Key words: Soil erosion, erosion control technique, sloping uplands, sandy loam, water quality, NPS pollution, BMPs
Detachment, transport, and deposition are basic processes that occur on upland areas (Foster 1982). Detachment occurs when the erosive forces of rainfall drop impact or when flowing water exceeds the soil's resistance to erosion. Detached particles are transported by the splash and flow of raindrop. Deposition occurs when the sediment load of eroded particles exceeds its corresponding transport capacity. The relative importance of these fundamental processes depends on whether the processes are occurring on inter-rill or rill areas and in the levels of the controlling variables. Eroded soil particles generally move downslope, flowing into rills and gullies. Understanding the soil erosion mechanism is very important to design the soil erosion measurement system and develop the soil erosion control techniques. This paper aims to describe the major factors affecting upland soil erosion processes, characteristics of sloping upland culture, the impact of soil erosion on water quality, soil erosion measurement methods, and the soil erosion control techniques in Korea.
Factors Affecting Soil Erosion
Detachment of soil particles is a function of the erosive forces of raindrop impact and flowing water, the susceptibility of the soil to detachment, the presence of material that reduces the magnitude of the eroding forces, and the management of the soil that makes it less susceptible to erosion. Transport is basically a function of transport forces of the transport agent, the transportability of the detached particles, and the presence of material that reduces the transport forces.
Either detachment or transport capacity may limit erosion and sediment load at a location on the slope. At a given location on a slope, if the amount of sediment made available for transport by the detachment processes is less than its transport capacity, then the sediment load moving downslope will be the amount of detached sediment available for transport. Conversely, if the available detached sediment exceeds the transport capacity, deposition occurs and the transport capacity controls the sediment load.
Hydrology, topography, soil erodibility, soil transportability, soil surface cover, incorporated residue, residual land use, subsurface effects, tillage, roughness, and tillage marks are the major factors that affect upland erosion processes. Among these factors, hydrology, soil erodibility, and soil transportability are not practically controllable. And thus, these factors are not modified or changed to control the soil erosion and the sediment production of a field.
Residual land use and subsurface effects are not commonly practiced to attenuate soil erosion because the effect of these factors is time-limited and is observed when a new crop field reclamation from a meadow or a forest is made. The complex root system of grasses and trees left in the soil surface and column during the reclamation can retard the soil erosion for up to three years. But with the decomposition of the roots, the residual land use and subsurface effects disappear.
Topography is somewhat controllable by building terraces. Terracing of a sloping field is a very effective sediment control method but it is an expensive alternative. Terracing can be applied where other best management methods are not assessed to be effective in controlling soil erosion and sediment discharge because of soil condition, steep slope, and long slope length. Terracing also can be effectively adopted where the land use is very intensive and the rainfall amount is large. Terraced paddy fields in many Asian countries are a representative example.
The rest of the major factors, including soil surface cover, incorporated residue, tillage, roughness, and tillage marks, can be practically managed to cut down the soil erosion and the sediment production of a field. Incorporated residue is the crop residue or mulch materials like corn stalks that are fully and partially buried. These materials increase the soil's organic matter content, act like grade control structures, and prevent rills from getting larger. Roughness of a crop field is related to tillage such as harrowing. Plowed and harrowed surface produce more sediment than just plowed surface because of the difference of roughness. No-till and reduced tillage practices definitely produce less soil erosion and sediment than conventional plow tillage. However, no-till and reduced tillage cannot be applied to all agricultural practices. Where no-till and reduced tillage are not appropriate, soil surface cover and tillage mark play a decisive effect in controlling soil erosion.
Tillage marks are grouped to up-and-down till and contour till. Contour till is generally known to be effective in reducing soil erosion. However, contour till often creates many gullies and produces a tremendous amount of sediment if the amount of rainfall is large enough to overflow the ridge. Once a ridge overflows and is destroyed, the stored water in the furrow is momentarily drained, destroying the ridges downslope, and consequently creating small and large gullies. Where the rainfall amount is relatively large, where the soil is sandy with a deep profile, and where the slope length is long with a relatively steep slope, contour till must be carefully practiced.
