Cross-scale interactions between surface components and rainfall properties. Non-linearities in the hydrological and erosive behavior of semiarid catchments

- Semiarid catchments with BSCs showed a sharp threshold in runoff generation.
- The threshold in runoff generation is controlled by total rainfall amount.
- In low magnitude rains, runoff generated in BSCs areas is reinfiltrated in vegetation.
- In high magnitude events, vegetation is unable of infiltrate all runoff from BSCs areas.
- In these events, runoff connectivity is increased, but no extra sediments were exported.

SummaryNon-vegetated areas in arid and semiarid ecosystems are usually covered by biological soil crusts (BSCs), which modify many surface properties and reduce soil erosion at local spatial scales. Nevertheless, runoff generated in these areas could increase downstream erosion, or on the contrary, supply water to downslope plants, which act as surface obstructions slowing down runoff, trapping sediments and thus reducing water erosion. However, their buffer capacity might be overwhelmed during some torrential rains, increasing catchment connectivity and modifying the hydrological behavior and subsequent water erosion measured at the catchment outlet. To progress in the knowledge of the effect of the interactions between BSCs and vegetation on runoff generation and water erosion at catchment scale, the long term hydrological response of semiarid badlands with an important cover of BSCs has been studied. This effect has been analyzed using piecewise regression analysis to find any possible break point in the runoff response of the catchment to rainfall, attributable to variations in the hydrological connectivity of the catchment. Once we found this break point, we used it to divide the database in two sub-databases, and we built two structural equation models (SEM) to identify the main hydrological drivers controlling runoff generation and water erosion at sub-datasets, corresponding with different stages of catchment connection. Our results show that during low magnitude rainfall events, runoff and erosion measured at the catchment outlet come essentially from physically crusted areas directly connected to the channel network and are controlled by rainfall intensity, whereas runoff generated in areas with BSCs is reinfiltrated downstream into local vegetation patches. However, during high-magnitude events, vegetated buffers are saturated, connecting areas covered by BSCs with the channel network and increasing the catchment efficiency in exporting water out of the system. Nevertheless, water and sediments connectivity is not always linked, and most sediments measured during both low and high-magnitude events are almost exclusively supplied by physically crusted areas, whereas, BSCs areas rarely yield sediments. The differences in the hydrological behavior of the catchment under low and high-magnitude rainfall events were found to be controlled by the interactions between source and vegetated areas, which in last turn control the hydrological connectivity at catchment scale.