Estudios y experiencias nacionales

The rapid rise and recession of the stormflow hydrograph observed in our tropical dry forest catchment was typical of the response observed in most tropical forest catchments (Elsenbeer et al., 1995a; Goller et al., 2005). Despite this rapid response, stormflow was overwhelmingly dominated by pre-event water (75 – 98%). The high pre-event contributions recorded from our catchment challenges the long held observation that stormflow in humid and semi-arid tropical catchments are composed of small (30 – 40%) fractions of pre-event water. In humid and semi- arid tropical catchments, the rapid translation of rainfall to runoff through surface and near- surface pathways reduces the mixing and displacement of old water stored in groundwater or deeper subsurface soil layers, resulting in low pre-event water in stormflow (Schellekens et al., 2004; Goller et al., 2005). Given the strong relationship between high event water and shallow flow pathways, the large pre-event water contributions in stormflow at our catchment suggests that storm runoff is likely generated from deep subsurface flow pathways.

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In most geochemical studies of runoff generation in tropical forest catchments, a large and rapid increase of K+ is typically observed during peak stormflow (Elsenbeer et al., 1995b; Sandström, 1996). The high K+ is often the result of flow over surface or through shallow subsurface

pathways, which is enriched with potassium from litter/organic matter (Elsenbeer et al., 1995a; Schellekens et al., 2004). The highest K+ recorded in the lysimeters within the upper 10 cm of soil at our research site suggests a similar leaching from litter and organic matter. However, the enriched K+ from this soil layer was not observed in the water sampled over the majority of the stormflow hydrographs, suggesting that runoff was not a result of flow over the surface or through shallow subsurface pathways. The depleted K+ in stormflow was most similar to the mean concentrations of K+ recorded in baseflow from the upper headwater basin and primary outflow channel. Likewise the Ca2+, Mg2+ and Na+ concentrations recorded during stormflow generally reflected the ion chemistry of the near-stream groundwater and baseflow from the primary outflow and upper headwater basin. As baseflow is defined as the portion of flow that originates from delayed subsurface flow or groundwater (Tallaksen, 1995), we suggest that the strong baseflow signature in stormflow originates from near-saturated subsurface soil water or transient groundwater sources. This hypothesis is supported by the geochemistry of the

underlying bedrock. The andesitic-basalt/basaltic-andesite that characterises this region is composed of K+, Na+, Mg2+ and Ca2+ ratios of 1:4:5:7 (Luhr and Carmichael, 1980; 1981). The baseflow from the upper basin and primary outflow and stormflow show similar mean

geochemical ratios of 1:3:5:7. Despite these results, we lack the hillslope groundwater tracer and hydrometric data to support direct groundwater contributions and suggest that future work in this catchment should test for groundwater contributions to runoff.

Previous work in this catchment has suggested that given the rapid increase in soil moisture above field capacity but below saturation during storm events, that storm runoff may be generated as mass or pressure wave translation through the unsaturated zone (Farrick and

Branfireun, 2014b). Torres et al. (1998) suggests that when soil is in the near-zero head pressure range, rainfall inputs can elicit a small increase in pressure head and a large increase in hydraulic conductivity. The resulting increase in the hydraulic gradient and hydraulic conductivity

generates pressure waves which produces a rapid response, displacement and discharges water in the saturated zone. For storms two (August 19) and four (September 11), the rapid increase in

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soil moisture at the 50 and 100 cm soil layers, coupled with large contributions of pre-event water, low K+ in storm water and Ca2+, Mg2+ and Na+ concentrations reflective of near-stream groundwater and baseflow sources, suggest that the displacement of stored water from the saturated zone occurs during these larger storm events.

Although high pre-event contributions and Ca2+, Mg2+, Na+ and K+ reflective of near-stream groundwater and baseflow were also recorded during storms one (August 7) and three

(September 9), the soil moisture response at depths below 50 cm lagged behind peak streamflow, suggesting a different runoff mechanism. In a catchment with soils of similar volcanic origin, Muñoz-Villers and McDonnell (2012) showed that despite a delayed response in the soil moisture at depths below 70 cm, runoff was composed of 72 – 99% pre-event water. Muñoz- Villers and McDonnell (2012) attributed the high pre-event to vertical preferential flow in near- stream areas, which bypasses the soil matrix and displaces near-saturated soil water or hillslope groundwater. Such a mechanism may occur for these two events as the maximum increase in near-stream groundwater occurred before peak stormflow. However, despite this rapid increase, the geochemistry reflective of near-stream sources was not observed until the recession limb (Storm 1) or just before peakflow (Storm 3), suggesting delayed contributions. It is important to note that these sampling wells were located in the upper half of the catchment (Figure 4.1) and the delayed contributions are likely a result of transport times from these locations.

Caution must be exercised when attributing runoff generation to direct contributions from the near-stream area. Chanat and Horberger (2003) suggest that substantial hillslope and

groundwater contributions may be masked by the higher ion concentrations in the near-stream or riparian zone. In our current study, this masking effect likely exists during storm one (August 7). Water collected at the 50 cm lysimeter had Ca2+ and Mg2+ values two times lower than the near- stream water (Table 4.2). Even if this hillslope water was discharged during the rising limb, the higher Ca2+ and Mg2+ in the near-stream water may mask hillslope contributions.

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4.5.3. The influence of catchment wetness on source area