WATER MANAGEMENT FOR ENVIRONMENTAL RESTORATION FLOWS IN THE BIG BEND REACH, RIO GRANDE/BRAVO

Due to high water demand, the scarcity of water, and the complexity of water allocation in the Rio Grande/Bravo basin, environmental flows have not been considered as an integral part of the water management in this basin. The Big Bend reach is located between the cities of Presidio/Ojinaga to Amistad reservoir, along the Rio Grande/Bravo main stream. Important environmental habitats such as the Big Bend National and State Park in the US, the Maderas del Carmen, Cañon de Santa Elena and Ocampo natural reserved areas in Mexico are ecologically threatened because of the lack of environmental water management policies. Several efforts have been undertaken by scientists, government agencies and NGOs to determine the environmental water requirements for this reach and strategies that can provide these flows. The objective of this presentation is to describe a water management policy that can conciliate environmental, human and international water uses in the Big Bend region. This policy considers mechanisms that reduce flooding and drought risks, and at the same time, met national and international water regulations. First, the system is characterized identifying water users, hydraulic infrastructure, and water allocation according to state, federal and international regulations. Second, a hydrograph for environmental flows is proposed; it has been derived from an historic analysis of hydrologic conditions prior dam alteration. Third, a water management policy to re-operate Luis L. Leon reservoir is proposed to provide environmental flows, all water sources are analyzed to quantify the releases from Luis L. Leon reservoir. Three types of natural flow regimes are considered: (1) base flows aimed to provide the normal flow regime in the region, (2) high flows to provide transversal connectivity between the side-banks, and (3) small floods aimed to re-widen the channel and connect the river longitudinally. The maximum annual restoration flow for the big bend region is 982X10⁶ m³/year [796 X10³ acre-foot/year]. Base flows and small floods are delivered mostly from Luis L Leon reservoir; high flows are supplied by the rest of the system. This policy supplies water for environmental purposes while effectively met the physical and legal constraints of the system, reducing the flooding and drought risk in Presidio/Ojinaga.