After reading this article you will learn about the measurement of soil in the runoff with the help of small plots and watersheds.
Runoff and soil loss from small rectangular plots of one soil type and one slope gradient can be collected in a tank placed at the bottom of the plot. Sometimes dividers are used to sample only a fraction of the runoff to measure the soil loss.
The disadvantage of the plot method is that natural overland flow from the neighbouring land is eliminated and the removal of soil after each rainfall changes soil characteristics affecting runoff.
A watershed is the natural unit of runoff, and is defined as a drainage area whose runoff flows past one point. There is no size limit to a watershed as such but for use in erosion studies the area has to be small enough to carry most of the soil that moves downhill past the outlet point of the watershed with the water during a rain.
The maximum size depends on the topography. The steeper the land, the larger can be the watershed. In flatter areas, especially where the slope gradient decreases downstream to form alluvium, the watershed should be so small that the outlet is above the alluvium; otherwise, too big a part of the eroded soil would be deposited before it reaches this point.
Where the effects of different types of land use or treatment are to be investigated, watersheds should be less than 20 acres in size. In these small areas, most of the soil that erodes during a heavy storm reaches the outlet. Most single land use watersheds studied are actually less than 10 acres in size.
The advantage of using a watershed is that both the cultural practices and the hydrologic behaviour are identical to those existing on large areas of land. On the other hand, most watersheds contain several soil types and erosion classes and a variety of slope gradients, lengths, and aspects. Consequently, the results cannot as readily be referred to specific soil and slope conditions as those from plots.
Since the total amount of runoff water form a natural watershed, even though the area is only an acre in size, is too great to place in a tank, a runoff measuring device and runoff sampler are used. The runoff is normally measured in a flume equipped with an automatic water stage recorder. The flume is calibrated, so that the rate of runoff for every height of water passing through it is known.
Various devices available to obtain aliquot sample of the runoff passing through the flume include. Indiana aliquot runoff sampler, Coshocton wheel, sludge box for the deposition of coarse fraction of the eroded material, and the silt sampler for the collection of a portion of the runoff water containing soil in suspension.
This suspended and dissolved material in the runoff water can be determined by multiplying the amount of runoff with concentration of the components expressed as:
kg of soil material/ha = kg of soil material/ ha-cm of runoff x cm of runoff.
The amount of soil in the runoff is determined by evaporating the aliquot to dryness. The resulting solid residue, including dissolved material, of the runoff is called total solids which is practically identical with soil. When erosion from a fertile soil is measured, the tonnes of total solids lost per acre do not give a picture of the seriousness of the loss.
In such a case, the concentration of plant nutrients in the runoff is determined. Since this concentration changes continuously and has no definite relationship to the rate of runoff, this determination is made either on an aliquot of the entire storm runoff, or on many individual samples taken throughout the storm.