The Effect Of Plot Length On Measurement Of Soil Erodibility

When using the empirical model, Universal Soil Loss Equation (USLE), to predict soil loss, Soil erobility (K) is an indispensable factor, which is defined in USLE as rate of soil loss per rainfall erosion index (R) as measured on a unit plot. Soil erodibility factors are best obtained from direct measurements on natural runoff plots of a long observing period. Time and economic factors have limited the establishment of long-term runoff plots and therefore the development of plot research with simulated rainfall was promoted. However, the size of unit plot (22.1 m in length and 5 m in width) is too large for simulating rainfall experiment; therefore, microplots of shorter length are employed. Unfortunately, erosion processes on microplots are different from those on unit plots because of the difference in slope lengths; consequently, the K values obtained on microplots are not necessarily representative. The purpose of this research is to examine the relationship of soil loss on different slope lengths and set up a slope length standard, below which the microplots are suitable for determine K values.For the above purpose, microplots of 7 slope lengths, 1m, 2.5 m, 4 m, 5.5 m 7 m, 8.5 m, and 10 m, were selected. Simulated rainfalll of Multi-intensity were employed to imitate a representative storm. A single rainfall, consisting of 12 periods, lasted 2 hours, with a total rainfall amount of 121 mm. Experiments for each slope length consisted of dry run and wet run, which were conducted 24 hours after dry run. During the rainfall, runoff samples, which were used to calculate the totle runoff and soil loss, were taken every 5 min. The flow velocity of runoff in both rill and interrill area were measured with dyeing method during the same period in every rainfall.By analyzing the data, the following conclusions were drawn:(1) On plots longer than 4m, sediment concentration and soil loss modulus per unit time pulse during the process of rainfall. This pulse is probably attributed to the formation and development of rill.(2) During the last 30-40 min of rainfall, the difference in soil loss modulus per unit time between plots of different slope length decreases. This decreasing is due to the crusting on surface, which prevents the soil from being detached by raindrop, and the decreasing in rainfall intensity, which is unable to detach and carry soil particles.(3) On the basis of single rainfall event, sediment concentration and soil loss modulus don't show a significantly increase from plots of 1 m to 2.5 m, while increase considerably with slope length above 4m. Moreover, the soil loss moduluses for wet run are small than dry run, and this can also be attributed to crusting.(4) From slope 0.5 m down slope from upper end of plot to 1.5 m, flow velocities on interrill area increase and level beyond 1.5 m, while flow velocities in rill become approximately stable beyond 5.5 m.(5) Within 10 m, 0.55405 is recommended for the exponent m for slope length factor, L, as defined in USLE, and this value can be used to calculate soil erodibilty with microplot experiment.There exist some drawbacks of this study and suggestions about future experiment. Firstly, experimenting slope lengths less than 4 m are not enough, so the effects of slope lengths on soil loss are not revealed sufficiently. Secondly, the longest testing slope length, 10 m, is still short compared to unit plot, and longer plots should be involved to establish the relationship between plots shorter than 10 m and units. Thirdly, the replications the treatment are few; more replications are needed to get a stable data. Finally, the results of this study are not compared with natural rainfall data, that restricts the extrapolation of this study to actual conditions.