Erosion is generally described in terms of average mass of soil lost per unit area from a plot, hillslope, or field. A simple direct means of measurement is to measure voided area in rills in a representative transect across the slope, collect bulk density samples from adjacent areas to determine mass per unit volume, multiply by the voided area, and divide by the total area the transect represents. Another technique is to drive stakes in a grid on the study area, mark ground line, and measure the distance the soil has eroded on each stake. Rods are sometimes driven in the ground and washers slipped over the rod. After rain events where soil particles are moved by raindrop splash, the washer will be left on a pedestal above the surrounding soil. Measurements of Cs137 and Pb210 concentrations in the soil surface have been used to estimate losses of surface soil since the atmospheric testing of atomic weapons ended in the early 1960s. Bordered plots can be installed and runoff and sediment can be collected in tanks, volume of the material measured, sediment concentration determined, and mass of soil lost calculated. Flumes can be used to determine discharge and sediment samples collected with an automated sampling device. Losses from watersheds can be collected in ponds or reservoirs and sediment deposits measured; this methodology is well adapted for long-term studies. Control sections where a structure or natural feature causes a condition of a direct relationship between water depth and discharge can be used, after calibration, with a water stage recorder to determine flow depth and a sediment sampler to determine sediment concentration in the flow. Soil loss from the upstream area can then be calculated. A number of handbooks are available for designing a measurement scheme for various conditions that may be encountered, whether low-cost systems for use in a developing nation with limited resources or high-technology systems for validating process-based models.
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