Modification of rain and run-off erosivity to predict erosion hazard level on The Manguan Sub-watershed’s Upper Slope

To protect natural resources and maintain the normal functioning of hydrological processes, the upper slope of the Manguan Sub-watershed should be designated as a conservation area. However, agricultural activities in the upstream have disrupted the watershed’s hydrological function, triggering high rates of erosion, sedimentation, and flooding. The aim of this research was to estimate the level of erosion hazard on Manguan Sub-watershed’s Upper Slope and its conservation directives to mitigate it. This study adopted a land unit approach to flow occurrence land units with the prediction of MUSLE (Modified Universal Soil Loss Equation) by integrating rainfall and run-off erosivity. The direction of land conservation will be evaluated through the perspectives of land rehabilitation and soil conservation. Based on the result, the prediction of erosion hazard levels is classified into four groups: Low, Moderate, High, and Extremely High. The Manguan Sub-Watershed’s Upper Slope has a significantly high level of erosion and necessitates soil conservation improvement measures on approximately 79% of its area.


Introduction
Soil erosion is a serious issue associated with land degradation, creating an enormous environmental hazard that seriously threatens global economic and social development [1].Loss of valuable land due to depleted soil layers is a consequence of erosion [2].The eroded material creates problems in numerous locations, such as sediment accumulation in rivers and reservoirs [3].The erosion and ensuing deposit of large quantities of sediment change the natural flow patterns of rivers, impact river channels, and contribute to sediment accumulation in reservoirs.These processes have the potential to decrease the carrying capacities of these reservoirs.
The Wonogiri Multipurpose Reservoir is one example.As a result of sediment accumulation, the present state of the Wonogiri Reservoir is marked by a 35% decrease in its storage capacity.It is estimated that the annual sediment deposition in the Wonogiri Reservoir equals the average sediment carrying capacity of the reservoir, which is 6.49 million m 3 , and the annual dredging volume is 0.09 million m 3 [4].According to the available data, the current sediment management practices must be modified to effectively resolve the current sedimentation rate.The average annual sediment yield in the Keduang Watershed is about 1.3 tons.This sediment accumulates in front of the dam's intake, disrupting reservoir operations [5].Attempting to clean out sedimented material through dredging is a short-term mission.The main issue that needs attention is the rate of the soil surface erosion [4].
Sedimentation material contributed by the Keduang Watershed is allegedly caused by the high rate of surface run-off [4] and erosion in the upstream area [6].Manguan Sub-Watershed's Upper Slope is a 1314 (2024) 012127 IOP Publishing doi:10.1088/1755-1315/1314/1/012127 2 micro section located in the upstream area of the Keduang Watershed.The upstream must become a conservation area that protects natural resources in its hydrological function [7].Manguan Sub-Watershed's Upper Slope are steep, and the local community has converted numerous parts from forest to agricultural land, thereby increasing the likelihood of surface erosion and impacting the downstream areas.It indicated that human activities in the Manguan Sub-Watershed's Upper Slope have a substantial impact on its sustainability [8].Hence, erosion management must be implemented to stop land degradation and sedimentation in downstream areas.
In erosion studies, researchers commonly use a few spatial approaches, including the land unit, landform unit, landscape unit, terrain unit, geomorphological unit, and land system approaches [9].The mentioned spatial unit techniques are commonly used in land resource studies [3].However, there is currently a lack of land resource studies within a watershed's context.The two main concepts in watershed studies are the hydrological concept, which involves conducting run-off studies to predict flooding, and the soil and water conservation concept, which involves conducting erosion studies to predict soil loss and the amount of sediment produced.These two concepts are still partially operational, so the concept of a watershed as an interconnected system has not been applied.The study of the connection between run-off and spatial units has not been considered, thus leading to a lack of comprehensive understanding regarding the factors contributing to the most significant erosion and the mechanisms through which it occurs [9].
The erosion prediction models that have gained the most widespread adoption are the Universal Soil Loss Equation (USLE) and the Modified Universal Soil Loss Equation (MUSLE).These models solely rely on either rainfall erosivity or run-off erosivity as input variables.Meanwhile, the erosion process can be associated with two main driving forces (erosivity): rain erosivity and run-off erosivity.Rain erosivity is a phenomenon that occurs when precipitation comes into contact with the soil's surface, leading to splash erosion.Then, run-off erosivity is due to rainwater flows over the soil's surface, leading to various forms of soil erosion, such as sheet erosion, rill erosion, and gully erosion.The study of rain erosivity and surface runoff is spatially related; an integral component cannot be separated from erosion studies because these two forces are responsible for losing topsoil layers.
Evaluation and inventory of surface run-off and soil erosion are essential for determining sustainable watershed conservation strategies [10].This study combines rainfall erosivity and run-off erosivity to predict erosion hazards on the Manguan Sub-Watershed's Upper Slope.The proposed method, which integrates erosivity variables, has the potential to generate a precise assessment of erosion in practical circumstances.Moreover, it can serve as a reliable basis for devising effective soil conservation measures and addressing issues related to sedimentation downstream.

