Some attributes of snow occurrence and snowmelt/sublimation rates in the Lesotho Highlands: environmental implications

Stefan Grab,Jonathan Linde,Hugo De Lemos

doi:10.4314/wsa.v43i2.16

Abstract

We present attributes of snow occurrence and dissipation rates (melt and sublimation) for the Lesotho Highlands, based on remotely-sensed MODIS images from 2003–2016. Multi-temporal imagery is used, with SNOMAP and NDSI algorithms applied to MODIS Rapid Response images. The spatial extent of snow loss was determined by daily repeat measurements of snow coverage, which was calculated from each filtered and trimmed MODIS SNOMAP image using the ArcGIS Spatial Analyst tool. Results indicate an average of 11.5 snowfalls per annum for the years 2003–2016; with snow longevity averaging ca. 10 days following individual snowfalls in mid-winter. Snow cover on the highest south-facing slopes persists longest during the months of June to August, in particular along the southern Drakensberg where it averages ca. 58 days during these 3 months. Mean daily melt increases by 1.6% per 1°C rise during the first 5 days post-snowfall, and by 3.3% per 1°C rise for 6 to 10 days post-snowfall. However, snowmelt rates are spatially highly variable given other factors such as wind deflation and wind-induced sublimation. The observed snow trends have important implications for biosystem functioning, regional climate and hydrology, earth surface processes, and rural livelihoods in the Lesotho Highlands.

Highlights

While southern African hydrological studies are internationally advanced in many of the sub-disciplinary components, including in particular catchment modelling (e.g. Govender and Everson, 2005; Vischel et al, 2008; Gibson et al, 2011) and groundwater studies (e.g. Levy and Xu, 2012; Schrader et al, 2014), very little work has focused on snow hydrology in the region
Snowfalls over the Lesotho Highlands typically occur from May to September each year, with occasional early and late events in March/April and October/November respectively
Over the last 14 years, an average of 11.5 annual snowfalls have been recorded across the mountain kingdom through MODIS satellite tracking, but, notably, the past 3 years have recorded an average of 13 snowfalls/annum

Summary

Introduction

While southern African hydrological studies are internationally advanced in many of the sub-disciplinary components, including in particular catchment modelling (e.g. Govender and Everson, 2005; Vischel et al, 2008; Gibson et al, 2011) and groundwater studies (e.g. Levy and Xu, 2012; Schrader et al, 2014), very little work has focused on snow hydrology in the region (e.g. snowwater equivalents, snowmelt/sublimation rates, contribution to groundwater recharge, etc.). Given that several mountain and highland regions of southern Africa receive appreciable quantities of snow during at least some years, it seems appropriate to begin addressing the hydrological role and impact of snow in the African sub-continent. The presence or absence of snow cover is an important indicator of climate change

Methods

Results

Conclusion