Rainfall Simulation Studies to Estimate Soil Erosion

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.

Where there is intensive land-use and heavy rainfall terracing is an effective method such as the paddy field in Asian countries. Incorporated residue is crop residue that is mulched into the soil such as corn stalks being buried to increase the organic matter content of the soil and which further operates like grade control structures thereby preventing rills from expanding. The use of no-till and reduce tillage practices result in less soil erosion and sediment than do convention methods of tillage or the use of plow tillage. However, it is not possibly to apply no-till and reduced tillage to all agricultural production initiatives.

Tillage marks are reported to be grouped to up-and-down till and contour till, which is generally effective in the reduction of soil erosion. There are however, many gullies and produced by contour as well as a great deal of sediment being created when the rainfall is heavy enough to cause ridge overflow. Ridge overflow results in the ridge being destroyed and downslope ridges are also destroyed and the consequence is the creation of small and large gullies.

The work of Ryan (1981) entitled "Sediment Measurement Techniques Used by the Soil Conservation Service of New South Wales, Australia "states that sediment monitoring is the responsibility of the Soil Conservation Service (SCS) of New South Wales Australia, as part of hydrological evaluation of land management strategies, including soil erosion control measures. In New South Wales sediment concentration in runoff is not constant with time, it fluctuates widely both between and during events Thus, a major difficulty in sediment monitoring is the impracticability of the operator being present for every event."(Ryan, 1981) The result is that the instruments must be utilized to provide the missing data. The SCS uses two instruments:

(1) The first system uses a sediment sampler to provide an estimate of both sediment concentration and total runoff. Total runoff is determined by dividing the Volume of stored runoff by the known proportion of runoff flow the sampler extracts; and (2)The second system uses a sediment sampler to give an estimate of sediment concentration and a supplementary instrument to give the total runoff estimate." (Ryan, 1981) Each of the systems is characterized by the sediment concentration per unit volume of runoff being applied to the total runoff amount for computing total sediment yield for a runoff period. The runoff period can be part of or, the entire, runoff event." (Ryan, 1981)

The choice of the sediment sampling system and sampler is dependent upon the "objects of the specific programs." (Ryan, 1981) Ryan reports that sediment monitoring from basins "is only undertaken as a part of a research project so that the aim of that research will dictate thee sampler and sampling system requirements." (1981)

The SCS begin research into sediment rates in 1946 and the equipment most suitable at the time was the Gieb multislot divisor and storage tanks. Ryan states that in the early 1950s, research into the hydrological effects of land treatment in small basins was started the aim being to compare total soil loss and runoff between treatments. A Pomerene wheel sampler is used in these studies. In the late I960's and early 1970's SCS research began to involve larger basins, up to 500 ha. Initially single stage sediment samples were used to obtain runoff samples. These were replaced by pumping samplers. When high sediment concentrations are anticipated pumping samplers are used in conjunction with a sloping crest Crump weir or H. flume with a 14% sloping drop box." (Ryan, 1989)

Ryan discusses these systems using the criteria of:

(1) capacity;

(2) sample;

(3) accuracy;

(4) reliability;

(5) cost; and (6) hydraulic head. (1989)

The sampler is reported by Ryan to require "sufficient capacity for measuring sediment relatively large runoff events." (1981) In addition, there should be no variation between "the sediment particle size distribution of the sample and that of the total runoff. The sample's suitability is determined by the type of sample collected and stored.

A sampler that collects a composite sample for each runoff event would be suitable for a "Simple basin treatment comparison study, but for soil erosion process research, discrete samples on a time or discharge rate basis are needed." (Ryan, 1981) It is reported that the accuracy of runoff measurement is higher than for sediment yield determinations. The sampler must be capable of dependable, automatic operation between inspection visits." (Ryan, 1989)

Ryan (1989) reports that the cost per satisfactory measurement of sediment "should be as small as possible. This cost should include both installation and maintenance costs." (1981) Ryan additionally states that the hydraulic head "is a site factor, some samplers require a greater hydraulic head to operate and would be unsuitable for use on very flat terrain. The smaller the hydraulic head required by a sampler the better, provided the other criteria are satisfied."

