Article I. Effects of land use on soil properties and hydrological processes at the point, plot, and field scale in volcanic soils in the Turrialba Region of Costa Rica
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Deforestation and conversion to crop land and pasture in tropical regions has important effects on hydrological processes including increased flooding and reduced drought flows. Few studies have investigated the simultaneous effects of multiple land uses in tropical regions at different scales. Upscaling from point scale measurements remains a challenge due to soil heterogeneity. In this study, I measured soil properties and hydrological processes at four sites representing four common land uses in Costa Rica (forest, coffee, sugar cane and pasture) at the point scale, the plot scale (1 m3), and the field scale (1-4 ha). At the point scale, I measured bulk density, organic matter, saturated hydraulic conductivity (Ksat), cumulative infiltration, and soil moisture at the soil surface and with depth. At the plot scale, I performed mass balance experiments in triplicate using artificial irrigation to determine infiltration, runoff, lateral flow, change in storage, and percolation, for each land use type. At the end of these experiments, I used Brilliant Blue dye to characterize differences in flow paths between the land uses. At the field scale, streamflow was measured from June 2004 to December 2006 using an H-flume that was installed below each land use type. While most of the soil characteristics between the field sites were similar, bulk density at the forest site (0.7 g/cm3) was significantly lower than bulk density at the other field sites (1.0 g/cm3). Soil moisture at the surface in the sugar cane and pasture sites was consistently greater than in soils of the forest and coffee sites. 17 Saturated hydraulic conductivity at the forest and coffee sites was at least twice Ksat at the sugar cane and pasture sites. At the plot scale, sugar cane and pasture plots had greater surface runoff and lateral flow coefficients (>30%) than the coffee and forest sites (<15%) during the mass balance experiments. Preferential pathways, along with deeper, thicker roots, facilitated the dye reaching greater depths at the forest and coffee plots. At the field scale, runoff was minimal at all of the field sites during the dry season. During the wet season, soil compaction and root characteristics played a strong role in shaping runoff characteristics. The sugar cane site had the greatest runoff coefficient (7%), event frequency, volume and intensity. Runoff coefficients at the other sites were less than 5%. The interplay of compaction at the surface, presence of different root distributions, and reduced bulk density with depth affected infiltration and storage dynamics in response to high intensity rainfall that provided primary controls for ‘fill and spill’ runoff generation during the wet season. In addition, flow convergence due to lateral flow from upland areas and restrictive layers augmented the 'fill and spill' mechanism. As measurement scale increased from the point to plot scale, infiltration rates increased due to the inclusion of more macropores. At the field scale, the effect of land use on runoff generation was somewhat diminished because the measurement scale included a greater number of macropores. These findings have implications for groundwater recharge and runoff predictions which are described in Chapter 2.
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