Esmaiel Malek

Comparison of the Bowen ratio-energy balance and the water balance methods for the measurement of evapotranspiration

Abstract An experiment was conducted in a 36 ha alfalfa field in Logan, Utah, USA, during three growth-harvest cycles of alfalfa in April–July 1992. A Campbell Scientific (CS) Bowen Ratio System was used to measure continuously 20 min means of micrometeorological elements such as air and dew point temperatures at 1 and 2 m, solar and net radiation, wind speed and direction at 2 m, soil heat fluxes at 8 cm (two locations), and soil temperatures at 2 and 6 cm during the three consecutive growing cycles. The gravimetric method (for 0–10 cm) and the Campbell Pacific Nuclear (CPN) Hydroprobe (for 20–150cm) were used to measure soil moisture daily at 09:00 h. The 24 h evapotranspiration (LE, using the Bowen ratio-energy balance method) was computed by adding the 20 min mean LE values from 09:00 h until 09:00 h of the next day for comparison with the water balance method. Evapotranspiration measured by the water balance method was 98% of that measured by the Bowen ratio-energy balance method during the experimental period (r = 0.987).

Evapotranspiration from the margin and moist playa of a closed desert valley.

Two long term microclimate measurement stations with Bowen ratio capability have been used to study water cycling in a closed desert basin. Microclimate variables including the temperature and vapor pressure gradients were monitored continuously and were used to estimate the Bowen ratio, sensible and latent heat fluxes during 1986 and 1987. Despite having a water table that varied between the surface and 30 cm below the surface, the playa had little evaporation except after rainfall events. The very high osmotic pressure of the soil and salt crust caused most of the absorbed radiation to be partitioned to sensible heat. In contrast, along the margin the thin grass and brush cover transpired water freely, with the latent heat flux exceeding 60% of available energy for much of the season. The higher air temperatures above the playa raised potential evapotranspiration (ET) significantly higher than along the margin throughout the summer. The annual average actual ET of the playa was only 36% of the margin. During the drier summer period (May–October), this ratio decreased to 638 mm were lost from the margin groundwater supply. The 24-h solar and net radiation correlations were 0.80 and 0.94 for the playa and margin, respectively. The lower correlation for the playa resulted from the wide variation of albedo with surface moisture changes. The annual average albedo values for the playa and margin were 0.64 and 0.46, respectively.

Night-time evapotranspiration vs. daytime and 24h evapotranspiration

The continuous measurement of night-time ETa,n (sunset to sunrise), daytime ETa,d (sunrise to sunset) and 24h ETa,24h alfalfa evapotranspiration was carred out, using the Bowen ratio-energy balance method, in a closed mountainous semi-arid irrigated valley in northern Utah. The results indicate that ETa,nETa,24h was 2.93 mm173.87 mm, or about 1.7%, during a complete growing cycle of alfalfa in August and September 1991. The results also show that this ratio could be very high (about 14%) when the night-time wind speed is high.

Rapid changes of the surface soil heat flux and its effects on the estimation of evapotranspiration

Abstract An experiment was conducted during August and early September 1991 in the middle of an alfalfa field in Logan, northern Utah. Twenty-minute values of climatic factors, including soil heat flux at a depth of 8 cm G 8cm and average soil temperature at 2 and 6 cm, were measured using the Campbell Scientific CS Bowen Ratio System. Knowing the soil moisture content, the values of the surface soil heat flux G sur were computed. Short term (20 min) evaluations indicated rapid changes of G sur vs. G 8cm . It was shown that in addition to solar and net radiations, cloudiness, wind speed and direction also affect the surface soil heat flux. Statistical analysis showed that daily actual evapotranspiration ET a (measured on a 20 min basis) could be overestimated up to 41% if G 8cm were used instead of G sur in the Bowen ratio-energy balance (BREB) method. The results indicated that ET a2 / ET a1 = 1.54 (based on 20-min calculations for this experiment). The suffixes 2 and 1 refer to the applicatin of G 8cm and G sur in the BREB method, respectively. The ratio for potential evapotranspiration ET p was ET p2 / ET p1 =0.998. This means using the short term G 8cm instead of G sur results in an overestimated ET a . For the 24-h computation of ET p , either G 8cm or G sur can be used without any serious errors.

