Spatial and temporal variability of spontaneous grass cover and its influence on sediment losses in an extensive olive orchard catchment

The average annual percentage of cover ranged from 23% to 36%, with a coefficient of variation of 57% and 6%, respectively. On the seasonal scale, the cover varied between 0.2% and 50%. Accumulated precipitation during the precedent 15 days, the average of minimum temperature of the previous 60 days and the cumulated potential evapotranspiration of the same period showed a significant correlation with the mean cover percentage in the catchment. A simple multiple linear regression adjustment between the cumulated precipitation for the 15 previous days and the number of months since the vegetation was removed using herbicide or by summer conditions gave a determination coefficient of 77%. Moreover, a permanent spatial pattern was observed for periods characterized by abundant preceding rainfall. Only the apparent electrical conductivity of the topsoil (50 cm in depth) showed significant correlations with the spatial patterns of spontaneous grass. Finally, similar rainfall events taking place with different degrees of soil cover presented very different hydrological responses, which allowed us to quantify the contribution of spontaneous vegetation to sediment dynamics during humid periods.