Decline in Iran’s groundwater recharge

Domain: earth_science
Published: 2023-10-21
Editorial outcome: minor_revision

Abstract
Groundwater recharge feeds aquifers supplying fresh-water to a population over 80 million in Iran—a global hotspot for groundwater depletion. Using an extended database comprising abstractions from over one million groundwater wells, springs, and qanats, from 2002 to 2017, here we show a significant decline of around −3.8 mm/yr in the nationwide groundwater recharge. This decline is primarily attributed to unsustainable water and environmental resources management, exacerbated by decadal changes in climatic conditions. However, it is important to note that the former’s contribution outweighs the latter. Our results show the average annual amount of nationwide groundwater recharge (i.e., ~40 mm/yr) is more than the reported average annual runoff in Iran (i.e., ~32 mm/yr), suggesting the surface water is the main contributor to groundwater recharge. Such a decline in groundwater recharge could further exacerbate the already dire aquifer depletion situation in Iran, with devastating consequences for the country’s natural environment and socio-economic development.

Introduction
Securing groundwater is essential since it globally supplies approximately half of the irrigation water and most of the domestic water demands1. Given that 60% of the global population resides in regions that are expected to experience water scarcity and reduction in clean surface water availability by 20502, reliance on groundwater resources will inevitably further increase in the future. However, despite the importance of groundwater as one of the pillars of food and water security, extractions from aquifer systems exceed their natural recharge, leading to the rapid depletion of aquifers across the world3–5. Therefore, re-establishing the balance between the amount of groundwater withdrawal and recharge is essential to sustainably use groundwater resources, especially in (semi)arid regional scales such as Iran, which is known as a global hotspot for aquifers depletion6–8.

The imbalance between groundwater withdrawal and recharge (i.e., groundwater depletion) was first reported in some aquifers in Iran in the 1970s. However, unpublished national observations revealed groundwater depletion in some plains from as early as the 1950s. This coincided with the gradual replacement of Persian qanats, which were sustainable groundwater extraction systems and UNESCO World Cultural Heritage sites9, with (semi)deep wells. This shift was a consequence of the modernization in the agricultural sector8. Currently, Iran records the volume of groundwater withdrawal from over one million extraction points, including wells, springs, and qanats, through its national groundwater monitoring network that covers all 30 national hydrological basins (Fig. S1). However, obtaining ground-truth data on nationwide groundwater recharge remains challenging due to the site-specific nature of measurements and the complex interaction between various hydroclimatic and physiographic factors. Factors such as precipitation, evapotranspiration, topography, depth to the groundwater table, geology, and land use/cover (LUC) play a crucial role in this process10,11. In fact, spatial and temporal variability of groundwater recharge, as an index of groundwater availability12, is not accessible even locally in most parts of the world, including Iran. This scarcity of data indicates how rarely this variable is monitored. A possible way to obtain estimates of groundwater recharge over a large spatial scale is using global/regional hydrologic models. However, the recharge estimated by these models is not certain, especially in semi-arid regions such as Iran, where uncertainties in evapotranspiration and precipitation may lead to large errors in the results13,14, hindering the detection of regional imbalances between groundwater recharge and human withdrawals8.

Following the study conducted by Noori et al.8 on Iran’s groundwater depletion, which raised concerns about the lack of the country’s groundwater recharge data, we estimate its recharge using an extensive, nationwide database in all 30 hydrological basins of Iran from 2002 to 2017. In this study period, the country’s groundwater resources were largely depleted due to anthropogenic and natural drivers. Our results suggest a significant decline in the nationwide groundwater recharge, primarily attributed to unsustainable development practices. By elucidating the anthropogenic and natural drivers contributing to this decline in Iran’s groundwater recharge as a representative of a (semi)arid region in the world, we aim to foster a more comprehensive understanding of the regional and global impacts of these large-scale depletions. Our findings advocate for the urgent need for transformative changes in groundwater management and policies, especially in regions experiencing increasing hydroclimatic fluctuations.

Results
Estimated groundwater recharge
The estimated groundwater recharges for each aquifer, totaling 666 aquifers, aggregated in Iran’s hydrological basins from 2002 to 2017, are depicted in Fig. 1. The Method’s section details how these values were obtained. Our findings reveal a statistically significant decrease in nationwide groundwater recharge of about 35% (p value≤0.001) during the study period, which has resulted in a groundwater imbalance in many basins (Fig. 1). The annual change in groundwater recharge varies from –10.3 mm/yr in the Great Karoon basin to +1.9 mm/yr in the Anzali basin, averaging about –3.8 mm/yr across the country (Table 1). The average groundwater recharge across the country and for the study period is estimated at 39.6 mm/yr, which is slightly above the upper end of the range globally reported for (semi)arid areas (i.e., 0.2 to 35 mm/yr)15. However, although Iran is mainly under a (semi)arid climate, some areas are wet with annual mean precipitation exceeding 1000 mm/yr (Fig. S1). As such, the nationwide recharge rate is higher than the global range reported for (semi)arid areas.

The mean amounts of estimated groundwater recharge lie within –44% to +21% of the recharge reported previously for some individual aquifers in Iran16–20. Differences between the methods used in the previous studies and the estimation method developed for this study could explain inconsistencies in the results. The observed progressive decline in groundwater recharge could account for the reduction in the studied aquifers in 2022 compared to their previous levels. Moreover, previous studies have also expressed concerns regarding the diminished recharge in various aquifers across the country21–26. Overall, the existing literature, alongside the records of Iran’s water authorities (see Note S1), unanimously agree that groundwater resource overdrafts and declining recharge have not only pushed the country into a state of water bankruptcy but also posed a significant threat to the country’s socioeconomic development.

