Typhoon-induced changes in rainfall interception loss from a tropical multi-species ‘reforest’

Typhoon Haiyan made landfall in the central Philippines on 8 November 2013 as one of the strongest tropical cyclones ever. It also affected a 23-year-old multi-species ‘reforest’ at Manobo near Tacloban City on Leyte Island. As part of a larger hydrological investigation of the impacts of reforestation on streamflow response in the Tacloban area, gross rainfall (P), throughfall (TF; 24 roving collectors) and stemflow (SF; 12 trees) were monitored at Manobo between June 2013 and May 2014. Leaf Area Index (LAI) above each TF collector was measured regularly. Total rainfall interception losses (I) were determined using Gash's revised analytical model for three consecutive periods:(i) pre-Haiyan (baseline), (ii) post–Haiyan (damaged canopy), and (iii) after initial canopy recovery. Modeled I was 18% of P before disturbance, 12% for the period with the most extensive canopy damage, and 17.5% after initial canopy recovery. Stemflow was low, and weighted mean values accounted for 2.7%, 1.3% and 2.0% of P for the respective periods. Contrasts in period-average values of I reflected changes in LAI as well as wet-canopy evaporation rates. Storm-based TF fractions at the 5 m × 5 m sub-plot scale were inversely related to LAI, especially for small storms and low rainfall intensities. Inferred hourly rates of wet-canopy evaporation showed a strong positive relationship to hourly rainfall intensity during large storms. The revised analytical model was also run using pre-disturbance parameter values for the entire year to assess the overall effect of canopy damage on I. Estimated annual losses with and without canopy disturbance were 514 mm (15% of P) and 572 mm (17%), respectively. Thus, observed and inferred changes in rainfall partitioning after canopy disturbance and initial recovery were comparatively modest, likely because the measurement site was relatively sheltered from the winds during typhoon passage and re-foliation relatively rapid. However, given the predicted increase in occurrence of 'super-typhoons’ due to continued global warming and oceanic freshening, the structure of forests in affected regions can be expected to be modified, with potential consequences for rainfall partitioning and hydrological response.