Decline in Iran’s River Flows

Decline in Iran’s River Flows Mohsen Maghrebi University of Tehran Roohollah Noori (  roohollah.noori@oulu. ) University of Oulu Farzaneh Darougheh Ferdowsi University of Mashhad Rahman Razmgir Islamic Azad University, Mashhad Branch Hossein Farnoush Ferdowsi University of Mashhad Hamid Taherpour Ferdowsi University of Mashhad Seyed Mohammad Reza Alavai Moghadam Islamic Azad University, Mashhad Branch Alireza Araghi Ferdowsi University of Mashhad Ali Torabi Haghighi University of Oulu Bjørn Kløve University of Oulu Research Article Keywords: Anthropogenic disturbance, River discharge, Climate change, Iran Posted Date: July 19th, 2021 DOI: https://doi.org/10.21203/rs.3.rs-701372/v1 License:   This work is licensed under a Creative Commons Attribution 4.0 International License. Read Full License 1 Decline in Iran’s River Flows 2 Mohsen Maghrebi1, Roohollah Noori2,*, Farzaneh Darougheh3, Rahman Razmgir4, Hossein 3 Farnoush3, Hamid Taherpour3, Seyed Mohammad Reza Alavai Moghadam4, Alireza Araghi3, 4 Ali Torabi Haghighi2, Bjørn Kløve2 5 1 6 Iran 7 2 8 University, 90014 Oulu, Finland 9 3 College of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran 10 4 Department of Civil Engineering, Mashhad Branch, Islamic Azad University, Mashhad, Iran School of Environment, College of Engineering, University of Tehran, 1417853111 Tehran, Water, Energy and Environmental Engineering Research Unit, Faculty of Technology, Oulu 11 12 *Corresponding authors: 13 14 Roohollah Noori ([email protected]), ORCID: https://orcid.org/0000-0002-7463-8563 1 Decline in Iran’s River Flows 15 16 Abstract 17 This study examined changes in Iran’s river flows by applying regression and analysis of 18 variance methods to long-term ground-truth data. Evaluations were performed for the country’s 19 data-rich rivers, covering almost 97% of all rivers and including more than 35 years of 20 measurements. The results showed that about 56% of Iran’s rivers have experienced a negative 21 trend in mean annual flow that is approximately 2.5 times greater than that reported for world’s 22 rivers, leading to a shift from perennial to intermittent for about 20% of rivers in Iran’s sub- 23 basins. This reflects surface freshwater shortages in Iran caused by natural and, more 24 importantly, anthropogenic disturbances. It may even indicate the development of new 25 hydrological regimes which can have significant implications for future surface water storage 26 in Iran. This research improves understanding of changes in Iran’s river flows and provides 27 beneficial information for sustainable water resources management in the country. 28 Keywords: Anthropogenic disturbance; River discharge; Climate change; Iran. 29 Introduction 30 Iran’s river flow regimes have experienced a number of extremes in spatiotemporal conditions, 31 such as unprecedented flood and drought events, in the past few decades1. For example, 32 destructive floods occurred during winter and spring 2019, causing a national emergency to be 33 announced in 40% of the country and resulting in 76 casualties. At the other extreme, a 50% 34 reduction in precipitation in 2017 created a severe hydrological drought that negatively affected 35 about 90% of Iran2. 36 In recent decades, changes in Iran’s river flows have been detected and have been 37 attributed to climate change3,4 and, in particular, to extensive changes in land-use patterns and 38 agricultural water use5-7. Water use patterns have changed as a result of large-scale dam 2 39 construction, with 647 dams currently in operation and 146 dams under construction8. 40 Construction of an additional 537 dams across the country is planned. Therefore, Iran’s river 41 flows are now strongly regulated, with the aim of enhancing regional economies and securing 42 national self-sufficiency in food production. 43 The severity and frequency of extreme flood and drought events are expected to 44 increase as a consequence of climate variability and water and land-use changes in Iran, but 45 the impacts are expected to be unequally distributed across the country9. Therefore, a good 46 understanding of both the patterns and changes in Iran’s river flow regimes is needed to 47 mitigate the severe pressures on Iran’s surface water resources and to help plan water 48 management strategies, particularly with respect to hydropower production and water supply. 