Iovino F, Callegari G, Veltri A
Abstract: The processes through which forest systems influence the formation of runoffs can be traced to interception, evapotranspiration and infiltration which are the significant components of hydrological balance in woodland (Susmel,1967; Hibbert,1967; Blackie et al.,1980; Fattorelli,1987; Swank and Crossley Jr.,1987; Meunier et Mathys,1993). This work presents the results of a study on the effects of forest cover and, in particular, on the modifications brought about by thinning on the cycle of water and erosion. The research has been going on in an experimental basin of 139 hectares and extends from altitude of 975 m to 1300 m a.s.l. in the Sila Greca (Cosenza) since 1986 (39°28’N, 16°30’E). 83% of the basin is covered by plantations and natural populations of Calabrian Pine (Pinus nigra Arn. var. calabrica). Plantations, carried out on man made terraces, cover 60% of the whole watershed surface. Among these, 43% has stocking from 1,250 to 2,200 p/ha, with a cover of 56% and 79% respectiveley;17.5% is low density (an average of 600 p/ha), where Calabrian Pine mixed with Oak failed for pedological limitation. Natural stands cover 18% of the watershed, with a stocking ranging between 2,700 and 4,000 p/ha and a cover of 72% and 56%, respectively. The remaining 4% of the surface is covered by natural and artificial stands, of about 800 p/ha, as a result of fire damage. The basin belongs to the second group of the classification drawn up by McCulloch and Robinson (1993), according to which the hydrological behaviour of each individual basin is to be evaluated before and after intervention on the vegetation and any consequent deviations of hydrological parameters are attributed to changes in forest cover. In line with this methodology hydrological measurements were taken on the conditions of density and cover described above from 1986 to 1993. In 1994 these conditions were modified with selective thinning which involved the whole area of the woodland with the elimination of, on average, 50% of the number of plants and 30% of the basal area. The results obtained show how the conditions of forest cover before the intervention significantly affected the hydrological response of the basin. For example, whereas the most notable runoffs occur between January and March, the most consistent rainfall is recorded from October to December. This delay can be attributed to the saturation of the soil which takes place between August and January, a period in which there is little surface runoff. The results obtained allow us to detect the considerable laminating influence of the woodland through the soil’s absorption capacity. The basin behaves in such a way as to spread out in time its response to heavy rainfall and extend the time needed for the flood events to flow through the basin. Finally, the efficient working of the sytem is also shown by the average annual values of the runoff coefficient (0.17), which demonstrates how the system plays a crucial role in the hydrological cycle, provided it is in a good condition. Further research will compare the hydrological behaviour of the basin now with that prior to thinning through the use of a model which will evaluate the variations of the components and parameters of the peak flow diagrams in function of the changes in cover. The effects of cutting on the components of the hydrological balance, however, have already been shown from an analysis of the data acquired since 1994 in two sites located within the basin. The hydrological data concerns the collection and measurement of gross rainfall, throughfall, stemflow, runoff and erosion. The first experimental plot covers an area of 150 m2, exposed to the east, with a 30% gradient at 1,090 m a.s.l.. The plot has a density of 1,500 piante/ha.. The second experimental plot, following selective thinning from below, in which 50% of the plants, equivalent to 30% of basal area, was cleared, has a density of 867 plants/ha.. The test plot has 94% forest cover, while the second plot has 84%. The results enable us to show that average interception in the cleared plot is 47% of total rainfall, while in the plot where there was no intervention it was 58%. The stemflow value is 0.67% of total rainfall in the cleared area and 0.66% in the other. In both plots an increase in interception up to a threshold of 40mm has been observed. Surface runoff has not been measured in either plot.
Citazione: Iovino F, Callegari G, Veltri A (1999). Hydrological balance in reafforestations of pine . 2° Congresso Nazionale SISEF, Bologna, 20 – 22 Ott 1999, Contributo no. #c2.6.44