Sustainable use of water resources in a Wadi system facing climate change impacts and growing groundwater demand

Understanding current and possible future alterations of water resources under climate change and increased water withdrawal allows for better water and environmental management decisions in arid regions. This study aims at analyzing the impact of groundwater withdrawals and climate change on groundwater sustainability and hydrologic regime alterations in a Wadi system in central Iran. A hydrologic model is used to assess streamflow and groundwater recharge of the Halilrood 10 Basin on a daily time step under different scenarios over a model setup period (1979-2009) and for two future scenario periods (near future: 2030–2059 and far future: 2070-2099). The Indicators of Hydrologic Alteration (IHA) with a set of 32 parameters are used in conjunction with the Range of Variability Approach (RVA) to evaluate hydrologic regime change in the river. The results show that groundwater recharge is expected to decrease, and is not able to fulfil the increasing water demand in the far future scenario. The Halilrood River will undergo low and moderate flow alteration under both stressors 15 during the near future as RVA alteration is classified as “high” for only three indicators, while in the far future, 11 indicators lie in “high” range. Absolute changes in hydrologic indicators are stronger when both climate change and withdrawals are considered in the far future simulations, since 27 indicators show significant changes and RVA show high and moderate level of changes for 18 indicators. Considering the evaluated RVA changes, future impacts on the freshwater ecosystems in the Halilrood Basin will be severe. The developed approach can be transferred to other Wadi regions for a spatially20 distributed assessment of water resources sustainability.


Introduction
Water resources are important in arid regions and any alteration caused by natural or anthropogenic activities might have strong environmental and socio-economic impacts. This poses a serious threat to the sustainable development of water resources in different sectors (Oki andKanae, 2006 andPanahi et al., 2020). Hence, sustainable management of water 25 resources is vital especially in arid regions with limited water availability (Wu et al., 2013;Davijani et al., 2016;Yu et al., 2019).
Sustainable use of water resources should be jointly assessed with regard to surface water and groundwater. Groundwater is not only a valuable source of high-quality freshwater and plays a central role in sustaining water supplies and rural https://doi.org/10.5194/hess-2020-599 Preprint. Discussion started: 13 January 2021 c Author(s) 2021. CC BY 4.0 License.
Where NWUSj and NWUSi are the number of water use systems in the year j and i, respectively; Pj and Pi is population in the year j and i, respectively. The number of springs as a natural WUS is assumed to remain constant in the future. The annual average water withdrawal per WUS recorded for the model setup period is assumed to remain constant in the future and is used to linearly extrapolate the required groundwater withdrawal for each sub-basin for the future number of WUS (NWUSj) for 2045 and 2085 (Table 2). 130 (ii): The minimum and maximum amount of water required per person per day in Iran is about 0.135 and 0.300 m 3 , respectively (ISC, 2017(ISC, -2018. According to these numbers and the estimated population growth (Table 1), maximum and minimum water consumption in near and far future are estimated (Table 3).

Scenarios
To disentangle the impacts of climate change and population growth and its combined effects on future aquifer condition and 135 hydrologic regime, five scenarios were developed (Table 4). "NO-WUS" scenario is included, to assess the sole impact of climate change on the hydrologic regime under pristine conditions. It therefore represents a scenario where all anthropogenic extractions have ceased. "Constant-WUS" scenario is defined to investigate the impact of climate change on hydrologic regime and groundwater sustainability in the future simulations in comparison to the current condition by keeping the number of WUSs unaltered. The impacts of both climate change and WUSs on groundwater sustainability and hydrologic 140 regime are assessed under "Projected-WUS" scenario. To precisely indicate the impact of the sole water demand by the population on groundwater sustainability in near and far future, the maximum and minimum amount of water required per person is computed and considered under "Minand Max-Consumption" scenarios. These two scenarios are considered only on entire basin scale due to limited availability of information regarding population growth on smaller scales (e.g., villages).
Minimum and maximum water consumption is included in the Constant-and Projected-WUS scenarios. 145

Groundwater sustainability
Groundwater sustainability is assessed on two different spatial scales: on the sub-basin and on the entire basin scale.

Sub-basin scale
Groundwater sustainability (GWS) on the sub-basin scale is defined as the ratio of groundwater withdrawal (GWW) to groundwater recharge (GWR). 150 To provide an appropriate estimate and range of the future aquifer condition on the sub-basin scale, groundwater withdrawal for the model setup period and two future periods is estimated for two scenarios: Projected-WUS and Constant-WUS.

Entire basin scale
On the entire basin scale (entire Halilrood Basin), groundwater sustainability (GWS) is assessed by comparing the total groundwater recharge (GWR) over the entire basin to (i) the total projected groundwater withdrawal (GWW) from the WUSs under Projected-WUS scenario, (ii) the minimum, and (iii) the maximum water consumptions (Min-and Max-WC) estimated for the growing population under Max-and Min-Consumption scenarios.

