggala Regency, Central Sulawesi Province and soil analysis was carried out at the Laboratory of the Soil Science Unit of the Faculty of Agriculture, Tadulako University. This research was conducted from October 2019 to November 2019.
Several tools used in this research were including Meters, Plastic Bags, Label Paper, Clinometer, Hoes, Ring samples, soil drills, pH meters, Global Position System (GPS), compasses and physical and chemical analysis equipment in the laboratory. While the materials used were slope maps, land The research was carried out by using the descriptive explorative method, through a survey to determine the predicted value of erosion in each land unit in Watatu Village, Banawa Selatan District, Donggala Regency, Central Sulawesi Province. Data collection was done by collecting primary data and secondary data. Primary data was including length data, slope class, permeability, texture, structure, organic matter, soil bulk density. Secondary data such as rainfall data for the last five years were obtained from the Meteorology, Climatology and Geophysics Agency (BMKG), Palu.
Taking soil samples from each land use unit (LUU) to obtain land characteristics: soil depth, slope length, slope, and image interpretation of land use. Sampling partial soil as many as 21 samples at seven land-use units. Each land use unit consisted of 3 samples, and then they were composited into 1 sample for each land unit. While for intact soil sampling, 21 samples were taken for each land unit, then averaged.
The erosion hazard index was processed using the USLE (Universal Soil Loss Equation) by entering primary data and secondary data into the USLE equation as follows:

Plant management factors and soil conservation factors (CP)
The CP factor value was determined based on the form of land use adjusted to the C factor value for various forms of land use.

Tolerable Erosion (T)
Tolerable erosion level measured by the equation put forward by [8], as follows:

Erosion Hazard Index (EHI): EHI = A / T Note:
A = amount of potential erosion (ton ha -1 yr -1 ) TSL = Tolerable erosion (ton ha -1 yr -1 )  From the calculation of the rain erosivity index (R) was using the average monthly rainfall data for the last five years at the BMKG station at Mutiara Sis-Aljufri Airport, Palu, the Watatu Village area ( Table 2). It has a rain erosivity value of 1258.92 cm/year, the highest monthly erosivity (R) was in June, about 209.38 cm/month, so in that month there was a possibility of soil erosion with a considerable enough potential. Then, the lowest R-value was in October, about 65.787 cm/month, at that month the chance of erosion was relatively low.   Table 3 above, it can be seen from the classification of soil erodibility values at land use unit of 1 -6, which were categorized as low soil erodibility. Meanwhile, land use unit 3, 5, and 7 were categorized as very low soil erodibility values. The difference in soil erodibility values on land use in Watatu Village was caused by soil properties, such as texture, permeability, structure, and organic matter.

Soil Erodibility (K)
Then, the permeability and organic matter can change over time as a result of changes in land use management. These soil properties influence each other in determining the level of soil erodibility in a land unit. The presence of organic matter on the soil surface can inhibit the surface flow rate and provide a more ample opportunity for water to be infiltrated; as a result the surface runoff becomes small [9]. Soil with a fine sand structure also has a high infiltration capacity, but if there is a runoff, the fine grains will easily be caught [10]. Fine sand and dust are soil particles affect the sensitivity of soil to erosion [11].  Table 4 shows the highest LS value was found in mixed garden land of land use unit 5 about 1.97, with a slope of 0.06% and a slope length of 137 m. At the same time, the LS value was low in the paddy field with a slope of 0.03 and a slope length of 26 m. The greater slope of the slope, the greater the number of soil grains that splashed down the slope by the collision of raindrops [12,13]. If the slope of the soil surface becomes twice as steep, the amount of erosion per unit area will be 2.0 to 2.5 times as much.  Table 5, it can be seen that the paddy field (land use unit 1) and cocoa plantation (land use unit 4) have the same CP value, was about 0.004. Then, at mixed gardens (land use unit 2, 3, 5 and 6) had the same CP value about 0.02 of each. Whereas at land use unit 7 (shrub) had a value of 0.3. The impact of vegetation on surface runoff is the interception of rainwater, reducing the speed of surface flow, and the destructive power of rain and surface flow, the influence of roots, organic matter from plant debris that falls on the ground, biological activities related to vegetation growth [14][15][16]. Also, it affects the stability of the structure of soil porosity and respiration, which results in reduced groundwater content [17]. Based on Table 6, the analysis results obtained the potential erosion value and the actual erosion value. The highest potential erosion was found in mixed garden land use (land use unit 5) with a value of 173.60 ton ha -1 yr -1, and the lowest was in shrubs (land use unit 7) about 45.82. Whereas the highest actual erosion was found in mixed garden land (lang use unit 5) with a value of 34.72 tonnes ha -1 yr -1 and the lowest was in paddy field (land use unit 1) with a value of 0.28 tonnes ha -1 yr -1 . This may occur due to various factors that influence the erosion, one of which is the absence of plant management and conservation measures (CP). Rachman, et.al (2003) stated that soil and plant management that accumulates plant debris has a good effect on Soil quality, such as improve the soil aggregate stability, soil resistance, and soil resistance to the destructive power of raindrops [18].  Table 7 and 8, which has been described, the results of the classification of the erosion hazard index in Watatu Village were high and moderate. The highest classification of erosion hazard index was found in the mixed garden (land use unit 2), cocoa plantation (land unit 4), and mixed garden (land use unit 5), while the classification of moderate erosion hazard index was found in paddy field (land unit 1) and mixed garden (land use unit of 3, and 6) and shrubs (land use unit 7).

Tolerable Erosion (T) and Erosion Hazard Index (EHI)
Land-use patterns and soil management measures cause the difference in the erosion hazard index, so it is essential to take conservation actions to overcome the occurrence of more significant erosion. The potential for erosion that occurs in the Palu watershed in the existing conditions is 19,241,205.99 tonnes -1 yr -1 or 10,689,558.88 m 3 yr -1 or around 62.09 tonnes ha -1 yr -1 categorized of class III erosion hazard (moderate) with a loss of soil layer thickness of 3.45 mm yr -1 and after controlling the amount of erosion decreased by 14,829,860.24 or 8,238,811.34 m 3 yr -1 or by 47.85 tonnes of ha -1 yr -1 , with a classification of erosion hazard level II (mild) and the loss of soil layer decreased to 2.66 mm yr -1 [19].

Conclusions
The classification of the erosion hazard index in Watatu Village was high and moderate. The highest classification of erosion hazard index was found in the mixed garden (land use unit 2), cocoa garden (land use unit 4) and mixed garden (land use unit 5), while the classification of moderate erosion hazard index was found in paddy field (land use unit 1), mixed garden (land use unit 3 and 6), and shrubs (land use unit 7). It is necessary to carry out agricultural land conservation efforts, must pay attention to land conservation principles following the conditions of the agricultural land. Thus, the erosion occurred does not get bigger. Besides, expected to plant crops and carry out land management that is suitable for sloping areas