A Novel Assessment of Hydrological Drought in the Central Plateau Basins of Iran Using the New Groundwater Storage Product

Abstract

Background and Objective: Groundwater drought, defined as the persistent deficit in groundwater storage, is a major subcategory of hydrological drought that directly affects drinking water, irrigation, and industry. The Central Plateau of Iran faces chronic water stress due to climate variability and increasing demand. This study evaluates groundwater drought conditions and the long-term trend of groundwater storage anomalies (GWSA) across the Central Plateau basins using the G3P satellite product, based on GRACE/GRACE-FO gravity data with a spatial resolution of 0.5° and monthly coverage from April 2002 to September 2023. Employing the new G3P product can reduce field monitoring costs and, instead of relying on limited observational data, provide a more comprehensive picture of groundwater status. The Central Plateau is facing a severe water crisis due to both climate change and declining available water resources on one hand, and population growth with increasing demand on the other. Considering the key role of groundwater resources in supplying drinking water, agriculture, and industry in this arid and semi-arid region, precise assessment of their changes is of critical importance.

Materials and Methods: Groundwater variations were analyzed using G3P satellite data, retrieved from the G3P Data Portal for the period 16 April 2002 to 16 September 2023. G3P data are derived from GRACE and GRACE-FO satellite gravity observations and provided monthly with a spatial resolution of 0.5° (~50 km). By integrating field and reanalysis datasets such as GLDAS, G3P estimates surface water storage components—including soil moisture, surface water, snow and ice, and vegetation—then calculates groundwater volume using the water balance equation. Python and Excel were used to process GWSA and the Groundwater Drought Index (GDI). In this study: (1) basin-averaged GWSA time series (in millimeters of water equivalent) and (2) the GDI, based on normalization of monthly anomalies relative to the reference climatology, were calculated to determine the onset, duration, and severity of groundwater drought.

Results and Discussion: The Central Plateau basins of Iran exhibited a significant negative trend in both GWSA and GDI during 2002–2023. The decline intensified from around 2011 and worsened after 2016. Spatially, the most pronounced decreases occurred in southern and central sub-basins such as Abarkooh–Sirjan, Tashk–Bakhtegan–Maharloo, and the Darreh Anjir desert, as well as in the provinces of Yazd, Kerman, Isfahan, and northern Fars. In contrast, northern sub-basins (Semnan, Tehran, and parts of Khorasan) showed milder negative trends. Seasonally, deficits were most notable from late winter through spring, aligning with precipitation patterns and peak withdrawal periods. The GDI confirmed recurrent and prolonged groundwater drought episodes in the past decade. Months of January, February, March, April, May, June, and December exhibited negative trends, with February (–66.81 mm), March (–65.78 mm), and January (–54.76 mm) recording the steepest declines. Among the basins, the Salt Lake watershed showed the lowest decline (–2.21 mm), while Abarkooh–Sirjan experienced the steepest decrease (–22.21 mm) during the study period.

Conclusion: Groundwater storage across the Central Plateau of Iran has declined continuously since 2002 and has intensified markedly since 2016. The absence of any sustained positive trend highlights a prolonged crisis of groundwater depletion, particularly in the southern and central basins. Routine use of satellite-based GWSA offers cost-effective, basin-scale monitoring to support early drought warning and strategic water resource planning in data-scarce environments. The findings demonstrate that all regions of the Plateau are experiencing persistent groundwater decline, with the most severe reductions in Yazd, Kerman, Isfahan, and northern Fars, while northern regions such as Semnan, Tehran, and parts of Khorasan also show decreasing trends, albeit less severe. The Abarkooh–Sirjan, Tashk–Bakhtegan–Maharloo, and Darreh Anjir basins are facing the steepest declines.

Keywords: Groundwater storage, drought, watershed, Central Plateau of Iran, G3P