ECOHYDROLOGY OF THE MIDDLE RIO GRANDE RIPARIAN ZONE IN NEW MEXICO

The Middle Rio Grande (MRG) is a large river flowing from north to south through central New Mexico. While historically meandering, the MRG was straightened by the US Army Corps of Engineers following catastrophic flooding in the Albuquerque reach during the early 1940s. Currently, riparian corridors along the MRG are choked with vegetation supported by a relatively shallow water table (1-4 m below ground surface) that is maintained by a network of riverside drains, reservoir releases, and waste water return flow. Native Rio Grande cottonwood (Populus deltoides ssp wislizeni) are restricted to areas with a reliably shallow aquifer, and they experience crown dieback in the southern MRG where the depth to the water table can exceed 3 m. Non-native saltcedar (Tamarix chinensis), on the other hand, has not shown any restriction to ET caused by excessive groundwater depth. On the contrary, evapotranspiration rates measured from saltcedar increase with declining groundwater depth. Through their direct access to groundwater, desert riparian phreatophytes maintain a local ET to precipitation ratio in excess of unity, and this excess ET enables evaporative cooling of cottonwood, and to a lesser extent saltcedar, canopies. Ecological restoration along the MRG, in the form of removing saltcedar and other undesirable, non-native species alters local energy balance by allowing more sunlight to reach the soil and eliminating morning and early-afternoon evaporative cooling. This modified energy balance at a restoration site, involving the transformation of the site from a heat sink to a heat source, is accompanied by a significant increase in ecosystem stomatal resistance (r_s). The same increase in rc is observed during drought, although r_s in all MRG species is small and limited by vapor pressure deficit (VPD). The interplay of climate, vegetation, and groundwater in the MRG illustrates a rich assemblage of hydrological and ecological processes at work in a large, highly-engineered river and riparian ecosystem.