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Rheinischen Friedrich-Wilhelms-Universität Bonn (2015)

Soil water availability in agriculturally used wetlands of East Africa

Böhme, Beate

Titre : Soil water availability in agriculturally used wetlands of East Africa

Auteur : Böhme, Beate

Université de soutenance : Rheinischen Friedrich-Wilhelms-Universität Bonn

Grade : Doktor der Agrarwissenschaften (Dr. agr.) 2015

Résumé partiel
Wetlands contribute increasingly to the food needs and income generation of the rural population in East Africa. The transformation of wetlands into farmland has gained momentum due to a growing food demand, upland shortages and increasing climate variability. A wetland’s agricultural production potential is largely determined by its soil fertility as well as the water availability in the rooting zone of crops. In this Dissertation, I assessed the spatial and temporal soil water availability, and related moisture status to hydrogeomorphological conditions and types of land use. Based on an initial classification of wetlands in the region, the present work focussed on two representative agriculturally used wetlands. Tegu is an inland valley wetland of about 10 ha in the humid tropical highlands (1720 m a.s.l.) south of Mount Kenya, Kenya. Malinda is a floodplain located in the plains of Mkomazi River in the sub-humid tropical lowlands (360 m a.s.l.) at the western leeward side of the Usambara Mountains, Tanzania. While both sites are mainly used for crop production, unused portions remain under natural wetland vegetation. Hydrometeorological state variables, comprising Frequency Domain Reflectometry (FDR)-based soil moisture contents, groundwater levels and stream discharge, were monitored between 2009 and 2011. Thereby, different landscape positions and land use types in the wetlands were covered. A site-specific calibration procedure for the FDR system yielded a higher measurement accuracy for the Tegu site than that achieved using the manufacturer’s calibration function. Groundwater level and the duration of soil saturation were closely related to the landscape position along and across the Tegu inland valley, thus determining the suitability of specific locations within the wetland for crop cultivation. In the bowl-shaped valley head with no pronounced stream channel, the cultivation of forage grass (Pennisetum purpureum) prevailed. In the floodplain-like lower section with an extensive network of drainage/irrigation channels, taro (Colocasia esculenta) was widespread and generally cultivated in association or rotation with upland crops, mainly maize (Zea mays) and vegetables. The Tegu valley bottom is agriculturally used although soil moisture contents are often too high, which bears the risk of crop failure due to flooding. The application of a semi-distributed rainfall-runoff model yielded an overestimation of the stream discharge during the long dry season of the hydrological year 2010/2011, indicating the impact of water abstraction for irrigation.

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