Remotely-Sensed Regional-Scale Evapotranspiration of a Semi-Arid Great Basin Desert and its Relationship to Geomorphology, Soils, and Vegetation

Abstract Landsat thematic mapper data are used to estimate instantaneous regional-scale surface water and energy fluxes in a semi-arid Great Basin desert of the western United States. Results suggest that it is possible to scale from point measurements of environmental state variables to regional estimates of water and energy exchange. This research characterizes the unifying thread in the classical climate-topography-soil-vegetation relation —the surface water and energy balance— through maps of the partitioning of energy throughout the landscape. The study was conducted in Goshute Valley of northeastern Nevada, which is characteristic of most faulted graben valleys of the Basin and Range Province of the western United States. The valley comprises a central playa and lake plain bordered by alluvial fans emanating from the surrounding mountains. The distribution of evapotranspiration (ET) is lowest in the middle reaches of the fans where the water table is deep and plants are small, resulting in low evaporation and transpiration. Highest ET occurs in the center of the valley, particularly in the playa, where limited to no vegetation occurs, but evaporation is relatively high because of a shallow water table and silty clay soil capable of large capillary movement. Intermediate values of ET are associated with large shrubs and is dominated by transpiration.

Annual mesoscale study of water balance in a Great Basin heterogeneous desert valley

We studied the annual mesoscale water balance in northeastern Nevada, USA, in a Great Basin heterogeneous semi-arid desert valley (the Goshute Valley) at 40°44′N, 114°26′W, witgh elevation of 1707 m above mean sea-level. This north-south-oriented flat valley has an area of about 1113 km2 and is partially covered mostly by sagebrush, greasewood, shadscale, desert molly, cheatgrass, and winter fat bushes. Five Bowen ratio stations measured the incoming and outgoing (reflected) solar radiation, net radiation, air temperatures and moisture at 1 and 2 m, the aggregated (soil + vegetation) surface temperature, wind speed and direction at 10 m, soil heat flux at 8 cm (three locations at each station), soil temperatures at 2 and 6 cm above each soil flux plate, and precipitation every 5 s averaged to 20 min throughout the valley from 1 May 1993 to 30 September 1994. Locations of stations were based upon the vegetation types and percentage of coverage by bushes. The topsoil (10 cm) moisture content was measured either by time domain reflectometer or gravimetric method at least once a week. We used the Bowen ratio energy balance (BREB) method for the measurement of 20 min evapotranspiration throughout the experiment. During the dry water year 1993–1994 (beginning from 1 October) the average amount of aggregated (soil + bushes) evapotranspiration (ETa) among stations measured by the BREB method was almost equal to the average total precipitation for the entire valley (160.9 mm vs. 157.7 mm, respectively). Variations of precipitation among stations (ranging from 173.7 mm at Station 2 to 130.5 mm at Station 1) were attributed to winter orographic effects and summer thermal lows. ETa ranged from 181.2 mm at Station 3 to 142.7 mm at Station 2. Variations were related mostly to vegetation types and percentage of the soil coverage. All stations showed slight water losses (ETa greater than precipitation) in the dry water year 1993–1994, except at Station 2, where water gain was about 31.1 mm. The 1993–1994 water losses at Stations 1, 3, 4 and 5 were 29.7 mm, 7.5 mm, 8.1 mm and 2 mm, respectively. The additional water for the process of evapotranspiration at these stations may be due to extraction of moisture from the air during the cool early mornings throughout the water year or to extraction of moisture from the soil. A set of relationships between the incoming solar and net radiation, surface and 2 m air temperatures, and surface and 8 cm soil heat fluxes among stations was also developed. This study showed how the mesoscale water balance was achieved during a dry water year in a desert valley. The findings could be applied to similar valleys.