Indicators of Hydrologic Alteration (IHA)
Changes in the hydrologic regime of the Halilrood River that are caused by climate change and groundwater withdrawal are not only a challenge for the water sector (e.g., small-scale farming), but also decrease groundwater levels and threaten the 160 Jazmorian wetland ecosystem by reducing its water availability. The hydrologic alteration is analyzed under following scenarios: No-WUS, Constant-WUS, and Projected-WUS.
Numerous hydrologic indicators have been developed to describe different components of the flow regime. A set of 32 hydrologic indicators were used to assess changes in the hydrologic flow regime (Richter et al., 1996). The indicators are categorized into five groups; Group1: Magnitude of monthly water conditions, Group2: Magnitude of annual extreme 165 discharge events with different durations, Group3: Timing of annual extreme water conditions, Group4: Frequency and duration of high and low pulses, and Group5: Rate and frequency of water condition changes ( Table 5) Where DAi is the degree of hydrologic alteration of the ith IHA; Roi and Rei are the number of observed and expected repetitions in the scenario period for the ith IHA falling within the RVA target range. Rei is defined as: Where is the proportion of a single indicator's values falling within the RVA target range in the near and far future, i.e. = 0.5 is the suggested RVA target range between the 25th and 75th percentile values. Rt is the total number of values for each indicator in the near and far future (30 years period), i.e. Rt = 30 (Richter et al., 1997;Zhang et al., 2019).
To evaluate the magnitude of change for each indicator, Richter et al., (1998) divided DAi (absolute value) into three classes: 0-±33% represents no or low alteration (L), ±33%-±67% represents moderate alteration (M), and ±67%-±100% represents high alteration (H). The direction of change is shown by positive RVA, where the indicator becomes more stable within the RVA targets and negative RVA, where the indicator is moving towards an upper or lower alternative state.

Groundwater sustainability
Groundwater sustainability assessment is evaluated on the sub-basin and entire basin scale. 190

Sub-basin scale
The SWAT model of the Halilrood Basin is divided into 285 sub-basins, however, WUSs are located only in 73 sub-basins corresponding to almost 33% (around 2385 km 2 ) of the total area of the Halilrood Basin. 31 of all 73 sub-basins with WUSs represent a sustainable state (GWR>GWW) in the model setup period, however, in 42 sub-basins (17% of the total area) the amount of extracted water from groundwater is higher than GWR. The rate of GWW to GWR is greater than 2 in 22 sub-195 basins and 5 in 8 sub-basins.
The impact of climate change on groundwater recharge is assessed in the future periods for Constant-WUS scenario ( Figure   2b and d). In the near future (Figure 2b), the number of sub-basins with a sustainable state (GWR>GWW) decreases from 31(model setup period) to 26, while the unsustainable subbasins (GWW<GWR) covering an area of 1211 km 2 (model setup period) increases to 1419 km 2 (20% of the total area). In the far future ( Figure 2d), 25% of the entire basin (55 sub-basins) 200 reach an unsustainable state, where GWW/GWR ratio is higher than 2 in 24 sub-basins and among these above 5 in 9 subbasins.
As shown in Figure 2c