49 Some recent studies have investigated potential river flow trends in individual sub- 50 basins in Iran4,10,11. However, generalisation of findings from these studies to national scale is 51 difficult, as the studies include regional differences and cover different periods. To our 52 knowledge, trends in river flow at national scale in Iran remain largely unclear, despite good 53 coverage of river flow at multiple monitoring stations. In order to address this knowledge gap, 54 in the present study trends in Iran’s river flows during recent decades were analysed. 55 Knowledge of river flow changes is imperative, since surface water supplies around five billion 56 cubic metres (km3) of water to around three million hectares of wetlands in Iran, meeting 44% 57 of the water needs in urban areas and the agricultural sector and recharging up to 10 km3 of 58 subsurface resources in Iran12. The analysis was based on monthly streamflow measurements 59 in all 30 sub-basins in Iran, obtained at 139 stream gauging stations that mostly have more than 60 35 years of records (Fig. 1). Spatiotemporal trend analyses were performed on monthly, 61 seasonal and annual time scales, to determine past streamflow variations across the country. 62 The overall aim of the study was to pictures out changes in Iran’s river flows for sustainable 63 water resources management in Iran. 3 k µ k k # ## # # k # ### k k k k ## ## k# k k# # k k k k # #k k k k kk k k k k k k k k k k k k k k k k k k k k k k k k k k kkkk kk kkkkk k kk k # k 20010 - 141634 141635 - 373416 # k 946652 - 3001184 k 3001185 - 8693706 k k k k k k k k # k # k k k k k # k k k k k k k k k # ## ### # ## # k k k k k k k k k k ## # # #k k# # # k k k k # ## # # # ## ## k k k k k k ## k k Persian Gulf Selected hydrometry station k k 280 k # k k # 420 k k Sub-basin 0 70 140 # k k Bareland 64 # k kk k k # k k k k # k k k k k Dense forest k k k k k k k k k k k Desert # k k k k k k k k k k k k k Agriculture # k k kkk k k k k 373417 - 946651 k k k k k k # # # k # # # # # k k # k # k k k # k k k k k k k k k k k k k k #k k kkk k k k k k kk k k k k k k k k k k k k k # k # # k ## ## k k # #k k # # # Population (2016) #k k k k k k k kkk k k k k k k k # ##k ## # # ##k k k k k # # #k # # kkkk# k k kkk # # k # k k k k kk k k k k # ## k k k # # k# ### ## # # # # # ## k k k # # # # k ## k ## k k k # k k k k # k k k Caspian Sea k k k k k # k Kilometers 560 k # k Sea of Oman 65 Figure 1: Location of the 139 selected hydrometric stations (orange triangles) in Iran, main 66 land-use and sub-basins with population distribution across Iran. 67 Results and Discussion 68 Decline in annual discharge. Figure 2 shows mean, maximum and minimum annual stream 69 flow, and indicates how their coefficient of variation (CV) varied across Iran (the greater the 70 diameter of a circle, the higher value of the index). The south-western rivers flowing into the 71 Persian Gulf and the western rivers flowing into the Caspian Sea were found to have the highest 72 annual discharge among all the rivers in Iran. These regions include most of Iran’s perennial 73 rivers. Likewise, the lowest annual river discharge, equal to or near zero, was observed in the 74 southern and eastern sub-basins, indicating the dominant location of intermittent rivers in the 75 country. These intermittent rivers play an important role in human life and ecosystem 76 sustainability by recharging aquifers and nourishing lakes, wetlands, marshes and dam 77 reservoirs in the arid regions13. The lowest CV was observed for annual river flows in the 78 Caspian Sea region, where a Mediterranean climate prevails. For example, stations 19051 and 4 79 18106 in this region had CV of 21% and 22%, respectively (Table S1). The highest values of 80 CV, representing high internal variability, were found in the east, north-east and central parts 81 of Iran, where the climate is classified as arid and semi-arid. Stations 41243 and 16079 in this 82 region had CV of 586% and 254%, respectively (Table S1). Caspian Sea Caspian Sea Maximum Average m3/sec 0.04 - 5.60 5.61 - 21.12 21.13 - 66.67 66.68 - 150.79 m3/sec 0.08 - 11.88 11.89 - 34.40 34. 41 - 96.63 96. 