Spatial variability in stable isotopes from Lesotho surface waters: insights into regional moisture transport

Abstract

Precipitation in Lesotho is highly spatially variable, a feature of the high altitude and rugged topography. The hydroclimate dynamics, despite being critical to the water security of Lesotho and adjacent South Africa, are poorly understood. Ratios of oxygen and hydrogen isotopes in meteoric water are excellent tracers of hydroclimatic processes. This study presents the first analysis of stable isotopes from surface waters in Lesotho, and an investigation into the moisture sources. Our results demonstrate considerable variability in isotope values. There are statistically significant relationships between both oxygen and hydrogen isotopes and the altitude of the site and source of rivers sampled, and with hydrogen isotopes and longitude. The meteoric water line for the Lesotho samples is most closely aligned with that of the Global Network of Isotopes in Precipitation (GNIP) station at Harare, in Zimbabwe. The meteoric water line for Windhoek is more closely aligned to the Lesotho samples than the more proximate Cape Town or Pretoria meteoric water lines, which would more closely represent the South African winter- and summer-rainfall zones respectively. HYSPLIT back-trajectory air parcel analysis supports these findings, demonstrating a frequent continental anticyclonic track through southern Zimbabwe. Deuterium excess values vary widely, although are most likely related to processes during moisture transport rather than differences in moisture source. These findings are of particular importance in the context of the future water security of both Lesotho and South Africa, especially as the poleward displacement of the westerly moisture corridor has raised concerns for winter precipitation in the region.

Classifying moisture sources associated with snowfall in the mountains of Lesotho

“…The methodology employed in this study is informed by previous research on air parcel trajectories for moisture sources in mountainous regions globally (eg. Li et al 2022), in Southern Africa (Leketa and Abiye 2020) and specifically in Lesotho (Fitchett et al 2024). We do not argue that this is the only approach to the analysis of air parcel trajectories or moisture sources, but have applied this approach as it has been applied and tested against isotope records for the regions of interest (Leketa and Abiye 2020;Fitchett et al 2024), and in regions of similar topographic landscapes (Li et al 2022).…”

“…These back-trajectories are set to the location of Afriski, 28°49'22'' S, 28°43'41'' E. The 72-hour back-trajectory timeframe is a result of the three-day moisture residency in the atmosphere of colder alpine regions (Li et al 2022). While a variety of studies indicate that 10 days is the residence time of moisture in the atmosphere, studies in localized highly mountainous regions such as this one use a shorter 72 hour back trajectory (Laderach and Sodermann 2016;Nieto and Gimeno 2019;Li et al 2022;Fitchett et al 2024). In addition, latitude, ocean-atmospheric dynamics of each ocean, and orographic uplift can shorten the lead time of moisture in the atmosphere emphasizing 72 hours as the most representative residence time of moisture in the atmosphere (Bookhagan and Burbank 2010; Gimeno et al 2021;Escobar et al 2022).…”

“…This elevation at 500m, 1000m and 2000m is configured by the topographical information of Lesotho, to produce a valid backtrajectory (Li et al 2022;Rose et al 2020;Bailey et al 2021). These altitudes are informed by the literature on precipitation in the region and on precipitation in mountainous regions (Leketa and Abiye 2020;Li et al 2022;Fitchett et al 2024). Similarly, using three or less altitudes in the HYSPLIT model is sufficient to provide a valid result (Rapolaki et al 2020;Rose et al 2020).…”

“…This is because this study was not concerned with pressure gradients between the different elevations that moisture followed along each pathway, referring to isobaric changes and isentropic changes as this process is assumed based on the orographic nature of the snowfall (Bookhagan and Burbank 2010; Li et al 2016). Relative humidity was consulted and modelled sparingly to simply illustrate that not all air parcels are moisture-bearing however, this has few implications for the bigger picture in relation to the broad region from which snow-bearing moisture originates (Rapolaki et al 2020;Fitchett et al 2024). Non moisture-bearing parcels are included in the trajectories without relative humidity being measured along the pathway.…”

“…Non moisture-bearing parcels are included in the trajectories without relative humidity being measured along the pathway. Isolating the saturated pathways, which occur in varying proportions, would not contribute to defining the most representative pathway of snowfall bearing moisture in Lesotho (Rapolaki et al 2020, Fitchett et al 2024). In addition, the model was set to run back-trajectories only once or at zero hours as the data used was not hourly but based on the snowfall record compiled from Instagram (Fitchett et al 2024).…”