2.3. Emergy indices Developing indices relating exposure, sensitivities and the associated adaptive capacities of the ecological economic system is important for the interpretation of vulnerability to extreme climate events. This research develops five emergy indices which can be used to assess and compare vulnerability caused by different events at different cities within the case study area (Table 1). The potential impact is a combined effect of exposure and the sensitivity of the system due to an environmental perturbation. The index of potential impact (PI) is the product of the intensity of exposure(IEx) and sensitivity (Sen), which is represented by the emergy of accumulated assets. The intensity of exposure (IEx) is the ratio of runoff (exposure; Ex) to rainfall (potential exposure; PEx).