CHARACTERISTICS AND IMPORTANCE OF RILL AND GULLY EROSION: A CASE STUDY IN A SMALL CATCHMENT OF A MARGINAL OLIVE GROVE

Measurements of gullies and rills were carried out in an olive orchard microcatchment of 6.1 ha over a 4-year period (2010-2013). No tillage management allowing the development of a spontaneous grass cover was implemented in the study period. Rainfall, runoff and sediment load were measured at the catchment outlet. The objectives of this study were: 1) to quantify erosion by concentrated flow in the catchment by analysis of the geometric and geomorphologic changes of the gullies and rills between July 2010 and July 2013; 2) to evaluate the relative percentage of erosion derived from concentrated runoff to total sediment yield; 3) to explain the dynamics of gully and rill formation based on the hydrological patterns observed during the study period; and 4) to improve the management strategies in the olive grove. Control sections in gullies were established in order to get periodic measurements of width, depth and shape in each campaign. This allowed volume changes in the concentrated flow network to be evaluated over 3 periods (period 1 = 2010-2011; period 2 = 2011-2012; and period 3 = 2012-2013). The cumulative precipitation values were 610 mm, 219 mm and 406 mm for period 1, 2 and 3, respectively, whereas the sediment ratios of rill erosion to total sediment yield were 44% and 118%, respectively, with a negative value (as a result of the volume reduction of rills and gullies) in period 3 when the total load was equal to 0.4 t ha(-1). The increased development of the gully network was associated to intense events with a return period of the maximum intensity in 30 minutes greater than 2 years. A high dependency on the sequence of events according to their intensity values could explain the sedimentary budget in the catchment. Most of the events generating runoff in the catchment could contribute to interrill and rill erosion, but only very intense events would allow the runoff to transport sediment to the outlet. In addition to the grass cover, control measures in the gullies would substantially improve the sustainability of the farm. These gullies are important sediment sources, particularly in autumn when there is no vegetation cover and intense rainfall events occur.