Soil surface cover is believed to be the most effective method to reduce soil erosion and sediment discharge. Once the soil surface is covered or mulched with crop residue or permeable material, raindrop impact is sharply reduced, soil particles minimally occurs, soil pores are not clogged, and thus, the soil's permeability and infiltration are maintained. Because of these consequential processes, surface runoff is significantly reduced, and the transport capacity of runoff is also sharply dropped, resulting in less soil erosion. Table 1 shows the differences of runoff and infiltration between bare soil and covered plots.
The soil was sandy loam (weathered granite) and the cover material was a nonwoven fabric. The infiltration test was carried out on 1 m x 1 m x 0.5 m runoff plots with a 10% slope using a ladder-type rainfall simulator developed by the USDA Soil Erosion Laboratory at Purdue University, USA. The test was composed of the first 30-minute rainfall simulation (dry run), 30-minute break, and the second 30-minute simulation (wet run). The rainfall intensity was 60 mm/hr, which is about the five-year recurrent rainfall intensity in Korea.
The differences of surface runoff, percolation, and soil retention between the bare soil and the 100% covered plots were very large. The simulated rainfall intensity of 60 mm/hr could be considered a downpour. However, no surface runoff was observed at the 100% covered plots while 71.8% of surface runoff occurred in the bare soil plots. This meant that only 28.2% of the rainfall infiltrated into the soil and the rest (71.8%) was drained through surface runoff, causing devastating floods and producing tremendous muddy water and sediment.
The 100% covered plots with a nonwoven fabric showed no surface runoff but 100% rainfall infiltration. This meant that there would be no soil erosion and no sediment generated in spite of the rainfall intensity of 60 mm/hr.
The amount of percolation collected at the bottom of the bare soil plots was 16.2% of the rainfall while that of the covered plots was 62.0%. Also, the soil retention of the bare soil plots was only 11.9% while that of the 100% covered plots was 38.0% of the rainfall. This meant that a percolation of 62.% showed that rainfall was drained throughout the subsurface flow. However, it does not cause floods nor muddy water problems in the receiving waters because percolation takes time and releases the water slowly into the receiving waters for more than a 24-hour period and the water is clean. Also, the high soil retention rate of 30.0% of the covered plots compared with the 11.9% of the bare soil plots meant that the covered plots could hold 3.2 times more rainwater in the soil profile and thus, provide more water to crop growth and increase drought resistance. It is well proven that surface cover is the most effective among the best management practices to reduce soil erosion and sediment from upland cultures.
The tested soil is widely distributed throughout the alpine agricultural areas in the uppermost watersheds of large rivers in Korea. The muddy water and the sediment from the alpine agricultural lands during the monsoon season cause severe water quality problems in every river originating from the areas.
It would be best for soil and water engineers, policy makers, and farmers to fully understand the 11 major factors affecting upland soil erosion to develop technologies and policies for the reduction of soil erosion and implement the technologies and the policies.
Impact of Soil Erosion on Water Quality in Korea
NPS pollution in agriculture has been recognized as one of the major sources in the degradation of the nation's water quality. The total agricultural land area of Korea in 2002 was 1,862,622 ha and composed of 1,138,408 ha (61.1%) of paddy and 724,214 ha (38.9%) of upland. The annual average pollutant discharge, so called "unit load" from the paddy and the upland is 1.59 kg/km2 and 2.30 kg/km2 for BOD, 9.44 kg/km2 and 6.56 kg/km2 for T-N and 0.24 kg/km2 and 0.61 kg/km2 for T-P, respectively (MOE 2004).
T-P load from the upland is estimated to be 2.5 times higher than that from the paddy. By considering that T-P is the limiting factor in Korea for river and reservoir eutrophication, experts believed that the impact of NPS discharge from the upland would be even greater than that from the paddy. However, pollutant discharges, including soil erosion from the upland, have not been intensively studied, and limited data are available.