IV. Measurements of Soil Erosion (models and other techniques)

The work of Stroosnijder (2003) entitled "Measurement of Erosion: Is It Possible?" reports that erosion measurements are used for the purpose of:

(1) determining environmental impact;

(2) designing policies and programs;

(3) planning conservation; and (4) optimally allocating resources.

The literature is reported to mention four cases:

(1) the large temporal and spatial variation of erosion;

(2) the paucity of accurate erosion measurements;

(3) the problem of extrapolating data from small plots to higher scales; and (4) the conversion of erosion into production and monetary units (impact). (Stroosnijder, 2003)

Measurements are needed to "develop, calibrate, and validate that technology" and additionally it is important to note that measurement techniques are differing in their "accuracy, equipment and personnel cost." (Stroosnijder, 2003) The techniques that are the most accurate and many times the most expensive are stated to fail to always serve the measurement purpose. (Stroosnijder, 2003)

Stroosnijder (2003) states that there are at least five reason that erosion measurements are taken:

(1) assessment through an erosion inventory;

(2) scientific erosion research;

(3) development and evaluation of erosion control technology;

(4) development of erosion prediction technology; and (5) allocation of conservation resources and development of policies and regulations. (Stroosnijder, 2003)

Assessments are stated to be carried out for planning of control of erosion at the watershed scale and it is stated that an erosion inventory generally uses a mixture of two technologies:

(1) direct measurements; and (2) the use of erosion prediction technology. (Stroosnijder, 2003)

Characteristics of measurements techniques for erosion inventory are:

(1) not so accurate;

(2) cheap and fast so that many spots can be measured. (Stroosnijder, 2003)

It is stated that characteristics of erosion measurement techniques for scientific erosion are:

(1) more accurate; and (2) aimed at causes and effects of erosion. (Stroosnijder, 2003)

Stated advantages of erosion measurements in the laboratory include:

(1) control of the range of dependent variables;

(2) use advanced and automated equipment; and (3) repeat measurements. (Stroosnijder, 2003)

Advantages of field research include:

(1) proper scale;

(2) realistic soil and plant characteristics; and (3) temporal changes in environmental variables.( Stroosnijder, 2003)

Erosion processes are stated to be "manyfold" and different processes operate at different scales, spatial and temporal. Measurements must be fitted to the scale. There are five relevant spatial scales reported:

(1) the point (1m2) scale for interrill (splash) erosion;

(2) the plot (,100 m2) for rill erosion;

(3) the hillslope (,500m) for sediment deposition;

(4) the field, 1ha) for channels and (5) the small watershed (>60 ha for spatial interaction effects. (Stroosnijder, 2003)

There are reported to be two relevant temporal scales:

(1) the single rainstorm for the design (strength) of erosion control technology; and (2) the annual average for conservation planning. (Stroosnijder, 2003)

It is reported that different aims make a requirement of different scales as show in the following table labeled Figure 1

Figure 1

Matrix of Scales and Aims

Aim/Scale Assessment Scientific Areal Lines Prediction Policies

Research Conservation technology

Point x x x

Plot x x x

Hillslope x x x

Field x x x

Watershed x x x

Source: Stroosnijder (2003)

V. Rainfall simulation and Soil Erosion Using Plots

The plot (

(1) long plots (4-10m),

(2) an artificial furrow with rain; and (3) the same as 2 with supplementary upstream flow. (Stroosnijder, 2003)

Precautions are needed in measuring correctly: rill erosion is the sediment measured at the bottom-end of the rill (furrow) minus the interrill erosion. When conservation practices are evaluated that control interrill and rill erosion the 'normal' width and length of a plot that is: width = 2-25 m and length= 10-25 m. These plots are in: 3 replications (with the same soil type and slope steepness) There is stated to be a question as to whether the plots should be bound or left unbounded. The negative aspect of bounded plots is stated to be that "there is no inflow at the top-end of the plot and the drawback of unbounded plot is that the source areas from where the runoff and sediment comes from is not known." (Stroosnijder, 2003)