Methods
The study was conducted on the upper slopes of the Manguan Sub-watershed and carried out from August 2021 to February 2022.It requires field surveys, documentation, laboratory tests, and map analysis to collect data regarding precipitation, soil type, slope, land use, and soil conservation techniques.This data will be utilized to predict erosion and assess erosion hazards.
This study used flow occurrence land units as its unit analysis.The flow occurrence land units are obtained by superimposing 3 (three) maps: land unit, contour, and flow occurrence border maps.After that, the type of flow occurrence can be determined, which includes a singular flow occurrence (if the run-off passes through only one land unit within an occurrence border) and a dual flow occurrence (if the run-off passes through two or more land units within an occurrence border) [11].Manguan Sub-Watershed's Upper Slope consists of 1029 flow occurrence land units, 161 of which are singular flow occurrences and 868 of which are dual flow occurrences.The distribution of flow occurrence land units in the Manguan Sub-Watershed's Upper Slope can be seen in Figure 1.Furthermore, the steps of analysis were:

Actual Erosion
The estimation of actual erosion was conducted by Modified Universal Soil Loss Equation (MUSLE) methodologies.The actual soil erosion prediction for a singular flow occurrence land unit can be determined by using equation: Meanwhile, actual soil erosion predictions for a dual flow occurrence land unit can be determined by using equation: Where A is the amount of soil erosion (tons/ha/year), R is the rainfall erosivity index, Rm is the run-off erosivity index, Rm1/Rmn is the additional run-off erosivity index, and K is the soil erodibility index, LS is the slope index, C is the vegetation cover index, and P is the soil conservation technique index.

Erosivity of Rainfall.
The estimation of erosivity, particularly the average annual precipitation, is obtained from a study conducted by Bols (1987) in the regions of Java and Madura.The annual EI30 is calculated by accumulating the monthly EI30 values.The rain erosivity calculation formula is defined as follows:  30 = 6,119 ( 1,21 ) ( −0,47 ) ( 0,53 ) Where EI30 is the average annual erosivity (tons), RAIN is the average yearly precipitation (cm), DAYS is the number of rainy days per year (days), and MAXP is the average maximal precipitation in 24 hours for one month that year (cm).

Erosivity of Run-Off.
Run-off erosivity is calculated using the equation according to Williams (1977): Where Rm is the erosivity of the run-off (metric tons/occurrence), Q is the run-off (m 3 /second), and V is the run-off volume (m 3 ).

Erodibility Index (K).
The K value calculated using the equation formulated by Wischmeier and Smith: Where K is the erodibility index, M is the percentage of magnificent sand and silt (0.1-0.05 and diameter 0.05-0.02mm) x (100a percentage of clay), a is the percentage of organic matter, b is the code soil structure, c is the soil-permeability class.To find out the soil structure code and soil permeability class, see Table 1 and Table 2.

Slope Index (LS).
The slope index is calculated using the equation: Where LS is the slope factor, m is 0.5 for slopes of 5% or more, 0.4 for slopes of 3.5-4.9%,0.3 for slopes of 3.5%, C is 34.71, α is the slope angle, and l is the slope length (m).

