عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Water scarcity is a physical metric that refers to the volumetric abundance of water supply and consumption during normal periods. It is typically calculated as a ratio of human water consumption to available water in a specific area. Addressing the scarcity of freshwater (blue and green) is vital for sustainable water resources management (Liu et al., 2017). Blue water (BW), freshwater flowing in groundwater, rivers, lakes or other surface water bodies, is directly used for human consumption (Veettil and Mishra, 2016) and green water (GW) is the portion of freshwater stored in the unsaturated soil layer and vegetation canopy that is available indirectly (Veettil and Mishra, 2016). The BW footprint is the amount of consumptive water used by humans. On the other hand, the GW footprint refers to the indirect use of freshwater by humans to produce goods and provide services and, therefore, it is equal to actual evapotranspiration from an agricultural area (Falkenmark and Rockstr?m, 2006; Gerten et al., 2011; Hoekstra et al., 2011; Kounina et al., 2013; Liu et al., 2017). The sustainable development goal 6 in 2030 Agenda, adopted by heads of nations, relates to water scarcity in target 6.4, which was monitored by water stress indicator 6.4.2 (Vanham et al., 2018). The indicator 6.4.2 is the ratio of total freshwater withdrawn by all sectors to the water availability (total renewable freshwater resources minus environmental flow requirement) in a given region (Liu et al., 2017; Vanham et al., 2018).
The objective of this study is to assess the scarcity and vulnerability of blue and green water resources over three watersheds of Karaj, Latian and Mamlu Dams (Tehran and Alborz Provinces) using Soil and Water Assessment Tool (SWAT) during 1995-2013.
Blue and green water resources are quantified using SWAT that is calibrated for the three watersheds during 1995-2013. The SWAT is developed and parametrized using ArcSWAT 2012 interface. The watershed is delineated using a 30-m digital elevation modeling data, resulting in 8 sub-basins. The major land uses over the study area are pasture (~92.7%) and agriculture (~6.4%). Soil layers are Leptosols (83.7%), Regosols (13%) and Solonchak (3.3%) (FAO/IIASA/ISRIC/ISSCAS/JRC, 2009; Iranian Water Resources Management Company, 2018). Each watershed was classified into five slope ranges of 0-10, 10-20, 20-40, 40-60 and >60%, and divided into 203 hydrological response units (HRUs). Daily data from ten climatic stations were used by SWAT to capture the spatial precipitation variation within the study area. The precipitation lapse rate of 356 mm/km was included in the model for ten elevation bands that are defined at each sub-basin. The temperature lapse rate of -6.5 ?/km was included for snowfall modeling. The minimum and maximum monthly snowfall rates were 1 and 8 mm, respectively, for all sub-basins based on the long-term observed snowfall rates at Tehran and Abali climatic stations within the watershed. Daily outflow of the Latian dam is included in SWAT during the operation period of 1991-2013. Potential evapotranspiration and surface runoff were calculated using the Hargreaves and SCS curve number methods, respectively. Scarcity was evaluated using the ratio of GW footprint to available GW and vulnerability was evaluated using the ratio of GW footprint to historical low available GW in specific sub-basin and month (Veettil and Mishra 2016). The GW footprint and availability are equal to actual evapotranspiration (ET) and initial soil water content (?SW?_i) in HRU output of the SWAT, respectively (Veettil and Mishra 2016). Scarcity was evaluated using the ratio of BW footprint to available BW and vulnerability was evaluated using the ratio of BW footprint to historical low available BW in specific sub-basin and month (Veettil and Mishra 2016). The BW footprint is equal to consumptive water use within a sub-basin and available BW is the volume of water that can be abstracted from a river without affecting the river-dependent ecology (Hoekstra et al. 2011; Veettil and Mishra 2016). The presumptive standard method allows using 20% of the river flow for consumption and leaving 80% for sustaining the environment (Veettil and Mishra 2016). Thus, available BW in a sub-basin is equal to 20% of the long-term mean monthly discharge of the river.
The results for calibration (1995-2007) and validation (2008-2013) periods indicate that the SWAT simulates the daily discharge well at eight hydrometric stations. The results reveal that the annual scarcity and vulnerability indices for GW were 0.388 and 0.66, while for BW were 0.65 and 1.04, respectively. The watersheds of Karaj and Mamlu dams experience minimum and maximum BW scarcity and vulnerability and on the other hand, the maximum and minimum GW scarcity and vulnerability over the study area, respectively. The scarcity and vulnerability assessment of water resources (blue and green water) in a given watershed can highlight the ecological hotspots (regions under water stress) and provide analysis for sustainable water resources planning and management. For example, BW allocation and conveyance among sub-basins can reduce the water stress in ecological hotspots.