Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Geology

Supervisor

Dr. Brian Branfireun

Abstract

The lack of understanding regarding the controls that govern runoff generation in tropical dry forests represent a critical gap in the hillslope and catchment hydrology literature. Tropical dry forests account for approximately 42% of the global tropical forests, but represent less than 1% of the forest hydrology literature. Three complementary studies were undertaken in a small tropical dry forest watershed, Mexico, to assess the controls that govern the retention and release of a rainfall in the catchment as runoff. In the first study, the high soil surface hydraulic conductivities, absence of a water repellent surface and low rainfall intensities during the wet season allows most of the incoming rainfall to percolate through the near-surface soil layers, suggesting that runoff is generated through a subsurface flow mechanism. In the second study, it was found that two different thresholds were required for streamflow activation and stormflow generation. The long dry period depletes the stores of soil water. Only after the soil storage deficit in the upper metre is satisfied, is streamflow activated from the catchment. Once streamflow became persistent, the stormflow response was almost entirely governed by the rainfall event characteristics and not antecedent soil moisture conditions. The third study used a combination of isotopic, geochemical and hydrometric measurements to describe the water flow pathways, source areas and residence times of stream water in this catchment. It was shown that runoff produced during storm events were composed primarily of old water that likely originated from either deep subsurface soil layers or groundwater and the source areas expanded, likely through sub-basin connectivity, as catchment wetness increased through the wet season. Given the arid climate of the watershed and known hydrological literature regarding runoff generation in tropical forests, it was hypothesised that runoff in this catchment should be delivered from surface or near-surface sources. However, this dissertation has shown that the combination of deep, permeable soil on steep slopes have a stronger influence on runoff generation in this catchment than climate. These three studies have therefore demonstrated the importance of characterising the physical controls that govern runoff generation in forests that are data poor.

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