Connectivity of overland flow by drainage network expansion in a rain forest catchment

Abstract

Soils in various places of the Panama Canal Watershed feature a low saturated hydraulic conductivity (K s ) at shallow depth, which promotes overland-flow generation and associated flashy catchment responses. In undisturbed forests of these areas, overland flow is concentrated in flow lines that extend the channel network and provide hydrological connectivity between hillslopes and streams. To understand the dynamics of overland-flow connectivity, as well as the impact of connectivity on catchment response, we studied an undisturbed headwater catchment by monitoring overland-flow occurrence in all flow lines and discharge, suspended sediment, and total phosphorus at the catchment outlet. We find that connectivity is strongly influenced by seasonal variation in antecedent wetness and can develop even under light rainfall conditions. Connectivity increased rapidly as rainfall frequency increased, eventually leading to full connectivity and surficial drainage of entire hillslopes. Connectivity was nonlinearly related to catchment response. However, additional information on factors such as overland-flow volume would be required to constrain relationships between connectivity, stormflow, and the export of suspended sediment and phosphorus. The effort to monitor those factors would be substantial, so we advocate applying the established links between rain event characteristics, drainage network expansion by flow lines, and catchment response for predictive modeling and catchment classification in forests of the Panama Canal Watershed and in similar regions elsewhere.

Quantification and interpretation of suspended-sediment discharge hysteresis patterns: How much data do we need?

“…Due to the absence of a riparian zone storage of bedload material is limited. Moreover, bank erosion is restricted to a few locations within the catchment (Zimmermann et al, 2012(Zimmermann et al, , 2014. All slopes in LC are covered with flowlines ( Fig.…”

“…1e) that extend the channel network (Fig. 1c) are, in contrast to channels, weakly incised microtopographic features on hillslopes where overland flow tends to concentrate (Zimmermann et al, 2013(Zimmermann et al, , 2014. The catchment is covered with tropical semi-deciduous forest (Foster and Brokaw, 1982) which is N 130 years old (Foster and Brokaw, 1982;Kenoyer, 1929).…”

“…According to FAO criteria (FAO, 1998), soils are classified as Eutric Cambisols (Baillie et al, 2007) and reach depths of 0.3 m on some ridges and steep slopes and~1 m elsewhere. At the end of the dry season (May) soils in LC show cracks up to 2 cm wide and 10 cm deep (Dietrich et al, 1996;Zimmermann et al, 2014) due to a substantial admixture of smectite in the clay fraction (Grimm et al, 2008). The first rains of the wet season saturate the soils quickly, and by July relative saturation in the upper soil has increased to more than 60% (Zimmermann et al, 2012).…”

“…However, the hydrological characteristics of our study area are not exceptional as forest catchments prone to overland flow are common in the Panama Canal Watershed (cf. Supplementary matrial in Zimmermann et al, 2014) and elsewhere in the tropics (cf. Fig.…”

“…1 in Zimmermann et al, 2012). For more in-depth descriptions of the hydrological characteristics of Lutzito catchment we refer to previous works (Godsey et al, 2004;Loos and Elsenbeer, 2011;Zimmermann et al, 2012Zimmermann et al, , 2013Zimmermann et al, , 2014.…”