Riparian zones are important for ecological purposes and ecosystem processes, and are valued for aesthetic, recreational, cultural, and historical reasons. The declining integrity of cottonwood-dominated riparian systems in the Middle Rio Grande (MRG) of central New Mexico has been evident for several decades, of which the largest cause has been the severe alterations riparian hydrology. While cottonwood germination responses to changing flood regimes have been well studied, the response to changing groundwater dynamics - and the suitability of groundwater regimes in the MRG - is less well understood. This study used pressure transducer groundwater datasets installed in the Rio Grande riparian zone by the Bosque Ecosystem Monitoring Program (BEMP) to investigate groundwater behavior in the MRG and its impact on bank storage, cottonwood recruitment, and native riparian integrity. A relational database of BEMP’s groundwater data was constructed, and its utility was assessed. It was concluded that BEMP’s data are largely accurate, with some exceptions. Time series analysis of the data indicated that riparian groundwater responds rapidly to changes in streamflow, and that bank storage is transient and does not extend far from the river channel. This may be caused by agricultural drains, which induce an uncharacteristic permanent hydraulic gradient sloping away from the river. This gradient intercepts bank storage and causes rapid groundwater recessions after high discharge events. At all study sites but one, groundwater recession is controlled directly by the rate of discharge decline, and often exceeded the maximum rate tolerable by cottonwood seedlings. A single successful cottonwood recruitment event in 2009 at one of the sites was captured in the pressure transducer record. Groundwater observations from this event indicate that cottonwood seedlings can tolerate relatively rapid recession rates, as long as these rates are not prolonged, or are interspersed with slower or negligible rates. Ultimately, the primary difference between present day conditions and when large-scale recruitment events occurred is the flow regime of the Rio Grande and loss of high-discharge flood events.
Riparian zones, cottonwood, riparian systems, Middle Rio Grande, hydrology, Bosque Ecosystem Monitoring Program, BEMP, groundwater
Heller, Tom. "Surface-water groundwater interactions in the Middle Rio Grande, NM Implications for bank storage and native species." (2018). https://digitalrepository.unm.edu/wr_sp/155