Groundwater recharge ratio
The countrywide groundwater recharge ratio (Fig. 2), defined as the fraction of recharge to precipitation, declined from 21% in 2006 to 14% in 2017, averaging about 17% during our study period, closely approximating the average groundwater recharge ratio at the global scale (i.e., 16%)3. The combination of declined recharge and over-exploitation of aquifers has resulted in approximately 422 out of 609 Iranian plains being in a critical/prohibited state. In this context, the term critical/prohibited signifies a condition where digging new wells is prohibited, except for drinking water use. Meanwhile, many of the free plains (i.e., where drilling new wells is legal) are located in desert or mountainous regions, where the aquifers have poor potential for groundwater usage. However, on a smaller geographical scale, i.e., in basins, the average recharge ratio during our study period varied from 3.3% (in Anzali basin) to 39.8% (in Bakhtegan basin) (Fig. 3). In general, the average recharge ratio is lower in the basins with the lowest groundwater depletion such as Anzali and Haraz basins8, which are located in the wet region in the north of Iran. On the contrary, the average recharge ratio is higher in the basins with the highest decrease in groundwater storage, such as Bakhtegan and Salt Lake basins27, which are located in relatively dry zones in the central and eastern parts of Iran, where the groundwater resources dominantly contribute to the supply of water for human and environment (up to 90%). Interestingly, both Anzali and Haraz basins, which had the lowest recharge ratio, were the only basins that demonstrated a decreasing trend in groundwater electrical conductivity during the study period8. This decline in electrical conductivity could be attributed to the reduced input of contaminants through infiltration into the groundwater28.

Main drivers of declined groundwater recharge
While Noori et al.8 and Ashraf et al.27 showed that the current extraction rates are unsustainable, other hydroclimatic and anthropogenic drivers (e.g., precipitation, evaporation, and changes in the LUC processes at the surface) governing the decline in aquifer recharge need to be further investigated. To better understand the impact of the natural and anthropogenic drivers on the decline in Iran’s groundwater recharge, the changes in the annual mean precipitation across the country and each basin were investigated during the 2000 to 2018 period (Fig. S2). Our results suggest a non-significant change in annual mean precipitation in Iran (p value≤0.05), in line with the results reported by Moshir Panahi et al.29 for the period from 1986 to 2016. No significant change was also detected in the annual mean precipitation in 26 out of the 30 basins investigated in this study (p value≤0.05). Similarly, no significant change was observed for the evapotranspiration reported in Iran, for the period from 1986 to 201629. Precipitation and (potential) evapotranspiration are the main natural drivers of groundwater recharge. Other natural-based controls on recharge include, among other factors, geology and topography30; however, these factors remained relatively stable in Iran over the observation period. This suggests that human interventions have dominantly impacted the decline in Iran’s groundwater recharge. It is noteworthy that insignificant changes in natural drivers do not necessarily mean that recharge generation mechanisms remain unchanged. For example, snow contributes more than rainfall to groundwater recharge31,32. The study conducted by Safarianzengir et al.33 highlighted a strong correlation between the decline in groundwater and a decreasing trend in snowfall in Iran (2000–2019). Recent observations also reported a decline in snowfall in different parts of Iran during the last decades34–36.

However, our results reveal the average amount of nationwide groundwater recharge was 39.6 mm/yr during the study period, which is more than the reported average annual runoff in Iran, i.e., about 32 mm/yr, for the same period (2001–2016)29, suggesting the national surface water resources are likely seeping into the groundwater basins. This finding is further supported by Saemian et al.37 and Maghrebi et al.38 studies, which reported that Iran’s surface water resources, as the main contributor of groundwater recharge, have largely declined during the last decades, mainly due to human interventions. Approximately 56% of Iranian rivers have undergone a decline in stream-flow, which is around 2.5 times higher than values reported for the world’s large rivers. Around 20% of the country’s permanent rivers have transformed into seasonal rivers, and former seasonal rivers have dried up and/or become narrow streams, leading to the development of new river and fluvial regimes38. Anthropogenically dwindling renewable water has also resulted in shrinkage of almost all of the national lakes, wetlands, marshes, and ponds8,39. This, in turn, is likely to have resulted in a reduced aquifer recharge from surface water bodies. A decline in irrigation return flows that can be attributed to the technological progresses of agricultural practices and improvements in the efficiency of irrigation water-use during the last two decades, as suggested by Noori et al.8, could also contribute to the decline in the country’s groundwater recharge—a concept known as pendulum swing40. Maghrebi et al.41 proposed that one of the key factors contributing to the failure of government plans for aquifers restoration through modernization of the irrigation systems was the decline in groundwater recharge, primarily caused by reductions in irrigation return flows. This reduction in recharge led to a significant increase in the nationwide groundwater deficit, escalating from 106 km3 to more than 140 km3 during a 12-year period. Intensive and progressive land subsidence across the country during the last two decades has also contributed to reduced soil permeability due to compaction, thereby impacting the capacity for groundwater recharge42–45. In support of an effect of land subsidence, Iran is ranked among the countries with highest rate of land subsidence globally25. As a result, aquifers’ capacities in Iran are declining due to countrywide land subsidence. In this scenario, the soil compaction reduces its permeability and hence the aquifers’ ability to be recharged, leading to a decline in both the rate and the total volume of recharge. Another factor influencing groundwater recharge is the changes in LUC, as several regions in Iran have experienced extensive deforestation, desertification, and rapid urbanization46–51.

Implications of reduced groundwater recharge
Reduction in groundwater recharge combined with the nonrenewable groundwater discharge from the country’s aquifers8,27,52 further contributes to decline in groundwater storage and shrinking groundwater table. The current declining trend in groundwater recharge and the reducing trend in groundwater table reported by Noori et al.8 and Ashraf et al.27 persist through this century under status of shrinking renewable water and the quo water and environmental resources management in Iran. It has resulted in a gradual exacerbation of Iran’s water and food security and makes the country’s landscapes prone to a wider and more severe range of already experienced disasters, such as desertification, dust storms, landslides, land subsidence, sinkholes, droughts, floods, and fires. Additionally, it leads to a decrease in soil fertility, biodiversity, and an increase in greenhouse gas emission. From a regional perspective, such a decline in groundwater recharge has devastating consequences for the country’s exports of agricultural products to the neighboring countries, compromising regional food security, given that Iran is one of the major food producers in the Persian Gulf region.