Tolerabel Soil Erosion
Tolerable Soil Erosion (TSL) is calculated using the equation formulated by Hammer (1981) as follows: Where TLS is the allowable erosion (tons/ha/year), d is the adequate soil depth (cm), fd is the soil depth factor value, W is the land use value (400 years), and BD is the soil density (cc/gr).

2.3.Erosion Hazard Level
The method developed by Hammer (1981) is used to determine the analysis of erosion hazard levels, particularly the soil erosion hazard index [12], whose categories are determined by the following equation: The erosion hazard index, labeled as EHI, while A is actual soil erosion, and TSL refers to the threshold of erosion tolerance.The categorization of the erosion hazard index is presented in Table 3.

Prediction of Actual Soil Erosion
The erosion quantified in this study refers to an erosion phenomenon impacted by land cover dynamics and human land management practices.The erosion calculations used in this study were based on the Modified Universal Soil Loss Equation (MUSLE).The equation presented herein integrates the erosive effects of rainfall and run-off.Actual erosion prediction results for each flow occurrence land unit on the Manguan Sub-Watershed's Upper Slope range from 0.67 to 76,107.15tons/ha/year, averaging 1625.49tons/ha/year.The actual erosion value is determined by its influencing factors.As is wellknown, land conservation techniques and land cover are erosion-controlling factors [9].The lower the erosion control factor index is, the lesser the erosion value.In other words, if the erosion control factors are in optimal condition, the amount of erosivity can be reduced.In the research area, the CP index ranges from 0.0004 to 0.35.Rice fields with effective bench terrace conservation methods demonstrate the lowest CP index value.In contrast, the settlement characterized by awful bench terraces shows the highest CP score.
It is well acknowledged that only 20% of rice fields are utilized, indicating that other land uses have a higher erosion value.The actual erosion value in different land uses is influenced by rain erosivity, run-off erosivity and steep slopes.One of these cases is the utilization of land for settlement.The relationship between a settlement location's steepness and the total erosion amount is directly proportional.The high erosivity rating of settlements is attributed to their significant runoff volume, which accounts for around 40% of the total rainfall.In contrast, the distribution of gardens, fields, and moors within the research area is almost 40%.Dryland farming is highly vulnerable to erosion due to the inability of the CP factor to mitigate the substantial erosivity force effectively.
According to the analysis, the predicted annual erosion rate on the Manguan Sub-Watershed's Upper Slope is estimated to be 1,672,628.91tons/ha/year.The total land area under consideration reaches 2,589.62 hectares, whereas the quantifiable volume of erosion observed amounts to 7,381,363.98tonnes.In 2012, the estimated average erosion rate for the Keduang Watershed was around 44 tons/ha/year, resulting in a cumulative erosion of 1.9 million tons/year [13].Furthermore, in 2016, the average erosion prediction outcomes for the Keduang Watershed indicated a rate of 232.9 tons/ha/year, resulting in a cumulative erosion estimate of 9.8 million tons/year [14].These results suggest that the erosion observed on the Manguan Sub-Watershed's Upper Slope in 2021 is almost comparable to the overall erosion followed by the Keduang Watershed in 2016.Figure 2 presents a comparative analysis of past erosion forecast outcomes.

Figure 2. Comparative Analysis of Past Erosion Forecast Outcomes
This study incorporates a modified erosivity factor, which combines the aspects of rainfall (R) and run-off (R) to estimate the erosion levels accurately.The erosivity value for each unit of land is observed to be higher, potentially leading to an overestimation.The increased erosivity value is due to the substantial amount of run-off.In the event of recurrent run-off, the erosive potential of a given land unit will impact the land unit situated underneath it, resulting in an increased erosive capacity for the latter land unit positioned along the run-off direction.The observed erosion on the Manguan Sub-Watershed's Upper Slope indicates the necessity of implementing soil conservation measures in the upstream Keduang Watershed.Potential issues can be effectively resolved by addressing the primary causes of land degradation and subsequent sedimentation.