Almost all rivers and reservoirs in Korea have experienced severe water quality degradation after the monsoon season because of soil erosion and the resulting muddy runoff. The muddy runoff is not directly drained to the sea but is stored in large reservoirs created by dams. The stored muddy water that holds various pollutants is then slowly discharged, for more than 14 weeks in some cases, to downstream, which increases turbidity, gives adverse impacts to the ecosystem, and decreases the safeness of water use. Because of this, the Korean government in 2004 initiated a comprehensive measure to cut down soil erosion coming from the muddy runoff from the sloping agricultural lands in the alpine areas of 400 meter above the mean sea level. However, the lack of soil erosion control engineers and experts to design, implement, and educate soil erosion control workers is one of the biggest obstacles to succeed in the plan in the short term.
Critical factors that can be economically controlled need to be identified and experimentally tested to develop and apply new or existing soil erosion control technologies. Table 2 shows some of the measured or estimated soil erosion data in Korea. Depending on the researchers, the measurement index is different. It is assumed that "SS" stands for suspended solids and does not include bed loads; "soil loss" stands for estimated average soil loss by the USLE; and "sediment" stands for both suspended solids and bed loads at the edge of a field or at the outlet of a watershed.
Current water quality degradation that the rivers in Korea have been experiencing may not be caused by the small amount of soil loss of about 5,000 kg/ha/yr. The water quality degradation is thought to be caused by soil loss much higher than 5,000 kg/ha/yr. It is well represented by the average watershed SS load of 876-3,962 kg/ha/yr and 8,100 kg/ha/yr. These SS loads were measured at the outlet of the respective watershed that was composed of uplands, forests, paved roads, residential areas, and others. Because forest, paved road, and residential land uses do not generate large soil erosion, it could be easily imagined that much of the SS load of 8,100 kg/ha/yr, for example, came from uplands that had steep slopes with a sandy soil texture. And thus, the sediment load at the edge of an upland of this watershed would be much higher than the 8,100 kg/ha/yr measured at the mouth of a watershed. The watershed called a "punch ball watershed" is one of the most notorious watersheds that drain bad muddy water every monsoon season in Korea.
Based on the rainfall, field, crop, and management method, sediment load at the edge of a field for radish and potato culture were measured up to 81,600 kg/ha/yr and 29,600 kg/ha/yr, respectively. A runoff plot study that had four 5 m x 30 m runoff plots on 28% slope, sandy loam soil, and Chinese cabbage culture, equipped with a rainfall simulation system, produced the sediment of 18,000-72,000 kg/ha during a 40-minute, 45 mm rainfall simulation (Choi et al. 2005). The plot size was relatively smaller than the actual field size in the alpine area. If the same condition was assumed in the runoff plot study, it would be expected that the sediment load at the edge of the alpine agricultural area might well be more than a few hundred thousand tons per hectare per year. This is the major cause of muddy runoff and water quality degradation in Korea.
Soil Erosion Measurement Methods in Korea
Soil erosion can be measured by the three types of experiments, as follows: small-size runoff plot test, runoff plot erosion test, and field-size erosion test.
The small-size runoff plot test uses plots of 1 m x 1 m, for example. It does not measure the actual amount of soil erosion but the effect of experimental treatments. Common experimental treatments are the percent of soil surface cover both by residue and vegetation, slope, rainfall intensity, the type of soil, the depth of impermeable layer or groundwater level, and the degree of compaction by tractor. This test is usually equipped with a rainfall simulator to measure the exact effect of the treatment under a tightly controlled environment. Experimental results are applied to describe the basic soil mechanisms and develop conceptual soil erosion models. This test also measures the effect of organic farming on water quality. Whether or not organic farming is environmentally friendly remains debatable. If organic farming was not environmentally friendly, especially in terms of water quality, what would be the alternatives to maintain agricultural productivity and minimize the NPS pollutant discharges? A few national laboratories and universities are performing this basic soil erosion test in Korea.