64 - 216.88 216.89 - 636.09 Persian Gulf 150.80 - 295.77 Sea of Oman Sea of Oman Caspian Sea Caspian Sea CV (%) Minimum m3/sec 20.9 4 - 38.49 0.00 - 1.96 1.97 - 5.84 5.85 - 11.95 11.96 - 29.35 29.36 - 128.40 38.5 0 - 53.64 53.6 5 - 69.31 69.3 2 - 91.32 83 Persian Gulf 91.3 3 - 586.35 Persian Gulf Sea of Oman Persian Gulf Sea of Oman 84 Figure 2: Mean, maximum and minimum annual stream flow, and coefficient of variation (CV) 85 at the selected hydrometric stations across Iran. Increasing circle diameter indicates higher 86 variability. 87 Figure 3 shows the trend obtained on analysing the stream flow data at annual scale. 88 The result revealed negative trends for annual river discharge at 78 of the stream gauging 89 stations investigated (56%), positive trends at 12 stations (9%) and null trends at 49 stations 90 (35%) (p–value <0.1). On global scale, around 22% of the world’s rivers are reported to be 5 91 showing significant decreasing trends in annual discharge and 9% are showing increasing 92 trends14. Thus, the number of rivers showing a decreasing trend in annual discharge is 2.5 times 93 greater in Iran than in the rest of the world14. Although both natural and anthropogenic driving 94 forces have led to the severe decline in Iran’s surface water resources15, anthropogenic forces 95 dominate16. According to the Deputy Director of Planning for Water and Water Resources at 96 the Iranian Ministry of Energy, Hedayat Fahmi, by 2018 about 20% of Iran’s perennial rivers, 97 such as the Zayandeh-Rud and Simineh-Rud rivers, had transformed into intermittent 98 (seasonal) rivers and most former seasonal rivers had dried up or become narrow streams. This 99 is a result of Iran’s unsustainable development plans, which aim to grow the economy, 100 infrastructure and the agriculture sector, regardless of the country’s renewable water and 101 environment-ecosystem resilience12,16. 102 In the present analysis, the greatest change in annual discharge was seen at station 103 21237 (+2.87 m3/s/yr) and the smallest change at station 21191 (-1.53 m3/s/yr) (Table S1). A 104 decreasing trend in annual discharge was observed in rivers located in the central part of Iran, 105 highlighting the overall decline in the streamflow availability in this region. Man-made impacts 106 of development, such as extensive inter/trans-basin water diversion policies, are the main 107 reason from the decline in river flows in central Iran12. There was also a decreasing trend in 108 annual discharge of rivers that are the main source of Iranian inland lakes (Lakes Urmia, 109 Zarivar, Parishan, Shadegan, Namak, Bakhtegan, and Maharlu), all of which are suffering from 110 a continuous decline in their water level9,17,18. Therefore, our findings confirm that Iranian 111 lakes/wetlands are drying up, which is an alarming finding for Iran’s water resources 112 authorities. The greatest negative change in annual discharge was observed in rivers located in 113 the north-west, near the Caspian Sea, and in the south-west, near the Persian Gulf, which 114 constitute the main stream flow into these water bodies (Fig. 3). This can add to environmental 115 problems in these regions, as river waters play an important role in balancing water salinity 6 116 and transporting organic matter. This eco-hydraulic condition will reduce the amount of land- 117 based nutrients in surface sediments of these water bodies and will affect the land-based 118 component of aquatic zones19. Large dams and reservoirs impounded along perennial rivers 119 have contributed dramatically to the negative change in annual discharge in these regions20. 120 Simultaneously, climate change impacts in terms of decreasing precipitation in western Iran21 121 have reduced annual discharge to the Caspian Sea and Persian Gulf4,22,23. # # # # # ## # # # " "" # " # " " # # # ## # # # # -0.04 - 0.00 # ## " # # -0.12 - -0.05 -0.25 - -0.13 " # # # # # # 85 170 " ## # # # # 0 # NT # " " " # # # 0.35 - 2.87 " " " # # # # # 122 # 0.05 - 0.34 # # # 0.01 - 0.02 0.03 - 0.04 # #" # Rate of annual discharge (m3/sec/yr) 0.00 # " " # # ## " " "" # µ " # # "" " # ## # # " # " " # "# "" "" ## " " " # " " # "## # # # # # # # Caspian Sea " # # ## # "" " # # # # # ## # # # " " -0.89 - -0.26 -1.53 - -0.90 Persian Gulf " Sub- basin 340 510 " " Sea of Oman 680 Kilometers 123 Figure 3: Spatial map of annual river discharge trends. Triangles show stations with a 124 statistically significant trend (p–value <0.1), with blue and red triangles indicating increasing 125 and decreasing rates, respectively, and triangle size indicating rate magnitude. Black rectangles 126 represent stations with no significant trend (p–value ≥0.1). 