The soil properties that are needed for mapping unit as input for the water erosion model WEPP include those shown in the following table labeled Figure 2

Figure 2

Soil Properties Needed for Each Mapping Unit as Input for the Water Erosion Model WEPP

Source: Stroosnijder (2003)

It is stated that measurements "should be taken frequently over sufficient long duration." (Stroosnijder, 2003) Frequency is stated to be the "number of times measurements are taken during the measuring campaign and duration is the length of time that measurements are taken." (Stroosnijder, 2003) Because it is expensive to take erosion measurements "the duration of measurements and their spatial coverage are often limited" which may result in a bias in the measurements because:

(1) There is often large inter-annual variation and large intra-seasonal due to different erodibility of the soil, crust formation, vegetation cover;

(2) There is large spatial variation: upland erosion may never show up in a stream because of sedimentation or large sediment load in a stream may be solely due to bank erosion; and (3) Changing wind may return sediment where it came from in a previous storm. (Stroosnijder, 2003)

Equipment used in taking erosion measurements should be:

(1) Properly constructed and with manual;

(2) Calibrated;

(3) Installed;

(4) Operated;

(5) Maintained; and (6) Handled by trained operators. (Stroosnijder, 2003)

Most equipment is not available at the commercial market but instead is made by researcher at the local level meaning there is not much standardization. (Stroosnijder, 2003) It is stated that Beven (2001) holds that it is not possible to build a catchment level hydrological model since there is a lack of measuring techniques that are adequate and states that it is better to use collective intelligence gained from years of laboratory and field experiments in the development of an approach that has collective induction as its bases rather than deduction. (Stroosnijder, 2003, paraphrased)

The work of Hicks (2001) entitled "A Summary of Techniques for Measuring Soil Erosion" reports a study commissioned for the purpose of reviewing techniques for monitoring soil erosion. Included in the report is the technique of measurement of soil erosion from runoff plots. This is described as a hydrological technique that involves the establishment of bounded plots to collect surface runoff with flow-collecting devices at outlet. This technique can be modified to trap sediment and calculate depth or volume of surface erosion.

The application was found usable and to be limited in that it is in applicable in determining the area depth and volume and is limited to surface erosion and is measured indirectly by extrapolation backwards from measured volume. The technique is reported to be scientifically defensible and is an excellent technique for measuring soil erosion based on hydrology combined with sediment-trapping devices. There are methodological problems in that all sediment may not be trapped.

The technique is reported as statistically valid if a large number of measurements are collected and carefully interpreted. The technique is also described as interpretable although it does take time to collect a large number of measurements as well as taking time to process the information gained from the measurements of soil erosion through use of its technique.

This technique is sensitive and detects change over short durations and as well detects change within small areas. The technique is replicable in that plots can be maintained or re-established. Setting up this method requires advance set-up and calibration and as well continuous monitoring is needed for a lengthy period in order to obtain enough data. In terms of the availability of historical information, it is reported that comparable data is rarely available. There are some local impacts due to soil and vegetable disturbance at the plot site however, these impacts are stated to be on an acceptable level. This technique is characterized by a high instrument cost and moderate staff cost.

The work of Coix-Fayos, et al. (2006) states that soil erosion plots of varying types and sizes are used widely for investigation the geomorphological processes related to erosion of soil. It is reported specifically that this field method "…has provided a variety of results, depending on the characteristics of the plots, on their suitability to reflect the ecosystem's characteristics and on the objectives of each particular research. The coupling of real soil loss at patch and slope scale within a landscape and the values obtained by field plots depend, among other things, on how good the methodology performs over a set of ecosystem properties, such as those related with temporal and spatial scale issues, disturbance and representation of natural conditions, and the ability to account for the complexity of ecosystem interactions..As regards the spatial and temporal scale of measurements, topics such as the exhaustion of available material within closed plots in long-term measurements, the different erosion processes operating (and measured) at different spatial scales and the problems and alternatives of extrapolation of the results from larger to smaller scales, are the main causes of variation between measurements.

