Introduction
Climate change creates changes in precipitation amount and pattern, temperature and evapotranspiration, which influences water resources availability, increases floods and droughts frequency and intensity, leads to degradation of the environment, which serves the water quality and leads to the dynamic change of runoff. On the other hand, land cover change and population increase impact on hydrological system too. Understanding the hydrological response of watersheds to physical change (land use) and climate change (precipitation and temperature) and quantifying the water balance change due to environmental change are very important for water resources sustainable development and an important component in water resources management and planning. This research aims to manage water supply and demand in the future under change conditions at a river basin-scale. In line with this goal, the situation of water supply and demand in the Gorganrud River basin was evaluated under the conditions of climate change, land use change and population change. The Gorganrud river basin is located in the northeast of Iran in the three provinces of Golestan, North Khorasan and Semnan, with an area of about 11,220 square kilometers. The studied area is an important source of water supply for the domestic, agricultural, industrial and environmental sectors in Golestan province, so it is important to investigate the water resources condition in this area in the future period.
Materials and methods
To project the climatic condition in future the data related to the CanESM2 large-scale global climate model under two scenarios of RCP2.6 and RCP8.5 were prepared. Then, these data were downscaled using Statistical Down Scaling Model (SDSM4.2) for local climatic gauging stations. Twelve climatic stations were considered for precipitation and nine for minimum and maximum temperatures. Land use maps for 1999, 2009 and 2017 were prepared using Landsat satellite images and IDRISI software. Then, land use changes were predicted using the Land Change Modeler (LCM) tool in the TerrSet software. The population size and water demand for domestic, industrial, agricultural, and environmental sectors were predicted for the future period. Then, HEC-HMS SMA was calibrated and validated using rainfall and runoff data of 2007-2009 and January 2013- March 2014, respectively.
The calibrated and validated hydrological model was used to simulate the hydrological components (runoff, groundwater recharge, and volumes of inflow, outflow, and storage in dam reservoirs) under climate and land use changes. Finally, the value of the Water Supply Stress Index (WaSSI) was calculated under two climate change scenarios, RCP2.6 and RCP8.5.
Results and Discussion
Investigations show that the weighted average of the Water Supply Stress Index has increased from 1.003 for the period 1966-2006 to 1.033 for the period 1966-2011. The study of climate change shows changes in precipitation, minimum and maximum temperature in different climate stations under both RCP2.6 and RCP8.5 scenarios. It should be noted that the impact of climate change scenarios is not the same in all climate stations. The prediction of land use change indicates the reduction of agricultural lands and their conversion into residential areas. In addition, the conversion of dense forests to semi-dense forests and rangelands is significant. The forecast of population changes and water demand indicates the increase of these variables. The comparison of the average annual rainfall in the past (2006-2017) and the future (2020-2040) at the Aqqla hydrometric station shows that this variable will have values of 207.038 and 210.8 million cubic meters under RCP2.6 and RCP8.5 scenarios, respectively which is significantly more than its amount in the last period with the amount of 179.64 million cubic meters. The total water supply components for the RCP2.6 scenario is equal to 930.49 and for the RCP8.5 scenario is equal to 935.85 million cubic meters. The sum of water demand components according to RCP2.6 scenario is equal to 1495.04 and according to RCP8.5 scenario it is equal to 1379.2 million cubic meters. The median value of WaSSI in the future under two climate change scenarios RCP2.6 and RCP8.5 was predicted to be 1.61 and 1.45, respectively.
Conclusion
In this research, the impact of climate change, land use change and population change on the water resources of the Gorganrud River Basin was investigated. By calculating the Water Supply Stress Index, it was found that based on the used index, the lack of water security in the Gorganrud River Basin will be a challenge in the future. In the management of water resources, it is necessary to focus on both sides of water supply and demand but, since it is possible to manage water supply less, it is necessary to focus more on water demand management. In this regard, we should look for solutions that are more likely to be implemented. Water demand management measures in the agricultural sector such as changing the cropping pattern, changing the cropping date, reducing agricultural sector waste and reducing evaporation from the soil surface are among the appropriate solutions in the field of water demand management. In addition, the observation of land use changes and especially the increase in the area of residential areas and the decrease in the area of dense forests in this research is a warning for policy makers, decision makers, managers and executive departments in order to further protect natural resources and manage the development of residential areas. The methodology and outputs of this research can be useful for decision makers and managers of water resources in the conditions of change and uncertainties related to it, in the Gorganrud River Basin and similar conditions. |
