Budget:

$31,400.00

Objectives . Methodology . Results

Comments:

Please send your comments/questions by email to Dr. Srinivasan or Dr. Ramanarayanan

Objectives:

• To develop a GIS to model the transport of sediment and toxic materials in the International Rio Grande/Rio Bravo watershed in cooperation with the different agencies representing different scientific methodologies.

• To better understand the dynamics of the ecosystem with several diverse issues.

Methodology:

An assessment of the Rio Grande/Rio Bravo watershed poses special challenges. The watershed spreads over eight states of US and Mexico complicating the protection and management of natural resources within the basin. The river faces several water-quality problems such as elevated fecal coliform, nutrients, radio nuclides, salinity, etc. In addition, cooperating across political boundaries to manage this international watershed can be a complex task.

Based on an existing binational toxic substances study of the river and a proposed sediment coring project, it was decided to perform a GIS analysis of sediment and toxic substances transport in the watershed. Sediment coring was performed in the Elephant Butte Reservoir of New Mexico and the International Amistad and Falcon Reservoirs (Texas/Mexico) and samples were analyzed. Baseline data and historical water-quality, sediment, and fish-tissue data were compiled to create a seamless GIS. Creation of the base data comprised the largest part of project time since the originating scales were slightly different and the classification systems were inconsistent between the US and Mexico. The landuse, land cover, hydrology, climate and soils data combined with the monitoring information formed the foundation of the analysis and transport modeling.

For a detailed analysis, the SWAT model is being used in GRASS environment. SWAT requires a specific land use/land cover classification which gives details not provided in simple classification schemes and also specific information about soil erosion potential and temperature commenly found in the United States Department of Agriculture (USDA)'s STATSGO Database; however Mexico uses the Food and Agriculture Organization's soil classification. To use the SWAT model effectively, a methodology is being developed for converting existing data layers from two different classification schemes to a consistent scheme and also the SWAT model is being modified to accommodate simpler inputs.

Another simple analysis using a simplified model developed at the University of Texas will also be performed. This six-step approach utilizes a combination GRID-Network approach in an ARC/INFO environment. A grid of one square kilometer cells will be laid over the digital elevation models and the drainage network of the watershed willl be defined using them. Mean annual runoff will be calculated from a combination of precipitation grids and flow gaging station measurements. Watersheds for each gaging station will be defined and by assuming constant precipitation across the entire gaging station drainage area, mean annual runoff for each cell in the drainage network will be calculated.

The expected mean concentration (EMC) values developed by USGS and the Natural Resources Conservation Service (NRCS) will be used to predict runoff associated with specific land uses and a new grid of EMC values will be developed by determining the land use in each cell and applying corresponding EMC value to that cell. A grid of estimated pollutant loading for each cell will be created and in-stream loadings will be calculated by summing the loading for each upstream cell in the drainage network. These in-stream loadings will be compared to the observed values.

The difficulty in applying this method to this international watershed is that the EMC values developed for typical land uses in the US will not necessarily correlate with land uses in Mexico. Also, this approach does not account for soil erosion potential and assumes that all loadings are conservative.

Results:

Preliminary results from the sediment coring of the three reservoirs revealed interesting results. Laboratory analysis showed levels of several toxic parameters far below the expected values. It was found that the rates of sedimentation in these reservoirs were far higher than expected and as much as ten times higher than the rates observed in most reservoirs in the US. Given the serious water quality issues in the southern and southeastern portions of the basin, the sedimentation rates will likely raise for water-supply planners.

The coring results from the Elephant Butte Reservoir showed a drastic decrease in radio nuclide deposition in the early 1970s. This decrease seemed to correspond with the closing of the Cochiti Reservoir in 1973. Therefore, it is proposedto take a core from Cochiti Reservoir to supplement the analysis.

Based on the preliminary analysis of bed-sediment data, the focus of the toxic substance analysis was narrowed to DDE/DDT, chlordane, polychlorinated biphenyls,arsenic, mercury, and radio nuclides which also were highlighted as potential pollutants in the basin by previous studies.

Further analysis will be performed in two phases: phase one will look at the entire Rio Grande/Rio Bravo basin; phase two will compare a reference watershed to a small "problemshed" within the basin. With large volume of flow and relative lack of intense population density throughout the basin, it is expected that only a few problem areas will be shown by the analysis and that dilution will confound the results so drastically as to mask most concerns. Therefore, analysis of smaller drainage areas is proposed. The Devils River Watershed which has remained relatively unimpacted will serve as the reference watershed for the basinand it should function as an appropriate background site for the basin. The Manadas Creek Watershed and the Arroyo Colorado Watershed will be studied as potential "problemsheds" for more detailed analysis.

Further analyses are being conducted and the results will be reported in future.