**1. Introduction**

Water is an essential resource for life on Earth. In recent decades, because of growth in the population and in technologies, demands for water resources have been increasing. Due to groundwater's physical properties, it has a special role in human life, and it is one of the most important natural resources for nations, especially in a dry and semi-dry climate like that of Iran. Water shortages in Middle Eastern countries cause a variety of problems and lead to economic and consequently political instability, and there is a high frequency of conflicts due to disputes over groundwater [1–3]. The annual precipitation in Iran is 273 mm, which is less than one-third of the world's mean annual precipitation [4]. Because of the recent drought, the rate of precipitation has reached its lowest point (the minimum yearly precipitation) in more than 40 years, and the average surface run-off in this period is 42% less than the long-term average. In Iran, 72% of precipitation evaporates and transpires and only 22% of precipitation flows in as surface water source [5]. The temporal and spatial distribution of rainfall in Iran is not uniform. About 75% of the nation's precipitation falls on the southern coast of the Caspian Sea, and only 25% of the precipitation falls during plant growth season [4,6]. In addition, because of the relatively high temperature, about half of the annual precipitation evaporates, so there are few permanent streams in Iran. Furthermore, population growth and inappropriate spatial population distribution is currently causing the 20 most populous Iranian cities to experience a medium to extremely high overall water risk.

**Citation:** Safdari, Z.; Nahavandchi, H.; Joodaki, G. Estimation of Groundwater Depletion in Iran's Catchments Using Well Data. *Water* **2022**, *14*, 131. https://doi.org/ 10.3390/w14010131

Academic Editor: Alban Kuriqi

Received: 8 November 2021 Accepted: 31 December 2021 Published: 5 January 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Deep beneath the ground, groundwater is unseen and protected from evaporation and pollution, and there is easy access to pumped wells and access to groundwater any time and everywhere it is needed. All of these factors make people inclined to use more groundwater in Iran [7,8]. Iran's sources of groundwater include wells, springs and underground aqueducts known as qanats. Groundwater amounts to 60% of the country's total supply and is consumed almost entirely by the agricultural sector [9]. Iran is one of the world's largest consumers of groundwater [10], and the vast majority of the population lives in areas that are highly dependent on groundwater for drinking and irrigation. Continuing the business-as-usual approach in depleting aquifers will expose Iran to food and water risks as well as social and political security issues. On average, more than half of the design capacity of Iran's reservoirs was empty from 2003 to 2017 due to intensive water extraction in the agricultural sector [11]. Groundwater overdraft has contributed to different socioecological problems, including the drying up of wetlands, desertification, sand and dust storms, weakening water quality, frequent occurrences of floods, and climate migration [12–14]. It has also increased the desertification and salinization of land and has also degraded groundwater quality due to natural processes such as saltwater intrusion [15–18]. Land subsidence due to groundwater depletion is now a manmade hazard to vital infrastructure and residents on vulnerable plains. The increasing stress on rural livelihoods and increasing tensions among groundwater users worsen food and water security risks [19], and create issues related to the migration of rural populations to urban areas. In addition, rapidly growing demand for crops has put pressure on authorities to give permission to dig new wells. There are many illegal wells in aquifers in Iran, so there is no appropriate control over the withdrawals of water there. The number of wells has dramatically increased in the past decade in Iran. While the number of groundwater extraction points increased by 84.9% from 546,000 in 2002 to over a million in 2015.

All of these issues make groundwater management a high-priority matter in Iran, but groundwater monitoring has not been carried out well in some regions. Groundwater withdrawal statistics are sometimes outdated and measured using inconsistent methods [20,21]. The acquisition of accurate data represents a major challenge mainly due to the hidden nature of groundwater. The spatial and temporal variability of groundwater data ought to be good enough for proper water management studies. However, availability of the time series of parameters of Groundwater Storage (GWS) is usually limited and uncertain [22]. Collected well water-table data are being used to produce the time series of GWS. The distribution of observation wells is usually not uniform and there are regions with sparce data. Better water level monitoring and better assessment and forecasting of water resources would help government agencies allocate water more efficiently among competing needs [23].

Relatively good groundwater data records exist in Iran. The Iran Water Resources Management Company has collected data from a large number of observation wells for 40 years that are used in different hydrological studies. But these data are not well organized, and a huge part of this study was to collect all these data and put them into a database to finally allow the calculation of GWS changes from well data. Despite its importance, there have been few hydrological studies carried out on an entire water basin using GWS observational data in Iran. In this study, we used the water level of all 17,865 observation wells between 2002 and 2017 to estimate nationwide groundwater depletion in Iran.

The main objective of this study is to collect, assess, and evaluate all observation well data, producing a nationwide database, and allocating those data across water basins in Iran to constitute the time series of groundwater data for a final estimation of GWS changes using in-situ hydrological data.

This study provides a statistical analysis of the major groundwater characteristics using a rich ground-based dataset (2002 to 2017) to determine the groundwater depletion in all 32 sub-basins of Iran. The investigation of the temporal trend and spatial distribution of groundwater depletion provides valuable information for the effective management of groundwater storage across Iran and offers insights to other countries facing similar water security issues.

In the following sections, the distribution of observation wells in Iran's main water basins is explained and the analysis method is presented. The results are manifested in the spatial dependence of groundwater storage and the time series of GWS changes. The time series of GWS changes across Iran's main basins are analyzed and the trends of GWS changes across main basins and sub-basins are presented. The last section is devoted to concluding remarks and outlooks.
