Bjerre et al. (2026) Groundwater responses to decadal rainfall variability in semi-arid South Africa

Identification

• Journal: Journal of Hydrology
• Year: 2026
• Date: 2026-01-29
• Authors: Elisa Bjerre, Søren Jessen, Raphael Schneider, K. Vullholth, Matthys A. Dippenaar, Trine Enemark, Rena Meyer, Jason Hallowes, Torben O. Sonnenborg, Thokozani Kanyerere, Karsten Høgh Jensen
• DOI: 10.1016/j.jhydrol.2026.135022

Research Groups

• Institute of Geosciences and Natural Resource Management, University of Copenhagen, Copenhagen, Denmark
• Geological Survey of Denmark and Greenland (GEUS), Copenhagen, Denmark
• Water Cycle Innovation, Randers, Denmark
• Department of Geology, University of Pretoria, Pretoria, South Africa
• Institute for Biology and Environmental Sciences, Carl von Ossietzky Universität Oldenburg, Oldenburg, Germany
• Ekosource Insight, Johannesburg, South Africa
• Department of Earth Sciences, University of the Western Cape, Bellville, Cape Town, South Africa

Short Summary

This study investigated groundwater responses to climate variability in semi-arid South Africa using climate and groundwater indices, revealing that groundwater levels are strongly dependent on antecedent rainfall over 7-8 year accumulation periods, aligning with decadal climate cycles despite no significant trend in total annual rainfall.

Objective

• To document long-term trends and variability in rainfall, temperature, and groundwater levels in a semi-arid catchment in the Limpopo region, South Africa, based on in-situ observations.
• To investigate responses of groundwater levels to decadal rainfall variability.

Study Configuration

• Spatial Scale: Hout/Sand River catchment, Limpopo Province, South Africa (7722 km²).
• Temporal Scale: Analysis of long-term trends and variability from 1940 to 2022, with specific data types spanning: rainfall (1940–2021), temperature (1948–2022), and groundwater levels (1969–2022).

Methodology and Data

• Models used: Standardized Precipitation Index (SPI), Standardized Groundwater Index (SGI), Mann-Kendall test, Sen's slope, Pearson correlation, Kolmogorov-Smirnov (KS) test, Ebisuzaki test.
• Data sources:
  • Daily rainfall data from 13 rain gauges (South African Weather Service (SAWS), Department of Water and Sanitation (DWS)).
  • Daily minimum and maximum air temperatures from 2 climate stations (SAWS, DWS).
  • Groundwater data from 83 monitoring wells (National Groundwater Archive, DWS).
  • Land cover data derived from satellite imagery (Landsat 5, Landsat 8, Sentinel 2) (Department of Forestry, Fisheries, and the Environment (DFFE)).

Main Results

• Total annual rainfall showed no significant long-term trends, but rainfall patterns intensified, characterized by higher daily rainfall intensity, longer dry periods, and shorter wet periods.
• Temperature extremes at the Mara climate station increased significantly, with the annual hottest day (TXx) increasing by 0.37 °C per decade and the coldest day (TXn) by 0.27 °C per decade from 1948–2022.
• Groundwater levels exhibited substantial decadal-scale variability, with the strongest correlation between the Standardized Groundwater Index (SGI) and the Standardized Precipitation Index (SPI) occurring at 7- and 8-year accumulation periods (Pearson r = 0.80, empirical p-value = 0.0001).
• Groundwater anomalies displayed multi-year to decadal recovery and decline periods, with abrupt rises coinciding with flood years, indicating non-linear recharge and potentially low aquifer storage capacity.
• While irrigated area doubled between 1990 and 2014, decadal climate variability was identified as the primary driver of observed groundwater trends, potentially amplified by irrigation and other demands.

Contributions

• Advances the conceptual understanding of groundwater responses to large-scale climatic patterns, crucial for assessing future water availability under climate change.
• Addresses a significant knowledge gap by providing an observation-based study on climate oscillations and groundwater in Africa, a region previously underrepresented in global reviews.
• Offers a novel investigation of multi-decadal trends and hydro-climatic interactions using long-term in-situ observations in a data-scarce semi-arid region.
• Emphasizes the critical need for systematic and regular long-term groundwater monitoring and the consideration of decadal climate variability for effective water resource management and drought preparedness.

Funding

• Ministry of Foreign Affairs of Denmark through Danida
• Project: “Enhancing Sustainable Groundwater Use in South Africa (ESGUSA)”
• File no. 21-M08-KU

Citation

@article{Bjerre2026Groundwater,
  author = {Bjerre, Elisa and Jessen, Søren and Schneider, Raphael and Vullholth, K. and Dippenaar, Matthys A. and Enemark, Trine and Meyer, Rena and Hallowes, Jason and Sonnenborg, Torben O. and Kanyerere, Thokozani and Jensen, Karsten Høgh},
  title = {Groundwater responses to decadal rainfall variability in semi-arid South Africa},
  journal = {Journal of Hydrology},
  year = {2026},
  doi = {10.1016/j.jhydrol.2026.135022},
  url = {https://doi.org/10.1016/j.jhydrol.2026.135022}
}