Artificial recharge is undoubtedly a promising engineering solution to recover depleted groundwater resources globally. Iran is also making efforts to complete a national plan that aims to artificially recharge groundwater up to about 1 km3. However, it should be noted that this amount is considerably less than the deficit in the country’s groundwater resources (see Note S1). Iran’s water resource management experiences highlight that engineering solutions alone cannot prevent the alarming depletion of groundwater resources for the country41, particularly when other factors such as countrywide land subsidence can hinder the effectiveness of recharge efforts. Our primary suggestion for facilitating the restoration of the depleted aquifers is prioritizing and reducing the water demand instead of relying on nonrenewable groundwater use. This recommendation requires adequate water governance backed by efficient conservation policies that are not currently implemented in Iran. The poor governance of hydro-environmental resources, characterized by unsustainable LUC planning, inequitable allocation rules, institutional ineffectiveness, and poor economic conditions, makes it challenging to rehabilitate groundwater resources through reduction in abstraction and implementation of efficient artificial recharge strategies. To address this concern, the existing top-down governance structure approach should be replaced with a cooperative and bottom-up approach that considers the interests of local stakeholders, particularly farmers, as the main consumers of groundwater resources.

Methods
Study area and data
Iran’s groundwater monitoring network has been gradually established since the early 1970s (Fig. 4 and Note S2). The network covers 492 out of 609 plains (Fig. S3), including 666 aquifers (Fig. S4), and contains 12,293 active piezometers (Fig. S4) that measure the groundwater level at monthly intervals. The total area of aquifers is around 272,000 km2 and the monitoring network covered more than 96% of this area (i.e., 261,000 km2) by the end of 2017. The remaining area (i.e., ~11,000 km2), includes 133 relatively small and less important aquifers distributed across the country5.

Although the groundwater levels were measured in active piezometers from the early 1960s, the comprehensive database of Iran’s groundwater resources was established from the water year of 2001–2002, when ~78% of the aquifer areas became covered by the groundwater monitoring network53. However, this database does not include groundwater recharge. To address this knowledge gap, we utilized the national groundwater database to calculate the groundwater recharge across Iran in each of the 30 basins during 2002 to 2017 period. This time interval was selected as our study period due to the data availability and adequate coverage of the country’s aquifers using the groundwater monitoring network (Note S2).

Groundwater recharge calculation
Groundwater recharge includes any penetrated water into the saturated zone54. We used the water budget method, i.e., Eq. (1)15, to estimate groundwater recharge in each aquifer across Iran.

Recharge = Q off + \Delta S (1)

where, Q off equals egressed water from the aquifer (positive flux out of aquifer), and \Delta S is the groundwater storage change (negative when storage decreases) (see, Fig. S5).

In this study, the representative hydrograph was determined for each aquifer. Then the annual change in groundwater storage was estimated by considering the aquifer area, specific yield, and the annual average change in the groundwater level. Given the large number of aquifers investigated in this study (i.e., 666), we aggregated the calculated recharge to obtain values for each hydrological basin and for the entire country. Details about the aggregated data are given in Note S3 and also supplemented in https://zenodo.org/record/8382150.

Egressed water from the aquifers (i.e., Q off) includes all anthropogenic groundwater withdrawals and natural groundwater losses. Anthropogenic groundwater withdrawals are fully measured based on sampling campaigns and field inventories in over a million wells and qanats on approximately a five-year cycle during wet and dry periods in Iran. The same measurements as the 5-year field campaigns are done twice a year during wet and dry periods at a small but representative set of selected extraction points in different aquifers. Natural groundwater losses can take place through different routes such as natural springs, evapotranspiration, and outgoing lateral fluxes from the aquifers. Fortunately, the national groundwater database includes the annual groundwater discharge through natural springs, which is measured as frequently as the anthropogenic groundwater withdrawals and thus also based on sampling campaigns and field inventories. Evaporation through the saturated zone is usually negligible when the depth to the groundwater table is large (>3 m depth). We supposed that the groundwater loss through evaporation, including transpiration, is negligible because the groundwater depth was more than 3 m in almost all the aquifers8. Groundwater losses through aquifer discharge to surface waters are also negligible due to the severe decline in the countrywide groundwater table during the last five decades (>20 m)55. This hypothesis gains further support when data show both a widespread decline in Iran’s river flows38 and drying out the national wetlands and inland lakes8,56. The natural groundwater losses through outgoing lateral groundwater fluxes are not considered due to the lack of data, which introduces uncertainty to the recharge estimate for individual aquifers. However, for the nationwide recharge estimate, these losses can be reasonably ignored by disregarding cross-border groundwater fluxes. It should be noted that incoming lateral fluxes from the adjacent aquifers are represented in the recharge itself. Another issue is that surface water basins are determined by topography, and they do not necessarily match with regional aquifers, for which the territory is identified by underground geology. As a result, depending on how aquifers and surface drainage basins are situated in a given region, there might be additional fluxes, both incoming and outgoing to a given aquifer, which represent how the aquifer interacts with adjacent drainage basins. Lack of data in this regard limited our ability to consider such detailed information in our study. However, only 6 out of 666 studied aquifers were shared in two adjacent drainage basins. Therefore, we expected that this issue would have a minimal impact on our results.

In summary, the simplifications implemented in this study may introduce uncertainties in the results. To address this issue, we conducted an uncertainty analysis that accounts for the impacts of such simplifications in our study.

Data analysis
Groundwater data processing, including aggregation, division, multiplication, normalization, and statistical analysis, for 666 national aquifers during the study period (2002–2017), was performed using PivotTable in Microsoft Excel environment, ArcGIS, and MATLAB. All figures were drawn in ArcGIS and MATLAB environments, Microsoft Office software, and Grapher program.

For a description of the change in groundwater recharge and precipitation, the magnitude of temporal trends was calculated by Sen’s slope estimator method57. A Mann–Kendall test58,59 was also applied to differentiate between significant and insignificant trends in both groundwater recharge and precipitation. These statistical analyses were done by MAKESENS 1.0 software, introduced by the Finnish Meteorological Institute60.

Data uncertainty
The results obtained in this study heavily rely on the data quality of the national groundwater database. Factors including poor information on aquifer type (e.g., confined or unconfined aquifer types), specific yield, and hypotheses made can potentially introduce errors/uncertainties in the calculated groundwater recharge for Iran and in each of the 30 basins. Detailed information on the data and the corresponding sources of errors/uncertainties is given by Noori et al.8.

