Short-term versus medium-term monitoring for detecting gully-erosion variability in a Mediterranean environment

This study investigates how medium-term gully-development data differ from short-term data, and which factors influence their spatial and temporal variability at nine selected actively retreating bank gullies situated in four Spanish basin landscapes. Small-format aerial photographs using unmanned, remote-controlled platforms were taken at the gully sites in short-term intervals of one to two years over medium-term periods of seven to 13 years and gully change during each period was determined using stereophotogrammetry and a geographic information system. Results show a high variability of annual gully retreat rates both between gullies and between observation periods. The mean linear headcut retreat rates range between 0.02 and 0.26 m a(-1). Gully area loss was between 0.8 and 22 m(2) a(-1) and gully volume loss between 0.5 to 100 m(3) a(-1), of which sidewall erosion may play a considerable part. A non-linear relationship between catchment area and medium-term gully headcut volume change was found for these gullies. The short-term changes observed at the individual gullies show very high variability: on average, the maximum headcut volume change observed in 7-13 years was 14.3 times larger than the minimum change. Dependency on precipitation varies but is clearly higher for headcuts than sidewalls, especially in smaller and less disturbed catchments. The varying influences of land use and human activities with their positive or negative effects on runoff production and connectivity play a dominant role in these study areas, both for short-term variability and medium-term difference in gully development. The study proves the value of capturing spatially continuous, high-resolution three-dimensional data using small-format aerial photography for detailed gully monitoring. Results confirm that short-term data are not representative of longer-term gully development and demonstrate the necessity for medium-to long-term monitoring. However, short-term data are still required to understand the processes - particularly human activity at varying time scales - causing fluctuations in gully erosion rates. Copyright (C) 2011 John Wiley & Sons, Ltd.