Effects of land cover on runoff coefficient

Land cover is considered to have significant influence on the hydrologic response of a river basin. In this study, we assessed how changes in land cover over time affected flood behaviour from 1988 to 2005, in the Upper Ping River Basin, northern Thailand. We correlated the types of land cover with rainfall-runoff behaviour for smaller and larger flood events taking place during this period. To quantify land cover, nine Landsat 5 TM images taken during the dry season (January or February) were obtained and processed to examine inter-annual land cover changes. From the networks of daily read rainfall data and stream gaugings available across the basin, 68 rainfall and 11 runoff stations were selected to evaluate peak flow rate and runoff coefficient for flood events. For individual sub-catchments, strong non-linear correlations were found between the overall runoff coefficient and peak flow rates for flood events. These runoff coefficients to peak flow relationships varied from year to year with different land cover for each sub-catchment. From these relationships within a particular sub-catchment, we determined relationships between different types of land cover and runoff coefficient for the 2, 5, 10 and 15 year Annual Recurrence Interval (ARI) peak flood events. We found that runoff coefficient increased with increasing forest proportion for these specified peak flood conditions, on nine out of eleven sub-catchments. On the other hand, the runoff coefficient associated with these peak flood events decreased as agricultural and disturbed forest areas increased. The influence of land cover on runoff coefficient was, however, found to be very different between smaller (lower than similar to 2 year ARI) and larger flood events (larger than similar to 2 year ARI). Runoff coefficient is higher for high forest cover during larger flood events; but for smaller flood events, runoff coefficient is lower when forest cover is high. This is due to the fact that for smaller flood events, rainfall loss rate for the forest area is normally higher than that of the non-forest area according to higher evapotranspiration and soil moisture capacity. Forests have proved to potentially offer flood mitigation benefits for smaller flood events. However, for larger flood events the situation of the basin can be different, especially on a basin with higher antecedent soil moisture or even under saturation stage. Antecedent soil moisture from the previous storms could be better retained within the forest area than the non-forest area due to deeper rote zone and higher soil moisture holding capacity of the forest area compared to non-forest area. For the larger flood events, forest area tends to produce more runoff than non-forest area as found in this study. These findings gave us a more thorough understanding of the effect of land cover types on flood behaviour at different stages of soil moisture conditions, and the severity of storm events. It can be useful for land use and flood management of the river basin. (C) 2011 Elsevier B.V. All rights reserved.