• Water security risks in a watershed are modelled using InVEST’s water yield model. • The impacts of future climate and land use changes on water stress are analyzed. • Water yield is negatively affected by climate change and positively by land use change. • Future water supply is less than the operating flow of a newly constructed dam. • Spatially differentiated conservation efforts are identified to ensure water security. Water security, a key policy objective for sustainable development, is under stress as a result of land use and climate change, especially in (semi-)arid areas like Iran. Land use change alters surface runoff and affects basin-wide hydrological processes and water consumption, while climate change modifies precipitation and temperature patterns and consequently evapotranspiration and water supply. In this study, water yield, supply and consumption are simulated in a watershed draining into the Caspian Sea in northern Iran, using the water yield model in the Integrated Valuation of Environmental Service and Tradeoffs (InVEST) tool. The novelty of this study is found in the combined modelling of the impacts of climate and land use change scenarios on water security, translating these results into a water stress indicator, and estimating the associated economic costs of reduced future water supply. The results show substantial spatial variation of the negative impacts of water supply and future water security across the watershed, further increasing the pressure on its inhabitants, their economic activities and ecological values. The estimation of the economic costs of increased water insecurity allows us to inform policy and decision-makers about future investments in climate adaptation and mitigation.

• Basins in the Canadian Prairies have varying contributing fractions of their areas. • Caused by the variable storage of water in depressions. • The effects of the spatial and frequency distributions of depressions are quantified. • Will lead to the development of improved hydrological models for the region. Runoff in many locations within the Canadian Prairies is dominated by intermittent fill-and-spill between depressions. As a result, many basins have varying fractions of their areas connected to their outlets, due to changing depressional storage. The objective of this research is to determine the causes of the relationships between water storage and the connected fraction of depression-dominated Prairie basins. It is hypothesized that the shapes of the relationship curves are influenced by both the spatial and frequency distributions of depressional storage. Three sets of numerical experiments are presented to test the hypothesis. The first set of experiments demonstrates that where the number of depressions is small, their size and spatial distributions are important in controlling the relationship between the volume of depressional storage and the connected fraction of a basin. As the number of depressions is increased, the areal fractions of the largest depressions decrease, which reduces the importance of the spatial distribution of depressions. The second set of experiments demonstrates that the curve enveloping the connected fraction of a basin can be derived from the frequency distribution of depression areas, and scaling relationships between the area, volume and catchment area of the depressions, when the area of the largest depression is no greater than approximately 5% of the total. The third set of experiments demonstrates that the presence of a single large depression can strongly influence the relationship between the depressional storage and the connected fraction of a basin, depending on the relative size of the large depression, and its location within the basin. A single depression containing 30% of the total depressional area located near the outlet was shown to cause a basin to be nearly endorheic. A similar depression near the top of a basin was demonstrated not to fill and was therefore unable to contribute flows. The implications of the findings for developing hydrological models of large Prairie drainage basins are discussed.