Scarcity and vulnerability assessment of blue and green water resources for three watersheds of Karaj, Latian and Mamlu Dams using SWAT

“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 given area. Addressing the security 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). Green water (GW) is the portion of fresh water stored in the unsaturated soil layer and vegetation canopy that is available indirectly (Veettil and Mishra, 2016). The BW footprint is amount of consumptive water use by humans. The GW footprint refers to the indirect use of freshwater by humans to produce goods and services and, therefore, it is equal to actual evapotranspiration from an agricultural area (Falkenmark and Rockström, 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 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 The indicator 6.4.2 is the ratio of total freshwater withdrawn by all sectors to the water availability (total renewable freshwater resources minus environmental flow requirement) in a given region (Liu et al., 2017;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 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 the observation period 1995-2013.

Hydrologic model: Blue and green water resources are quantified using SWAT that is calibrated for three watersheds during the observation period 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. Dominant land uses are pasture (~92.7%) and agriculture (~6.4%) over the study area. Soil layers are Leptosols (83.7%), Regosols (13%) and Solonchak (3.3%) (FAO/IIASA/ISRIC/ISSCAS/JRC, 2009; Iranian Water Resources Management Company, 2018). Thes watershed is classified into five slope ranges of 0-10%, 10-20%, 20-40%, 40-60% and >60% and divided to 203 hydrological response units (HRUs). Daily data from ten climatic stations are included in SWAT to capture the spatial precipitation variation within the study area. The precipitation lapse rate (356 mm/km) is included in the model for ten elevation bands that are defined at each sub-basin. The temperature lapse rate (-6.5 ℃/km) is included for snowfall modeling. Minimum and maximum monthly snowfall rates are 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 (1991-2013). Potential evapotranspiration and surface runoff are calculated using the Hargreaves and SCS curve number methods, respectively. Scarcity and vulnerability assessment for green water: Green water footprint refers to indirect use of freshwater by humans to produce goods and services and it is equal to actual evapotranspiration from an agricultural area (Hoekstra et al. 2011). The green water scarcity and vulnerability are calculated using the following equations (Veettil and Mishra 2016): 〖GW〗_(scarcity(i,t))=〖GW〗_(footprint(i,t))/〖GW〗_(availability (i,t)) (1) 〖GW〗_(vulnerability(i,t))=〖GW〗_(footprint(i,t))/〖GW〗_(availability(P30)(i,t)) (2) in which 〖GW〗_(footprint(i,t)) is the green water footprint, 〖GW〗_(availability (i,t)) the available green water and 〖GW〗_(availability(P30)(i,t)), the historical low available green water in sub-basin i during time t. The green water footprint and availability are respectively equal to actual evapotranspiration (ET) and initial soil water content (〖SW〗_i) in HRU output of the SWAT (Veettil and Mishra 2016). Scarcity and vulnerability assessment for blue water: Blue water scarcity and vulnerability are calculated using the following equations (Veettil and Mishra 2016): 〖BW〗_(scarcity(i,t))=〖BW〗_(footprint(i,t))/〖BW〗_(availability (i,t)) (3) 〖BW〗_(vulnerability(i,t))=〖BW〗_(footprint(i,t))/〖BW〗_(availability(P30)(i,t)) (4) in which 〖BW〗_(footprint(i,t)) is the surface water footprint, 〖BW〗_(availability (i,t)) the available surface water for consumption and 〖BW〗_(availability(P30)(i,t)) the historical low availability of surface water in sub-basin i during time t. Surface blue water footprint is the amount of consumptive water use (Rodrigues et al. 2014; Veettil and Mishra 2016). The 〖BW〗_availability is the amount of water which can be abstracted from a river without affecting 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). 〖BW〗_(availability(i,t))=Q_((i,t))-〖EFR〗_((i,t)) (5) 〖EFR〗_((i,t))=0.8Q_(mean(i,t)) (6) where Q_((i,t)) is the river flow (m3/s), 〖EFR〗_((i,t)) the environmental flow requirement (m3/s) and Q_(mean(i,t)) the long-term mean monthly discharge in sub-basin i.

Results for calibration (1995-2007) and validation (2008-2013) periods indicate that SWAT simulates well the daily discharge at eight hydrometric stations. Results reveal that annual scarcity and vulnerability indices for green water are 0.388 and 0.66, respectively, while scarcity and vulnerability indices for blue water are 0.65 and 1.04, respectively. The watersheds of Karaj and Mamlu Dams respectively experience minimum and maximum blue water scarcity and vulnerability, but they respectively experience maximum and minimum green water scarcity and vulnerability over the study area. 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, therefore, provide analysis for sustainable water resources planning and management. For example, blue water allocation and conveyance among sub-basins can reduce the water stress in ecological hotspots.