Land use change effects on runoff generation in a humid tropical montane cloud forest region

Abstract

Abstract. While tropical montane cloud forests (TMCF) provide critical hydrological services to downstream regions throughout much of the humid tropics, catchment hydrology and impacts associated with forest conversion in these ecosystems remain poorly understood. Here, we compare the annual, seasonal and event-scale streamflow patterns and runoff generation processes of three neighbouring headwater catchments in central Veracruz (eastern Mexico) with similar pedological and geological characteristics, but different land cover: old-growth TMCF, 20 yr-old naturally regenerating TMCF and a heavily grazed pasture. We used a 2 yr record of high resolution rainfall and stream flow data (2008)(2009)(2010) in combination with stable isotope and chemical tracer data collected for a series of storms during a 6-week period of increasing antecedent wetness (wetting-up cycle). Our results showed that annual and seasonal streamflow patterns in the mature and secondary forest were similar. In contrast, the pasture showed a 10 % higher mean annual streamflow, most likely because of a lower rainfall interception. During the wetting-up cycle, storm runoff ratios increased at all three catchments (from 11 to 54 % for the mature forest, 7 to 52 % for the secondary forest and 3 to 59 % for the pasture). With the increasing antecedent wetness, hydrograph separation analysis showed progressive increases of pre-event water contributions to total stormflow (from 35 to 99 % in the mature forest, 26 to 92 % in the secondary forest and 64 to 97 % in the pasture). At all three sites, rainfall-runoff responses were dominated by subsurface flow generation processes for the majority of storms. However, for the largest and most intense storm (typically occurring once every 2 yr), sampled under wet antecedent conditions, the event water contribution in the pasture (34 % on average) was much higher than in the forests (5 % on average), indicating that rainfall infiltration capacity of the pasture was exceeded. This result suggests that despite the high permeability of the volcanic soils and underlying substrate in this TMCF environment, the conversion of forest to pasture may lead to important changes in runoff generation processes during large and high intensity storms. On the other hand, our results also showed that 20 yr of natural regeneration may be enough to largely restore the original hydrological conditions of this TMCF.

Interactions between payments for hydrologic services, landowner decisions, and ecohydrological consequences: synergies and disconnection in the cloud forest zone of central Veracruz, Mexico

“…3000 mm, water yield (P + CWI -ET) was much higher for pasture (2185 mm; with negligible CWI) than for mature cloud forests (1730 mm; with annual CWI = 45 mm) or regenerating cloud forests (1985 mm; with annual CWI = 38 mm) (Muñoz-Villers et al 2015). Despite higher annual water yield, baseflows from the pasture during the late dry season were 35% and 70% lower than flows from the mature and regenerating cloud forests, respectively; these differences were attributed to a combination of lower topsoil infiltration capacity, gentler topography, and lower soil and groundwater storage capacity in the pasture compared to the forested catchments (Muñoz-Villers and McDonnell 2013). Maintaining dry-season baseflow is a critical hydrologic service for downstream communities, which in the past, have experienced water scarcity during very dry years in Veracruz (Tejeda-Martínez 2005).…”

“…Forest conversion to more intensive forms of land use is often accompanied by soil compaction and degradation, which can reduce infiltration rates and increase rainfall-runoff responses (Molina et al 2007, Zimmermann and Elsenbeer 2009, Bathurst et al 2011, Ghimire et al 2013). At our Veracruz site, despite generally higher topsoil bulk density and much lower saturated hydraulic conductivity under pasture compared to regenerating and mature cloud forests (0.49 ± 0.06, 0.45 ± 0.11, and 0.25 ± 0.17 g cm -3 , and 30 ± 14, 615 ± 690, and 777 ± 931 mm h -1 , respectively) (Muñoz-Villers et al 2015), prevailing rainfall intensities at this elevation rarely exceeded topsoil infiltration capacities to generate flood-producing rates of overland flow (Muñoz-Villers and McDonnell 2013). However, during the largest rainfall events, peak flows from the pasture catchment were up to two times greater than those from the forested catchments (Muñoz-Villers and McDonnell 2013), which suggests that beyond some threshold of rainfall amount and intensity, cloud forests would provide important hydrologic buffering against flooding.…”

Advancing catchment hydrology to deal with predictions under change

“…Land use will also create hydrologic differences between otherwise similar catchments (similar in pedology, geology, and climate). For example, differences between mature forest and pasture landscapes in neighboring catchments in Mexico become clear under more intense storms (once every 2 yr) (Muñoz-Villers and McDonnell, 2013). Under those conditions, the infiltration capacity of the pasture is exceeded and event water contributions are much higher than for the forested landscapes even though both overlay rather permeable volcanic soils.…”

Moving sociohydrology forward: a synthesis across studies

“…For example, several studies considered the effects of land use change altering the hydrologic regime, through increased irrigation in the Heihe River in China (Zhang et al, 2014), alterations to the water cycle through tile drainage (Yaeger and Sivapalan, 2013), irrigation from groundwater in the midwestern US (Zeng and Cai, 2014), and deforestation in eastern Mexico (Muñoz-Villers and McDonnell, 2013). The oneway nature of influence in these studies possibly results from a timescale separation between the rapid timescales of human intervention in the water cycle and the longer timescales on which these interventions alter agricultural productivity.…”