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Article

Comparison of Long-Term Changes in Non-Linear Aggregated Drought Index Calibrated by MERRA–2 and NDII Soil Moisture Proxies

1
Department of Earth Sciences, University of Venda, Thohoyandou 0950, South Africa
2
Department of Geography and Environmental Studies, University of Zululand, KwaDlangezwa 3886, South Africa
*
Authors to whom correspondence should be addressed.
Academic Editor: Hossein Tabari
Water 2022, 14(1), 26; https://doi.org/10.3390/w14010026
Received: 9 November 2021 / Revised: 8 December 2021 / Accepted: 13 December 2021 / Published: 23 December 2021
This study aimed at evaluating Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA–2) and Normalized Difference Infrared Index (NDII) soil moisture proxies in calibrating a comprehensive Non-linear Aggregated Drought Index (NADI). Soil moisture plays a critical role in temperature variability and controlling the partitioning of water into evaporative fluxes as well as ensuring effective plant growth. Long-term variability and change in climatic variables such as precipitation, temperatures, and the possible acceleration of the water cycle increase the uncertainty in soil moisture variability. Streamflow, temperature, rainfall, reservoir storage, MERRA–2, and NDII soil moisture proxies’ data from 1986 to 2016 were used to formulate the NADI. The trend analysis was performed using the Mann Kendall, SQ-MK was used to determine the point of trend direction change while Theil-Sen trend estimator method was used to determine the magnitude of the detected trend. The seasonal correlation between the NADI-NDII and NADI-MERRA–2 was higher in spring and autumn with an R2 of 0.9 and 0.86, respectively. A positive trend was observed over the 30 years period of study, NADI-NDII trend magnitude was found to be 0.02 units per year while that of NADI-MERRA–2 was 0.01 units. Wavelet analysis showed an in-phase relationship with negligible lagging between the NDII and MERRA–2 calibrated NADI. Although a robust comparison is recommended between soil moisture proxies and observed soil moisture, the soil moisture proxies in this study were found to be useful in monitoring long-term changes in soil moisture. View Full-Text
Keywords: drought; drought indices; evaporation; soil moisture; SQ-MK; trends analysis; water resources; wavelet analysis drought; drought indices; evaporation; soil moisture; SQ-MK; trends analysis; water resources; wavelet analysis
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MDPI and ACS Style

Mathivha, F.; Mbatha, N. Comparison of Long-Term Changes in Non-Linear Aggregated Drought Index Calibrated by MERRA–2 and NDII Soil Moisture Proxies. Water 2022, 14, 26. https://doi.org/10.3390/w14010026

AMA Style

Mathivha F, Mbatha N. Comparison of Long-Term Changes in Non-Linear Aggregated Drought Index Calibrated by MERRA–2 and NDII Soil Moisture Proxies. Water. 2022; 14(1):26. https://doi.org/10.3390/w14010026

Chicago/Turabian Style

Mathivha, Fhumulani, and Nkanyiso Mbatha. 2022. "Comparison of Long-Term Changes in Non-Linear Aggregated Drought Index Calibrated by MERRA–2 and NDII Soil Moisture Proxies" Water 14, no. 1: 26. https://doi.org/10.3390/w14010026

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