127 Most eastern and south-eastern trans-boundary rivers were also found to show a 128 negative trend in annual discharge (except for station 53013 – Table S1). In addition to climate 129 change impacts, harnessing of rivers in neighbouring countries (Afghanistan) for agriculture 130 and hydropower to enhance economic development is also influencing inflow from the trans- 7 131 boundary rivers to Iran, and this impact might even outstrip those of climate change in the 132 region24. The decline in annual discharge in the eastern and south-eastern trans-boundary rivers 133 has reduced crop yields25, increased inland migration26, lowered the water level in Lake 134 Hamun27 and, more importantly, exacerbated international disputes between Iran and 135 Afghanistan28. In north-east Iran, which is one of the country’s main agricultural hubs, a 136 negative trend in river discharge has increased groundwater abstraction for agricultural 137 development. This could be the reason for the high rate of groundwater loss in these areas 138 reported by Ahmadi et al.28. Future flow manipulation in the Harirud river in Afghanistan 139 would add to the negative trend in annual discharge in north-east Iran. In north-west Iran (Lake 140 Urmia and Aras sub-basins), all hydrometric stations investigated showed a negative or no 141 trend in annual river discharge. According to a study by Ashraf et al.15, the Lake Urmia sub- 142 basin has experienced the greatest rate of decline in water storage in Iran, an effect dominated 143 by anthropogenic activities. Therefore, impacts of human interventions and climate change are 144 contributing to severe negative trends in river discharge in these regions30,31. In the west of the 145 country, where the rivers mostly flow out of Iran (Fig. 4), a negative trend in river annual 146 discharge was observed mainly due to river flow regulation, as the region has the highest 147 concentration of dams in Iran. This can pose a threat to downstream terrestrial and aquatic 148 ecosystems and exacerbate the small-scale storms caused by drying up of small wetlands in 149 Iran and Iraq4,32. 150 Projected change and variability in climate indices using climate models indicate a 151 general decreasing trend in precipitation in Iran in the future9,15. Anthropogenic impacts of 152 development and policies to achieve national self-sufficiency in crop production to provide 153 food for Iran’s growing population by harnessing running waters would exacerbate the impacts 154 of climate change on Iran’s rivers5. Therefore, the increased pressures on Iran’s water resources 155 can be expected to continue or even accelerate in the future. Studies by Milly et al.33 and 8 156 Haddeland et al.34 found a decrease in projected runoff in the Middle East region, including 157 Iran. Therefore, a shift toward sustainable water and land management is required to mitigate 158 the negative effects of water shortages across Iran. Azerbaijan Turkey # Turkmenistan # # ## Caspian Sea #### # # # # ## # # ## # ### ## # # # ## # ## # # # # # # # # # ## # # # # # # # # ## ## # # # # ## # Iran # ## # Iraq # # # # # # # # # # # ## # ## ## # # # # # # ## # ## # # Kuwait # # # # # Saudi Arabia # µ # # Selected Hydrometric Station Sub-basin River & Lake # # # ### # # # # # # Afghanistan # # # # ## Persian Gulf # Bahrain Qatar Pakistan # ## ## # # # Sea of Oman 159 160 Figure 4: Iran’s river network with information about surface inflows or outflows in trans- 161 boundary rivers. 162 Decline in seasonal discharge. Figure 5 shows the results of trend analysis on river discharge 163 in Iran on a seasonal time scale. The results revealed that the seasonal average river flow change 164 in summer was -0.02±-0.12 m3/s/yr. The highest rate of seasonal discharge in summer was 165 observed at station 42009 (0.64 m3/s/yr) and the lowest at station 17041 (-0.32 m3/s/yr). 