Advection Influences on Evapotranspiration of Alfalfa in a Semiarid Climate

Advective enhancement of crop evapotranspiration (ET) occurs when drier, hotter air is transported into the crop by wind, and can be an important factor in the water balance of irrigated crops in a semiarid climate. Thirteen days of moderate to extremely high ET rates of irrigated alfalfa (Medicago sativa L.) were evaluated using energy balance and atmospheric coupling models to examine the magnitude of ET enhancement due to advection. Alfalfa ET was measured using precise, monolithic weighing lysimeters. The average ET of the selected days was 11.3 mm d, with ET exceeding 15 mm d on 3 d, with mean 24-h vapor pressure deficit (VPD) of 2.1 kPa and 2-m wind speed of 4.4 m s. Evapotranspiration due to available energy (net radiation1 soil heat flux) was fairly stable at an average of 6.6 mm d whereas advected atmospheric deficits and sensible heat flux (H) added as much as 10.5 mm d to ET, with H providing an average of 42% of the energy used in ET. Overnight ET losses due to continued H flux gains and VPD resulted in ET losses as large as 3.0 mm. Advective enhancement of ET plays a significant role in the water balance of the semiarid region of the southern High Plains. PENMAN (1948) advanced the theoretical and experimental aspects of evapotranspiration (ET) science with his general equation for the rate of ET from open water, bare soil, and grass. He merged two separate theories concerning evaporation by recognizing that ET estimation required both a thermodynamic equation for surface energy balance and an aerodynamically based vapor transfer equation, making ET a function of the meteorological elements of solar irradiance, air temperature, vapor pressure, andwind (Monteith, 1998). Monteith (1965, 1981) expanded the applicability of Penman’s original equation to other surfaces by adding variable resistances to the fluxes of momentum, heat, and water vapor through the plant–atmosphere system based on surface characteristics such as a crop’s stomatal and aerodynamic resistances, which came to be known as the Penman–Monteith (P-M) model. The P-M model is now widely used in many variations, e.g., McNaughton and Jarvis (1983) and Allen et al. (2005). The McNaughton-Jarvis (M-J) model separately calculated the P-M model’s energy balance and vapor transfer terms, but also added aweighting, or decoupling, factor V which ranged between 0 and 1 (McNaughton and Jarvis, 1983). This factor, which was based on the ratio of surface and aerodynamic resistances, determined the partitioning of ET between the energy balance and vapor transfer terms based on the degree of decoupling from the regional atmospheric conditions. Their equation was written as 2lE 5 V[D(Rn 1 G)/(D 1 g)] 1 (1 2 V)[(rcpVPD)/(g rs)] [1] where lE is latent heat flux, Rn is net radiation, andG is soil heat flux, all inWm with fluxes toward the surface positive in sign; D is the slope of the saturation vapor pressure-temperature curve (kPa 8C); l is the latent heat of vaporization (J kg); r is air density (kg m); cp is the specific heat of air at constant pressure (1013 J kg 8C21); VPD is the vapor pressure deficit (kPa); g is the psychrometric constant (kPa 8C); rs is surface (crop and soil) resistance (s m) to vapor transport; and V is defined as: V 5 {1 1 [g/(D 1 g)](rs/ra)} 21 [2] where ra is aerodynamic resistance (s m ). In a system where ra is very large compared with rs such that V tends toward 1, latent heat flux is determined principally by the energy balance term, [D(Rn 1 G)/(D 1 g)]. The ET rate is effectively “decoupled” from regional atmospheric conditions, implying that the saturation deficit is controlled by physical processes at the surface, and ET is in “equilibrium” (ETeq) with available energy (AE), or Rn 1 G. Conversely, when ra is small compared with rs such that V tends toward 0, latent heat flux is increased beyond that supplied by AE by the vapor transfer term, (rcpVPD)/(g rs), with the saturation deficit imposed on the surface by the state of the air passing over it (Monteith, 1998). Regionally determined vapor and heat concentrations are advected to the surface by vigorous turbulent mixing by wind, resulting in an “imposed” ET rate (ETimp). Advection is the transport of an atmospheric property (e.g., vapor, heat) solely by the mass motion of the atmosphere expressed in terms of wind and the atmospheric property and its gradient (Rosenberg et al., 1983). McNaughton and Jarvis (1983) considered the impact of dry or moist air advection on the local equilibrium saturation deficit that would result in either the enhancement or depression of the ET rate. They reported that, for extensive areas of short grass or crops with wet or dry surfaces, the advection component was found empirically to be typically about one-fourth of the radiation component. Advective enhancement of ET also occurs when sensible heat flux (H, in W m) transfers energy toward USDA–Agricultural Research Service Conservation and Production Research Laboratory, P.O. Drawer 10, Bushland, TX 79012. Mention of trade or manufacturer names is made for information only and does not imply an endorsement, recommendation, or exclusion by USDA-ARS. Received 1 Feb. 2006. *Corresponding author (jtolk@ cprl.ars.usda.gov). Published in Agron. J. 98:1646–1654 (2006). Agroclimatology doi:10.2134/agronj2006.0031 a American Society of Agronomy 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: AE, available energy (Rn 1 G); DOY, day of year; ET, evapotranspiration; ETeq, equilibrium evapotranspiration; ETimp, imposed evapotranspiration; ETimp-night, nighttime imposed evapotranspiration; LAI, leaf area index; VPD, vapor pressure deficit. R e p ro d u c e d fr o m A g ro n o m y J o u rn a l. P u b lis h e d b y A m e ri c a n S o c ie ty o f A g ro n o m y . A ll c o p y ri g h ts re s e rv e d .