Spatiotemporal Analysis of Vegetation Cover Change in a Large Ephemeral River: Multi-Sensor Fusion of Unmanned Aerial Vehicle (UAV) and Landsat Imagery

Bryn E Morgan,Douglas T Bolger,James T Dietrich,Jonathan W Chipman

doi:10.3390/rs13010051

Abstract

Ephemeral rivers in arid regions act as linear oases, where corridors of vegetation supported by accessible groundwater and intermittent surface flows provide biological refugia in water-limited landscapes. The ecological and hydrological dynamics of these systems are poorly understood compared to perennial systems and subject to wide variation over space and time. This study used imagery obtained from an unmanned aerial vehicle (UAV) to enhance satellite data, which were then used to quantify change in woody vegetation cover along the ephemeral Kuiseb River in the Namib Desert over a 35-year period. Ultra-high resolution UAV imagery collected in 2016 was used to derive a model of fractional vegetation cover from five spectral vegetation indices, calculated from a contemporaneous Landsat 8 Operational Land Imager (OLI) image. The Normalized Difference Vegetation Index (NDVI) provided the linear best-fit relationship for calculating fractional cover; the model derived from the two 2016 datasets was subsequently applied to 24 intercalibrated Landsat images to calculate fractional vegetation cover for the Kuiseb extending back to 1984. Overall vegetation cover increased by 33% between 1984 and 2019, with the most highly vegetated reach of the river exhibiting the greatest positive change. This reach corresponds with the terminal alluvial zone, where most flood deposition occurs. The spatial and temporal trends discovered highlight the need for long-term monitoring of ephemeral ecosystems and demonstrate the efficacy of a multi-sensor approach to time series analysis using a UAV platform.

Highlights

Rivers and their adjacent riparian forests are dynamic environments where hydrological, geomorphological, and ecological processes interact [1,2]
The combination of high spatial resolution unmanned aerial vehicle (UAV) imagery and high temporal resolution satellite imagery facilitated the analysis of the structure of the Kuiseb riparian vegetation across longitudinal and temporal gradients, revealing an overall increase in fractional vegetation cover of 33% over the last three-and-a-half decades
Fractional cover was highest in the stretch where the river runs dry, highlighting the importance of this terminal alluvial zone in controlling the structure of the riparian forest

Summary

Introduction

Rivers and their adjacent riparian forests are dynamic environments where hydrological, geomorphological, and ecological processes interact [1,2]. Dryland riparian ecosystems are increasingly under pressure from anthropogenic disturbances [6,7,8,9], climate change [10,11,12,13], livestock grazing [2,14,15,16,17], and invasive species [18,19,20] Amongst other impacts, these pressures can alter the hydrologic regime [6,8], the key control on vegetation growth and distribution in the riparian zone [4,21]. Changing flood patterns can lead to a shift from perennial to annual communities [9], decreasing species richness [22], changes in plant functional type [23], and decreases in woody cover coupled with increases in herbaceous species [21] Such changes in forest composition can further alter the ecohydrological and geomorphological structure of the riparian landscape

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Discussion

Conclusion