Runoff and Erosion Risks Indicators on the Main Soils of the Mediterranean Mountains of Occidental Rif Area (Morocco)

摘要

In Mediterranean mountains of Morocco, erosion problems are varied and dangerous: sheet erosion (1 to 5 t/(ha · year), rill erosion (10 to 100 t/(ha · year), gully erosion (100 to 300 t/(ha · year), mass movements (up to 10000 t/(ha • year), and tillage erosion (1 to 60 t/(ha · year). Linear erosion being more important than sheet erosion on steep slopes, runoff energy is more important on the mountains than rainfall energy. Therefore, a simple irrigator was used to simulate 50-mm rainfalls at 75-90 mm/h on 1-m~2 microplots, in order to measure propounding rainfall (Pi) and stable infiltration rate (If) on 64 plots representative of various soils and land uses, on 15 to 40% hillslopes. Significant correlations (R = 0.70 to 0.95) were observed (ⅰ) between stable infiltration (If) and soil properties (topsoil organic matter and stable macro-aggregate contents, cohesion, bulk density), and chiefly (ⅱ) between stable infiltration and soil surface status (% of surface covered by stones, litter and crusts). In contrast, there was no significant relation between infiltration rate, texture (topsoil sand, clay and clay + loam contents), initial soil moisture or resistance to shearing stress (scissometer), probably because the range of variations was too limited. These data emphasise the possibility to evaluate runoff and erosion risk in mountains areas taking into account land use (forest and Pinus plantations, overgrazed bush land, i.e. matorral, cereals and cannabis), soils (If on sandy clay fersiallitic soils > If on loamy clay brown vertisols > If on sandy lithosols), slopes and indicators like prepounding rainfall, sealing crust, topsoil water-stable macro-aggregate content and compaction.