An uncertainty analysis was conducted by determining the lower and upper limits of the 95% confidence interval for magnitude of the trends in groundwater recharge calculated using Sen’s slope estimator53 (see the Methods section for details). Table 1 shows the results of the uncertainty analysis for the calculated groundwater recharge across 30 basins and the countrywide values. By considering a significant level of 0.05 (i.e., \alpha = 0.05), the lower and upper limits of the confidence interval show the acceptable deviation from the magnitude of the trends for groundwater recharge across different basins and for the entire country.

Data availability
The raw data used in this study are publicly available via Data Archive of the Iran Water Resources Management Company (IWRMC) at https://stu.wrm.ir/register.asp. Alternatively, the processed data presented in this paper are supplemented in https://zenodo.org/record/8382150.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review of “Decline in Iran’s Groundwater Recharge” by Noori et al.\nThis is an interesting study that shows how not only the groundwater levels have been declining in\nIran’s aquifer, but also that groundwater recharge has been dwindling. This is fact would provide a\ndouble-jeopardy for Iran’s water resources and the economy and wellbeing that depends on it.\nI think the study, given that is based on a wealth of data, and Iran being an important depletion\nhotspot, is worth being published in Nature Communications after some consideration.\n1. The way the declining recharge is coupled to groundwater development as framed as a vicious\ncycle is flawed. The reduced discharge is likely separate from this, as the authors show: it likely\ncomes from declined streamflow infiltration (concentrated recharge), reduced return flows (which\nmay for instance be caused by more efficient irrigation technology) and increased potential\nevapotranspiration (climate change). This should be made more clear in the abstract while the\nterm vicious cycle avoided later on since it suggests a positive feedback between groundwater\ndecline and reduced recharge, while one expect a negative feedback in fact: lower groundwater\ntables reduce soil wetness and therefore enhance groundwater recharge.\n2. The causes behind declined recharge should be further explored. 1) How is streamflow changing\nover time (there must be measurements), what are the causes and are these declining streams\nalso the ones that are infiltrating with respect to groundwater levels? 2) Have irrigation\ntechnologies changed over the period, i.e. become more efficient? 3) What happened to\ntemperature and therefore potential evapotranspiration (atmospheric demand) over time? This\nwould shed light on which of these causes is most important and where.\n3. The data underlying the methods are quite limitedly displayed in the paper. I understand the\nraw data are available under request, but the aggregated results for each aquifer could be\nprovided in the SI. Thus, for each aquifer it would be good to show figures with in each figure the\ntime series of Q_pumping, Q_discharge_from_springs, delta_S and finally the resulting Recharge\nwith a regression line fitted through it (based on the equation in the Methods). Also, these time\nseries data should be made available through some repository. Finally, the coordinates of wells\nand springs and their characteristics (including average groundwater level and Sen’s slope) could\nbe presented in a large table in the Supplementary. This would make the paper and methods as\nFAIR as possible.\n4. There is a discussion on uncertainties, but no uncertainty analysis is presented. It must be\npossible using a simple uncertainty analysis (assuming statistical independence) based on\nuncertainties in aquifer average Q_pumping, Q_discharge_from_springs, delta_S to obtain\nuncertainty estimates of Recharge. Or, if trends as estimated the standard deviation of the Sen’s\nslope reported.", "reviewer_2": "This manuscript evaluates the decline in groundwater recharge in the Iranian basin from 2002 to\n2017 by use of the Iran groundwater monitoring network. Overall, the authors provided novel and\nimportant results for the public, scientific community, and policymakers. The manuscript is written\nwell and it is in good shape, and I recommend acceptance of this paper in Nature Communications,\nafter addressing the concerns listed below.\n1. In the estimation of groundwater recharge, various sources of uncertainty accumulate such as\nsurface water and groundwater connections, evapotranspiration, and local geology and land use.\nThe authors addressed surface water and evapotranspiration in the method section. I would like to\nask the authors' justifications about local geology and land use in groundwater recharge\nestimation.\n2. The estimation of groundwater recharge, could be varied based on the climate of the study\narea. For example, the groundwater recharge rate has more important in arid and semi-arid\nregions due to limited precipitation and their susceptibility to drought. So, it is recommended to\ndistinguish the results for the estimation of groundwater recharge in Iran’s basin based on the\ndifferent climates in Iran.\n3. Based on a previous study conducted by Ashraf et al. 2021, excessive groundwater withdrawal\nin Iran has leading in decreases soil stability in Iran basins, would you please justify how this\nchange can affect groundwater recharge in the study area?\n4. What are the authors' suggestions to policymakers and water managers in Iran to take\nmeasures in advance to improve groundwater recharge in Iran?\nReference: Ashraf, S., Nazemi, A. & AghaKouchak, A. Anthropogenic drought dominates\ngroundwater depletion in Iran. Sci Rep 11, 9135 (2021). https://doi.org/10.1038/s41598-021-\n88522-y", "reviewer_3": "\"Decline in Iran’s Groundwater Recharge\" tries to provide a new understanding about the massive\ngroundwater depletion in Iran, this time from the lens of declining recharge. The issue of regional\ngroundwater depletion in the MENA region in general and and Iran in particular is a known crisis\nsince the GRACE data have become available. As moved forward, GRACE findings have been\nvalidated through in-situ information. Restating and reframing this issue from the new perspective\nof recharge that can result in a hydrological explanation of this phenomenon, therefore, are both\nrelevant and timely and can provide a better insight about the nature of groundwater depletion in\nIran and globally.