Prediction of Tolerable Soil Loss (TSL)
The TSL (Tolerable Soil Loss) represents an indicator for assessing the necessity of implementing soil conservation measures.Erosion is a geological process that plays a significant role in shaping the Earth's surface.Soil erosion is an inevitable occurrence that cannot be mitigated.If the erosion rate is beyond the threshold of tolerable soil loss, it might pose a significant risk to the land.The analysis results indicate that the amount of TSL per flow occurrence land unit in the Manguan Sub-Watershed's Upper Slope varies between 5.88 tons/ha/year and 82.35 tons/ha/year.In contrast, the average allowed erosion rate is 42.68 tons/ha/year.The forest shows the highest TSL value due to a relatively thick soil depth.
On the other hand, dry agricultural land presents a lower TSL value due to its extensive cultivation practices, reducing soil solum depth.The permissible soil loss on each unit of land significantly influences the erosion hazards index.The erosion hazards index increases as the acceptable erosion value diminishes.The mean TSL value observed in this study shows a significant deviation from the average erosion predicted value.
Nevertheless, the TSL value is an irreversible factor beyond the control of human intervention.The TSL value serves as a point of reference for assessing the erosion hazard level and is the basis for making wise choices on conservation strategies.If a land experiences more substantial erosion, it necessitates implementing conservation measures.On the contrary, the observed decay is comparatively lesser in volume when compared to the threshold sediment loss (TSL), indicating no immediate need to implement conservation strategies.According to an analysis of the Erosion Hazard Level on the Manguan Sub-Watershed's Upper Slope, it has been determined that approximately 79% of the land needs to be taken care of by conservation measures.

Erosion Hazard Level
In addition to human land management practices, the percentage and type of land cover are aspects of land use that have a significant impact on the erosive value [15].The erosion hazard level analysis results for the Manguan Sub-Watershed's Upper Slope were categorized as low, moderate, high, and extremely high.The following is a description of each category: Low.The classification of low erosion hazards implies that the observed erosion levels are either equal to or below the threshold of tolerable soil erosion.This category encompasses an area of 535.5 hectares, predominantly found in rice fields, forests, and arid agricultural land filled with shrub vegetation.The illustrated land in Figure 3 has a gentle slope, which can be effectively mitigated in terms of erosivity through appropriate land cover vegetation (Figure 3b and 3c) and effective conservation techniques (Figure 3a).Even though the slope is typically steep, the erosivity of run-off on shrubs and forest land use does not significantly erode the land.In other words, in substantially enhancing the erosivity of run-off, the CP index has a more significant influence than the slope index.Splash erosion is the predominant form of erosion in this category.As runoff forces are not apparent, conservation measures are not a top priority.

Moderate.
The moderate-level implies that the actual erosion is significantly higher than tolerable erosion, necessitating the implementation of land conservation strategies.The moderate-level an area of 8 459.6 hectares.Moderate erosion is frequently observed in settlement, vegetable plantations, moor, and ricefields.The illustrated area in Figure 4 shows gentle slope characteristic, yet it requires adequate erosion resistance due to poor vegetation cover (Figure 4a and 4b) and ineffective soil conservation techniques (Figure 4c and 4d).In this category, the CP index has not been able to significantly reduce the erosivity value, thereby increasing the erosive force.The reason is that run-off erosivity acts a significant role in soil erosion.Another characteristic under this category refers to the phenomenon when actual erosion surpasses the tolerable erosion.However, there is an opportunity for improvement in the selection of plant types and the application of conservation strategies, although to a limited extent.

High.
This erosion hazard level is classified within the index range of 4 to 10.This category is characteristic of exceeding the tolerable erosion value, hence necessitating the enhancement of land conservation strategies.It spans an area of 358.02 hectares and is predominantly found on settlement and cassava crops.The area shown in Figure 5 represents a steep slope and existing land conservation techniques are inadequate, making it unable to endure the forces of erosion.The magnitude of potential damage due to run-off erosion increases significantly.River and gully erosion is a common manifestation of erosion processes in this category.Improving the construction of soil conservation techniques is essential, complemented by the implementation of annual crop cultivation in settlements and the adoption of more effective cover crops in agricultural environments.

