Two causes for runoff initiation on microbiotic crusts: Hydrophobicity andpore clogging

Hydrophobicity and pore clogging are suggested as two mechanisms responsibl e for generating runoff over microbiotic crusts overlying dune sand, Althou gh natural microbiotic crusts in the Hallamish dune field (Negev Desert, Is rael) did not show any hydrophobicity, that was not the case with natural c rusts subjected to long periods of continuous wetness in the lab. Monoalgal crusts, grown in the lab, also showed high hydrophobicity when dry. The hy drophobicity vanished, however, once the surface was wetted. Runoff on mono algal lab-grown crusts was obtained when (i) the dry crust exhibited hydrop hobic properties and (ii) the wetted crust no longer showed water repellenc e. Although runoff generation caused by hydrophobicity is expected, it is s uggested that runoff initiation when the crusts do not exhibit any hydropho bicity stems from the high water absorption and swelling of the exopolysacc haride cyanobacterial sheaths causing pore clogging. The multi-layered stru cture of the crust and filament migration to the surface may enhance pore c logging. The experiments and their interpretation are supported by reinterp retation of published data.