Assessing plot‐scale impacts of land use on overland flow generation in Central Panama

Abstract

Land use in Panama has changed dramatically with ongoing deforestation and conversion to cropland and cattle pastures, potentially altering the soil properties that drive the hydrological processes of infiltration and overland flow. We compared plot-scale overland flow generation between hillslopes in forested and actively cattle-grazed watersheds in Central Panama. Soil physical and hydraulic properties, soil moisture and overland flow data were measured along hillslopes of each land-use type. Soil characteristics and rainfall data were input into a simple, 1-D representative model, HYDRUS-1D, to simulate overland flow that we used to make inferences about overland flow response at forest and pasture sites. Runoff ratios (overland flow/rainfall) were generally higher at the pasture site, although no overall trends were observed between rainfall characteristics and runoff ratios across the two land uses at the plot scale. Saturated hydraulic conductivity (K s) and bulk density were different between the forest and pasture sites (p < 10 −4). Simulating overland flow in HYDRUS-1D produced more outputs similar to the overland flow recorded at the pasture site than the forest site. Results from our study indicate that, at the plot scale, Hortonian overland flow is the main driver for overland flow generation at the pasture site during storms with high-rainfall totals. We infer that the combination of a leaf litter layer and the activation of shallow preferential flow paths resulting in shallow saturation-excess overland flow are likely the main drivers for plot scale overland flow generation at the forest site. Results from this study contribute to the broader understanding of the delivery of freshwater to streams, which will become increasingly important in the tropics considering freshwater resource scarcity and changing storm intensities.

Stream chemical response is mediated by hydrologic connectivity and fire severity in a Pacific Northwest forest

“…Finally, while slopes, soils, and underlying geology are similar between the two burned watersheds, forest age, land‐use history, and drainage area differ greatly (Table 1). Such factors are likely to influence runoff efficiency and the hydrologic flow paths from the landscape to the stream (Birch et al, 2021; Bush et al, 2020; Zema et al, 2021) but given that we did not have additional measurements (e.g., pre‐ and post‐fire canopy interception) these factors cannot be normalized.…”

Stormflow response and ‘effective’ hydraulic conductivity of a degraded tropical Imperata grassland catchment as evaluated with two infiltration models

“…Repeated cycles of slash-andburn cultivation, as well as more intensive forms of agricultural cropping and grazing, have resulted in reductions in topsoil organic matter content, soil faunal activity and macroporosity, and an increase in bulk density (Martinez & Zinck, 2004;Shougrakpam et al, 2010;Recha et al, 2012;Zwartendijk et al, 2017;Toohey et al, 2018). The associated decline in soil infiltration capacity typically leads to increased occurrence and amounts of infiltration-excess overland flow (IOF) in regions and/or periods with high rainfall intensities (Chandler & Walter, 1998;Ziegler et al, 2004;Molina et al, 2007;Ghimire et al, 2013;Bush et al, 2020). IOF, in turn, causes accelerated erosion, as well as higher runoff peaks at the headwater catchment scale (Ziegler et al, 2009;Liu et al, 2011;Recha et al, 2012;Ribolzi et al, 2017;Birch et al, 2021aBirch et al, , 2021b, which exacerbates flooding and sedimentation problems downstream (Bruijnzeel, 2004;Sidle et al, 2006;Valentin et al, 2008;Yin et al, 2019).…”