BOWEN-RATIO ESTIMATES OF EVAPOTRANSPIRATION FOR TAMARIX RAMOSISSIMA STANDS ON THE VIRGIN RIVER IN SOUTHERN NEVADA

A Bowen ratio energy balance was conducted over a Tamarix ramosissima (saltcedar) stand growing in a riparian corridor along the Virgin Rive r in southern Nevada, Measurements in two separate years were compared and contrasted on the basis of changes in growing conditions. In 1994 , a drought year, record high temperatures, dry winds, and a falling w ater table caused partial wilt of outer smaller twigs in the canopy of many trees in the stand around the Bowen tower. Subsequently, evapotr anspiration (ET) estimates declined dramatically over a 60-day period (11 mm d(-1) to <1 mm d(-1)). In 1995, the Virgin River at the Bowen t ower area changed its course, hydrologically isolating the Tamarix sta nd in the vicinity of the tower. In 1996, a 25% canopy loss was visual ly estimated for the Tamarix growing in the area of the tower. Higher soil temperatures relative to air temperatures were recorded in 1996 i n response to this loss in canopy, With a more open canopy, thermally induced turbulence was observed in 1996, On day 160 of 1996, a 28 degr ees C rise over a 9-hour period was correlated with increased wind spe eds of greater than 4 m s(-1). Subsequently, higher ET estimates were made in 1996 compared to 1994 (145 cm versus 75 cm), However, the ener gy balance was dominated by advection in 1996, with latent energy flux exceeding net radiation 65% of the measurement days compared to only 11% in 1994, We believe this advection was on a scale of the floodplai n (hundreds of meters) as opposed to regional advection, since the maj ority of wind (90%) was in a N-S direction along the course of the riv er, and that a more open canopy allowed the horizontal transfer of ene rgy into the Tamarix stand at the Bowen tower. Our results suggest tha t Tamarix has the potential to be both a low water user and a high wat er user, depending on moisture availability, canopy development, and a tmospheric demand, and that advection can dominate energy balances and ET in aridland riparian zones such as the Virgin River.