The effect of forests on flow and flood lamination decreases as the magnitude and intensity of torrential events and the watershed surface increase, thus resulting negligible when extreme events affect large catchments. However the effect of forests is advantageous in case of major events, which occur more often, and is particularly effective in soil erosion control. As a result, forests have been extensively used for watershed management and restoration, since they regulate water and sediments cycles, preventing the degradation of catchments.

Flood peak data for focus catchments in Costa Rica, Ecuador, Chile and Argentina are analyzed to test the hypothesis that, as the size of the hydrological event increases, the effect of forest cover on the peak discharge becomes less important. Previous research suggests that this hypothesis may hold for small catchments (less than 1 km(2)) but the pattern is less clear for large catchments. The principal study results are for small paired catchments (0.6-10 km(2)) with different forest covers (forest/pasture) in highland Ecuador and a small (0.35 km(2)) plantation catchment in southern Chile subjected to logging. The former were analyzed by comparing the corresponding peak discharges for given rainfall events, the latter by comparing the relationships between peak discharge and rainfall event size for the pre- and post-logging periods. In all cases there is relative or absolute convergence of the responses as discharge increases, with convergence likely for flood return periods of around 10 years. More limited data for larger catchments which have undergone either deforestation or afforestation (131 km(2) in Costa Rica and 94-1545 km(2) in Chile) suggest that the percentage change in forest cover must exceed 20-30% to provoke a measurable response in peak discharge; convergence of peak discharge response at high flows (return periods of around 5 years) for the different forest covers may then be observed. For a 12.9-km(2) snowmelt-affected catchment in Tierra del Fuego, Argentina, extreme floods require rain-on-snow events but the data are not sufficient to quantify the complex relationship between forest cover, event return period and peak discharge. In general, forest cover is unlikely to reduce, significantly, peak discharges generated by extreme rainfall but may still offer substantial mitigation benefits for moderate (i.e. more frequent) rainfall events. (C) 2011 Elsevier B.V. All rights reserved.

Through a systematic modelling analysis for focus catchments in Costa Rica (131 km(2)), Ecuador (10 km(2)), Chile (0.35 km(2)) and Argentina (12.9 km(2)), the hypothesis is tested that, as the size of the hydrological event increases, the effect of forest cover on the peak discharge becomes less important. For each focus catchment, a 1000-year synthetic rainfall time series was generated, representative of the current climate. This time series was used to run SHETRAN hydrological models for each catchment with two contrasting land use scenarios (generally with and without a forest cover). The corresponding maximum daily discharges for the contrasting scenarios were then compared to show the extent to which the two responses converged as the size of the peak discharge increased. For a given forest catchment discharge there could be a range of larger non-forest catchment discharges, depending on antecedent soil moisture content. The simulations show consistently for the rainfall dominated sites that the width of this range either remains constant or narrows as discharge increases, indicating either relative or absolute convergence of the responses. The pattern is more difficult to distinguish for a snowmelt regime but a relative convergence of response still appears possible. The results therefore support the test hypothesis. However, the pattern is complicated by factors such as catchment scale, soil depth, antecedent moisture content and land management. Forests may also still offer significant flood mitigation benefits for moderate (and more frequent) rainfall events and they protect against soil erosion and sediment transport for a wide range of events. (C) 2011 Elsevier B.V. All rights reserved.

Through a systematic modelling analysis for focus b a ins in Costa Rica (131 km ), Ecuador (10 km), Chile (0.35 km) and Argentina (12.9 km ), the hypothesis is tested that, as the size of the flood peak increases, the effect of land use on the peak becomes less important. For each focus basin, a 1000-year s ynthetic rainfall time series was generated, representative of the current climate. T his time series was used to run SHETRAN hydrological models for each basin with two c ntrasting land use scenarios (generally with and without a forest cove r). The corresponding maximum daily discharges for the contrasting scenarios were then compared to show the extent to which the two responses converged as the size of the peak discharge increased. For a given forest basin discharge there could be a ran ge of larger non-forest basin discharges, depending on antecedent soil moisture c ontent. The simulations show consistently for the rainfall dominated sites that the width of this range either remains constant or narrows as discharge increases, indicat ing either relative or absolute convergence of the responses. The pattern is more d ifficult to distinguish for a snowmelt regime but a relative convergence of respo n e still appears possible. The results therefore support the test hypothesis. Howe ver, the pattern is complicated by factors such as basin scale, soil depth, antecedent moisture content and land management. Forests may also still offer significan t flood mitigation benefits for moderate (and more frequent) rainfall events and th ey protect against soil erosion and sediment transport for a wide range of events.