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The sustainability of dynamic natural systems often depends on their capacity to adapt to uncertain climate-related changes, where different management options may be combined to facilitate this adaptation. Salt marshes exemplify such a system. Marsh sustainability under rapid sea level rise requires the preservation of transgression zones - undeveloped uplands onto which marshes migrate. Whether these uplands eventually become marsh depends on uncertain sea level rise and natural dynamics that determine migration onto different land types. Under conditions such as these, systematically diversi ed management actions likely outperform ad hoc or non-diversi ed alternatives. This paper develops the first adaptation portfolio model designed to optimize the bene fits of a migrating coastal resource. Results are illustrated using a case study of marsh conservation in Virginia, USA. Results suggest that models of this type can enhance adaptation benefits beyond those available via current approaches.

Cities have been investing heavily in recent years to augment their water supply, focusing on either stormwater harvesting or desalination. A more optimal strategy is to consider all sources of water jointly, thereby allowing for hedging of supply risks. A portfolio model of urban water supply is derived, consisting of reservoirs, stormwater harvesting and de- salination, which takes into account the uncertainties of water ows and rainfall as well as the relative costs of associated with the three types of water sources. Calibrating the model to Melbournes existing water sup- ply system, a signi cant result is that optimal contributions are dependent on current water stocks, thereby providing insights into the appropiate- ness of historical water augmentation decisions. It is found that, with the exception of reservoirs, observed contributions deviate from optimal contributions for stormwater harvesting and desalination with the results suggesting a need for future investments to target stormwater harvest- ing ahead of desalination technology. Moreover, the optimal portfolio is found to vary throughout the year with desalination being the preferred supplementary supply source to reservoirs during summer months, while harvested stormwater is the preferred choice during winter. This result has implications for augmentation investments to mitigate seasonal water supply shortages.