Mechanisms for biocrust-modulated runoff generation – A review

The complex interactions between biocrust properties and rainfall characteristics give rise to partitioning of infiltration and runoff. In this review we examine primary mechanisms for runoff generation due to water repellency (WR), pore clogging (PC), soil texture and structural features including surface roughness. The primary hypothesis is that while WR plays an important role in runoff generation in humid regions, PC is a dominant process for runoff in arid and semiarid regions even under moderate rainfall intensities. Evidence suggests that the amount and properties of biocrust extracellular polymeric substances (EPS) dictate the rheological properties and swelling characteristics of the biocrust, which, in turn, govern the degree of pore clogging (due to surface swelling) and the resulting surface saturation. Changes in species composition of the main photoautotrophic component of the biocrust control the amount and properties of the embedded EPS. Disturbance or warmer climate may affect runoff generation via changes in biocrust species composition. These changes may not be necessarily long-lived when the disturbance is ceased or climate conditions revert to ‘original’ long term state.