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Universität Hamburg (2019)

Quantifying the effect of vegetation and land use on soil hydrological functioning along a regional transect in southern Africa

Landschreiber Lars

Titre : Quantifying the effect of vegetation and land use on soil hydrological functioning along a regional transect in southern Africa

Quantifizierung der Effekte von Vegetation und Landnutzung auf bodenhydrologische Funktionen entlang eines regionalen Transekts im südlichen Afrika

Auteur : Landschreiber Lars

Université de soutenance : Universität Hamburg

Grade : Doctoral degree in natural sciences 2019

The catchment of the Okavango River extends over three countries and two hydro-climatic zones in Southern Africa, from the dry sub-humid headwaters in the central highlands of Angola over the semi-arid Kavango region in northern Namibia to its semi-arid inland delta region in Botswana. For flora, fauna and not the least the human pop-ulation, the river system is a reliable source of livelihood in an environment otherwise characterized by very limited water supply. An increasing population growth exerts direct pressure on this resource by abstraction for human consumption, irrigation purposes or livestock farming, whereas indirect pressure effects due to change of land use cause modifications of the hydrological cycle. The change-related reactions and consequences for soil-water balances and dynamics are analysed based on three over-arching questions : 1.) How does agricultural land use expansion change soil water balances in a dry sub-humid landscape in Angola ? 2.) How is the influence of different vegetation and agricultural land use character-istics on soil water balances in northern Namibia ? 3.) How does bush encroachment alter soil water dynamics and potential ground-water recharge in the semi-arid highlands of central Namibia ? Three regional case studies along the hydro-climatic gradient of the catchment describe 1.) pristine conditions with grassland and woodlands as well as 2.) scenarios characterized by smallholder dryland agriculture and commercialized agriculture under irrigation. Methodically, a detailed pedological survey comprising static soil-hydrological parameters, time series of soil-water contents and potentials as well as vegetation analysis are combined with a long-term soil-hydrological simulation based on land-use scenarios. General findings show an increase in deep drainage by an additional 29 % of the precipitation input for the study sites in Angola and northern Namibia with conversion from pristine conditions to subsistence agriculture. For the irrigation scenarios, the key finding is that application of integrated soil-hydrology irrigation management will lead to a 30 % to 60 % lower water abstraction in contrast to the evapotranspiration demand calculations. An increasing bush encroachment leads to complete suppression of bottom fluxes in the central Namibian highlands. The soil-hydrological elasticity of the pristine ecosystems in Angola and northern Namibia are able to support medium-to-high crop densities provided there is sufficient nutrient supply. On the other hand, the ecosystem drought resilience of the thornbush savannah in central Namibia decreases with increasing bush encroachment caused by diminished soil water buffers. The hydrological effects of land-use changes have multiple impacts on upstream/downstream relations, where the higher drainage rates in the headwater regions lead to improved water supply in downstream regions over the dry season. Increasing bottom fluxes are concomitant with an increased nutrient leaching, and mitigation measures - e.g. vegetation management in an agroforestry system in headwater regions - decrease the formerly positive effects of downstream hydrograph changes. In contrast to flood and leaching control by conservational vegetation management in areas generating river run-off, debushing measures in parts of the catchment affected by dry spells can support groundwater recharge and increase the ecohydrological resilience by restoring soil water buffers. This study illustrates the possibilities of how a combined empiric-conceptional based approach can contribute to an in-depth knowledge of fundamentals and consequences of land use and land cover change in an area of tension between contrasting soil water utilization in drylands and subsequent hydrological processes. Especially in regions with limited data, this approach is able to support measures for a sustainable land use and ecosystem management.

Mots clés  : Bodenkunde , Hydrologie , Trockengebiet , Angola , Namibia , Afrika , Landnutzung , Modellierung , Bodenwasser , Wasserhaushalt


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