Use of remote sensing indicators to assess effects of drought and human-induced land degradation on ecosystem health in Northeastern Brazil

Authors: MARIANO, D.A. - University Of Nebraska; SANTOS, C.A. - Federal University Of Campina Grande; WARDLOW, B. - University Of Nebraska; Anderson, Martha; SCHILTMEYER, A.V. - University Of Nebraska; TADESSE, T. - University Of Nebraska; SVOBODA, M. - University Of Nebraska

Submitted to: Remote Sensing of Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/27/2018
Publication Date: 8/1/2018
Citation: Mariano, D., Santos, C., Wardlow, B., Anderson, M.C., Schiltmeyer, A., Tadesse, T., Svoboda, M. 2018. Use of remote sensing indicators to assess effects of drought and human-induced land degradation on ecosystem health in Northeastern Brazil. Remote Sensing of Environment. 213:129-143. https://doi.org/10.1016/j.rse.2018.04.048.
DOI: https://doi.org/10.1016/j.rse.2018.04.048

Interpretive Summary: Land degradation due to both drought and human activities is a significant concern in the drylands of Northeastern Brazil, leading to soil erosion, reduced agricultural productivity, and in extreme cases, desertification. In addition, degradation may impact local evapotranspiration (ET) and rainfall rates, creating a positive feedback leading to intensification in the degradation cycle. This paper examines remote sensing indicators of land degradation and climate feedbacks on the regional water cycle. These remote sensing indicators include trends in satellite-derived leaf area index (LAI; biomass amount), evapotranspiration (ET; vegetation water use), and albedo (surface reflectance). Of these, LAI trends were most related to known areas of land degradation and therefore provide the best approach to identifying degraded areas regionally. The analyses also attempted to attribute land degradation as drought or human-induced (including urbanization, grazing intensification) by incorporating satellite-derived rainfall data, and found drought was the most significant driving factor on average over the region of study. Feedbacks of degradation on ET were identified through causality analyses, indicating that a positive cycle toward intensifying degradation is possible. The study raises attention regarding the roles of drought and land degradation in desertification risks faced in Northeastern Brazil.

Technical Abstract: Land degradation (LD) is one of the most catastrophic outcomes of long-lasting drought events and anthropogenic activities. Assessing climate and human-induced impacts on land can provide information for decision makers to mitigate associated effects. The Northeastern region of Brazil (NEB) is the most populous dryland on the planet, making it a highly vulnerable ecosystem especially when considering the lingering drought that started in 2012. The present work consisted of detecting trends in biomass [gross primary productivity (GPP)] and albedo anomalies as indicators of land degradation in NEB. Both GPP and albedo data were derived from MODIS (Moderate Resolution Imaging Spectroradiometer)/Terra sensor at 8-day temporal and 500m spatial resolutions. For precipitation z-scores, we relied on Climate Hazards Group InfraRed Precipitation with Station (CHIRPS)-v2 10- day temporal and 5km spatial resolution data. For detecting trends, we applied linear regressions on time series of MODIS GPP and albedo images. Trend analysis was performed for the periods ranging from 2002-2012 (no severe droughts) and 2002-2016 (including the last drought). The first analysis highlighted the human-induced LD whereas the last detected drought induced LD. About 4.5% of the area has undergone human-induced degradation whereas drought was responsible for 13%, although, not mutually exclusive. As reported in the literature and official data, grazing intensification was the primary driver for human-induced LD. GPP trends were more pronounced and had a stronger signal than albedo, therefore, is considered more efficient on mapping LD. Finally, the effects of LD on evapotranspiration anomalies [Evaporative Stress Index (ESI)] were assessed by impulse-response analysis as a way to link land surface feature changes to the hydrological cycle. Albedo had a slightly stronger impulse than GPP on evapotranspiration (ET) whereas precipitation played a minor role. These relations are very site-specific, and land surface features (biomass and albedo) showed to have a more substantial influence on ESI in severely degraded areas. Although, this can be a mirroring effect that, in turn, intensifies degradation itself. We conclude that, in fact, drought led to severe LD in NEB and the degradation cycle has a positive feedback derived from ET reduction resulting in an increased net moisture deficit. The study warns of the desertification risk that NEB is facing and the need for the authorities to take action to mitigate degradation and drought effects on both traditionally surveyed and new found degraded areas.