\nI have a couple of points for authors' considerations and response for the next round of review:\n1) Equation 1 (Recharge = ± ∆S) presents the core conceptual/mathematical framework with\nwhich the recharge is calculated across Iran's basins based on the changes in aquifer storage and\n𝑄𝑄𝑄𝑄𝑄𝑄𝑄𝑄\noutgoing fluxes. I have a couple of concerns here. First, I am not able to understand the reason for\n± sign for changes in the storage. If Qoff is an outgoing flux and recharge is an incoming flux then\nwe should have only + sign for changes in the storage. Please clarify. In addition, second, it seems\nthat we have a more severe issue as there might be some other terms in the aquifer balance\nequation that are not seen here. Most importantly, the groundwater/surface water interaction and\nthe incoming and outgoing fluxes from one aquifer to adjacent aquifers are not represented in this\nequation. Authors mentioned in the text that the Qoff consider both human and natural outfluxes\nfrom the aquifers, but what about incoming fluxes beyond basin recharge, for instance as return\nflow from irrigated lands? I can understand that this may be represented in the recharge itself, but\nthe problem is still there. The key issue is that surface water basins are determined by topography\nand they do not necessarily match with regional aquifers, for which the territory is identified by\nunderground geology. As a result depending on how aquifers and surface drainage basins are\nsituated in a given region, there might be additional fluxes, both incoming and outgoing to a given\naquifer, which represent how the aquifer is interacting with adjacent aquifers and/or drainage\nbasins. It should be clarified how these terms are seen here, and if they are not seen what the\nimplications of this major simplifications would be.\n2) Authors noted to a number of works that have used in-situ data to estimate groundwater\ndepletion in Iran. There are even more studies that use satellite estimates and indirectly estimated\nthe recharge. Is the estimates of recharge coming from this work confirming previous estimates of\nrecharge? If the estimates are different, it should be clarified to what extent, in which temporal\nand spatial scales, and more importantly why. I believe the value of this work will be much more if\nsomehow the estimates made in here are positioned with respect to previous studies.\n3) Without more information, I would be a bit sceptic about the conclusion made that the conclusion made that the decline\nin the recharge is solely due to unsustainable management of water resources. There is no doubt\nthat unsustainable management of water resources have a significant role in here, but it does not\nmean that if the course of unsustainable actions are reversed today, then the natural recharge\nwould be replenished. Numerous reports on Iran's land subsidence point to the fact that aquifers'\ncapacities in Iran (not only their storages) are declining too. In that case, due to the soil\ncompaction, infiltrability and accordingly the rate and the total volume of recharge declines too.\nTherefore, even if water management decisions are changed, still the recharge cannot be\nrecovered, because past mismanagements affected the natural capacity of recharge today (and\nperhaps forever). Another issue is that recharge is not only affected by the decision related to\nwater resource management per say. Activities related to land use and broader environmental\nmanagement, e.g. urbanization, deforestation, etc., can also affect the rate of recharge\ntremendously. I found the argument of the authors rather unconvincing that vegetation and land\ncover are rather unchanged during the study period, as Iran are among nations with fastest rates\nof land cover changes due to deforestation, desertification and aggressive urbanization. As a\nresult, the effect of vegetation, and land use/management should not be neglected. Also even if\nthe amount of P and E have insignificant changes, it does not necessary means that\nrunoff/recharge generation mechanisms remain unchanged. For instance form of P (snow vs. rain)\ncan be important for recharge. The issue of vegetation and changes in the infiltrability, which are\nalso noted above, are two other evidence showing that insignificant changes in P and E are not\nstrong evidences for insignificant changes in natural mechanisms that generate\nrecharge."}, "author_response": "RESPONSE LETTER TO COMMENTS ON “Decline in Iran’s Groundwater Recharge”\nMANUSCRIPT NUMBER: NCOMMS-23-07744.R1\nResponse to Comments of Reviewer #1\nGeneral Comment\n• This is an interesting study that shows how not only the groundwater levels have been\ndeclining in Iran’s aquifer, but also that groundwater recharge has been dwindling. This is\nfact would provide a double-jeopardy for Iran’s water resources and the economy and\nwellbeing that depends on it. I think the study, given that is based on a wealth of data, and\nIran being an important depletion hotspot, is worth being published in Nature\nCommunications after some consideration.\n• Response\nWe would like to thank Reviewer #1 for their positive evaluation of our work and their\nvaluable feedback. We have carefully considered your feedback and revised our manuscript\naccordingly to address all your comments.\nComment #1\n• The way the declining recharge is coupled to groundwater development as framed as a\nvicious cycle is flawed. The reduced discharge is likely separate from this, as the authors\nshow: it likely comes from declined streamflow infiltration (concentrated recharge),\nreduced return flows (which may for instance be caused by more efficient irrigation\ntechnology) and increased potential evapotranspiration (climate change). This should be\nmade more clear in the abstract while the term vicious cycle avoided later on since it\nsuggests a positive feedback between groundwater decline and reduced recharge, while one\nexpect a negative feedback in fact: lower groundwater tables reduce soil wetness and\ntherefore enhance groundwater recharge.\n• Response\nThank you for your comment.\nIn general, we agree that it may exist negative feedback between groundwater decline\nand reduced recharge: lower groundwater tables reduce soil moisture and therefore\nenhance groundwater recharge. This feedback is known to be strong in relatively wet\nenvironments (e.g., most of Europe). However, most of our study area is in semi-arid,\narid and hyper arid environments. In such environments, this feedback is not very\nstrong. For example, groundwater level in many wells in the south of Fars, Kerman,\nEsfahan, Semnan, Hamedan, Razavi Khorasan, and Tehran provinces have dropped\ndown from around 10s to 100s of meters during the last two decades. Now, some\ndrinking wells in Fars province are even more than 500 m deep. This changes are very\nsignificant, but they do not influence the moisture level at the surface. Hence, this\nspecific feedback is very week.\nFurthermore, reduction in soil moisture does not necessarily enhance groundwater\nrecharge. If there is available water for recharge, lower moisture levels might increase\nrecharge. However, that is not the case in our study area. Simply, there is no water left\nfor recharge. Long-term monitoring data shows that, approximately 56% of the Iranian\nrivers have undergone a decline in streamflow, with the decline being around 2.5 times\nhigher than that reported for the world’s large rivers. Around 20% of the country’s\npermanent rivers have transformed to seasonal rivers and former seasonal rivers have\ndried up and/or become narrow streams. Anthropogenically dwindling renewable\nwater has also resulted in shrinkage of almost all national lakes, wetlands, marshes,\nand ponds, with some of these water bodies passed the point of no return and\nundergoing complete annihilation. In addition, groundwater depletion has resulted in\nintensive and a progressive land subsidence across Iran during the past two decades.