A search of available digital products for the construction of data layers necessary for hydrologic model parameterization found that suitable data exists for the subject area within the United States, but that data is relatively sparse in Mexico (see Appendix A for list of data sources). Primary map layers for GIS-based model parameterization include soils, vegetation, land use, hydrology, topography, and climate. Data for each of these subjects are available for the entire study area, although it should be noted that the sources for these data are varied, which will lead to some difficulty in assembling appropriate continuous thematic coverages. The majority of Mexico-based data is appropriate for relatively small-scale mapping (greater than 1:50,000), while higher resolution data covering the United States would allow for a application at a larger scale. This represents another significant barrier to cross-border implementation of hydrologic models; the lack of consistency in map standards and accuracy between the two countries.

Topographic data is of critical importance to hydrologic modeling, since it influences infiltration, flow direction, velocity, detention/retention, and, hence a host of interrelated hydrologic properties. Digital elevation models (DEMs) have been created to a high degree of accuracy for the entire United States: 7.5 minute quadrangles are freely available, which are suitable for mapping at a 30 meter resolution, or 1:24,000 scale. Such high resolution maps do not exist in Mexico as yet. A DEM of the State of Sonora, Mexico has been created at a 1:50,000 scale, which is more equivalent to the United States 15 minute quadrangle DEM, while the Mexican Sate Baja California has only been mapped at a 1:250,000 scale, equivalent to 1 degree United States quadrangles. While it is desirable to map at the finest resolution possible, the United States data may be altered to match the scale of the Mexican data. Furthermore, modeling at a large scale (1:24,000) may be impractical for a basin the size of the Lower Colorado River.

High quality, large scale soil information, is generally lacking in both the United States and Mexico. The STATSGO database of soil surveys is applicable throughout the United States portion of the study area, and it has been successfully linked with hydrologic models. While digital maps of soil properties exist within Mexico, soil properties have not been catalogued at a high level of detail. This lack of data regarding soil properties may be a hindrance to accurate modeling of infiltration and runoff-runon relationships. Inaccuracies in model results associated with mis-attribution of soil properties will be reduced due to the large study area, which limits the influence of small perturbations in the system, and the application of large storm events which overwhelm the system response. However, the determination of soil physical properties in Mexico and the correlation of soil properties between the two countries represent significant data needs.

A host of vegetation maps and biotic community maps exist within both the United States and Mexico. Generalized vegetation maps for the United States and Mexico exist at a small scale (1:100,000), and larger-scale maps exist within the United States. However, a lack of continuity across the border and the small size of large-scale mapping projects will pose a challenge for the assembling of accurate, consistent vegetation data. The use of remotely sensed data for mapping vegetation indices and determining vegetation communities would alleviate these issues. The North American Landscape Characterization project has mapped the entire study region at 60 meter resolution, and would provide a basis for determining land cover change with the addition of updated satellite imagery. Large-scale land cover classification of the entire study area using operational satellite data is relatively expensive, but given the importance of obtaining an accurate representation of land use and land cover change, the acquisition and integration of these data into a thematic database is critical.

Data currently available concerning spatially distributed data within the Lower Colorado River basin are of sufficient quality and coverage to warrant the development of a geographic information system (GIS) database. This GIS could be used to edge-match data in the United States with its Mexican equivalent, extrapolate physical properties to areas not currently mapped, and provide a link to the hydrologic models for the parameterization of these models. There are, however, several areas of research that will need to be addressed in order to create a seamless cross-border database of watershed characteristics.

Mapping of watershed characteristics was completed by State and Federal officials with a lack of international cooperation and scientific collaboration. As a result, mapping units within the two countries are different from one another, which creates two geographic hurdles: edge-matching and the development of a single mapping standard. The creation of a single mapping standard will necessitate the involvement of scientists familiar with the individual data sets, which implies a need for cooperative relationships with the various agencies and universities responsible for the creation of the original map sets. Once a single mapping standard has been developed, the edge-matching of polygons across the international border can be completed.

Enclosed in Appendix A is a list of currently available data for the Lower Colorado River. Included is the appropriate scale at which this data may be applied, but conspicuously absent is any indication of data quality or the manner in which this data was collected. The question of data quality and accuracy must be addressed for each thematic layer not created specifically for this project, and these findings accounted for in the modeling effort and data analysis.

Most of the available mapsets are suitable for relatively small-scale mapping and modeling efforts (smaller than 1:100,000). Data sets within the United States are generally mapped at a larger scale or higher resolution than the contemporary maps in Mexico. Generally speaking, the smallest scale should be used when integrating maps, which implies either improving the quality of the data in Mexico or downgrading the data in the United States to match that of Mexico. If the data needs for modeling the Lower Colorado River basin call for data at a larger scale than is currently available, research must be directed to address this conflict. Localized regions within the study area have been more intensively surveyed and mapped at a finer resolution; such projects could be used to improve the quality of the surrounding regions. This integration of large-scale, high resolution thematic data with smaller-scale data represents an opportunity for research.

Data: 7.5 minute quadrangles

Scale/Resolution: 30m resolution; 1:24,000

Coverage: complete

Cost: free download from the USGS

Format: SDTS

Data: 7.5 minute digital line graphs (boundaries, transportation, hydrography, hypsography)

Scale/Resolution: 1:24,000

Coverage: complete along Colorado River; sparse data to East along border; more complete on California side

Cost: free download from the USGS

Format: SDTS

Data: Intermediate-scale hydrologic unit map (USGS cataloging system of watersheds, basins, geographic regions)

Scale/Resolution: 1:250,000

Coverage: complete

Cost: free download from the USGS

Format: Arc/Info & SDTS

Data: Monthly precipitation

Scale/Resolution: 2 decimal places

Coverage: Blythe, CA

Cost: free download from CA Department of Water Resources

Format: Text file

Data: Climate divisions of the United States by the National Climatic data Center for display of seasonal precipitation, temperature maps

Scale/Resolution: 1:2 million

Coverage: complete

Cost: free download from the USGS

Format: Arc/Info or SDTS

Data: State Soil Geographic generalized state soil surveys (STATSGO)

Scale/Resolution: 1:250,000 (minimum mapped area ~1544 acres)

Coverage: complete

Cost: free download from NRCS

Format: Arc/Info, GRASS, DLG, DOS-DLG

Data: Major roads in California

Scale/Resolution: 1:24,000

Coverage: Arizona

Cost: free download from ALRIS

Format: Arc/Info

Data: Natural vegetation of Arizona (from Arizona Game & Fish DepÆt)

Scale/Resolution: unknown

Coverage: Most of Arizona; complete coverage in SW Arizona

Cost: free download from ALRIS

Format: Arc/Info

Data: GAP Landsat TM

Scale/Resolution: 90m resolution

Coverage: California

Cost: free download from CA GAP site

Format: unknown