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 River 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 water table caused partial wilt of outer smaller twigs in the canopy of many trees in the stand around the Bowen tower. Subsequently, evapotranspiration (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 tower area changed its course, hydrologically isolating the Tamarix stand in the vicinity of the tower. In 1996, a 25% canopy loss was visually estimated for the Tamarix growing in the area of the tower. Higher soil temperatures relative to air temperatures were recorded in 1996 in 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°C rise over a 9-hour period was correlated with increased wind speeds 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 energy 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 floodplain (hundreds of meters) as opposed to regional advection, since the majority of wind (90%) was in a N–S direction along the course of the river, and that a more open canopy allowed the horizontal transfer of energy into the Tamarix stand at the Bowen tower. Our results suggest that Tamarix has the potential to be both a low water user and a high water user, depending on moisture availability, canopy development, and atmospheric demand, and that advection can dominate energy balances and ET in aridland riparian zones such as the Virgin River.

[1]  J. Cleverly,et al.  Plant water relations of Tamarix ramosissima in response to the imposition and alleviation of soil moisture stress , 1997 .

[2]  G. S. Campbell,et al.  Water use by saltcedar and by replacement vegetation in the Pecos River floodplain between Acme and Artesia, New Mexico , 1987 .

[3]  J. Gash,et al.  The average surface resistance of a pine forest derived from Bowen ratio measurements , 1975 .

[4]  I. S. Bowen.  The Ratio of Heat Losses by Conduction and by Evaporation from any Water Surface , 1926 .

[5]  Daniel C. Bowman,et al.  Evapotransportation, Crop Coefficients, and Leaching Fractions of Irrigated Desert Turfgrass Systems , 1992 .

[6]  K. Mcnaughton.  Evaporation and advection II: evaporation downwind of a boundary separating regions having different surface resistances and available energies , 1976 .

[7]  L. Fritschen.  An energy budget analysis of water use by Saltcedar , 1979 .

[8]  Dennis Baldoechl.  Canopy-atmosphere water vapour exchange: Can we scale from a leaf to a canopy? , 1989 .

[9]  Todd E. Dawson,et al.  Determining water use by trees and forests from isotopic, energy balance and transpiration analyses: the roles of tree size and hydraulic lift. , 1996, Tree physiology.

[10]  O. E. Leppanen.  Evapotranspiration from rapidly growing young saltcedar in the Gila River Valley of Arizona , 1981 .

[11]  K. G. McNaughton,et al.  Stomatal Control of Transpiration: Scaling Up from Leaf to Region , 1986 .

[12]  M. Novello,et al.  Geodesic motion and confinement in Gödel's universe , 1983 .

[13]  Stanley D. Smith,et al.  Water Use by Tamarix Ramosissima and Associated Phreatophytes in a Mojave Desert Floodplain , 1996 .

[14]  A. Salal,et al.  The Effect of applied water on the water use of saltcedar in a desert riparian environment , 1997 .