\nIran is ranked among the countries with highest rate of land subsidence globally. As\nsuch, aquifers’ capacities in Iran (not only their storages) are also declining, primarily\ndue to the countrywide land subsidence. Due to soil compaction, the infiltrability\ndecreases, which subsequently leads to a decline in the rate and total volume of\nrecharge (Lines 148-199 and Text S1).\nComment #2\n• The causes behind declined recharge should be further explored. 1) How is streamflow\nchanging over time (there must be measurements), what are the causes and are these\ndeclining streams also the ones that are infiltrating with respect to groundwater levels? 2)\nHave irrigation technologies changed over the period, i.e. become more efficient? 3)\nWhat happened to temperature and therefore potential evapotranspiration (atmospheric\ndemand) over time? This would shed light on which of these causes is most important\nand where.\n• Response\nThank you for your comment.\nWe have now made a revised and enhanced discussion on the underlying cause and\nprocesses behind the declined recharge (Lines 148-199).\n1) Our revised manuscript provides additional discussion on the countrywide\ndeclining streamflow, and surface water resources, and elaborated on the main\nunderlying causes of this in Iran by referring to the recent studies conducted by\nSaemian et al. (2022) and Maghrebi et al. (2023) (Lines 172-183).\n2) Yes. To relieve Iran’s water crisis, the government invested about US$1.5 billion\nin modernizing the country’s irrigation systems, aiming to restore groundwater\nresources. At the start of the program (in 2011), Iran’s groundwater storage had fallen\nby about 106 billion cubic meters since 1965. But the government investment and\nmodernization of irrigation systems did not result in improving this deficit. By 2022,\nfield measurements indicated that groundwater deficit had increased to about 140\nbillion cubic meters. However, the increased competition for the consumption of the\nwater saved by the program resulted in intensified agricultural development and even\nhigher water demand. The failure of the program can be attributed to the development\nof bare lands, reductions in groundwater recharging by returning flows from irrigation,\nand changes in cultivation patterns across the country. The manuscript has now\nrevised to reflect these issues in our discussion (Lines 183-191).\n3) You are correct about the possible effects of climatic conditions on the recharge.\nThe manuscript has elaborated on this issue in depth, and we discussed the changes in\nevapotranspiration and even in the form of precipitation in Iran by referring to the\nrecent conducted studies (Lines 148-171).\nComment #3\n• The data underlying the methods are quite limitedly displayed in the paper. I understand\nthe raw data are available under request, but the aggregated results for each aquifer could\nbe provided in the SI. Thus, for each aquifer it would be good to show figures with in each\nfigure the time series of Q_pumping, Q_discharge_from_springs, delta_S and finally the\nresulting Recharge with a regression line fitted through it (based on the equation in the\nMethods). Also, these time series data should be made available through some repository.\nFinally, the coordinates of wells and springs and their characteristics (including average\ngroundwater level and Sen’s slope) could be presented in a large table in the\nSupplementary. This would make the paper and methods as FAIR as possible.\n• Response\nThe raw data used in this study are publicly available via Data Archive of the Iran\nWater Resources Management Company (IWRMC) at https://stu.wrm.ir/register.asp.\nAlternatively, the data presented in this paper are now supplemented in\nhttps://zenodo.org/record/8179231\nUnfortunately, we did not have access to the coordinates of springs, Qanats, and wells.\nThus, we could not present spatial distribution of these extraction points in Iran (more\nthan one million points).\nComment #4\n• There is a discussion on uncertainties, but no uncertainty analysis is presented. It must be\npossible using a simple uncertainty analysis (assuming statistical independence) based on\nuncertainties in aquifer average Q_pumping, Q_discharge_from_springs, delta_S to obtain\nuncertainty estimates of Recharge. Or, if trends as estimated the standard deviation of the\nSen’s slope reported.\n• Response\nThank you for your constructive comment.\nWe have conducted an uncertainty analysis and introduced this to the manuscript\n(Lines 234-250 and Table 1).\nResponse to Comments of Reviewer #2\nGeneral Comment:\n• This manuscript evaluates the decline in groundwater recharge in the Iranian basin from\n2002 to 2017 by use of the Iran groundwater monitoring network. Overall, the authors\nprovided novel and important results for the public, scientific community, and\npolicymakers. The manuscript is written well and it is in good shape, and I recommend\nacceptance of this paper in Nature Communications, after addressing the concerns listed\nbelow.\n• Response\nWe would like to thank Reviewer #2 for their positive evaluation of our work and the\nconstructive feedback that helped us to shape our paper well. We have carefully considered\nyour helpful feedback and revised our manuscript accordingly.\nComment #1\n• In the estimation of groundwater recharge, various sources of uncertainty accumulate such\nas surface water and groundwater connections, evapotranspiration, and local geology and\nland use. The authors addressed surface water and evapotranspiration in the method section.\nI would like to ask the authors' justifications about local geology and land use in\ngroundwater recharge estimation.\n• Response\nThank you for your comment.\nLocal geology can be considered as a rather stable control over the observation period\n(Lines 162-165). However, land use/cover was subject to continual changes during\nthe study period. We added new discussion in the revised manuscript to elaborate on\nthe influences from land use/cover (Lines 197-199). In addition, we added further\ndiscussion about the other significant drivers of declined recharge, relevant to the case\nof Iran, e.g. land subsidence (Lines 191-197) and changes in the form of precipitation,\ni.e., declined snowfall (Lines 166-171).\nComment #2\n• The estimation of groundwater recharge, could be varied based on the climate of the study\narea. For example, the groundwater recharge rate has more important in arid and semi-arid\nregions due to limited precipitation and their susceptibility to drought. So, it is\nrecommended to distinguish the results for the estimation of groundwater recharge in Iran’s\nbasin based on the different climates in Iran.\n• Response\nThank you for your comment.