The runoff plot erosion test commonly measures the amount of soil erosion driven by rill and inter-rill erosion, verifies and modifies the soil erosion mechanisms derived by the small-size runoff plot test, collects model input data for USLE, for example, derives soil erosion models, and measures the effect of agricultural management practices, vegetative filter strips, and other best management practices on water quality. The standard plot size, used to derive the USLE, measures 72.6 feet (22.1 m), with a variable width on a 9% slope. The width is not specified but 13.3 feet (4.1 m) or wider is preferred to allow the development of rill and inter-rill erosion patterns. The surface of the plots is carefully prepared to have a uniform slope. The test can be performed with or without a rainfall simulation system. However, it is strongly recommended that it be equipped with a rainfall simulation system to minimize experimental errors and to measure the effect of treatments as precisely as possible. A runoff plot is bordered with steel strips driven into the ground and an endplate at the bottom end of the plot. A gutter or a trough is placed to collect and guide the runoff to an H-flume or other flow measurement device. If a large amount of sediment is expected from the plots, a drop box is used instead of an H-flume. A drop box is a kind of flume designed to drain the sediment without depositing in the flume by causing turbulent flow in order to stably measure the runoff. Runoff samples are taken by an automatic sampler for distributed samples or a Coshocton wheel sampler for composite samples. The Rural Development Administration (RDA) and Kangwon National University have been performing the runoff plot experiments in Korea.
The field-size erosion test is performed on a field that is bordered by earthen berm and equipped with runoff measurement and sampling devices. The sizes of field tests vary depending on the objectives of a test and the local land uses. This test can represent the actual effect of experimental treatments. However, fields are very much site-specific, which means that there is no same field that has the same soil texture, and concave and convex topographies unless the surface is manually prepared. However, because the objective of a field test is to measure the actual effect of treatments under the same local land use and cultural conditions, manual preparation for a field test is not recommended. The results of a field test also are site-specific and are difficult to generalize. Numerous test results are needed to avoid risks, biases, and misrepresentations in using the results for a policy development or for public releases. Depending on the field size and condition, runoff and soil erosion measurement system must be installed. Because the test is weather-dependent, automatic flow measurement and sampling system is preferred. Flume or drop box should be large enough to measure the runoff that can occur under a severe storm event. This test has not been intensively performed and experienced engineers and experts are very rare in Korea.
Soil erosion test plots should be equipped with runoff measurement and sampling systems. For flow measurement, H-flume, long throat flume, drop box, and V-notch weir are commonly used. Water level or/and velocity of these devices are measured by various types of water level gauges and flow meters. Runoff volume is either computed by using a rating curve or measured directly by a flow meter. Runoff samples are taken manually or by a sampler. Also, runoff samples can be taken by time or runoff volume. Recent devices can compute runoff rate real time and take runoff samples by preset time interval or flow volume. One disadvantage of these pieces of equipment is that they are expensive. If distributed samples are not required, a composite sampler such as Coshocton wheel sampler can be effectively used. Soil erosion tests are generally weather-dependent and they need an automatic measurement system. If a field plot size is large and heavy sediment is expected, a sediment basin is placed where the runoff is collected before a flume could measure the runoff rate.
Soil Erosion Control Techniques
Numerous soil erosion control techniques, including the best management practices (BMPs), have been developed in many advanced countries. These techniques are basically based on the control of the major factors affecting soil erosion and have greatly contributed in cutting down soil erosion to meet the allowable soil erosion (or tolerable soil loss, T-value) criterion. Korea has not established the allowable soil erosion standard but the USA has set up the allowable soil erosion of 11 t/ha/yr in the 1960s (Mutchler et al. 1994). It is based on the assumption of an average 1 mm sheet erosion of a soil surface. However, the allowable soil erosion can be adjusted by a local government to meet the local water quality criteria. For example, Hudson (1981, quoted from Shin and Kim 2001 and reference not listed) proposed the allowable soil loss of 2 t/ha/yr for common agricultural fields and 1 t/ha/hr for water quality-sensitive area by NPS pollution.
