Ecological Water Requirement (EWR) Analysis of High Mountain and Steep Gorge (HMSG) River—Application to Upper Lancang–Mekong River

As “corridor” in the south–north and “barrier” in the west–east direction, Lancang River, the upstream of Lancang–Mekong River, has an obvious spatial–temporal characteristic and unique regional attributes. Recently, the hydropower development of the mainstream along Lancang River has disturbed the regional ecosystem to have unstable factors, and threatened the ecosystem health. This paper used the couple model of Grading Coefficient of ecological water requirement (GCEWR) and the ecological runoff (ER) to simulate the ecological water requirement (EWR) of Lancang River, in a broad sense, this method belongs to hydrology–ecology methodology. In the GCEWR–ER, We adopted ecological characteristic indexes (ECI) and hydrological characteristic variables (e.g. variance index) to calculate the GCEWR, and used three methods to calculate the basic variable (e.g. ER) of EWR: the first method directly used annual average runoff as ER; the second method was used frequency method and took year as basic time unit, and the third method took season (e.g. flood season, non-flood season) as the basic time unit to evaluate ER. Finally, in order to demonstrate applicability of this developed methodology, this paper adopted GCEWR–ER method to calculate the EWR of Lancang River in the Longitudinal Range-Gorge Region. By the systematic analysis of the results, we could get the minimum, satisfying and optimal EWR for the Lancang River, which were 142.53 × 10⁸, 286.46 × 10⁸ and 385.96 × 10⁸ m³. The three EWR respectively occupied 25.10%, 50.46% and 67.98% of the average measured run-off (567.75 × 10⁸ m³) of the Lancang River, and respectively occupied 18.63%, 37.45% and 50.45% of the natural run-off (765 × 10⁸ m³) of the Lancang River.