Bowen Ratio Estimates of Evapotranspiration for Tamarix ramosissima Stands on the Virgin River in Southern Nevada
D.A. Devitt, A. Sala, S.D. Smith, J. Cleverly, L.K. Shaulis and R. Hammett
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 based on 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 sixty day period (11 mm day-1 to < 1 mm day-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 wind functions were observed in 1996. On day 160 of 1996, a 28C0 rise over a nine 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 vs. 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. We conclude that Tamarix can 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.
MATERIAL AND METHODS
A field study to quantify Bowen ratio ET estimates of Tamarix ramosissima (Ledeb) was conducted during 1994 - 1996 along the lower Virgin River (southern Nevada). The site was located in the floodplain near the northern boundary of Lake Mead National Recreational Area (36o 35' N, 114o 20' E, elevation 380 m). The site consisted of a monospecific stand of mature Tamarix thickets growing on raised river sediment deposits within the 1200 m wide floodplain. At the experimental area, the height of the Tamarix was approximately 4 m. Annual rainfall in this region is usually less than 10 cm per year, with maximum air temperatures recorded in July of 50C0 and annual potential evapotranspiration of 220 cm (Devitt et al. 1989). Depth to the water table at the site varied throughout the year and from year to year with values ranging from 2 to >3 m.
A Bowen ratio energy balance (BREB) approach was used to estimate canopy level ET by Tamarix (Bowen 1926). An 8 m Bowen tower was erected over a dense full canopy stand of Tamarix, approximately 312 m from the desert edge. Local fetch requirements were typically met, as prevailing wind direction was N-S, along the riparian corridor, where Tamarix canopy formed a fairly continuous cover over the width of the floodplain.
The instrumentation used was based upon the design commercially available from Campbell Scientific (Logan, UT). A net radiometer (REBS, Seattle, WA) was mounted at 4 m above the canopy. Vapor pressure, air temperature, wind speed and wind direction were measured at 1.5 and 4 m above the canopy. Vapor pressure above the canopy was measured in air samples taken alternately at each sensor height, from the end of an arm extending 1.3 m from the main tower structure. The air samples were aspirated to a single relative humidity/ air temperature sensor (Humicap, Vaisala, Sweden) after passing through a mixing chamber. Air temperatures were measured with fine wire chromel-constantan thermocouples (0.025 mm), mounted at the end of the arms. Wind speed and direction were measured with a RM Young wind sentry wind set (Traverse City, MI). Soil heat flux was estimated from measurements taken with a soil heat flux plate mounted at 8 cm below the soil surface (REBS, Seattle, Washington) and changes in soil heat storage estimated from changes in soil temperature above the plate. Soil heat transfer calculations were based on measured soil bulk densities and soil relative water contents. All sensors were operated by a CR10 data logger (Campbell Scientific, Logan,Utah), and data were recorded as 20 min averages.
Surface flow in the Virgin River (USGS, Littlefield AZ, gauging station, 57.9 km upstream from Lake Mead) was obtained as 1 day total stream discharges (USGS 1997). The electrical conductivity of the river water averaged 2.84 dS m-1. Complete chemical analysis of the water is reported elsewhere (Devitt et al. 1997a).
Data were analyzed with descriptive statistics and regression analysis. Significant results were reported only when the regression coefficient had a p < 0.05.
1) During the three year monitoring period of this study, surface flow in the Virgin River (Fig. 1) remained below 24.7 x 106 m3 (20,000 acre feet) per month except during a five month period in 1995 when a flood event occurred in March (123.3 x 106 m3 ;>100,000 acre feet). In 1995 the accumulated yearly flow was 440.3 x 106 m3 (357,000 acre feet) , which was typical of high flow years on the Virgin River which occur on the average every 5 years.
2) The flood event in 1995 caused the river to change its course, thereby subjecting the Tamarix near the Bowen tower to decreased moisture availability in 1996.
3) Canopy loss in 1996 was visually estimated to be 25%. To determine if this estimated canopy loss associated with the change in the course of the river had any significant impact on heat transfer within the canopy, the relationship between air temperatures (1.5 m above the canopy) and soil temperatures (6 cm depth) were compared in both years (Fig. 2). Air temperatures were higher than soil temperatures during the active growing period (days 120-300) in 1994. However, in 1996 no clear separation in air and soil temperatures could be observed.