A few guidelines for soil erosion control in Korea have been proposed. In the province of Jeju, land use is grouped into three categories: absolute conservation area, moderate conservation area, and sustainable development area ( Table 3). The moderate conservation area is again subgrouped into three classes. It is based on the soil loss from 50 m-long sloped field (Yoon et al. 1997). Ha et al. (2004) proposed a general guideline based on the size of soil erosion and compared with OECD standards ( Table 4). These are only guidelines and have no legal binding authority. The allowable soil erosion standard that has legal binding authority is the most powerful and effective tool to enforce and persuade farmers to adopt the erosion control techniques.
Soil erosion control techniques are theoretically simple and easy but practically dirty, tough, time-consuming, laborious, controversial, and costly. Also, soil erosion techniques are very much site-specific. One technique can be successfully applied to reduce the soil erosion on a site but success cannot be guaranteed on another site if it is not modified to reflect site-specific characteristics. Soil erosion control strategies can be approached in three ways: administrative system approach, public relations and training, and technical approach.
Administrative System Approach
Administrative system approach is a key to the success of soil erosion control in Korea. The system should be well organized and supported by laws and the members of the system should well understand the soil erosion processes and the factors affecting soil erosion as well as hydrology and hydraulics. Also, the government should set up the allowable soil erosion criterion in terms of the amount of sediment discharge at the edge of a field. It is believed that the local administrative offices in Korea are well established and they provide excellent public services to the local citizens for general and routine affairs. But because the office personnel is mostly composed of non-engineers who do not understand engineering principles and practices related to soil erosion control, there may be many trials and errors in developing and implementing policies to reduce soil erosion and muddy runoff from the uplands. As a matter of fact, the policies have not been satisfactory.
Many measures must be carried out to successfully control soil erosion in Korea. One of the most urgent measures for the Korean government to carry out is the education of public domain workers who practically execute the government budget in planning and performing soil erosion control projects. Also, private consultants and construction engineers who are willing to design and perform soil erosion control projects must take soil erosion control training courses offered by workshops and institutions that have a specialty in soil erosion control. Because government budgets have been executed by the two groups who do not understand soil erosion control techniques, the soil erosion plans by local administrative offices could not be carried out satisfactorily.
Setting up of the allowable sediment at the edge of a field is also one of the most urgent tasks. Based on the allowable sediment, BMPs can be effectively chosen and practiced. If the set of BMPs to reduce soil erosion from a field cannot meet the allowable sediment discharge, the field can be forced to change the land use to produce less soil erosion. For example, land uses of vegetation cultures to row crop cultures and eventually to grassland or forest can be forced. In this case, farmers or landowners must be compensated for their losses by the land use changes by the government.
Soil erosion control projects must be continuously supported by a large government budget. In most cases, soil erosion control structures are not permanent ones and may be destroyed, buried, lost, or damaged by runoffs and floods caused by severe storm events. Those soft and hard soil erosion control structuresshould be continuously reinforced and repaired to maintain the designed purposes. Minor maintenance operations need to be done primarily by farmers and landowners. And if the maintenance works are beyond the landowner's ability, local construction engineers may be hired by the local administration to fix the problem. Land purchase or lease for the soil erosion structures, regular and irregular maintenance works, and incentives to farmers for the loss of land productivity and the cooperation of maintenance works need money, not small but large budget every year. The Korean government announced a comprehensive plan to reduce soil erosion and muddy runoff from the alpine uplands by the end of October 2004. More than 2.2 million US dollars will be invested from 2006, for 10 years, according to the plan. The success of the plan is largely dependent on the conditions described above.
