Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Titre : Forecasting productivity losses caused by sheet and rill erosion in semi arid rangeland : A case study from communal areas of Botswana.

Auteur : Biot, Y.

Université de soutenance : University of East Anglia

Grade : Doctor of Philosophy (Ph.D.) : 1987

Résumé
Public awareness of the potential consequences of land degradation has recently been heightened by tragic famine in the highlands of north-east Africa. Despite concerted efforts in the field erosion monitoring and forecasting, the empirical evidence and scientific methodology needed for the assessment of future productivity declines caused by erosion are lacking. It is the purpose of this thesis to develop a mathematical modelling technique for the prediction of productivity trends of agricultural land in regions where the technical resource base is weak, and where projections are needed fast. Two models are proposed : THEPROM, a theoretical model which incorporates the most important components and pathways of the erosion/productivity system and allow the study of interactions which would not necessarily become clear in experimental work, and EPROM, a parameter-efficient summary model for use as a field tool. EPROM consists of a simple balance sheet of productivity losses caused by erosion and gains from rock weathering and organic matter production. The Available Water Storage Capacity (AWSC), a measure of quantity (soil depth) and quality (Available Water Capacity) of soil, is proposed as an index of the intrinsic productivity of land. Procedures for the calibration of EPROM are outlined and tested in a case study of rangeland degradation in the Hard Veld of Botswana. The results of the case study confirm the choice of the productivity index and the input model. An alternative output model fits the observed values of productivity losses better. The productivity decline of a soil with average soil depth is simulated using a numerical integration method. The minimum allowable productivity level is derived from economic considerations, and the ’residual suitability’ or ’Soil Life’ of the range is calculated which is defined as the residual productive lifespan of land, in years. It is predicted that the present stocking rates in the study area are sustainable for another 400 to 500 years, unless other climatic and/or vegetation changes occur. It is concluded that the productive potential of communal rangeland in the Hard Veld, Botswana, is not at risk from water erosion. The results of the case study demonstrate that the calibration of EPROM is fairly easy, cheap and fast and that the forecast is reliable as long as average soils over large areas are considered. The technique, however, is not accurate enough to determine productivity trends at the plot level.

Mots clés : Soil erosion/crop output Agricultural engineering Agronomy Plant diseases Horticulture Applied mathematics Agricultural engineering Agronomy Applied mathematics

Annonce : EThOS (UK)

Proquest Dissertations & Theses