Method Development for Including Environmental Water Requirement in the Water Stress Index

The overuse of human demand for water threatens the capacity of rivers or watersheds to retain their ecosystem services. Environmental water requirement (EWR) needs to be taken explicitly into account when assessing the impact from freshwater use on freshwater resources. Thus, we propose two perspectives for incorporating EWR into the Demand to availability (DTA) ratio of the water stress index (WSI); including it as a separate demand along with anthropogenic demands (Index 1) or reserving it from the available water (Index 2). It is expected that the more demand increases, the more pressure is put on the lowest priority sector. Index 2, by definition, gives the first priority to EWR whereas this is not so for Index 1. Thus Index 2 provides a more conservative approach in terms of the environmental protection and Index 1 allows a more flexible prioritization approach. Both these indices, however, allow flexibility of changing the EWR based on specific circumstances and context, thus making the evaluation more appropriate for water resource planning at both policy and implementation levels.