Extremely High.
The erosion hazard level classified as "very high" is characterized by an index above 10.The actual erosion value significantly exceeds the acceptable erosion threshold, indicating the necessity for land conservation actions.This category encompasses a land area of 1,236.5 hectares and mainly occurs in settlements, teak plantations, precipitous cassava plantations, and vegetable crops.The shown area in Figure 6 illustrates the attributes of highly steep slopes covered with vegetation and soil conservation techniques incapable of enduring erosive forces (categorized as poor).The current land situation at this level is a cause for significant concern since the relatively low CP index does not adequately mitigate the erosion forces at play.The erosivity of run-off shows a clear correlation with the state of highly steep and land cover condition.It was further revealed that forest-covered land and open fields generate contrasting levels of erosion, as evidenced by the run-off generated by these two land uses [16].The phenomenon is distinguished by the erosion of soil, resulting in the exposure of tree roots.Gully erosion is the dominant form of erosion.Land in such circumstances is primarily focused on implementing intensive soil conservation practices, including constructing bench terraces, cultivating plants resilient to runoff, and maybe undertaking reforestation efforts.
Based on the previous explanation, it is known that the area of very high erosion hazards dominates due to the characteristics of the land on the Manguan Sub-Watershed's Upper Slope, which is very susceptible to erosion.Such as land use for arid agriculture with minimal cover crops and poor soil conservation practices.Additionally, the massive erosivity force cannot be controlled by erosion control factors.So, the actual erosion produced is extensive.Meanwhile, the TSL value on dry agricultural land and residential areas must be improved.It is caused by intensive land cultivation so that the depth of the soil solum becomes shallow, and if large-scale erosion occurs, the land is vulnerable to degradation.

Figure 7. The Manguan Sub-Watershed's Upper Slope Erosion Hazard Level Map
To inventory erosion problems for sustainable watershed management, a distribution of erosion hazard levels is needed to facilitate the provision of targeted conservation guidance.The distribution of erosion hazard levels on the Manguan Sub-Watershed's Upper Slope is shown in Figure 7.Meanwhile, the proportion of area can be seen in Figure 8.A r e a of E r os i on H a z a r d L e ve l ( H a ) Based on Figure 7, it can be seen that the Manguan Sub-Watershed's Upper Slopes are primarily affected by an extremely high likelihood of erosion.The Manguan Sub-Watershed's Upper Slope are characterized by slope classifications dominated by steep and very steep classifies.Wonokeling Village, with a land area of 297.7 hectares; Bubakan, with a land area of 293.2 hectares; Sanan, with a land area of 265.5 hectares; and Wonorejo, with a land area of 192 hectares, have a very high risk of erosion, according to Figure 6.This region is located upstream of the watershed, characterized by arid land use and vegetable cultivation.Figure 8 demonstrates that the land in the study area requires conservation treatment to safeguard resources and resolve erosion issues, which can have downstream effects.Conservation practices, including substantially improved soil conservation construction and ground cover crop planting, are required.It is highly recommended that land in the upstream region be reforested to become a protected area that improves the hydrological function of the watershed.

Conclusions
Modifying the erosion prediction model, which combines rainfall erosivity and run-off erosivity, indicates that the Manguan Sub-Watershed's Upper Slope has a significantly high level of erosion, comparable to the erosion forecast outcomes observed in the Keduang Watershed.The Manguan Sub-Watershed's Upper Slope necessitates soil conservation improvement measures on approximately 79% of its area.
Erosion model calculations that combine rain erosivity and runoff reveal which land must be prioritized for land conservation restoration or enhancements.The sedimentation problem is an effect of problems with surface erosion.If the issue of surface erosion is not addressed, sedimentation will continue to pose a threat.Erosion hazard calculations are an initial endeavour to collect problems that occur on land, which are then used as a reference to provide conservation guidance that is precise in location and the actions recommended.

Figure 1 .
Figure 1.The Flow Occurrence Land Units Map of The Manguan Sub-Watershed's Upper Slope

Figure 8 .
Figure 8. Proportion of Erosion Hazard Level in the Manguan Sub-Watershed's Upper Slope

Table 1 .
Code of Soil Structure

Table 2 .
Class of Soil Permeability Factor C is determined by extracting land cover attributes and determining factor C values based on a coefficient table (in Asdak, 2020).

Table 3 .
Category of Erosion Hazard Index