4) With a more open canopy, thermally induced wind functions were observed to occur in 1996. On day 160 of 1996, a 28C0 rise over a nine hour period was correlated with increased wind speeds of greater than 4 m/s-1 , with the most dramatic rise occurring between the hours of 1000 and 1400 (Fig. 3).
5) Evapotranspiration (ET) estimates (Fig. 4) generated from energy balance calculations were averaged for each month and plotted with average net radiation (Rn) values to determine if significant change in ET occurred that could not be explained by energy input (Fig. 5). Rn curves developed for both years were not significantly different (p=0.001).
6) In June of 1994, record high temperatures and dry winds associated with falling water tables caused partial wilt of outer smaller twigs in the canopy of many trees in the stand around the Bowen tower (Devitt et al. 1997a). Following this stress period, ET estimates fell rapidly over the next 60 days. Maximal ET values in June were in excess of 11 mm/day, but by August, values had dropped to below 1 mm/day, showing no correlation with net radiation values. In 1996, even though canopy loss occurred, significantly higher ET rates were estimated for the summer months. The ET estimates in 1996 paralleled the change in net radiation.
7) Yearly estimates of ET were 75 cm in 1994 and 145 cm in 1996. Average values for January - March, 1994, were extrapolated from the data; minimal error was believed to occur with these estimates since the trees were without leaves during this time period
8) Complete energy balances were plotted for four separate days in 1994 that represented the time period over which ET significantly declined in response to water stress. Days as close to those selected in 1994 were chosen in 1996 for comparison (Fig. 6 and 7). On day 161 in 1994 latent energy (LE) exceeded net radiation (Rn) between the hours of 1200 and 1500 and ET was estimated at 11.2 mm/day . By day 184 (early July), ET estimates had dropped to 6.4 mm and LE was less than Rn between the hours of 0900 and 1600. A significant decline in ET (2.8 mm) occurred by day 212 (late July), with sensible heat exceeding LE between the hours of 0900 and 1600. Finally, by day 232 (mid-August), ET had declined to a value of 0.4 mm day-1 with sensible heat dominating the energy balance. In 1996, on day 160 ET was estimated at 14.7 mm and LE was significantly greater than Rn between the hours of 1000 and 1800. On day 187, ET was 8.2 mm and LE exceeded Rn between the hours of 1100 and 1700. By day 211, ET was still high at 8.2 mm and LE exceeded Rn between the hours of 1100 and 1800. Finally, by day 230 ET was estimated at 6.4 mm, with LE still exceeding Rn between the hours of 1300 and 1800.
9) Advection was inferred whenever LE exceeded Rn. The percentage of days (between days 120 and 300) in which daily total estimates of LE exceeded Rn was 11% in 1994 and 65% in 1996. On those days in which LE exceeded Rn , LE surpassed Rn by an average of 35 + 22% in 1994 and 36 + 30% in 1996. Although the riparian area was surrounded by dry desert areas with sparse vegetation, the prevailing wind pattern was in a north-south direction along the river course. Between the hours of 0600 and 2000, wind originating from either an east (60o -120o) or west (240o-300o) direction was observed to occur only 10 % of the time on a monthly basis in 1994 and 7 % in 1996.
This study has demonstrated that advective conditions are important in riparian corridors dominated by Tamarix in the arid regions of the Southwest. Because of advective conditions, ET estimates generated in this study should be used with extreme caution. Inability of the Bowen method to take into account incoming advective heat (horizontal energy flow), which would need to be added to the energy balance, leaves such ET estimates questionable. Results from this study, however, indicate that Tamarix can significantly reduce transpirational loss under limited water availability (1994 year), that Tamarix will reduce canopy volume (1996 year) in response to changes in river course (1995 year), and that ET estimates can be fueled by greater energy transfer within these open canopies. A significantly higher number of advective days occurring in 1996 may have been associated with advection originating from within the floodplain.
We conclude that Tamarix ramosissima can 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 balance approaches to ET in the arid regions of southern Nevada.
This study was funded by the Las Vegas valley Water District. The authors thank Mr. Jeff Piorkowski, Mr. Matt Ronshaugen, Ms Kimberly Mace, Mr. Peter Ross and Dr. Tim Ball for their assistance.
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