Tropeano-1984-Earth_Surface_Processes_and_Landforms

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SOIL EROSION PROCESSES 263 transported. In the course of heavy rainfall (which often occurred in the area under control see Table 111) quick deepening or widening of rills, piping phenomena, etc. may produce an excess of erosion or mass movement processes on a small scale. It is evident, in this case, that the correlation between soil loss and rainfall characteristics is not immediate. Abnormal erosion rates in Plots A 1-2-3 and C 3 during 1981 can be explained only by taking into account the occurrence of erosional processes like those detailed above. In the period from May to December 1982, a second plot was studied near and parallel to the first one at Mongardino, on a slope apparently in the same physical conditions. Nevertheless, the total amount of sediments collected exceeded by 21.7 per cent the quantity trapped in the same period at the outlet of the previous plot. Also, in this case the incidence of fortuitous variables had been strong. At Albugnano, since the beginning, soil losses and runoff had been measured separately for the three physically homogeneous plots, lying on the same stretch of slope. The total difference between the maximum and minimum value, over the entire survey period, was 32.7 per cent of the maximum value. Such difference may be understood if we consider that vegetative cover (grass and weeds) in the second year developed over the whole surface, though somehow more abundantly on the less eroded plot. In addition, there is no doubt that the propensity of a soil to be eroded is variously affected by farming practices-such as ploughing, tillage, and the use of plastic film mulches-and may vary greatly, not only from one place to another, but also within the same plot. The results of analytical comparisons suggest that the best forecast of soil loss can be obtained by direct measurements of runoff. The expected amount of sediments, in each individual event, may be computed by making use of the regression equations reported in Table VI.3. Based on the data collected in the survey period, we observed that almost all erosive activity in the study area was concentrated in the warm months, being strongly affected by showers (Table IV). Although the total depth of such rains is slight if compared to the total annual rainfall depth, parameters such as ‘intense rainfalls’ and their mean intensity prove to be highly significant. In fact, Figure 4 shows that the intense rainfall-soil loss relationship remained quite uniform over the two years in the Mongardino area-the most important in this context. The Albugnano area is an exception inasmuch as basic changes occurred in land treatment and vegetative cover from 1981 to 1982. Soil humidity before a storm is an important parameter controlling the start of overland flow. No measurements of this variable, neither on the infiltration into the slopes, were made; however, in the intervals between rainy periods in the warm season it was always observed that the soil surface quickly dried in all three areas, subjected to strong insolation and high temperatures. In this respect, the data recorded by the C.N.R. Istituto per la Meccanivazione Agricola at Albugnano are significant. During 1981, in the warmest season (from 1 June to 31 August) the mean value of the maximum daily temperature was of about 26°C with a minimum average air moisture of 59 per cent. In the same period, in 1982, the same daily values were 27°C and 58 per cent respectively. Under such conditions it can easily be seen that the soil surface may be considered in an almost steady state of dryness from one storm to another, in the very period when erosion is most important. CONCLUSIONS The rainfall depths for each month of the two-year survey (as compared with the mean monthly values supplied by a long series of pluviometric surveys at stations very close to the sites under study), and the runoff and soil loss data (Figure 5) show that the sites differ markedly from one another in both runoff and soil loss values. Such differences can only be explained by taking into account the different cultivation techniques in the three areas. At Albugnano, in the first year a sequence of fully-mechanized operations led the soil to an excessive weakness and to an increased seepage capacity, while the surface was kept bare of vegetation. This resulted in abnormal erosion values and relatively low runoff. In the second year, no tillage or herbicide treatment was made and the vegetative cover established itself widely. Keeping in mind the slight difference in total rainfall depths between 1981 and 1982, we observed (Table V) that after only one year soil losses were reduced to less than 1/7 of their former values. At Mongardino, where no tillage was carried out but full herbicide treatment was made in the course of the
Page 1 and 2: EARTH SURFACE PROCESSES AND LANDFOR
Page 3 and 4: SOIL EROSION PROCESSES 255 EXPERIME
Page 5 and 6: SOIL EROSION PROCESSES 257 Figure 2
Page 7 and 8: Table 111. Storm events and related
Page 9: SOIL EROSION PROCESSES 261 Table V.
Page 13 and 14: SOIL EROSION PROCESSES 265 Table VI

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