Use of Environmental Isotopes to Investigate the Interaction between Water Resources in the South Phuthiatsana Catchment, Lesotho

Código

I-EBHE0093

Autores

Thabang Phori, Seifu Kebede, Khahliso Leketa

Tema

WG 1.14: Droughts in Mountain Regions

Resumo

Lesotho is known as the Southern African Water Tower due to its water resources emerging at high altitudes and being a primary source for the Orange-Senqu River. However, there is limited information available about catchment hydrology in the country. This highlights the need to improve our knowledge of this critical resource to protect it for current and future generations. Isotopic techniques can be used to evaluate catchment hydrology, complementing conventional methods. Therefore, it is essential to identify and mitigate any potential sources of recharge to facilitate policy implementation. The future availability of water depends on the river's response to climate change and population pressure, making it crucial to determine the interactions between surface and groundwater on a catchment scale. This study aims to investigate these interactions and contribute to filling the knowledge gap in hydrology in the country. Data has been collected from precipitation, springs, boreholes, wetlands and rivers across the catchment at varying altitudes. Additional data has been collected from several projects across Lesotho, including the International Atomic Energy Agency's Global Network of Isotopes in Precipitation/Rivers (GNIP/R) dataset not yet published. Daily precipitation data has been used to develop the local meteoric water line (LMWL), while GNIR data has been used to create the local evaporation line (LEL). Both datasets are located within the study area and used to assess catchment hydrology. Furthermore, the created LEL displays isotopic fractionation. Most of the smaller streams are fed by sources with relatively high humidity but undergo evaporation in as they approach confluences in the lowlands area. These are second and third-order, are positioned downstream of the reservoir at an elevation below 1800m and are affected by increased evaporation. Upstream of the reservoir, the main river demonstrates a depletion in heavy isotopes. This region experiences cooler climatic conditions and is situated at altitude above 1800m. Moreover, the reduction may be influenced by the river's width and discharge. Wetlands in the uplands (Sponge) are depleted, indicating a more humid climate, whereas those in the lowlands are enriched. The data suggests that diverse geographical regions will experience varying impacts due to climate change. It is thus imperative to implement tailored government interventions to effectively address the distinct challenges within each specific area of the catchment.