Discussion
The spatio-temporal variations of ratio of groundwater use to groundwater recharge in the Halilrood Basin is compromising groundwater sustainability in the near and far future. These challenges are expected to be more severe when both climate 265 change and population growth are considered in our scenarios. In addition, groundwater sustainability on the sub-basin scale for the Projected-WUS scenario as compared to Constant-WUS shows that the increases in groundwater withdrawal and consumption exacerbate the negative impact of climate change on groundwater sustainability. To predict future groundwater withdrawal, we used population growth as the main driver. However, increases in number of days with zero flow coincide with higher temperature and evapotranspiration rate, and shifts in the precipitation regimes caused by climate change 270 (Mahmoodi et al., 2020b). While we considered this reduction in water availability, the changing climate may lead to increasing irrigation requirements and may put the existing water use systems under additional pressure as similarly revealed in Toews and Allen (2009).
The rising water demand and WUSs will cause a decline of groundwater levels, due to the imbalance between the groundwater recharge under climate change and estimated groundwater withdrawal in the future. This is not only resulting in 275 an unsustainable groundwater use on sub-basin level and in the entire basin, but also changes the hydrologic regime and (summer) of the Eskandari Watershed in central Iran due to human activities (abstraction of groundwater and surface water for irrigation purpose).
The predicted unsustainability of groundwater could be even more intense if we focus on the changes projected for the 285 magnitude and timing of annual extreme conditions, in which base, minimum and maximum flows are projected to decrease and a 4 months shift is expected for minimum flows to occur earlier (shift from Sep to June). This could lead to a higher groundwater withdrawal in summer season when the surface water does not meet the rising demand.
In the near and far future, monthly flows and annual extreme flows are expected to decrease. However, the different magnitude of changes under the three WUS scenarios (No-, Constant-, and Projected-WUS) indicated that the influence of 290 climate change on the flow regime alteration is stronger than groundwater withdrawals. This is in agreement with previous studies e.g., Döll and Zhang (2010) and Shahid et al., (2018). In addition, the similar results for timing, frequency and duration of extreme hydrologic parameters under all three scenarios also showed that their changes are mainly caused by climate change.
Since the Halilrood River is the most important source of water in the region, the significant changes in hydrologic alteration 295 indicators may have an impact on the ecosystem of the Wadi and Jazmorian wetland (water presence, area of water body, water depth, and wetland species). We are expecting smaller inundated area and shallower water body in Jazmorian wetland under climate change condition and groundwater withdrawal, as 27 hydrologic regime indicators are representing substantial alterations since out of 32 RVA 12 are classified as "high" and 15 as "moderate". Simultaneously, the availability of water for the wetland is reduced since, among 23 IHA considered for the magnitude of monthly flows and annual extreme flows, 300 21 IHA have indicated significant changes and 15 IHA show high and moderate levels of alteration based on the RVA approach. Moreover, we expect lower water availability in future for the wetland due to increases in the number and duration of low pulses and number of days with zero flow as well as decreases estimated for the number and duration of high pulses. The significant alteration in falling rates, coinciding with alteration in the magnitude of flows, might influence soil moisture in the wetland and consequently change the distribution of the plants by an intensification of drought stress on plants, 305 preventing wind and water erosion in the Jazmoriam wetland. In summary, hydrologic regime alteration caused by climate change and groundwater withdrawal, will contribute substantially to the ecological change of the wetland and hence, influence the freshwater ecosystem of Wadis in central Iran according to our RVA analysis.
Assessing the streamflow regime changes using IHA in conjunction with RVA, provide a proxy on initial ecological responses to the hydrologic regime changes without having to explicitly investigate ecological indices or building ecological 310 models. However, in order to understand detailed ecological consequences and to identify hydrological thresholds for sustaining the complete or parts of the wetland ecosystem, an in-depth study involving ecological indicators and species requirements is nevertheless needed.The RVA approach enables researchers to link and track the hydrologic and ecological responses to the desirable implementations or ecosystem research efforts. Since, the RVA targets were set as the median ±25th percentile of the baseline data for each hydrologic indicator, the high variation of the streamflow data in Wadi systems might lead to a high range of RVA targets. Therefore, we recommend a combination of RVA approach and a statistical method such as ANOVA to test the level of alteration and their significance in different hydrologic indicators.

Conclusions
The spatio-temporal variation of groundwater sustainability and the streamflow alteration in the near and far climate changeimpacted future have been assessed under five different scenarios: (i) no groundwater withdrawal (ii) unaltered present-day 320 groundwater withdrawal (iii) an increase in groundwater withdrawal (iv) minimum-, and (v) maximum water consumption.
Our findings show that: 1) The significant reduction estimated for groundwater recharge under climate change coincides with rising demand from WUSs and water consumption.
2) The growing groundwater withdrawals in the future exacerbates the impact of climate change on the sustainable use of 325 water resources in the Halilrood Basin.
3) A sustainable state is possible for the entire Halilrood Basin in near and far future if only consumptive water use is considered. However, several sub-basins would still be extremely unsustainable. Hence, water provisioning from sustainable to unsustainable sub-basins would be required.

4) The impacts of climate change and groundwater withdrawals on the freshwater ecosystems in the Jazmorian wetland 330
Basin are expected to be intensified as a considerable alteration is estimated in the hydrologic regime of the Halilrood River (27 IHA indicators show significant changes in the far future and among these the RVA is classified as "high" and "moderate" for 18 IHA).
The combined results show that climate change has a stronger impact on hydrologic regime alterations and consequently on the freshwater ecosystem in the near and far future as compared to groundwater withdrawals in Halilrood Basin. The 335 presented results are useful for long-term planning which is required for a sustainable water resources management under changing future conditions.

Author contribution
NM, JK, and PDW developed the study design. NM performed the model simulations with essential support of JK who carried out the EURO-CORDEX data processing. All authors substantially contributed to the interpretation of the results. 340 NM wrote the initial draft of the manuscript and all co-authors discussed the results and revised the work carefully.

Competing interests
The authors declare that they have no conflict of interest.