\nAlthough our method proposed for estimation of recharge is applicable for all climate\nconditions, the results presented in this study, in agreement with the previous studies,\nshow a spatial variation based on the aridity of specific regions. The average\ngroundwater recharge across the country and for the entire study period (2002-2017)\nwas estimated at 39.6 mm/yr, which is just above the upper end of the range reported\nfor (semi)arid regions in the world (i.e., 0.2 to 35 mm/yr) by Scanlon et al. (2002).\nHowever, although Iran is mainly covered by (semi)arid lands, some areas are wet\nwith annual mean precipitation exceeding 1000 mm/yr. Therefore, the nationwide\nrecharge rate is slightly higher than the range reported for (semi)arid regions in the\nglobe (Lines 98-104). In addition, we have now added further discussion to the revised\nmanuscript, to elaborate on and distinguishing the difference between the estimated\nrecharge rates in dry and wet regions of Iran (Lines 133-138).\nComment #3\n• Based on a previous study conducted by Ashraf et al. 2021, excessive groundwater\nwithdrawal in Iran has leading in decreases soil stability in Iran basins, would you please\njustify how this change can affect groundwater recharge in the study area?\n• Response\nThank you for bringing this point to our attention.\nWe appreciate the suggested useful paper, which can certainly help us to further justify\nand sense check our results. Soil stability, sinkholes, landslides, and land subsidence\ncan affect groundwater recharge. We have now added a full discussion about the\nchange in the soil form (mainly land subsidence) and its impacts on the declined\ngroundwater recharge in Iran (Lines 191-199, 220-223, Text S1).\nComment #4\n• What are the authors' suggestions to policymakers and water managers in Iran to take\nmeasures in advance to improve groundwater recharge in Iran?\n• Response\nWe have now added a new paragraph to the manuscript, which suggest possible\nmeasures to policymakers for enhancing groundwater recharge (Lines 217-233).\nResponse to Comments of Reviewer #3\nGeneral Comment\n• “Decline in Iran’s Groundwater Recharge” tries to provide a new understanding about the\nmassive groundwater depletion in Iran, this time from the lens of declining recharge. The\nissue of regional groundwater depletion in the MENA region in general and Iran in\nparticular is a known crisis since the GRACE data have become available. As moved\nforward, GRACE findings have been validated through in-situ information. Restating and\nreframing this issue from the new perspective of recharge that can result in a hydrological\nexplanation of this phenomenon, therefore, are both relevant and timely and can provide a\nbetter insight about the nature of groundwater depletion in Iran and globally. I have a couple\nof points for authors' considerations and response for the next round of review:\n• Response\nWe would like to thank Reviewer #3 for their positive evaluation of our work and their\nconstructive feedback that helped us to better shape our paper. We have carefully\nconsidered your helpful feedback and revised our manuscript accordingly.\nComment #1\n• Equation 1 (Recharge = (cid:1843)(cid:1867)(cid:1858)(cid:1858) ± ∆S) presents the core conceptual/mathematical framework\nwith which the recharge is calculated across Iran’s basins based on the changes in aquifer\nstorage and outgoing fluxes. I have a couple of concerns here. First, I am not able to\nunderstand the reason for ± sign for changes in the storage. If Qoff is an outgoing flux and\nrecharge is an incoming flux then we should have only + sign for changes in the storage.\nPlease clarify. In addition, second, it seems that we have a more severe issue as there might\nbe some other terms in the aquifer balance equation that are not seen here. Most\nimportantly, the groundwater/surface water interaction and the incoming and outgoing\nfluxes from one aquifer to adjacent aquifers are not represented in this equation. Authors\nmentioned in the text that the Qoff consider both human and natural outfluxes from the\naquifers, but what about incoming fluxes beyond basin recharge, for instance as return\nflow from irrigated lands? I can understand that this may be represented in the recharge itself,\nbut the problem is still there. The key issue is that surface water basins are determined by\ntopography and they do not necessarily match with regional aquifers, for which the territory\nis identified by underground geology. As a result depending on how aquifers and surface\ndrainage basins are situated in a given region, there might be additional fluxes, both\nincoming and outgoing to a given aquifer, which represent how the aquifer is interacting\nwith adjacent aquifers and/or drainage basins. It should be clarified how these terms are\nseen here, and if they are not seen what the implications of this major simplifications would\nbe.\n• Response\nThank you for your comment.\nWe agree with the point that you raised and have revised Equation (1) accordingly\n(Line 274).\nAs you correctly mentioned, the groundwater/surface water interaction, the incoming\nfluxes from one aquifer to adjacent aquifers, and the incoming fluxes beyond basin\nrecharge (e.g., return flow from irrigated lands) are be represented in the recharge\nitself. We also agree with you regarding the natural difference between the\nhydrological basins and aquifers. Unfortunately, lack of data made it impossible to\nconsider such detailed information in our study. However, it is worth noting that only\n6 out of 667 studied aquifers were shared between two adjacent basins. Therefore, this\nissue have a minimal impact on the results presented in this study. We have now\nclarified this issue in the text of the revised manuscript. In addition, we carried out an\nuncertainty analysis that would somewhat cover such simplifications in our study\n(Lines 234-250, 279-282, 289-290, 299-313, Table 1, Text S3, and Figure S5).\nComment #2\n• Authors noted to a number of works that have used in-situ data to estimate groundwater\ndepletion in Iran. There are even more studies that use satellite estimates and indirectly\nestimated the recharge. Is the estimates of recharge coming from this work confirming\nprevious estimates of recharge? If the estimates are different, it should be clarified to what\nextent, in which temporal and spatial scales, and more importantly why. I believe the value\nof this work will be much more if somehow the estimates made in here are positioned with\nrespect to previous studies.\n• Response\nThank you for your comment.\nWe have compared our findings with the results of relevant studies conducted in some\nindividual aquifers in Iran. In addition, we discussed the underlying reasons for the\npossible inconsistences that may exist between our results and those reported by\nprevious studies (Lines 105-119 and Text S1).\nComment #3\n• Without more information, I would be a bit sceptic about the conclusion made that the\ndecline in the recharge is solely due to unsustainable management of water resources. There\nis no doubt that unsustainable management of water resources have a significant role in\nhere, but it does not mean that if the course of unsustainable actions are reversed today,\nthen the natural recharge would be replenished. Numerous reports on Iran's land subsidence\npoint to the fact that aquifers' capacities in Iran (not only their storages) are declining too.