Digital photogrammetric analysis for the assessment of erosion rates in a sample catchment of central Italy

The digital photogrammetric analysis represents a novel and powerful tool for the evaluation of spatial and temporal distribution of denudation processes and their rates. This method was applied in a selected test watershed of central Italy, the Upper Orcia River Valley (Tuscany), which is highly representative of more widespread areas that are highlysusceptible to erosion processes in the Mediterranean environment. It consists of a small catchment extending about 4.4 km2, where accelerated slope dynamics related to concentrated surface runoff affect the dominant clayey substratum. As a result, typical badland landforms well-known as “calanchi” and “biancane” occur, often coupled with and mass movements consisting of soil creep, solifluction and various types of landslides. All these processes are enhanced by the seasonally-concentrated rains and repeated land use changes. A digital photogrammetric analysis of landforms was performed on three series of aerial photos which refer to the period comprised between 1976 and 2003, by using a Z-Map digital photogrammetric workstation. In order to obtain the best image orientation (both in terms of inner and outer orientation) a differential GPS survey was carried out and about 70 ground control points were located. Three high resolution Digital Elevation Models (DEMs with 2x2m pixels) of the test area were extracted from the aerial photographs dating to 1976, 1994 and 2003, respectively. The quantitative evaluation of the sediment loss rate was computed by overlaying these DEMs. In addition, interpretation of air photos allowed to map the main changes in denudation processes. On the basis of a comparison of these maps, important morphological changes occurred during the whole period (1976–2003) were mapped. The areas affected by “calanchi”, as expected, highlighted the most severe landform modifications. In particular, a maximum headwater retreat reaching up to 10-15 cm yr-1 was recorded. An average erosion rate of 1.2 cm yr-1, was calculated for the period 1976-2003 in the whole test area, with a sediment loss of about 230 t ha-1 yr-1. The proposed method can be considered very useful to better locate denudation landforms and associated processes and to assess their rates. Moreover, it can be used for sustainable land management, planning of erosion control measures and calibration of regional prediction models in Italy, as well as in other Mediterranean environments.