The disturbance and inadequate representation of natural conditions, such as the heterogeneity, continuity and connectivity of factors and processes, are also sources of variation in the results of specific measurements. In short, the key factor is the difficulty to encapsulate the complexity of system interactions and to represent these interactions by means of field plots. The complexity concept is translated in the connectivity of water and sediment fluxes in the landscape and the interaction between processes and patterns of vegetation and surface components operating across scales." (Coix-Fayos, et al., 2006)

The work of Bagatelle and Ferro (2001) entitled "Plot-scale Measurement of Soil Erosion at the Experimental area of Sparacia" reports that in order to obtain good quaility soil loss data from plots "…equires knowledge of the factors that affect natural and measurement data variability and of the erosion processes that occur on plots of different sizes. Data variability was investigated in southern Italy by collecting runoff and soil loss from four universal soil-loss equation (USLE) plots of 176 m2, 20 'large' microplots (0.16 m2) and 40 'small' microplots (0.04 m2). For the four most erosive events (event erosivity index, Re > 139 MJ mm ha-1 h-1), mean soil loss from the USLE plots was significantly correlated with Re. Variability of soil loss measurements from microplots was five to ten times greater than that of runoff measurements. Doubling the linear size of the microplots reduced mean runoff and soil loss measurements by a factor of 2.6-2.8 and increased data variability. Using sieved soil instead of natural soil increased runoff and soil loss by a factor of 1.3-1.5. Interrill erosion was a minor part (0.1-7.1%) of rill plus interrill erosion. The developed analysis showed that the USLE scheme was usable to predict mean soil loss at plot scale in Mediterranean areas. A microplot of 0.04 m2 could be used in practice to obtain field measurements of interrill soil erodibility in areas having steep slopes."

The work of Hudson (1993) entitled "Field Measurement of Soil Erosion and Runoff" states that runoff plots are used best for demonstration of known facts including demonstrating to farmers that serious erosion is taking place or to show that erosion is much less from a plot which has a good vegetative cover than from a bare plot. (Hudson, 1993, paraphrased) Another valid use of field runoff plots is the use in comparative studies such as testing, demonstrating or getting an approximate indication of the effect on runoff or erosion of a simple comparison such as with or without surface mulch, or the amount of runoff at the top and at the bottom of a slope." (Hudson,1993) Another use is for obtaining data, which are to be used in constructing or validating a model or equation to predict runoff or soil loss. Stated as the classic example is that of the 'Universal Soil Loss Equation'. Problems stated to be associated with runoff plots include those as follows:

(1) Runoff plots are expensive;

(2) Runoff plots use a great deal of staff time at several different levels;

(3) Easy access to the site is important;

(4) Backup facilities are needed as well as the immediately operational staff.

(5) Laboratory facilities are required for handling samples and repairs to equipment. (Hudson, 1993)

There are also constraints on what can be investigated on small runoff plots. Hudson reports that the field-scale loss of soil from land with bench terraces is dependent on the probability of the structure failing. Included in the types of runoff plots are natural or simulated rainfall and it is reported that the cheapest and simplest method is installation of the plot and waiting for rain however, the "unpredictability of rain can make this frustrating." (Hudson, 1993)

Stated as an alternative is the use of artificially manufactured rain through the use of rainfall simulators. The primary advantages of use of a simulator is that it speeds up gaining results and the rainfall amount can be controlled. The disadvantage of simulators is stated to be due to the fact that simulators for large plots are expensive to build and have a high labor requirement for operation. The majority of plots are stated to have "boundaries which define the area from which the runoff and soil are being collected, but there are some cases where it is appropriate to use unbounded plots using what are usually referred to as Gerlach Troughs…" (Hudson, 1993) These consists of "a small collecting gutter which is let into the soil surface and connected to a small collecting container on the downstream side.

