Literature Review Chapter: Rainfall Simulation Studies to Estimate

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[. . .] 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 (<100 m2) for rill (= interrill) erosion is reported to be the sediment collection method that is best suited for this scale. It is reported that either the "total flow + sediment is collected during a limited period of time or only a fixed fraction of the flow+sediment is collected." (Stroosnijder, 2003) Various dividers exist according to the report and it is stated that rill erosion can be measured using:

(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… [END OF PREVIEW]

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