How economically and environmentally viable are multiple dams in the Upper Cauvery basin, India? A hydro-economic analysis using a landscape-based hydrological model

Abstract. The construction of dams threatens the health of watershed ecosystems. The purpose of the study is to illustrate how multiple dams in a basin can impact hydrological flow regimes and subsequently aquatic ecosystems that depend on river flows. The approach assesses the ecosystem services, including the tradeoffs between economic and ecological services, due to altered the flow regimes. It uses a previously developed model that integrates a landscape-based hydrological model with a reservoir operations model at basin scale and at daily time scale. The approach is unique not only because it offers the analysis of alterations in ecosystem services at daily scale but also because dams can be synthetically placed anywhere in the river network and the corresponding alterations in flow regimes simulated in a flexible manner. As a proof of concept, we analyse the economic and ecological performances of different spatial configuration of existing reservoirs in the Upper Cauvery River basin in India. Such a study is timely and being conducted for the first time, especially in the light of the calls to assess cascade of reservoirs in India and regions elsewhere where pre-dam data is unavailable. The hydrological impact of different configurations of reservoirs is quantified using Indicators of Hydrologic Alteration (IHA). Additionally, the production of two major ecosystem services that depend on the flow regime of the river, as indicated by irrigated agricultural production and fish species richness, is estimated, and a trade-off curve, i.e. a production possibility frontier, for the two services is established. Results show that smaller reservoirs on lower-order streams that maximize the economic value of water stored are better for the basin economy and the environment than larger reservoirs. Cultivating irrigated crops of higher value can maximize the value of stored water and, with lower storage, generate similar economic value than with lower value crops while reducing hydrological alterations. The proposed novel approach, especially when simulating synthetic spatial configurations of reservoirs, can help water and river basin managers to understand the provision of ecosystem services in hydrologically altered basins, optimize dam operations, or even prioritize dam removals with a balanced provision of ecosystem services.