There are various degrees of sophistication in the construction of the gutters and containers but expensive or complication construction is not justified because what is required is a large number of replications to overcome the variation which arises from the fact that, without any boundaries to direct or limit runoff into the collecting gutter the amount collected depends on the chance occurrence of minor depressions or rills." (Hudson, 1993) Hudson (1993) states that the size of plots must be related to the purpose of the trial and specifically:

(1) Micropolots of one or two square meters may be appropriate if the objective is a simple comparison of two treatments where the effect of the treatments is unlikely to be influenced by scale.

(2) Small-scale plots usually of about 11 m2 are most commonly used for trails of cropping practices, cover effects, rotations, and any other practice which can be applied to small plots in the same way as it would be on a field scale, and where the effect can be expected to be unaffected by plot size.

The plot size is dictated both "by the treatment and the system for collecting soil loss and/or runoff. Microplots may have collecting tanks, which store the whole of the runoff and soil, but it is important to measure extreme events, the size of the tank required becomes excessive for plots of more than a few square meters.

When designing the size and capacity of the collector system there are two factors that must be considered:

(1) it must be able to handle the maximum probable rate of flow; and (2) it must be able to store the maximum probable quantity of runoff. (Hudson, 1993)

The maximum probable rate of runoff can be computed "from the maximum probable runoff ratio from an already saturated soil and the probable maximum intensity over a short period of perhaps five minutes." (Hudson, 1993) Free discharge from the collector into the storage tanks is a requirement.

Hudson (1993) states that there is no standard for the "ratio of length to width" for plot sizes as short plots are not considered desirable since they may interfere with rill development however, the "relationships between erosion and length of the slope is debatable. The width should be suitable for the farming systems to be applied. For cropped plots, a buffer strip at the sides and top may be used to reduce border effects and also to allow access without walking on the plots."

Many materials have been utilized as plot boundaries, including earth banks, brick or concrete walls, timber planks, and strips of metal, asbestos cement or plastic. They are usually permanently installed for the life of the plots, but sometimes removable boundaries have been used to permit tractor or ox cultivation across the plot. Other points about boundaries are:

(1) Leakage in or out of the plot across boundaries is a common source of error. There should be a drain above the plot to diver surface water flowing down from higher ground;

(2) Plots should not have common boundaries otherwise one leakage affects the result of two plots. There should be a buffer strip between plots;

(3) Boundaries must be carefully installed, inserted deep enough to prevent leakage underneath, tall enough to prevent overtopping and lengths of timber or metal must either overlap or bet butted tightly together to prevent leaks;;

(4) S. small filet of earth heaped against the outside of boundaries stops surface water ponding or flowing against the boundary, but the soil must be drawn against the boundary wall without leaving a channel which could start a rill;

(5) When building earth banks, a channel should not be left where the soil has been taken from. The soil should be taken only from outside the plot. (Hudson, 1993)

It is stated that when large amounts of soil are eroded that the collecting trough "may prevent the natural development of a new profile down the slope -- that is the sill of the trough will be higher than the level which the soil would be if the trough were not there." (Hudson, 1993) This error can be avoided through construction with a sill that can be lowered as erosion progresses. Another potential error is noted to be "leakage underneath the collecting trough, and if this cannot be controlled by compaction of the soil below the collector, some form of cutoff wall should be installed, perhaps by inserting an impermeable membrane, or, if the collecting trough is of brick or concrete, by taking the foundations down deep enough to prevent seepage underneath." (Hudson, 1993)

The question of how it is known that an erosion model is working accurately is addressed in the work of Wainwright and Mulligan entitled "Environmental Modeling: Finding Simplicity in Complexity." Wainwright and Mulligan note the factor of variability in the data in the study of rainfall and its effects erosion of soil. Nearing (2000) relates the procedures for the way that erosion model evaluation can be conducted for the study with data uncertainty, which is a straightforward method, but one that involved detailed computations in achieving a 'rule-of-thumb' model. Erosion models for conservation planning are stated to fall into two basic categories:

(1) empirical; and (2) process-based. The erosion process is divided into rill and inter-rill components where the inter-rill areas act as sediment feeds to the rills, or small channel flows. The model is applicable to hillslopes and small watersheds." (Wainwright and Mulligan, 2004)

