Models of transboundary watersheds such as the Rio Grande/Rio Bravo (RGRB) along the US/Mexico border are receiving increasing attention as the difficult problem of water allocation between nations intensifies during times of drought. We present an interdisciplinary effort to develop a semi-distributed regional dynamic simulation model (DSM) for examining water issues in the Lower RGRB basin. The model is designed to inform decision-makers of the water demand and supply changes necessary to meet the needs of international obligations and growing populations in the short and long term. We focus on the Rio Conchos basin, which contributes approximately 70-80% of the surface flow in the lower RGRB basin. Irrigated agriculture has historically been the major user of water and irrigated acreage continues to expand, but it faces increasing competition from industrial development, maquiladoras, and rapidly growing residential water demand.

International agreements such as the Treaty of 1944 between the US and Mexico stipulate that the flows in the RGRB are equally split. Yet uncertainties remain due to vagaries in the legislation. For example, Mexico is required to provide an average of 350,000 AF/yr over a five-year cycle, unless “extraordinary drought” occurs, although the Treaty does not define extraordinary. The characterization of droughts poses a significant problem for hydrometeorologists and water resource engineers. Our simulation model incorporates drought indices developed to characterize droughts in semi-arid and arid regions and statistical approaches to examine the spatial influence of droughts. Special attention is paid to the performance of the DSS under drought conditions.

To examine the economic effects of precipitation variability and various structural and institutional changes to water use in the basin to meet Treaty requirements, we model agricultural, municipal, and industrial water demands linked directly to sectors of the regional economy using a regional input output (IO) model. IO models can be used to examine how changes in water deliveries to the agricultural or manufacturing sectors affect the level of output, employment, and wages in the regional economy. Management options examined include water banking and trading between irrigation districts, trading between Mexico and the US as well as irrigation standards, application efficiency, cropping patterns and traditional reservoir operations strategies.