Land use change effects on stormflow generation in humid tropical montane cloud forests

Tropical montane cloud forests (TMCF) are reliable sources of high quality water to the downstream, often densely populated, regions throughout the humid tropics. While TMCF provide critical ecosystem services, our understanding of runoff generation processes in these environments is quite rudimentary. Here, we present a new study aimed at identifying the dominant pathways, water sources and the mechanisms driving the delivery of water to the streams in an old-growth (MAT) headwater catchment in central Veracruz, eastern Mexico. We then compare this response with neighboring headwater catchments underlain by very similar volcanic soils and geology but with different land use cover: a 20-year-old naturally regenerating forest (SEC), and a heavily grazed pasture (PAS) catchment. We examined the runoff response across a sequence of storms during the wetting-up cycle of the 2009 wet season, using isotopes and chemical tracers and hydrometric field measurements. Total rainfall accumulated during the 6-week study period was ~1200 mm. Storm runoff ratios increased progressively from 9 to 41% for the MAT, 7 to 52% for the SEC, and 3 to 59% for the PAS. Hydrograph separation analysis also showed a clear progression through the wet season where the first storms had very low pre-event water percentages that increased progressively through time (pre-event water ratios increased from 33 to 98% (MAT), 35 to 95% (SEC), and 64 to 97% (PAS)). In general, the event/pre-event storm fractions were comparable between land cover types, with two notable exceptions: 1) for the first storm sampled under dry antecedent conditions, the forests had much higher event water contributions compared to the pasture, most likely due to seasonal differences in soils hydrophobiticy; and 2) for the largest event of our study period (110 mm in three hours), that occurred under very wet antecedent conditions, the event water contributions in the PAS (28%) were much higher than in the forests (2 and 7%, MAT and SEC, respectively). Overall, our findings suggest that in this environment, land use change effects on runoff seems to be minimal, except for rare, very high intensity events (typically occurring 1-2 times a year). Rather, subsurface streamflow appears to dominate at all three sites where, through the progressive wetting-up, activation of stored water increases and promotes high runoff response of old, stored water.