Rainfall Simulation Studies to Estimate Soil Erosion as Influenced by Rainfall Intensity and Slope in Four Distinct Soils
(1) To investigate the effect of slope angle and rainfall intensities on soil erosion under controlled conditions using four (4) distinct soil types; (2) To compare this data with that for a cropped plot; and (3) To highlight an approach at estimating erosion risk and nutrient loss.
Soil erosion or the wearing away of soil due to the effects of water, wind, tillage and other factors. Rain erosion is the wearing away of soil and this is known as 'splash erosion'. If the rainfall has sufficient intensity then the kinetic energy of raindrops as they hit the bare soil detaches and moves soil particles. Considerable amounts of soil may be moved by rainsplash however, the soil is stated to be "redistributed back over the surface of the soil" although there will be a small amount of downslope movement of the soil on steep slopes. Rainsplash erosion requires high intensity rainfall and has the most effect under "convective rainstorms in the world's equatorial regions." (Favis-Mortlock, 2005)
Rainfall also moves soil in an indirect manner through runoff in rills or small channels and gullies or larger channels that are unable to be removed by tillage. (Favis-Mortlock, 2005, paraphrased) The small amount of the rainfull that does not soak into the soil flows downhill under the influence of gravity and is known as "runoff or overland flow." (Favis-Mortlock, 2005)
I. Soil Erosion Processes and Factors Affecting Soil Erosion
There are two reasons for runoff:
(1) if rain arrives too quickly for it to infiltrate the runoff which results is then known as infiltration excess runoff or Hortonian runoff; and (2) Runoff may occur is the soil has already absorbed all the water it can hold. Resulting runoff is known as saturation excess runoff. (Favis-Mortlock, 2005, paraphrased)
As the runoff moves downhill, it is reported to be at first "a thin diffuse film of water which has lost virtually all the kinetic energy which it possessed as falling rain" therefore moving slowly and having lost it low flow power, and it reported to be "generally incapable of detaching or transporting soil particles." (Favis-Mortlock, 2005)
It is stated that the microtopograpy of the soil's surface tends to cause this overland flow to concentrate in closed depressions, which slowly fill: this is known as 'detention storage' or 'ponding'. Both the flowing water, and the water in detention storage, protect the soil from raindrop impact, so that rainsplash redistribution usually decreases over time within a storm, as the depth of surface water increases." (Favis-Mortlock, 2005) The microtopography of the surface of the soil is reported to have a tendency to cause this overland flow to "concentrate in closed depressions, which slowly fill: this is known as 'detention storage' or 'ponding'." (Favis-Mortlock, 2005)
Reported as well is that both the flowing water and the water in detention storage, "…protect the soil from raindrop impact, so that rainsplash redistribution usually decreases over time within a storm, as the depth of the surface water increases." (Favis-Mortlock, 2005) It is reported as well that there are "complex interactions between rainsplash and overland flow." (Favis-Mortlock, 2005)
Soil erosion is reported to occur "both incrementally, as a result of many mall rainfall or wind-blow events, and more dramatically, as a result of large but relatively rare storms. It is the large storms which produce the big hard-to-miss erosional features such as deep gullies. But while erosion due to small common events may appear insignificant on the field, its cumulative impact (both on the eroding field, and elsewhere) may, over a long timescale, be severe." (Favis-Mortlock, 2005)
Water erosion is stated to be comprised of a "complex hierarchy of processes" and this translates to mean that study of water erosion is over a wide range of spatial scales as this is how water processes occur. The occurrence of microrills and rills during rainsplash distribution occurs at the millimeters scale while rill erosion occurring on agricultural hillslopes is known to occur at a scale of meters to tens of meters and gully erosion occurs on a scale of hundreds of meters or possibly even on a scale of kilometers. It is reported that offsite impacts of erosion may affect areas that are large-scale and potentially hundreds of even thousands of square kilometers. Erosion at each spatial scale is reported to be "highly patchy. In areas that are severely eroded the soil loss rates experience great variation at each point on the landscape "as the vagaries of topography and land use concentrate erosive flows on a wide range of spatial scales. Obvious erosion in one field can be found side-by-side with virtually untouched areas; and within an eroded field, the severity of erosion can vary markedly." (Favis-Mortlock, 2005)
On-site impact is primarily noted in the reduced quality of soil due to the loss of nutrients in the upper layers of the soil which are generally rich and as well the water-holding capacity of the soil being reduced due to erosion. Areas that are impacted by soil erosion in affluent countries are able to mitigate these impacts through increasing the use of artificial fertilizer however, in poorer countries this is not an option. Soil erosion results in the upper horizons of the quality of the soil and diminishes the suitability of the soil for agriculture or other vegetation since the most nutrient-rich soil is the eroded upper horizons of the soil. As well, it is reported "the finest constituents of eroded soil tends to be transported furthest" and the soils that are eroded are reported to be "preferentially depleted of their finer fraction over time; this often reduces their water-holding capacity." (Favis-Mortlock, 2005)
The 'cream of the soil' is removed by erosion. Loss of quality of soil is reported as a problem that is long-term and the most serious impact on a global scale is the threat it presents to agricultural long-term productivity due to the damage caused on-site by erosion. The upper horizons of the soil are critically important for crops, which are greatly reliant on its part of the soil. Soil is redistributed soil and this results in thinner soils on "topographically convex areas within a field." (Favis-Mortlock, 2005)
Off-site effects are also noted in the work of Favis-Mortlock (2005) and these are stated to be the "movement of sediment and agricultural pollutants into water courses." This leads to "…silting-up of dams, disruption of the ecosystems of lakes, and contamination of drinking water. In some cases, increased downstream flooding may also occur due to the reduced capacity of eroded soil to absorb water." (Favis-Mortlock, 2005) This results in increased runoff that is likely to result in flooding downstream and damage to local property. Another major off-site impact is reported to result from the chemicals used in agricultural production which is stated to move with the erosion of sediment as it is moved. The chemicals are moved into watercourses and travel downstream into bodies of water polluting them. Where the inputs of agricultural chemicals are quite high as well as are the removal of such chemicals from drinking water.
The work of Choi, et al. (nd) entitled "Soil Erosion Measurement and Control Techniques" states that detachment of soil participles 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 participles and the presence of material that reduces the transport forces." Erosion and sediment load at a location on the slope. At given location on the slope, 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." (Choi et al., nd)
The major factors that affect upland erosion processes are such as hydrology, topography, soil surface cover, incorporated residue, residual land use, subsurface effects, tillage, roughness, and tillage marks are the major factors that affect upland erosion processes." (Choi et al., nd) There is reported to be no practicable way to control the erosion of soil and the sediment production of a field. Residual land use and subsurface effects are also not practiced commonly to addressed the erosion of soil since 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 systems of trees and grasses result in retardation of the erosion of soil for a period up to three years but as the roots decompose the residual land use and effect below the surface disappear. Terrace-building or the sloping field can somewhat control topography although this alternative is quite expensive.