Hydrologic alteration and potential ecosystemic implications under a changing climate in the Chitral River, Hindukush region, Pakistan

This study investigates changes in the hydrologic regime of the Chitral River, Hindukush – Karakoram – Himalayan (HKH) region, Pakistan. Different statistically based methods were used to assess climate change-induced hydrologic alterations that can possibly impact aquatic habitat in the study region. The hydrological model Hydrologiska Byråns Vattenbalansavdelning (HBV) was calibrated, validated, and applied to predict stream ﬂ ow in the Chitral River basin. The HBV model was forced with the ensemble of four general circulation models under different representative concentration pathway emission scenarios to generate future stream ﬂ ow under climate change conditions in the basin for the mid-twenty- ﬁ rst century. The results of this study show that hydrologic regimes in the study area, expressed by the magnitude, duration, frequency, timing, and rate of stream ﬂ ow, are likely to alter in the future. Positive (i.e., with increased frequency) hydrologic alteration is projected for most ﬂ ow parameters under all scenarios for the 2021 – 2050 period compared with values observed during the historical period (1976 – 2005). These hydrologic alterations might have impacts on ﬁ sh and migratory bird species in the study area. This research can be helpful in providing practical information for more effective water resources and aquatic ecosystem management in the HKH region. impacts on the riverine (ecohydrological) system under different future climatic conditions. The HBV-light hydrological model employed in this study showed a good stream ﬂ ow prediction ef ﬁ ciency. The average results from climatic multi-model ensemble projections in this large basin are used to feed the HBV-light and predict future stream ﬂ ow regimes more reliably. The IHA/RVA analysis re ﬂ ected a range of stream ﬂ ow characteristics. Natural stream ﬂ ow conditions were projected to change in the mid-twenty- ﬁ rst century. Our results suggest an increase in the alterations of all ﬁ ve IHA groups, representing the magnitude, duration, frequency, timing of daily stream ﬂ ow, and the rate of change of stream ﬂ ow. These alterations might have impacts on the most abundant ﬁ sh species in the Chitral River, especially during their spawning and development stages, as well as on the migration processes of migratory bird species that are common in the study area.


GRAPHICAL ABSTRACT INTRODUCTION
Climate has substantially changed compared to pre-industrial times and is projected to further alter considerably worldwide in the twenty-first century at the global scale (Hoegh-Guldberg et al., ). Changes have also been observed in Pakistan, and extreme climatic events are predicted to increase especially in the Hindukush-Karakoram-Himalayan (HKH) region of Pakistan. The HKH region has the most overstressed aquifers in the world and is a climate change hotspot; therefore, the predicted extreme events will pose additional threats to the region, already exposed to numerous and severe flood events (Lutz et al. ).  Oncorhynchusmykiss, Salmo trutta fario, Schizothorax plagiostomus, Diptychus maculates, Ptychobarbus conirostris, Racoma labiata, and Schizopyge esocinus (Rafique & Khan ). Flora represents another important source of biodiversity in the HKH region and plays a vital role in the catchment water cycle in terms of soil conservation and evapotranspiration fluxes. Both fish and plant biodiversity in the Chitral River are seriously exposed to ecological threats due to hydrological alteration. For instance, the Chitral River shows poor fish diversity associated with changes in water temperature and high turbidity (Rafique ; Bari et al. ). Moreover, deterioration of water quality in the Chitral River has been observed and is associated with physical factors depending on climate change (Nafees et al. ).
Despite the important ecological and societal implications of the climate change impacts in the Chitral River basin, previous studies so far have focused, as far as we know, only on hydrologic alterations related to changes in mean and extreme streamflow values but more detailed analysis of hydrological modifications induced by climate change and their effects on the riverine ecosystem in this region is missing and is highly needed for operational purposes. Therefore, overall, this study aims to identify and quantify the main sources of hydrologic alteration that might cause pressing effects on aquatic organisms in the Chitral River basin.

Study region and data availability
The Chitral River has a drainage area of 11,396 km 2 and is a transboundary river basin between Pakistan and Afghanistan ( Figure 1). Large parts of the basin are at high elevations and host wide glaciers. More than 50% of the catchment area is between 4,000 and 5,000 m a.s.l. (Hayat et al. ). Glacier and snowmelt waters (Burhan et al. a) make significant flow contributions to the Chitral River and serve as a critical resource for irrigation and hydropower generation. In addition, the river water is crucial for supporting domestic activities, playing therefore a vital role in the socio-economic development of the region.
Hydrometeorological data in the Chitral River basin were available for the period 1994-2012 (Table 1). A 90-m resolution digital elevation model was also available for the study area. Potential evapotranspiration was computed using the approach by Irmak et al. ().
The results from different General Circulation Models (GCMs, Table 2) have been averaged in order to have more reliable precipitation and temperature data inputs in the study region and to carry out more solid interpretation

Model development and performance assessment
The HBV model was calibrated by employing an automatic calibration method, i.e., the genetic algorithm and Powell optimization method (Seibert ). This method is based on a recombination of parameter sets and the best set of parameters resulting in the highest objective function (e.g. Nash-Sutcliffe Efficiency index (NSE)) was selected. This process was repeated several times to obtain the best-optimized set of parameters and improve the model efficiency.

Analysis of streamflow alteration
high alteration (60% < X < 80%); and severe or extreme alteration (X > 80%). The degree of hydrological alteration (HA) of each parameter was then calculated as follows (Richter et al. ): where HA is the degree of hydrologic alteration, obs is the number of years in which the observed parameter value fell within the target range, and exp indicates the number of years in which the value of the parameter is expected to fall within the target range. X is calculated using Equation

Alteration of monthly streamflow
The HA values of the 12 parameters of the IHA group 1 (monthly streamflow) for the Chitral River under the RCP 2.6, RCP 4.5, RCP 6.0, and RCP 8.5 emission scenarios are listed in Table S2.

Alteration of annual extreme flows
The HA values of the 12 parameters of the IHA group 2 (annual extreme flow conditions) and the two parameters of the IHA group 3 (annual extreme flow conditions) for the Chitral River under the RCP 2.6, RCP 4.5, RCP 6.0, and RCP 8.5 emissions scenarios are listed in Table S3.

Improved overall degree of hydrologic alteration
The improved overall degree of hydrologic alteration (X) for the five IHA groups and the four RCPs is reported in Figure 9, calculated using Equation (5). The highest overall X values were projected for group 4 (magnitude, frequency, and duration of annual extreme flows) for RCPs 6.0 and 4.5, followed by the first group for RCP 8.5. Consistently lower values were observed for group 5 for all RCPs. Groups 2 and 3 showed relatively similar X values.
Fourteen, 18, 9, and 16 parameters out of 32 indicators returned HA values of more than 100% in the post-impact period for the low (RCP 2.6), average (RCP 4.5), high (RCP 6.0), and extreme (RCP 8.5) emission scenarios, respectively. The maximum average alteration (323%) was returned for the RCP 6.0 emission scenario and the minimum average alteration (127%) was projected for the RCP 2.6 emission scenario in all IHA groups (Table S6).

CONCLUDING REMARKS
In this study, an assessment of hydrological alteration was performed in the flow regime of the Chitral River in the HKH region, Pakistan. The widely used methods of the IHA and the non-parametric RVA were adopted to characterize the degree of alteration and the possible future