Spatiotemporal Evolution of Flow Decline and Water Consumption in a Highly Regulated Transboundary River: The Lower Rio Grande/Bravo

Long-term flow variability in transboundary rivers reflects the combined influence of water availability, management, and usage across space and time. However, integrating historical flow with evolving water-use data remains challenging, as assessments are often conducted within national frameworks despite shared watersheds among at least two nations. Moreover, water discharge and usage data are often fragmented across multiple sources. This study presents a 125-year discharge record (1900–2025) from historical Water Bulletins and modern observations at 12 internationally operated gauging stations, delivering a spatiotemporal assessment of long-term flow evolution and water-budget components in the Lower Rio Grande/Bravo (LRG). Extending from below Falcon Dam Reservoir to the Gulf (~460 river km), the LRG is regulated between the U.S. and Mexico by three major dams (Falcon, Anzaldúas, and Retamal) and eight weirs below Anzaldúas. Supplying ~98% of regional water consumption for more than 2.8 million people at a binational scale, the river has been altered by anthropogenic activities since the late 1800s, with agricultural irrigation accounting for ~85% of current annual withdrawals. To address data discontinuities, flow records were evaluated using four representative 16-year hydrological regimes—natural (1900–1915), pre-dam (1935–1950), post-dam (1974–1989), and modern (2010–2025) —analyzed in conjunction with modern binational usage records across major water sectors (agricultural, municipal, and other uses). Below Falcon Dam, median annual inflow declined by ~64% in the last 125 years, while downstream outflow decreased by ~96%, resulting in a near 100% modern Gulf-delivery deficit relative to natural conditions (i.e., no freshwater delivery to the Gulf from the median basis). While binational water use is dominated by irrigation (~58% U.S., ~26% Mexico), downstream losses exceed what can be attributed to irrigation alone. Findings show upstream delivery reduction as the primary driver of long-term decline, accounting for 54.7% [31.8–91.4%] of total flow reduction relative to natural conditions, while binational water use represents a substantial but secondary contribution, accounting for 33.5% [30.5–43.8%], and unknown losses (e.g., evapotranspiration) follow with minor impacts. Insights from this work aim to inform sustainable, binational water management under increasing urban, agricultural, and climatic pressures across transboundary river systems.