Remote sensing vs. field-based monitoring of agricultural terrace degradation

Abstract

The degradation of agricultural terraces is considered a major challenge to soil and water conservation in steep-slope viticulture. Although terracing is a widespread conservation practice, its sustainability is threatened by adverse climatic and man-made conditions. Previous studies have shown the impact of terrace designs on the formation of runoff pathways, causing degradation processes on terrace platforms (e.g. sheet erosion) and walls (e.g. piping, landslides, collapse). This study evaluates a remote sensing versus a field-based approach to monitor hydrological processes responsible for terrace degradation, as tested in a north-Italian vineyard. The field-based approach was based on spatially measured Soil Moisture Content (SMC) using a Time Domain Reflectometry (TDR) instrument, which clearly revealed saturation hotspots around two damaged terraces in the study area. Moreover, these zones showed a particular cross-sectional SMC profile, with the highest saturation close to the terrace platform edges. The remote sensing approach was based on aerial imagery acquired by an Unmanned Aerial Vehicle (UAV) and photogrammetric reconstruction of the vineyard geomorphology, allowing terrain-based analysis and physical erosion modelling. In this approach, simulations indicated that terrace damages could be partly explained by the formation of preferential runoff pathways caused by the terrace design. This parallel methodology allowed a comparison of the merits and limitations of either approach, as done in light of published work. The occurrence of two SMC hotspots at terrace edges (and their non-typical cross-sectional profiles) could be better understood from simulated surface flow paths. While the causal relationship between heterogeneous soil saturation and terrace instability has been previously reported in literature, the novelty of the presented study is the use of topsoil SMC as an indicator of potential damages, favouring the scalability compared to fixed, local and often intrusive terrace sub-surface experiments. Remote sensing based approaches, however, tend to offer the most time-efficient solution on larger scales, and aerial acquisition of SMC distribution could thus potentially offer a powerful integrated methodology.