166 Negative rates of river discharge in summer were observed at 60 stations (43%), positive rates 167 at 10 stations (7%), and no significant change at 69 stations (50%). In autumn, positive trends 168 were seen at 17 stations (12.2%) and negative trends at 52 stations (37.4%). The maximum, 9 169 minimum and mean rate of river discharge in autumn was 2.2 (station 21237), -0.6 (station 170 17043) and 0.001±0.29 m3/s/yr, respectively. In winter, the calculations showed that the 171 maximum, minimum and mean rate of river discharge was 4.9 (station 21237), -0.2±0.9 and - 172 2.85 m3/s/yr (station 22023). In winter, 59 stations (42.4%) showed a significant decrease in 173 river discharge and 12 stations (8.6%) showed an increase. # # # # # # # # -0.60 - -0.18 # # # " "" "" # # " # # # ## # # # ## # # # # " # # # # # # # # ## # # # 0.07 - 4.00 -2.04 - -0.34 # # # ## # # # # 0.02 - 0.01 # 0.02 - 0.06 # # # -3.92 - -2.05 # # ## # # -0.03 - 0.00 -0.10 - -0.04 -0.33 - -0.11 # # # # # # # # " # # # # # " " Sea of Oman Spring " "" " " " " " " " " ## " "" # " # # # # # # # # # # # # # # ## # # Persian Gulf # # ## # # # #### # # # # # ## " " " """ " "" " " # # ## # # " ## " " " " " # " # # " "" # ## " "" Caspian Sea " """ " "" "# # " " "" ## # # " " # # # # # # # # # # # # -0.323 - -0.126 # # # # ## -0.125 - -0.044 # # #### # # # # # # # # #### " " " " " "" "" " # ##" " " Sea of Oman 0.37 - 2.25 ## # # -0.043 - -0.020 0.26 - 0.36 Sub-basin -0.61 " "" " " " # # # # ## # # " " " " " """" " " # " Persian Gulf 0.08 - 0.25 # # # # -0.17 - -0.10 0.03 - 0.07 # # # -0.09 - -0.04 "" " " " "" # # # -0.03 - 0.00 # # " # # # # ## # ## -0.019 - -0.008 # # # Sub-basin -0.007 - 0.000 # # # 0.38 - 0.65 # # # # 0.03 - 0.37 # # 174 0.02 # # NT # Winter " " Sea of Oman " " " # " """ # # ## # " # #" # " " " Caspian Sea " "# #" " " " " "#" # " "#" " # ## # # # -0.323 - -0.126 " " " " " " # ## # # # # ## # # -0.125 - -0.044 " # -0.043 - -0.020 " ### # # # ## -0.019 - -0.008 " " "" # # Sub-basin # # # 0.38 - 0.65 " "" " " " " #" Persian Gulf -0.007 - 0.000 # # # Summer 0.02 0.03 - 0.37 " " " " " "# " " " " # " " "" # " NT " " ## # # " Caspian Sea " " " "" " " # #"" #" " " "#" # " # "" "# " " # " " "" " " """ " #" " " # " " "" # " " #"" "# # " " #"# " " " Autumn " # 0.00 - 0.02 " NT """ " "# ## # ## # # # # " # " "" " " " #### # # # Caspian Sea " " """ "" " "" " # "" """ " " ## " " """ " # # ## # " "" Persian Gulf Sub-basin " NT " " " Sea of Oman 175 Figure 5: Spatial map of seasonal discharge trends in rivers in Iran. Triangles show stations 176 with a statistically significant trend (p–value < 0.1), with blue and red triangles indicating 177 increasing and decreasing rates, respectively, and triangle sizes indicating rate magnitude. 178 Black rectangles represent stations with no significant trend (p–value ≥ 0.1). 179 Maximum, minimum and average rates of river flow in winter season are 4 (station 180 21237), -3.92 (station 21191), and -0.33±0.92 m3/s/yr, respectively. During the spring season, 181 the negative and positive rates of river discharge are observed at 53.9% (75 stations) and 4.3% 10 182 (6 stations), respectively. The maximum decline in river’s discharge in this season took place 183 at the southwest and north of the country, where the climate change has had a profound effect 184 on snowmelt and river discharge capacity35. Maximum, minimum and mean rate of river flow 185 in spring was 4 (station 21237), -3.92 (station 21191), and -0.33±0.92 m3/s/yr, respectively. In 186 spring, negative rates of river discharge were observed at 75 stations (53.9%) and positive rates 187 at 6 stations (4.3%). The greatest decline in river discharge in spring was observed in the south- 188 west and north of the country, where climate change has had a profound effect on snowmelt 189 and river discharge capacity35. 190 Based on the results, the highest number of stations with a significant positive trend and 191 the lowest number with a significant negative trend occurred in autumn. This may be the result 192 of a positive trend in autumn rainfall in Iran during the past few decades36. More importantly, 193 water needs for agricultural crops, and thus exploitation of surface water resources, are low in 194 autumn. Increasing river discharge in autumn at regional/country scale has also been reported 195 by Lins and Slack37 and Birsan et al.38. The greatest negative trend in river discharge was seen 196 in spring. This is in line with previous findings for different regions of Iran, which show that 197 rivers with snowmelt sources are experiencing a decreasing trend in spring discharge4, 198 highlighting climate change impacts in terms of declining snowfall in the country39. 