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Water balance as a common approach has been used so far to qualitatively judge the water resources management in which unsustainability is referred for depleted groundwater storage and reduced river flow. Lack of a clearly defined path, universal methodology and specificity of a concept in literature to quantify the level of water resources management, applicable for different spatio-temporal scales, under supported environmental flow requirement are main criticisms for sustainable water resources management. Thus, this study aims to present a physical metric, based on water availability and supply concepts, to overcome mentioned challenges. It quantifies the level of management (either for surface or ground water) using the ratio of residual volume of available water (available water minus water supply) to initial available water in a given area. Management of nine rivers and Fumanat aquifer in Gilan Province of northern Iran (adopting as a case study) is quantified for the baseline period (1989–2012) and two future (2020–2050) scenarios, S1 (climate change) and S2 (simultaneous land-use and climate changes), under alternatives of available and improved water use efficiencies. The Soil and Water Assessment Tool is used to simulate river flow under two scenarios S1 and S2. Climate change study under three Shared Socio-economic Pathways (SSP1-1.9, SSP2-4.5, SSP5-8.5) indicates that temperature (precipitation) will increase (decrease) by 1.4–2.1 °C (2.9%–12.31%). Quantified management of rivers and groundwater is minimum in spring (−4.43 to 0.66) and summer (−0.613), respectively, and is maximum in autumn-winter (1) and winter (0.491), respectively. Quantified annual management in rivers (groundwater) is −1.278 to 1 (−0.07) and −0.772 to 1 (0.104) under available and improved water use efficiencies, respectively. Climate and land-use changes will reduce the level of management over the study area in which the reduction is higher for a greater level of warming and scenario S2. Quantified annual management of rivers (groundwater) in scenario S2 under available water use efficiency is −1.34 to 1 (−0.379), −1.45 to 1 (−0.654) and −1.55 to 1 (−0.947) for the SSP1-1.9, SSP2-4.5 and SSP8.5, respectively. Efficiency improvement increases management level under scenario S2; however, three rivers and groundwater will still suffer from mismanagement in the future. • Here, a physical metric is presented to quantify surface and ground water management. • It uses water availability and supply concepts under supported ecosystem flow requirement. • It can quantify impact of climate and land-use changes on water resources management. • It is highly recommended to use proposed metric for sustainable water management. [ABSTRACT FROM AUTHOR]