Comparing different approaches to measure erosion by concentrated flow in a rainfed agroecosystem in SE Spain: Field inventory vs RPAS

Concentrated-flow erosion and sheet erosion are main drivers of soil loss in Mediterranean agroecosystems with soft lithologies. Morphologies of concentrated-flow erosion were assessed in a terraced rainfed almond crop (Prunus dulcis Mill.) located on Tertiary marls in SE Spain, using two methodologies: i) field inventory and ii) photogrammetry through images taken with RPAS (Remotely Piloted Aircraft System). Both methods were applied to compare their results within one year after three major rainfall events. For the application of the field inventory, each morphology was divided longitudinally, measuring their dimensions (width, length and depth) to estimate the volume of mobilized material following the shape of each division. RPAS images were processed to obtain digital elevation models (DEMs) and orthophotos, which were subsequently used to identify erosional morphologies and calculate the volume of mobilized material through variation in height and resolution of the DEMs. Sampling for soil’s bulk density was carried out to convert volume into mass. Differences between second and first inventory or flight (period 1), and between third and second ones (period 2) were calculated to estimate sediment yield rates.

Erosional morphologies, identified with both methodologies, were: i) rills, mainly along terraces; ii) gullies, mainly on the edges of terraces; and iii) mass movements, only on the edges of the terraces. Moreover, a fourth form not described in previous studies was identified, called mixed erosion: a joint action of laminar and concentrated flow under extreme rainfall conditions. This form appeared at the base of gullies and, occasionally, connecting gullies.

In period 1, gullies showed higher median sediment yield than rills, both with field inventory (1,3 ± 1,7 y 0,22 ± 0,11 t ha^-1 y^-1, respectively), and photogrammetry (1,37 ± 1,01 y 0,24 ± 0,2 t ha^-1 y^-1, respectively). While, in period 2, mixed forms and gullies were more erosive than rills and mass movements, both with field inventory (8,8 ± 0,9, 2,4 ± 1,9, 0,54 ± 0,48 and 1,1 ± 0,8 t ha^-1 y^-1, respectively), and photogrammetry (1,7 ± 1,1, 1,2 ± 0,9, 0,23 ± 0,18 and 0,15 ± 0,11 t ha^-1 y^-1, respectively).

69, 16 and 50 % of the rills, gullies and mass movements, respectively, identified through inventory method were also identified with RPAS images. In contrast, all of the mixed erosional morphologies identified in the field survey were vissible in the RPAS images. The proportion of rills and gullies identified simultaneously with both methods increased in successive samplings, likely due to the increased intensity of the successive rainfall events, resulting in more easily identifiable features. Finally, for those morphologies identified with both methods, the RPAS sediment yield estimations were 40 – 50 % higher than those made by inventory method, probably due to the difference in resolution between methods.

The mixed form of erosion and the gullies showed a high erosive potential, involving a great environmental threat on this kind of agroecosystems. At a relatively low cost, RPAS, through its greater resolution, can help to get more reliable values of concentrated erosion rates in rainfed agroecosystems than field inventories.