199 Decline in monthly discharge. Figure 6 shows the results of trend analysis of river discharge 200 on monthly scale. It was found that areas adjacent to the Caspian Sea had the largest number 201 of stations with no significant trend in river discharge in all months. This is consistent with 202 previous findings of a lower impact of climate change on water resources in this region40. River 203 flows at stations located in the Lake Urmia basin showed negative or no trends (except for 204 Lighvan station (station 31019) in April, May and September), as a result of dam construction 205 and excessive agriculture development in this area. A study by Hassanzadeh et al.30 found that 206 declining inflow contributed about 65% to the drop in lake volume and water level. Considering 11 207 the negligible share of groundwater feed to Lake Urmia (~3%)41, decreased streamflow would 208 dramatically threaten the lake’s ecosystem. In the south-east of Iran (e.g. station 53013), 209 monthly river flow trends were negative or null. Decreasing access to fresh surface water 210 resources has been identified as one of the main reasons for an increasing number of 211 unauthorised wells and inland migration in these areas27. The decrease in river discharge in 212 north-east Iran in the cropping season has increased utilisation of groundwater resources, 213 resulting in the greatest rate of drop in groundwater level in these months42,43. 214 The number of stations with positive, negative or null trends, together with the rate of 215 river discharge at these stations are given in Table S1. The average rate of river discharge was 216 negative in all months except December. The greatest rate of decline in average discharge was 217 seen in April and May. The highest number of stations with a positive (negative) significant 218 trend in river discharge was seen in September and the lowest in May. The highest number of 219 stations with a negative significant trend in river discharge was seen in June and the lowest in 220 December. The number of stations with no significant trend varied from 53 to 79 stations in 221 different months of the year. 12 # " """ " " #" # # # " "" # Oct #" " #" " " # # " " # # # # ## # # ## # # # # ## " " """ " " " # " " " " # ## # # # # < 0.49 # < 0 .19 # 0.50 - 1.42 # # > -0.18 # 0.20 - 0.76 > -0.16 # -0.60 - -0.19 # # # # # #### # # # # # # # " " " #" # # " # # # " """ " "" " " # " " " " " " " # #" " " " # "" " # # # ## # ## ### # # # # # " "" # " " "" "" " "# ""# " " "" " ## # ## # ## # # ## # " # ## # " " " " " " " " # ## # # # # ## # # " " "" " # ### # ### ## -0.86 - -0.52 ## # 0.46 - 4.56 > -0.51 " # # # " -2.80 - -0.99 # " "" " " " " " " # # # # # ## #### # < 0.45 # # ## # # #"" " #""# " " # ### # Nov 0.19 - 0.57 > -0.98 # # # # # # # " " " " "" " " " "" " # # "" " " # # # #" < 0.18 # # # # # # # # #### # # " " " " "#"# " # "" "" " " # "# " " ## " " " " " "" " " # " """ " " " " # # ## # # # # ## # # # # ## # " # " " "" " " # ## "" # ## # # # # # # # " " " # " "" " "" " " -3.52 - -1.97 " "" # # # " # " # "" " " ## 0.98 - 3.04 > -1.96 # " # # # # # " #"" " " # # 222 # " " " " " " " " " " "" " " " " "" " " # " " " # " ## "" # # # # ## # # " "" " " " "" " # < 0 .97 # # # # # # " # # ##"# """ #" " ## # "# # ## " # ## # # " # # # " Dec # ## " " " " " "" " ## # # # # # #### # # # # # -3.38 - -1.91 " "" "" " "" " " " Jan # " " " " " " " " "" " " " ### " " " #" " " # # # # # # ## ### ## > -1.90 # " " " " " " " " " " " " "" # "" " "" " "" "# # "" " # " #" " " " "# " ## # 1.15 - 6.4 7 Feb " # " " "" " " "" " "" " "#"" "" " # # # # # # ### # # # " #"" "" # #" # ## ## # # # # < 1.14 " # "" ## # " "# " "# # # # # # # ### ## # " "" "" " "" " " -5.63 - -2.92 # #### # # # # # # # " " " " " " " " " " "" # " " " " > -2.91 # # # # # # # " " " " " # # " " " " 0.62 - 6.34 # -4.82 - -2.2 0 " "" < 0 .61 # ### # # ### ## # > -2.19 # " " # " " " """" # # # # " "# " " " " "" " " " " " " " """ " " " # Mar " "" # " "" # " "" " " # # " "" " # 0.02 - 0.18 " " " " " "" " " #"# "" " " # # # # # ## # # # # # # # # # ## # # # < 0 .0 1 " ## # # " ### # # " " " " "" " " " # # # # # # " # " """ " "" "" " # # ## # # # " " " Apr "" " " " "" " " " " ## " " ## " " " " # # """""# " " "" "" " " " ## " " " " " # " # # ## # # # -2.33 - -1.03 # -0.59 - -0.15 " " " "" " > -0.14 ## # # # ## ## ## ### # # # # # ## # # # # # ## ## # # # # # ### # ### # # # #### # # # ## # # # #### # # # # #### # 0.11 - 0 .53 > -1.02 " 0.32 - 1.07 " "" "" # # # # ## # # # " " " < 0.31 # # # # # # # #### # May # # # # ## # # ### # #### # # # ### # " "" " "" " " " "" " " " " # # # # < 0 .1 0 # " "" " " " " # " " " " " "" " "" " "# " "" "" " " " # # -0.42 - -0.23 # " "" " # " "" " # # ### # # # # > -0 .01 # # # # # 0.12 - 0 .68 "" ## # # # # ## ##### # # " " ##" # < 0.11 " " " "" " "" " # ## #" # # # Jun "" "" " Jul # "" """ " # "" " "" " " "" " # " ## # " # " " " " " ##" " " " " " " # # # ## ## ### # # "" " """" " # "" " " " # # # " # # # ## # ## ### ## # # ## # # # ## -0.84- 0.85 " " #" " # " " >-0.83 # " " 0.17 - 0.66 " # " " #" #" #" # " # " "" " " "" " #" # # #""" Aug " ## " " # ## # # ## " " " # # # # " " " # " # " # ## ## ## ### # ## # # # ## < 0 .1 6 # ## # # # # # # # # # # # ## ## # # # # # " " # # ## # # # " # " # # ## ## # # # # # "" # ## " "" " # # ## # # # # # # ### # # # " # " " " " " " ## # # # # ## # # "" " " #" " "" " "" "" "" " "# " " " "" " " " " " " # " #" " " "" " " " " # ""# " # # # # ## # # # # " # " " ""# " "" "" " "" """ " " " """ # # ## # ## # " " "" """ " " #" " " # " " " " # "# Sep " " "" " "" " "" #" " " "# " " # ## # #### " " " " # #"" " "" -0.3 6 - -0.17 " " " 223 Figure 6: Spatial map of monthly river discharge trends. Triangles show stations with a 224 statistically significant trend (p–value <0.1), where blue and red triangles indicate increasing 225 and decreasing rates, respectively, and triangle size indicates rate magnitude. Black rectangles 226 represent stations with no significant trend (p–value ≥0.1). 227 Conclusions 228 Increasing demand for water to secure food and water for Iran’s rapidly growing population 229 and extreme climate conditions have placed severe pressures on the availability of surface 13 230 water resources in the country. This study investigated changes in river flow in Iran over time 231 and space. The results showed that rivers located in regions with a Mediterranean climate 232 (mainly the Caspian Sea basin) had the lowest variations in inter-annual discharge, while rivers 233 in regions with an arid climate (east and central parts) showed the highest variations. On an 234 annual time scale, around 56% of the 139 hydrometric stations studied showed a decreasing 235 trend in river discharge, with the proportion indicating a decline being 2.5 times the global 236 average (about 22%). Such declines in Iran’s river flows pose serious threats to the country’s 237 water and food security and its environmental sustainability. In addition, severe declines in 238 river flows and shifts from perennial to intermittent rivers in some sub-basins of Iran may 239 indicate the development of new hydrological regimes, with significant implications for future 240 surface water storage. Considering the general decreasing trend in future precipitation 241 projected for Iran by climate models and the country’s ambitious development plans, regardless 242 of renewable water resources, the pressures on Iran’s water resources are expected to continue 243 or even increase in the future. Therefore, prompt action should be taken to mitigate the impacts 244 of natural and man-made driving forces on the country’s water resources sector. 245 Methods 246 Study are and data. The study area covered all of Iran’s territory. Iran is located in the world’s 247 dry belt, with mean annual precipitation of around 250 mm, which is less than one-third of the 248 global average. The overall morphology of the country in western and northern parts consists 249 of numerous mountains, while eastern and central parts consist of low and uniform terrain. The 250 presence of the mountains has led to the formation of diverse climates, so that Iran experiences 251 both an arid climate (east region) and a Mediterranean climate (coast of the Caspian Sea at the 252 north) at the same time of year. These physical conditions have led to an unequal distribution 253 of rainfall in the country, whereby around 70% of Iran’s surface area receives only about 31% 254 of total rainfall (Fig. S1). Annual potential evaporation ranges from 500 mm in the north-west 14 255 to 3750 mm in southern basins of the country. Mean annual temperature varies between 0 °C 256 in the north and 28 °C in the south of the country. Mean annual rainfall varies from 50 mm in 257 the centre and east to 1800 mm in the northern regions of Iran (Fig. S2). Mean annual runoff 258 in Iran is about 400 km3, with 270 and 130 km3 of evapotranspiration and renewable water 259 capacity, respectively44. 