\nIn that case, due to the soil compaction, infiltrability and accordingly the rate and the total\nvolume of recharge declines too. Therefore, even if water management decisions are\nchanged, still the recharge cannot be recovered, because past mismanagements affected the\nnatural capacity of recharge today (and perhaps forever). Another issue is that recharge is\nnot only affected by the decision related to water resource management per say. Activities\nrelated to land use and broader environmental management, e.g. urbanization,\ndeforestation, etc., can also affect the rate of recharge tremendously. I found the argument\nof the authors rather unconvincing that vegetation and land cover are rather unchanged\nduring the study period, as Iran are among nations with fastest rates of land cover changes\ndue to deforestation, desertification and aggressive urbanization. As a result, the effect of\nvegetation, and land use/management should not be neglected. Also, even if the amount of\nP and E have insignificant changes, it does not necessary means that runoff/recharge\ngeneration mechanisms remain unchanged. For instance, form of P (snow vs. rain)\ncan be important for recharge. The issue of vegetation and changes in the infiltrability, which are\nalso noted above, are two other evidence showing that insignificant changes in P and E are\nnot strong evidences for insignificant changes in natural mechanisms that generate\nrecharge.\n• Response\nThank you for your constructive comment.\nWe agree with your comment. Our statements could reflect the sense that the decline\nin the recharge is solely due to unsustainable management of water resources. Given\nyour insightful suggestions, we have now made a full discussion on the results and\nconcluded that “This decline is primarily attributed to unsustainable water and\nenvironmental resources management, exacerbated by decadal changes in climatic\nconditions. However, it is important to note that the former’s contribution outweighs\nthe latter” (Lines 28-31). We have now revise the manuscript to avoid this ambiguity\nfor the readers in different parts of the manuscript (Lines 28-31, 148-199, 220-223,\nText S1).\nWe agree that land uses/covers were changing during the study period. In addition,\nyou are correct about the impact of land subsidence on the decline of aquifers’\ncapacities in Iran. We have now revised the manuscript to address these issues in the\nmanuscript (Lines 191-199, 220-223, Text S1).\nAs you correctly mentioned, the form of precipitation can affect the groundwater\nrecharge. We have now discussed about this issue and enhanced our discussion with\nsome relevant studies that highlighted the positive relationship between the declined\nsnowfall and reduced groundwater recharge over Iran (Lines 166-171)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have given it their best to answer the questions I had. They also performed additional\nanalyses to provide the main causes of groundwater recharge decline, as well a providing\nadditional uncertainty analyses on the slope estimates.\nThe data are now available through zendo, which is important. It is a pity that the coordinates of\nsprings and quanats are not availabel, but then I urge the authors to alos provide teh shapefiles of\nthe aquifer boundaries and the names of the aquiifers the time series belong to,\nIn their description of the reasons behind reduced recharge they also mention more efficient\nirrigation technology leading to more agriculktural developement because of the additional water\nbecoming available. This is known as the pendulum effect and they could add a refernce to that\neffect if they agree.\nOtherwhise, I think this paper is ready to be accepted.", "reviewer_2": "The authors made a great effort to improve the manuscript during the revision.", "reviewer_3": "Thanks for the revision. The revised paper provides another line of evidence for how unsustainable\nresource management can cause drastic consequences on the Earth System process. Particularly\nfor the case of Iran, this work can be insightful given the depth and gravity of nation-wide water\ninsecurity. Kudos to authors!"}, "author_response": "RESPONSE LETTER TO COMMENTS ON “Decline in Iran’s Groundwater Recharge”\nMANUSCRIPT NUMBER: NCOMMS-23-07744.R2\nResponse to Comments of Reviewer #1\nGeneral Comment\n• The authors have given it their best to answer the questions I had. They also performed\nadditional analyses to provide the main causes of groundwater recharge decline, as well a\nproviding additional uncertainty analyses on the slope estimates. The data are now available\nthrough zendo, which is important. It is a pity that the coordinates of springs and quanats\nare not available, but then I urge the authors to also provide the shapefiles of the aquifer\nboundaries and the names of the aquifers the time series belong to.\nIn their description of the reasons behind reduced recharge, they also mention more\nefficient irrigation technology leading to more agricultural development because of the\nadditional water becoming available. This is known as the pendulum effect and they could\nadd a reference to that effect if they agree.\nOtherwise, I think this paper is ready to be accepted.\n• Response\nWe would like to thank Reviewer #1 for their positive evaluation of our work and their\nvaluable feedback.\nWe have now added the shapefile of the aquifer boundaries and their representative code\nto the reposited data in https://zenodo.org/record/8382150. In addition, we added shapefiles\nof the basins and primary basins to better support the readers in regeneration of our\npresented results in the manuscript.\nWe agree with the comment on the notion of pendulum swing, and we added a relevant\nreference:\nVan Emmerik, T.H.M., Li, Z., Sivapalan, M., Pande, S., Kandasamy, J., Savenije, H.H.G.,\nChanan, A. and Vigneswaran, S., 2014. Socio-hydrologic modeling to understand and\nmediate the competition for water between agriculture development and environmental\nhealth: Murrumbidgee River basin, Australia. Hydrology and Earth System Sciences,\n18(10), pp.4239-4259.\nResponse to Comments of Reviewer #2\nGeneral Comment:\n• The authors made a great effort to improve the manuscript during the revision.\n• Response\nWe would like to thank Reviewer #2 for the positive evaluation of our work and the\nconstructive feedback that helped us to shape our paper well.\nResponse to Comments of Reviewer #3\nGeneral Comment\n• Thanks for the revision. The revised paper provides another line of evidence for how\nunsustainable resource management can cause drastic consequences on the Earth System\nprocess. Particularly for the case of Iran, this work can be insightful given the depth and\ngravity of nation-wide water insecurity. Kudos to authors!\n• Response\nWe would like to thank Reviewer #3 for the positive evaluation of our work and the\nconstructive feedback that helped us to better shape our paper."}]

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