Introduction

1. Are depth to groundwater and soil moisture correlated? Yes, depth to groundwater and soil moisture are correlated at all depths (i.e. 25, 40, 55, 70, 85 and 100 cm below ground). Is soil moisture at flood sites more responsive to river flow than soil moisture at nonflood sites? … Perhaps (to be investigated) Abstract Over the past 50 years, volume and timing of the Rio Grande’s flow, including the annual flood pulse, have been altered due to damming and diversion of the river. As a result, the river is largely isolated from its riparian forest, or bosque, and the native cottonwood forest is aging, is not regenerating, and is being invaded by exotics. Restoration of native bosque may require restoring the annual flood pulse and thus the hydrologic connection between the river and its floodplain. To determine the current extent of hydrologic connectivity within a 160 km stretch of the Middle Rio Grande in New Mexico, we are investigating the relationship among river flow, groundwater depth and soil moisture at 4 flood and 4 nonflood sites. United States Geological Survey station data are used to monitor river flow at various locations within our study reach. Pressure transducers at each site measure groundwater depths throughout the year. Water content of the top 30 cm of soil at each site is measured using a water content reflectometer, and soil moisture from 25 to 100 cm below ground is measured using a neutron probe. Preliminary analysis of pressure transducer data suggests that flood sites have a higher water table than nonflood sites, and analyses of neutron probe data suggest that soil moisture is higher at flood sites. In addition, soil moisture and depth to groundwater are negatively correlated. Results will help us inform policy-makers on potential tools, such as managed seasonal flooding in years with adequate water availability, to help restore the native bosque and the hydrologic connectivity between the river and its floodplain. Is the water table higher at flood sites than at nonflood sites? Yes (P=0.002) Are groundwater levels at flood sites more responsive to river flow than groundwater levels at nonflood sites? …Perhaps (to be investigated) • Introduction • The Rio Grande’s flows are about half of what they were 60 years ago, and half of the wetlands in the Middle Rio Grande have been lost in just 50 years (Crawford et al. 1993). • Overbank flooding in late spring/early summer due to snowmelt runoff occurs in a very limited area only in years with above-average snowfall. The last major floods in the Middle Rio Grande that produced large-scale cottonwood establishment occurred in the springs of 1941 and 1942. • Cottonwood germination, which require sand bars and adequate moisture from high river flows (Braatne et al. 1996), has declined substantially. • Without changes in water management, the Rio Grande bosque (riparian forest) will be dominated by exotic species, namely salt cedar (Tamarix ramosissima) and Russian Olive (Elaeagnus angustifolia) over the next 50 to 100 years (Howe and Knopf 1991). • Restoration of native bosque along the hydrologically altered and highly controlled Rio Grande would require instituting managed late spring/early summer floods in years with good water availability that coincide with the historical peak in snowmelt discharge. Are soil moisture levels higher at flood sites than at nonflood sites? Yes • What distinguishes flood sites from nonflood sites? Is it… • Elevation of wells in relation to the river? • Distance of site to the river? • Soil type, hydraulic conductivity, stratigraphy? Downloading pressure transducer Groundwater well with pressure transducer Taking soil moisture readings with a neutron probe Literature Cited Braatne, J.H., S.B. Rood and P.E. Heilman. 1996. Life history, ecology and conservation of riparian cottonwoods in North America. In Biology of Populus and its Implications for Management and Conservation. Edited by R.F. Stettler, H.D. Bradshaw, Jr., P.E. Heilman and T.M. Hinckley. NRC Research Press, Ottawa, Ontario, Canada. pp. 57-85. Crawford, C.S., A.S. Culley, R. Leutheuser, M.S. Sifuentes, L.H. White, and J.P. Wilber. 1993. Middle Rio Grande Ecosystem: Bosque Biological Management Plan. U.S. Fish and Wildlife Service, District 2, Albuquerque, New Mexico, 291 pp. Howe, W.H., and F.L. Knopf. 1991. On the imminent decline of the Rio Grande cottonwoods in central New Mexico. Southwestern Naturalist 36: 218-224. Acknowledgements UNM Hydrogeoecology Group NSF Grant DEB-9903973 Middle Rio Grande Conservancy District Bosque del Apache National Wildlife Refuge City of Albuquerque Open Space Division New Mexico State Land Office Rio Grande Nature Center Contacts Jennifer F. Schuetz 505-277-5732 jschuetz@sevilleta.unm.edu Jean-Luc Cartron 505-277-5732 jlec@unm.edu Manuel C. Molles, Jr. 505-277-3050 molles@sevilleta.unm.edu Cliff N. Dahm 505-277-2850 cdahm@sevilleta.unm.edu Cliff S. Crawford 505-277-3411 ccbosque@juno.com Nonflood site with non-native understory of salt cedar and Russian Olive, older cottonwoods, and a large quantity of coarse woody debris and litter on forest floor Flood site with native understory of willow and other species, relatively healthy cottonwoods, and a small quantity of coarse woody debris and litter on forest floor