Bilancio idrologico in rimboschimenti di pino laricio

Hydrological balance in Calabrian pine artificiol stands. The processes through which forest systems influencethe formation of runoffs can be traced to interception. evapotranspiration and infiltration which are the significantcomponents of hydrological balance in woodland. This work presents the results of a study on the effecis of forest cover and.in particular. on the modifications brought about by thinning on the cycle of water and erosion. The research has been goingon 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). Some 83% ofthe basin is covered by plantations and natural populations of Calabrian Pine(Pinus nigra, Am. 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% respeetiveley; 17.5% is low density(an average of 600 p/ha), where Calabrian Pine mixed wilh 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. Theremaining 4% of the surface is covered by natural and artificial stands, of about 800 p/ha, as a result of fire damage. Thebasin belongs to the second group of the classification drawn up by McCulloch and Robinson (1993), according lo which thehydrological behaviour of each individual basin is to be evaluated before and after intervention on the vegetation and anyconsequent deviations of hydrological parameters are attributed to changes in forest cover. In line with this methodologyhydrological measurements were taken on the conditions of density and cover described above from 1986 to 1993. In 1994the e conditions were modified with selective thinning which involved the whole area of the woodland with the eliminationof, on avcragc, 50% of the number of plants and 30% of thc basaI area. The results obtained show how the conditions offorest cover before the intervention significantly affected the hydrological response of the basino For example, whereas themost notable runoffs occur between January and March. the most consistent rainfall is recorded from October lO December.This delay can be attribuited to the saturation of the soil which takes place between August and January, a period in whichthere is little surface runoff. Thc results obtained allow us to detect the considerable laminating influence of the woodlandthrough the soil's absorption capacity. The basin behaves in such a way as to spread out in time its response to heavy rainfalland extend the time needed for the flood events to flow through the basin. Finally, the efficient working of the sytem is alsoshown by the average annual value of the runoff coefficient (0.18), prior to thinning. After sylvicultural treatments, in theperiod between 1994 and 1998. the mean annual value of runoff coeffieient is 0.42. Differences between the values of runoffcoefficients are lower during the wet season than during the dry one. Further research will compare the hydrologicalbehaviour of the basin now with that prior to thinning through the use of a model which will evaluate the variations of theeomponents and parameters of the peak now diagrarns in funetion of the changes in cover. The effects of culling on thecomponents of the hydrological balance, however, have already been shown from an analysis of the data acquired since 1994in 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 ml , exposed to the cast. with a30% gradient at 1.090 m a.s.l.. The plot has a density of 1.500 lree/ha.. The second experimental plot, following selectivethinning from below, in which 50% of the plants, equivalent to 30% of basal area, was cleared, has a density of 867plants/ha.. The tesi plot has 94% forest cover, while the second plot has 84%. The results enable us to show that averageinterception in the cleared plot is 47% of total rainfall, while in the plot where there was no intervention it was 55%. Thedifference between both thesis is lower than that recorded in a previous work.The stemflow value is 0.52% of total rainfall inthe cleared area and 0.59% in the other. Surface runoff has not been measured in either plot.