Public Relations and Training
Theories, principles, and techniques to cut down soil erosion and muddy runoff from sloping uplands have been well established in environmentally advanced countries. However, adoption of these techniques is dependent on the citizens' support and the decision makers' intentions. When the general public asks its government to reduce sediment and other NPS pollutants to protect and conserve water quality, a decision maker can easily adopt the techniques and allocate the necessary budget. Other important variables for the successful implementation of sediment and muddy runoff reduction policies are the farmers and landowners. Unless they are willing to accept the soil erosion control techniques on their lands and in their agricultural management practices, no policies and techniques can be successful. Farmers can voluntarily accept the techniques only when they understand the impacts of sediment and muddy runoff from their lands on a receiving water body, when they feel a strong responsibility for the degradation of water quality, and when they are sincerely motivated to stop soil erosion in their lands. Hefty incentives to farmers can greatly help them decide to adopt the necessary soil erosion control techniques.
By providing various free incentives, low interest loans, and other benefits without any conditions, the Korean government has been helping farmers. However, it is strongly recommended that the government must ask farmers to take certain soil erosion control classes before they apply for government incentives. It is an easy way to educate and train farmers. Korea has a well-established agricultural extension service system. Extension service personnel generally have agricultural backgrounds but are not familiar with soil erosion control techniques. It is also strongly recommended that the extension service personnel be trained about the soil erosion control techniques so that they can discuss and educate farmers at the site. It should be kept in mind that the voluntary participation of farmers in the soil erosion reduction campaign is the best way, technically and economically, to achieve the goal of long-term soil erosion control.
Soil erosion control techniques are very much site-specific. It means that a technique may be successful on a site but may not work on other sites. Although numerous soil erosion techniques have been developed in many advanced countries, these techniques may not be directly applicable to agricultural fields in Korea because of the differences in soil, slope, crop, customary agricultural management, rainfall, and so on. However, because the basic theories, principles, and practices of soil erosion control techniques are the same, some of these can be applied in Korea with minor modifications and after verification experiments. Verification experiments are complex, laborious, and costly in most cases. Only a few researchers and institutions have been conducting these experiments in Korea.
BMPs for soil erosion control for plain and mild-sloped fields are not much different from those of other countries. However, BMPs for steep-sloped uplands in Korea may be quite different from those of other countries. The emphasis of this paper is placed on the soil erosion control techniques for the steep-sloped uplands in the alpine belts of 400 m above the mean sea level in the Korean Peninsula.
It is known that the dominant factors affecting soil erosion are land slope and length, amount of land cover, inherent erodibility of the soil, and rainfall characteristics. The alpine uplands in Korea unfortunately have more than sufficient conditions to meet the worst combination of the dominant factors. The slope and the length of the uplands are generally very steep and long, the sandy soil is very much erodible, the surface is never covered by residues, and rainfall is very intensive. It is no wonder that the uplands dump a huge amount of sediment and muddy runoff into receiving waters. As described earlier, a 5 m x 30 m runoff plot with 28% slope and 54 mm rainfall in 40 minutes produced about 72 t/ha of sediment. Controlling soil erosion is not a matter of improving soil quality by increasing organic matter content. It is a matter of controlling runoff. So, erosion control strategies must be directed to reduce or bypass surface runoff by all means.
The following BMPs are mainly focused on the reduction or the bypass of runoff and the removal of sediment during conveyance to a receiving water. Sediment basin and trap, terrace, drainage channel, check dam, weir (e.g., concrete drop structure and chute, gabion), and wetland are considered as hard BMPs. Surface cover, vegetative filter strip, tillage method and mark (e.g., no till, reduced till, contour till) are considered as soft BMPs. The hard and soft BMPs must be functionally combined to get the best results. These BMPs for soil erosion control can be schematically developed as shown in Fig. 1.
Soft BMPs. Surface cover and tillage method and mark are the two main BMPs to reduce runoff and erosion at the source. It is proven that the tillage mark of contour does not significantly reduce soil erosion. Contour tillage mark can work well to reduce runoff and erosion if rainfall is small. However, if rainfall is large enough to fill the furrow, the ridge is destroyed, the water in the furrow suddenly flushes downslope, consequently destroying the ridges downstream to form large rills and gullies, resulting in a huge sediment discharge. Therefore, contour tillage mark practices are not very effective when a large rainfall is expected.
