Introduction

Gullies are permanent erosional forms that develop when water concentrates in narrow runoff paths and channels and cuts into the soil to depths that cannot be smoothed over by tillage any more. They occur all over the world mostly in semi-arid and arid landscapes where high morphological activity and dynamics can be observed. Semi-arid climate conditions and precipitation regimes encourage soil erosion processes through low vegetation cover and recurrent heavy rainfall events. Torrential rains with irregular spatiotemporal distribution result in high runoff rates, as the crusted and dry soil surface inhibits infiltration. In addition, widespread land-use changes of traditional agriculture toward both more extensive use, such as abandoned agricultural fields used for sheep pasture, and less sustainable use, such as almond plantations, prepare the ground for soil crusting, reduced soil infiltration capacity, and increased runoff, which together aggravate the risk of linear erosion downslope and cause considerable offsite impairment such as reservoir siltation (Poesen et al., 2003).

In this context, gullies link hillslopes and channels, functioning as sediment sources, stores, and conveyors. From a review of gully erosion studies in semi-arid and arid regions, Poesen et al. (2002) concluded that gullies contribute an average of 50-80% of overall sediment production in dryland environments.

Gullying involves a wide range of subprocesses related to water erosion and mass movements, such as headcut retreat, piping, fluting, tension crack development, and mass wasting (Fig. 13-1), and it is the complex interaction of these subprocesses on different temporal and spatial scales that complicates reliable forecasting by gully erosion models (Poesen et al., 2003).

The evaluation of gully development rates under different climatic and land-use conditions provides important data for modelling gully erosion and predicting impacts of environmental change on a major soil erosion process. Numerous authors have investigated (non-ephemeral) gully development in different environments (e.g., Burkard and Kosta-schuk, 1997; Oostwoud Wijdenes and Bryan, 2001; Vandekerckhove et al., 2001; Archibold et al., 2003; Betts et al. 2003; Martinez-Casasnovas et al., 2004; Avni, 2005; Wu, Zheng, et al., 2008), but still both methodological problems and a lack ofcomparability across study areas exist.

Poesen et al. (2002, 2003) therefore stressed the need for more detailed and more precise monitoring and modelling of gullies. Lane et al. (1998) pointed out that the monitoring of the changes in form may provide a more successful basis for understanding landform dynamics than monitoring the process driving those dynamics, particularly when spatially distributed information on process rates can be acquired. In this context, remote sensing is an obvious choice for monitoring gully erosion, as it allows the rapid and spatially

FIGURE 13-1 Gully erosion in southeastern Spain. (A) Active headcut of small gully ~2.5 m wide and <1 m deep. (B) Large gully with remains of piping hole and fluted walls typical for higher dispersible sub-horizons. (C) Large gully filled with debris of collapsed sidewall. Ground photos taken by IM in 2006.

FIGURE 13-1 Gully erosion in southeastern Spain. (A) Active headcut of small gully ~2.5 m wide and <1 m deep. (B) Large gully with remains of piping hole and fluted walls typical for higher dispersible sub-horizons. (C) Large gully filled with debris of collapsed sidewall. Ground photos taken by IM in 2006.

continuous coverage of a site. However, the measurement precision and repeat rate attainable with conventional remotely sensed images are not able to correspond with the process magnitudes and dynamics that are required for recording and investigating the short-term spatial and temporal variability of gully retreat (Ries and Marzolff, 2003). Therefore, most gully monitoring studies using remotely sensed imagery resort to standard large-format aerial photographs with medium to small image scales, looking at medium- to long-term intervals (e.g., Burkard and Kostaschuk, 1997; Vandaele et al., 1997; Nachtergaele and Poesen, 1999; Vandekerckhove et al., 2003).

Bridging the resolution gap between terrestrial and conventional aerial photography, small-format aerial photography (SFAP) has proven to be an excellent tool for gully erosion monitoring in several research studies conducted by the authors (Marzolff, 1999; Marzolff et al., 2003; Ries and Marzolff, 2003; Marzolff and Ries, 2007; Marzolff and Poesen, 2009). The following sections summarize some of the work done with hot-air blimps and kites across semiarid landscapes in southern Europe and West Africa, illustrating the benefits of SFAP not only for quantifying but also for understanding gully erosion processes by selected examples.

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