Effectiveness of the GAEC standard of cross compliance retain terraces on soil erosion control
AbstractThe GAEC standard retain terraces of cross compliance prohibits farmers the elimination of existing terraces, with the aim to ensure the protection of soil from erosion. In the Italian literature there are not field studies to quantify the effects of the elimination or degradation of terraces on soil erosion. Therefore, the modeling approach was chosen and applied in a scenario analysis to evaluate increasing levels of degradation of stone wall terraces. The study was conducted on two sample areas: Lamole (700.8 ha, Tuscany) and Costaviola (764.73 ha, Calabria) with contrasting landscapes. The Universal Soil Loss Equation model (USLE) was applied in the comparative assessment of the soil erosion risk (Mg . ha-1 . yr-1), by simulating five increasing intensity of terrace degradation, respectively: conserved partially damaged, very damaged, partially removed, removed, each of which corresponding to different values of the indexes of verification in case of infringement to GAEC standard provided for by the AGEA rules which have come into force since December 2009 (Agency for Agricultural Payments). To growing intensity of degradation, a progressive loss of efficacy of terraces was attributed by increasing the values of the LS factor (length and slope) of USLE in relation with the local modification of the length and steepness of the slope between adjacent terraces. Basically, it was simulated the gradual return to the natural morphology of the slope. The results of the analysis showed a significant increase in erosion in relationship with increasing degradation of terraces. Furthermore, it is possible to conclude that the GAEC standard retain terraces is very effective with regard to the primary objective of reducing erosion. A further statistical analysis was performed to test the protective value of terraces against soil erosion in areas where agriculture was abandoned. The analysis was carried out by comparing the specific risk of erosion (Mg . ha-1 . yr-1) of polygons with land uses: forest and abandoned, with natural vegetation in evolution. In both areas, forest on totally degraded terraces is able to decrease erosion well below the tolerance threshold of 11.2 Mg . ha-1 . yr-1, in the same manner as conserved terraces do for other soil uses. At Lamole, the natural vegetation in evolution on completely degraded terraces is able to decrease erosion below the tolerance threshold. On the contrary, at Costaviola on the same soil use and level of terrace degradation, soil erosion remained above the tolerance threshold. This difference can be explained by considering that the average gradient of hillslopes (considered without terraces) is 65.4 % for Costaviola and 35.0 % for Lamole. From these findings it is possible to argue that terraces, although degraded, continue to play a role in the protection of soil against erosion in abandoned areas. Thus, they continue to exert a valuable environmental function in terms of production of public goods and services; in particular, in the decrease of hydrogeological risk.
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