Schepp, Claudia: Linking slopes to the wetland: Hillslope hydrology and associated nitrate transport in a tropical valley bottom wetland. - Bonn, 2022. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
Online-Ausgabe in bonndoc:
author = {{Claudia Schepp}},
title = {Linking slopes to the wetland: Hillslope hydrology and associated nitrate transport in a tropical valley bottom wetland},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2022,
month = nov,

note = {Valley bottom wetlands are regarded as a white hope for enhancing food security in East Africa. Due to their prolonged water availability, they buffer some of the effects of climate change and further offer fertile soils in otherwise frequently degraded landscapes. For West Africa it has been shown that these attributes are related to water and nutrient fluxes from the surrounding slopes into the valley bottom wetlands, while for East Africa the knowledge on hillslope hydrology and its relevance for water and nutrient transport to the wetland is still limited. The aim of this work is to investigate interflow processes on a saprolitic hillslope and to explore the agricultural relevance of water and nitrate inputs from the slopes to the wetland fringe.
The experiment was set up as a plot study in a representative small inland valley in Central Uganda and a multi-method approach was followed. The hillslope hydrology was investigated via electrical resistivity measurements as well as the monitoring of soil moisture along the toposequence. Furthermore, interflow was collected in trenches at the slope toe, while surface runoff was quantified on runoff plots at the lower slope position. In view of gaining a better process understanding and to formulate management recommendations, the influence of different land use types as well as rainfall characteristics on water and nitrate fluxes from the slopes to the wetland fringe was analysed. To that end, three different land use types were established on plots of 30 x 105 m² on the slopes, including crop production, semi-natural vegetation and bare fallow. The dynamics of nitrate formation and fluxes as well as the seasonal cumulative amount of bio-available nitrate were monitored along the toposequence under the three land use types. In addition, nitrate loads in the interflow as well as in the surface runoff were determined. The study revealed that due to the fill-and-drain mechanism interflow is not directed along the soil-saprolite interface but passes through the saprolite. A quick as well as a delayed interflow component develops, which contribute to the shallow aquifer in the wetland soils as well as to the aquifer in the valley sediments. The total rainfall amount during the rainy season as well as the distribution of rainfall over the season strongly influence interflow generation, with the delayed interflow component also carrying the signal of the previous rainy season. Land use type as well as the rainfall characteristics also impact the nitrate translocation along the slope. These factors are the key drivers of soil moisture conditions and the activation of vertical and lateral flow paths in the soil and in the saprolite, but also for the dynamics of nutrient uptake from the soil solution.
Regarding agricultural production, water from the slope is only one amongst several drivers of the soil water status at the wetland fringe. While there was an input of nitrate from the slopes to the wetland, low contents of seasonal bio-available nitrate hint to a quick loss of nitrate at the wetland fringe. Therefore, agronomic measures should focus on water and nutrient conservation along the slopes and at the cultivation of deep rooting crops at the slope toe in order to maintain water and nitrate in the soil-plant system and thus minimize losses.
The study highlights the complexity of subsurface flow processes in saprolitic environments and stresses the importance of a sound knowledge of the local hillslope hydrology to detect catchment-wetland interactions. This knowledge is the basis for pinpointing agronomic interventions, enabling a sustainable use of the natural resources in inland valley landscapes of East Africa and, hence, contributing to food-security in the region.},

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