Vegetation greening and browning trends in the Rio Grande – Rio Bravo basin from 2000 to 2020

The Rio Grande/Bravo (RG/B) is a transboundary river shared by the United States of America (USA) and Mexico. The RG/B River basin has a drainage area of more than 550,000 sq. km. It extends over three states in the USA (Colorado, New Mexico, and Texas) and five states in Mexico (Durango, Chihuahua, Coahuila, Nuevo León, and Tamaulipas). The watershed area is home to over ten million people. The region is under sustainability stress due to various challenges, including shifting land use and cultivation patterns, and climate change that affects the regional vegetation. We evaluate spatiotemporal dynamics of vegetation greening and browning using the trend and rate of change of annual median NDVI (Normalized Difference Vegetation Index) values using Landsat and MODIS data. Further, we have explored the climatic parameters (precipitation and temperature) using DAYMET and ERA5 products, and satellite-based precipitation estimates using CHIRPS and IMERG products. A non-parametric (Mann-Kendall and Theil-Sen) trend analysis is conducted to analyze the monotonic trend of NDVI and climatic parameters. The correlation between climatic parameters and NDVI is also analyzed to diagnose the modulating factor of vegetation greening/browning in different regions across the RG/B River basin. The data was used to delineate greening (increasing vegetation indices), and browning (decreasing vegetation indices) regions using the latest 21 years (2000 – 2020) data. Temporal anomalies were computed by normalizing the temporal deviation from average annual values with 1) Standard Deviation and 2) Range (max minus min). Overall, greening is seen in the southern part of the RG/B River basin (i.e., Mexico) and browning in most USA regions. However, we could also see slight greening effects on the headwater of the RG/B River in Colorado (USA). There are no clear statistically significant trends in precipitation change. However, a declining trend (statistically insignificant) is dominant across New Mexico (USA).