**5. Discussion and Conclusions**

The cyberinfrastructure and REON website contributed significantly to this study in portraying relevant characteristics of each of the North and Central Watersheds. The REON website not only collects distinct information into one single source but also allows the stakeholders within each watershed to assess the watershed characteristics. Therefore, this platform is an innovative tool that supports effective watershed characterization. ArcGIS automated hydrology tools have shown to have satisfactory results in delineating watersheds. Overall, the study showed that the watershed delineation process used provided acceptable results to characterize the North and Central Watersheds.

Although the HWMD watershed was not the highest regarding the urban areas, it is considered higher in NPS pollution with respect to the entire area of the North and Central Watersheds. Urban areas have more impact on the HWMD in comparison to the other watersheds regarding the overall watershed areas. This finding suggests that urban areas in this watershed are linked to the presence of bacteria and chlorophyll-a. Based on the water quality data obtained, only chlorophyll-a levels were higher than the other watershed levels. The high levels of chlorophyll-a relate to the HWMD watershed in extensive urban areas. Based on the total PS pollution found in the North and Central Watersheds, HWMD is the watershed to contribute a 3.66 ratio with respect to the watershed area. While this watershed has greater PS pollution than the other two watersheds, it is not particularly the most affected watershed with respect to the drainage area. The NPS and PS results for HWMD were consistent with the elevated levels of the water quality data analyzed from the SWQMIS database. Bacteria, total nitrogen, nitrate and nitrite, chlorophyll-a, ammonia, total phosphorus, and organic nitrogen in HWMD had significant values in this watershed compared to the other watersheds. In addition, the high pollutant loadings in this watershed correspond to the high flow values recorded. Therefore, more flow data are needed in the future to further support this characterization and make the proper connections between sources of pollution and pollutant loads.

The RVD watershed had a higher percent of 20.3% for ranches and was identified to be higher regarding the total area of the North and Central Watersheds as well. The water quality parameters associated with the presence of ranches are bacteria, ammonia, TP, nitrite, and nitrate. The results showed that the RVD watershed has greater bacteria levels in comparison to the other watersheds, which suggests ranches and the activities within these areas are causing high levels of bacteria. The RVD watershed pollutant loadings were generally low, but bacteria loadings were significant because of the high presence of NPS pollutants. Bacteria loading mean value corresponds to almost 12.3 MPN/km2/year.

The IBWCNF watershed was identified to have higher crop areas with 58.5% regarding the area as well as the overall area of the three watersheds, which suggests the presence of significant agricultural activities. Therefore, it was determined that agricultural runoff is prone to release higher levels of ammonia where this watershed was limited to carry high ammonia levels. This finding indicates a possible change in land cover from 2016 to 2020. In addition to ammonia, bacteria, TKN, TP, nitrite and nitrate, and chlorophylla are present in agricultural areas. The IBWCNF watershed has a greater presence of nutrient water impairments because of the high agricultural area. This finding suggests the high levels of nitrite and nitrate in this watershed correspond to agricultural lands. This watershed had the higher contribution of PS pollutants such as WWO, OSSFs, MS4s, and colonias among the watersheds. The sources contributing to the high levels of water quality concentrations were identified. Ammonia, nitrate, and nitrite primary sources can be related to WWO, MS4s, and colonias. The load concentration results showed the IBWCNF to have high bacteria and ammonia loads. This finding suggests that the presence of a significant contribution of OSSFs is linked to bacteria loadings.

To uncover which North and Central watersheds contributed the most to the LLM watershed impairment, a cyberinfrastructure was established along with an ample watershed delineation. Then NPS pollution, PS pollution, water quality concentrations, flow data, and pollutant loadings were enhanced to identify unique characteristics of the watershed.

HWMD and IBWCNF were the watersheds to contribute the most in water impairments to the LLM watershed. They were found to have significant loadings of water quality parameters as well as NPS and PS pollutant contributions. Urban areas, TLAP, and MSW were related to the high contribution of chlorophyll-a, TKN, and TP. OSSFs and colonias were linked to the major influence of bacteria concentrations and loadings of which the IBWCNF watershed possesses the most. These results along with the user-friendly cyberinfrastructure may assist stakeholders from the region in identifying the characteristics of watersheds and mitigate the sources of pollution. This study is essential in bringing awareness to the local communities that reside within these watersheds, especially the people who visit the LLM watershed. One of the limitations of this study was the acquisition of available data for such an extensive study area of more than 3000 km<sup>2</sup> . Additional flow data and water quality data could enhance the characterization as it was limited to only 8 samples for the HWMD and the RVD watersheds. Flow data are essential for determining the load concentrations and provide a better overview of the north and central watersheds' potential sources of pollution.

**Author Contributions:** Conceptualization, A.E. and A.M.; methodology, A.M., A.O. and A.E.; software, A.E. and C.F.; formal analysis, A.O. and L.N.; writing—original draft preparation, L.N., J.J., I.R.S.C. and A.M.; visualization, I.R.S.C.; supervision, A.M. and A.E.; project administration, J.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** Funding for this research was provided by the Texas Commission on Environmental Quality (Project Contract# 582-19-90196) and financed through grants from the U.S. Environmental Protection Agency (Federal ID# 99614623).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** The authors would like to thank Tim Cawthon (Texas Commission on Environmental Quality) for his support and contribution in data for the study. The authors also would like to thank Javier Guerrero and the Lower Rio Grande Valley Stormwater Taskforce members for their collaboration in the study.

**Conflicts of Interest:** The authors declare no conflict of interest.