Roels and Jonker (1985) reported a study that was designed to demonstrate soil loss sampling which was conducted on a submediterranean range and hillslope covering 8250 m2 on which there was no grazing or grass-burning. The study is reported to have been based on "…interrill, pre-rill and rill erosion measurements using Modified Gerlach troughs and involves random, systematic and cluster sampling procedures. A statistical method is introduced to test the representativity of the various erosion samples and to calculate the accuracy of the plot data for different sample sizes. Both the representativity and the accuracy of the erosion data are evaluated for separate storms ranging from frequent low magnitude to occasional high magnitude. A large scatter was found in the erosion data: soil loss estimates from samples which deviate by 50 to 100% from the (sub)population soil loss are the rule rather than the exception. Systematic soil loss sampling is more accurate than cluster sampling. However, even if large systematic samples are used and sampling is applied separately to each source area, samples still deviate systematically from their reference value. The results indicate that the soil loss data obtained from plots and quoted in current erosion research papers must be approached with caution. As a result of the large scatter in erosion data the samples for study have to be extremely large. This means that future erosion sampling will be both time-consuming and expensive." (Roels and Jonker, 1985)

The work of Jackson, Knoop, Szalona and Hudson (1986) entitled "A Runoff and Soil-Loss Monitoring Technique Using Paired Plots" reports a study using runoff plots that were "equipped with retention tanks" stating that this method has been used in successfully measuring long-term runoff and erosion rates." (Jackson, Knoop, Szalona and Hudson, 1986) The plot-retention tank technique of measuring runoff and erosion volumes is reported to be extremely accurate, amenable to replication and control, and inexpensive to install and maintain." (Jackson, Knoop, Szalona and Hudson, 1986) Additionally, data collected "can be compared using standard statistical techniques or analyzed using the Universal Soil Loss Equation (USLE), the SCS Curve Number rainfall-runoff technique, or other common runoff or erosion models." (Jackson, Knoop, Szalona and Hudson, 1986)

The primary disadvantages to using upland runoff plots for directly monitoring rangeland hydrological condition are:

(1) the low number of measurable events;

(2) equipment failures;

(3) improper site selection or plot installation; and (4) difficulties interpreting upslope processes in terms of off-site effects." (Jackson, Knoop, Szalona and Hudson, 1986)

The plot technique is considered most applicable "when upland soil loss and surface runoff are the issues being addressed by management." (Jackson, Knoop, Szalona and Hudson, 1986) The plots are stated to be constructed of "low cost and readily available materials, and are easily installed. Cost of materials per plot was reported to be approximately $125 at the time of the report in addition to $160 for the recording instrument. The time to install the four plots is stated at "approximately 10 person-days." (Jackson, Knoop, Szalona and Hudson, 1986) Each plot was reported to be 50 feet long by 10 feet wide with side and upper border wood planks set about 3 inches into the soil and supported by wooden surveyor stakes. The lower border is reported to be a "standard metal rain gutter set in the soil with its upper edge at ground level. The gutter is installed at an angle to the slope with a slight drop to insure movement of sediments through the gutter. A length of angled roof edging is placed in the soil above the gutter and attached so that it overhangs the gutter edge, providing stable runoff surface into the gutter. The gutter is covered with hardware cloth to prevent rodent nesting. The disturbed area above the gutter is treated with Celltite, a liquid soil sealer which hardened when sprayed on the soil." (Jackson, Knoop, Szalona and Hudson, 1986) The material list for runoff plots is stated as follows:

44 1 in. x 6 in. x 10 ft treated boards

2 bundles 18 in. surveyor stakes

3 lbs 8 penny galvanized nails

4 10 ft. metal rain gutters

12 10 ft. metal corrugated downspouts

4 10 ft. type AA angled roof edging

4 gutter end caps

4 gutter connecting sleeves

4 gutter corners

2 tubes latex caulk

12 ft. x 36 in. wide 1/2 in. mesh hardware cloth

Bailing wire

Twine

Fencing materials

Celltite soil sealer

4 mechanical float counters

4 100-200 gal stock water tanks (Jackson, Knoop, Szalona and Hudson, 1986)