260 During the last decades, surface water resource planning and management in Iran has 261 been based on political rather than hydrological units. In hydrological terms, the country 262 contains six primary water basins and 30 secondary basins or sub-basins (Fig. S3), equipped 263 with more than 1730 active gauging stations. Iran’s water resources monitoring network is 264 stronger than that in other countries in the Middle East. The first hydrometric stations in Iran 265 were established in the 1940s, in the vicinity of the capital city of Tehran and in the south-west 266 of the country. Currently, around 20% of all stations have a service life of more than 40 years 267 (Fig. S4). Approximately 22% of Iran’s border with neighbouring countries is made up of 26 268 major rivers, through which part of surface runoff enters or leaves the country (Fig. 4). 269 As mentioned, monthly river flow measurements were used in this study. Stream flow 270 data from 1731 stations were obtained from Iran Water Resources Management Company and 271 filtered based on the quantity and quality of measurements. Finally, 139 data-rich hydrometric 272 stations with proven data quality, located in 30 sub-basins across the country, were selected for 273 further analysis. Figure 1 shows the distribution of these 139 hydrometric stations in Iran, while 274 they are listed and their statistical characteristics are presented in Table S2. It is worth 275 mentioning that perennial rivers in Iran are mostly located in the north and west, while 276 intermittent rivers and streams are mostly located in central and eastern parts. 277 Trend analysis. The trend analysis methodology used in the present study is illustrated in Fig. 278 S5. Linear regression analysis followed by analysis of variance (ANOVA), as suggested by 279 Pinhas et al.45, was used to determine trends in river flow, rate of variation and its significance. 15 280 The methodology has the advantage of simplicity, while providing the necessary information 281 in terms of slope and variability, making it useful in practice. 282 To apply this methodology, a code was first assigned to each station and a number to 283 each sub-basin. The entire dataset was then categorised using the Pivot Table in Microsoft 284 Excel software, based on the assigned codes and sub-basin numbers. Linear regression and 285 ANOVA F-test were performed using the analysis Tool Pak in Microsoft Excel. The null 286 hypothesis in this approach is that all coefficients of each estimated regression are equal to 287 zero. Therefore, rejection of the null hypothesis means that the coefficients are not equal to 288 zero, i.e. the estimated regressions are meaningful. This happens when the estimated 289 significance level for the F-statistics < 0.1 (90% confidence level). 290 Thereafter, river flow time series at each hydrometric station were analysed at monthly, 291 seasonal and annual timescale. Finally, the Xlstat add-in solution was used to prepare 292 classification and regression trees of river discharge change across the sub-basins. Note that no 293 data reconstruction was performed in this study and all analyses were performed on the 294 statistical period of the hydrometric stations. 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Pinhas, M., Tzelgov, J., Ganor-Stern, D., 2012. Estimating linear effects in ANOVA 443 designs: The easy way. 444 https://doi.org/10.3758/s13428-011-0172-y Behavior 22 Research Methods, 44(3), 788-794. 445 Author contributions 446 Data Curation: M.M., R.N., F.D. and H.F.; Formal Analysis: M.M. and R.R.; Methodology: 447 M.M., R.N. and A.T.H.; Resources: R.N., A.T.H. and B.K.; Software: M.M., H.T. and 448 S.M.R.A.M.; Writing–Original Draft: M.M., R.N., and A.A.; Writing–Review and Editing: 449 R.N. A.A. and B.K. 450 Conflict of interest 451 The authors declare no conflicts of interest. 23 Supplementary Files This is a list of supplementary les associated with this preprint. Click to download. SciRepSM.pdf