Comparison of the Bowen ratio-energy balance and stability-corrected aerodynamic methods for measurement of evapotranspiration

SummaryThe Bowen ratio-energy balance (BREB) and the stability-corrected aerodynamic method were used to estimate turbulent fluxes of sensible and latent heat at an irrigated alfalfa site in a semi-arid valley in northern Utah, U.S.A., during August and September of 1991. Despite inclusion of a generalized stability factor, the aerodynamic method underestimated the daytime (sunrise-sunset) sensible and latent heat fluxes by approximately 30% in comparison with the BREB method. The sum of the aerodynamic estimates of sensible and latent heat seldom balanced the energy avaiable from net radiation and change in storage. Wind speed was low during the experiment (averaging 1.6 m s−1), and so a second analysis was run for data from daytime, non-rainy, turbulent conditions (wind > 1.5 m s−1). This showed that sensible and latent heat were still underestimated by approximately 30% in comparison with the BREB approach. This suggests that underestimation of sensible and latent heat fluxes by the aerodynamic method was not related to the wind speed conditions during the experiment. These results show that the stability-corrected aerodynamic model did not agree with the Bowen ratio method in this experiment. It appears unlikely that the discrepancies resulted from measurement errors. Perhaps the theoretical foundation of the similarity parameters (stability functions) in the aerodynamic model are not sufficiently generalized. The discrepancies found here confirm the necessity of calibration checks on the validity of aerodynamic estimates of the turbulent fluxes.

CONTINUOUS MEASUREMENT OF AERODYNAMIC AND ALFALFA CANOPY RESISTANCES USING THE BOWEN RATIO-ENERGY BALANCE AND PENMAN-MONTEITH METHODS

To investigate the alfalfa crop response to environmental factors, a Bowen ratio-energy balance method was used to evaluate short-term alfalfa canopy resistance. Continuous evapotranspiration (ETa) and the aerodynamic resistance (ra) for an alfalfa crop in each 20-min interval were calculated. Using the calculated ETa and ra and the Penman-Monteith approach, the bulk stomatal or actual canopy resistance (rc) was evaluated. The continuous 20-min resistances were computed for clear and partially cloudy sky conditions, and different average crop heights. The results show that this technique can satisfactorily be used to study the manner in which the aerodynamic and canopy resistances respond to short-term variations in weather elements such as photosynthetically active radiation (PAR), wind speed and atmospheric saturation vapor deficit.

Comparison of alternative methods for estimating ETp and evaluation of advection in the Bajgah area, Iran

Abstract This paper seeks to characterize the impact of advected energy on evaporation behaviour of irrigated lucerne in the arid region of southern Iran. A set of monthly coefficients are presented to account for enhanced rates of evaporation due to divergence in sensible heat flux caused by horizontal transport of dry, hot air over an irrigated field. The coefficients are presented for irrigation scheduling estimates based on Class A pan evaporation.

Calibration of the Penman wind function using the Bowen ratio energy balance method

Abstract The Bowen ratio energy balance (BREB) method was used to calibrate the wind function constants (Wf = aw + bwu2) in the aerodynamic term of the Penman equation for computation of the hourly and 24 h alfalfa-reference evapotranspiration (Etr) at an irrigated semi-arid valley in northern Utah during the 1991–1992 growing season. Statistical analysis of the 28 non-rainy days (when the conditions of alfalfa-reference evapotranspiration were met) showed that the hourly constants in the wind function were aw = 2.677 and bw = 0.127 (with u2 expressed in km h−1). The hourly relationship between the alfalfa-reference evapotranspiration estimated by the BREB method (Etr(BREB)) and that computed by the Penman method (Etr(Penman)) is Etr(Penman) = 0.976Etr(BREB), with the correlation coefficient (R) equal to 0.974. The daily (24 h) constants were aw = 1.00 and bw = 0.014 (with u2 expressed in km day−1). The 24 h relationship is Etr(Penman) = 1.009Etr(BREB), with R = 0.988. The wind function constants found here can be applied for computation of hourly and 24 h alfalfa-reference evapotranspiration and irrigation scheduling in this semi-arid valley.