Regulation of water flux from the ground to the atmosphere in riparian habitats. Cleverly, James^*,1, Dahm, Clifford¹, Thibault, James, McDonnell, Dianne^{1, 2}, Allred Coonrod, Julie², ¹ Department of Biology, Albuquerque, NM² Department of Civil Engineering, Albuquerque, NM

ABSTRACT- Riparian evapotranspiration (ET) has received a great deal of attention as rivers run dry through the current drought. In light of some of the grandiose claims involving ET from certain types of vegetation, long-term observations of the hydrologic, vegetative, and atmospheric constraints upon riparian ET contribute to the best possible management of water resources. Riparian plant water use depends upon groundwater dynamics in a species-specific manner. Tamarix chinensis (saltcedar) responds to groundwater decline with increased ET as greater soil volumes can be exploited by rapidly growing roots systems. Populus deltoides, on the other hand, has shown chlorosis, crown dieback, and loss of leaf area index (LAI) when the water table dropped below two meters. At LAI < 3, ET increased with LAI regardless of species composition. Limited returns were observed when LAI > 3 due to shading of lower canopy leaves. On a daily basis, local atmospheric conditions were most related to extremes in ET. Vapor pressure deficit (VPD), net radiation (R_n), and the friction coefficient (u_*) were positively correlted to daily ET, while sensible heat flux (H), cross-corridor wind speed (v), and temperature (T) were negatively correlated to daily ET. The strength of these relationships is dependent upon nearby topographical features, distinguishing nearby mesas from mountains. Both hydrologic and atmospheric constraints upon riparian ET are mediated through vegetation composition, physicality, plant stress responses.

KEY WORDS: atmospheric physics, ecohydrology, non-native species, riparian ecosystems