Modeling - Field Sampling: Principles and Practices in Environmental Analysis - page 185

different places, can be used in a series of experiments determining different
characteristics. When all the data are collected and all the analysis completed a relatively
simple multiple regression analysis can be carried out using common statistical computer
programs. (See Chapter 6.) The results of such an analysis will relate the various
variables and show which are important in predicting a characteristic of a contaminant in
soil.
The sorption of chromium (CrIII) was investigated by Stewart et al. [7] in such a
manner. The analysis showed that sorption of chromium was related to a soil's pH,
inorganic carbon, cation exchange capacity (CEC), and clay content. They were also able
to determine that the bioaccessability of CrIII was related to a soil's clay and its inorganic
carbon content. Both of these were represented by relatively simple equations called
models of CrIII sorption and bioaccessability in soil.
7.4. LANDSCAPE MODELS
One of the most used environmental soils models is the universal soil loss equation
(USLE) or the updated revised universal soil loss equation (RUSLE). This model has
been developed for the United States; however, it and similar equations are used in other
countries. The use of this equation involves two components. The first part is an equation
that allows the calculation of the estimated amount of soil lost (eroded) from a field in a
year under a set of described conditions. The second part is a t-value for the soil or soils
in the field or fields. The T-value is the acceptable loss of soil in tons per year.
The USLE takes the form of
A=R×K×LS×C×P
A is the amount of soil lost in tons per year. This figure is compared to the acceptable
loss of tons per acre of soil from the soil types present in the field. The acceptable level is
the amount that can be lost without adverse affects on the soil or the surroundings. It is
important to know that even under the best vegetative cover soil is lost by erosion every
year. This is called geologic erosion. Accelerated erosion is above this natural level and is
caused by man's activities. The T-values to which A is usually compared range between
1 and 3 tons per ha per year.
The R factor is a storm erosivity factor. It takes into account the intensity of rainfall,
which is a measure of the energy in the raindrops and the rate of rainfall. The energy in
rainfall depends on the size and speed of the raindrop when it hits the soil surface. The
size of the raindrop is the main factor because the speed is determined in large measure
by gravity. Basically this means that rains that come in short, heavy downpours, with
large drops are more erosive than long, gentle rains. The rate of rainfall is the amount of
rain per unit of time. Typically this figure is in cm/min (in the United States in./hr). The
higher the rate of rainfall, the more erosive a rainfall event will be.
R factors are measured or interpolated. A common way to represent R factors is as
lines of isoerosivity on a map of the area being studied. In the United States such maps
are available for selected areas, and the whole country. The lines are separated by 50 R