Reduced till and no-till practices can be good alternatives to reduce erosion from the steep-sloped uplands if grain crops such as corn, soybean, wheat, and barley were cultivated. But the major crops in the high mountain alpine fields are potato, Chinese cabbage, radish, carrot, and other vegetables. These crop cultures need conventional tillage and the surface is completely disturbed before transplanting or seeding is made every year, making the soil soft and easily erodible.
The last alternative left to reduce runoff and erosion at the source is surface cover. As shown in Table 1, the 100% covered sandy soil plots did not produce runoff while the bare plots released runoff of 71.8% of the provided rainfall. Soil retention and groundwater runoff also showed a large difference between the covered and the bare soil plots. The problem is that surface cover with the vegetable cultures is not easy and may not be economical. It is well proven that the source control of NPS pollution is the best both technically and economically. If so much soil is eroded every monsoon season and the water quality of the receiving water is sensitive, it is well worth applying the surface cover method. Loose rice straw mats can be used to cover the surface.
A vegetative filter strip (VFS) is an alternative to retain sediment in runoff. VFS can work well in the uplands where the slope is mild, and runoff does not form large rills. However, where the land size is small and concentrated runoff occurs like in the alpine uplands, VFS may not work well to remove sediment in the runoff. Munoz-Carpena and Parsons (2005) developed the VFSMOD-W that could estimate filter length, width, slope, and vegetation to meet a sediment reduction. But they recommended that the model be applied to smooth slopes, typically less than 10%. In the USA, VFS is required to remove 75% of sediment to meet the total maximum daily load (TMDL) criterion. Considering that a 5 m x 30 m runoff plot on 28% sandy loam soil produced sediment up to 72 t/ha/event, it could easily be imagined that no VFS could stand the sediment load. Before a VFS is made on a field, it is strongly recommended that it undergo the experiments on the width and VFS vegetation if it would be installed at the edge of steep-sloped uplands.
There are many other soft alternatives to reduce soil erosion from the uplands. One of them is the coir net that is woven with palm tree fiber. It takes 3-4 years for the net to decay in the wild environment. Various types and forms of coir net products are placed at the edge of a field and mulched on a slope to minimize soil erosion.
Adding huge amounts of soil has become customary in some uplands where crop rotation is not practiced and soil sickness is experienced. The depth of fresh soil layer added is sometimes deeper than 20 cm. Weathered granite soil (sandy or sandy loam soil) that contains practically no nutrients is usually quarried from a mountain and placed on the existing field. If soil is continually added, the elevation of a field becomes higher than the adjacent roads. In this case, rainfall runoff may be drained through the roads and the roadside soil may be severely eroded. To prevent these from eroding, a 20-30 cm-high concrete sill may be made along the roadside.
Hard BMPs. Terrace, sediment basin and trap, drainage ditch and channel, diversion and catch drain, grassed waterway, tile drain, grade stabilization (drop or chute) structure, and constructed wetland are some of the examples of hard BMPs that can be applied on steep-sloped uplands in Korea. The basic concept of these BMPs is the safe drain of surface runoff so that it does not form rills and gullies. If no rills and no gullies were formed, soil erosion would decrease drastically. Therefore, the BMPs need to be functionally combined to minimize the formation of rills and gullies and to drain the runoff to a channel where it can be discharged into the drainage system. This job may involve so much work and so many farmers and local engineers who do not have soil erosion control experiences and may not understand the nature of erosion control works. They may even think of giving up land cultivation altogether. These BMPs are not easy to do but they have to be done to stop erosion. Otherwise, the land uses may have to be changed to less erodible uses such as grassland or forest.
Terrace is a good alternative to remove sediment in runoff and safely drain surface runoff through drain pipes. A terrace in small-sized lands is similar to a small sediment basin that is mainly made of a small dry pool. A dry pool can be placed at the end or in the middle of a field where concentrated runoff passes or discharges. The dry pool receives runoff from the upper field and the runoff is drained directly or through drain pipe(s) to a drainage channel or nearby stream. While the runoff passes the pool, velocity is drastically reduced, and much of the sediment in the runoff, except the clayey particles, is deposited. The sediment removal rate of the small pool is amazingly high in sandy soil fields.
Catch drain, diversion, drainage channel, and sediment basin or trap can play key roles as a system in controlling sediment discharges. The slope of these waterways must be mild enough to decrease runoff velocity and to deposit the sediments in runoff in the waterway. If a slope is too steep and the water velocity is too high to deposit sediments, drop, chute or grade stabilization structures must be constructed to make the slope smooth. Where a concentrated flow flows into the waterway, a section of the waterway must be enlarged to accommodate the depositing sediments. It is because the runoff velocity decreases where it meets the waterway and the runoff sediments may suddenly block the waterway. Sediments do not evenly deposit over the wide area but they immediately settle down where the runoff losses its transport capacity as the velocity decreases. The enlarged section may be called a sediment trap or basin. It is believed that the waterway system of catch drain, diversion, drainage channel, and sediment basin can functionally work well and remove much of the runoff sediment. The author's experiment proved the sediment removal of more than 95% with a small sediment basin. One of the disadvantages of this approach was the need to remove and empty the basin and waterway after every severe rainfall event that produced a large sediment volume.
A drainage channel system in the upland area is not well established in Korea. The construction of catch drain, diversion, drainage channel, and sediment basin systems may require a large land size, farmer's agreement, and considerable budget. However, it also is understood that without the system, the reduction of sediment and muddy runoff from sloping uplands may not satisfy the water quality standards.
Sediment basin itself can remove sediment effectively if it is properly constructed and maintained. Because of intensive land uses in Korea, it is not desirable and economical to build a large sediment basin. But it is recommended that a sediment basin be built at the mouth of a field where runoff is concentrated and discharged. The size of the sediment basin may be designed to hold the sediment volume that can be discharged by two to three large rainfall events. After each sediment discharge event, the basin must be emptied for the next storm event. The design and construction manual of a sediment basin for upland cultures has not been established in Korea. However, references on simple and easy methods of designing a sediment basin are available (Ohio Department of Natural Resources 1996).
A favored design is the tile drain where interflow seeps to the surface during the monsoon season. Grassed waterway may be a good alternative to remove sediment in runoff while safely draining runoff. But because it takes a relatively large area and needs careful maintenance, it may not be accepted by farmers. Paddy as a constructed wetland may be an alternative if it is functionally combined with a drainage system that is composed of catch drain, diversion, drain channel, and sediment basin. An upland watershed is divided into small sub-watersheds. And at the mouth of each sub-watershed, a paddy or a cascade of paddies is prepared to accept the runoff from the drainage system. The paddy then further settles down the fine particles in runoff from the drainage system.
Soil erosion mechanisms, factors affecting soil erosion, the current status of soil erosion, and the impacts of soil erosion on water quality in Korea were briefly described. Small and large runoff plots and field-scale monitoring methods with or without a rainfall simulation were explained based on the researches performed in Korea. Soil erosion control techniques that can be applied to steep-sloped uplands in the alpine area were grouped into the following three categories: administrative system approach, public relations and training, and technical approach. Basic theories and principles for soil erosion control, unique characteristics of the upland cultures in the alpine area, and recommendations and suggestions to reduce soil erosion from sloping uplands were made for each category.
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Index of Images
Figure 1 Schematic Flow Chart for the Development of BMPS for Soil Erosion Control. (Esa Stands for Environmentally Sound Agriculture, and VFS for Vegetative Filter Strip.)
Table 1 Differences of Runoff and Infiltration between Bare Soil and Covered Plots
Table 2 Some Existing Soil Erosion Data in Korea
Table 3 Soil Loss Criterion in Jeju Province, Korea
Table 4 Proposed Soil Erosion Classes in Korea
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