Evapotranspiration Methods compared on a Sierra Nevada Forest Ecosystem Joshua Fisher Abstract Evapotranspiration, as a major component in terrestrial water balance and net primary productivity models, is often difficult to measure and predict. This study compared five potential evapotranspiration models applied to a ponderosa pine forest ecosystem at an Ameriflux site in Northern California. The Ameriflux sites are research forests across the United States, Canada, Brazil, and Costa Rica with towers measuring carbon, water, and energy fluxes out of the ecosystems. The evapotranspiration models ranged from simple temperature- and solar radiation-driven equations to physically-based combination approaches and included reference surface and surface cover-dependent algorithms. For each evapotranspiration models, results were compared against mean daily latent heat from half-hourly measurements recorded on a tower above the forest canopy. All models calculate potential evapotranspiration and thus overpredicted values from the summer seasons of 1997 and 1998. Development of a soil moisture function to connect potential with actual evapotranspiration resulted in significant improvement on three of the five models. A modified Priestley-Taylor method performed well given its relative simplicity.Introduction The terrestrial water cycle has become increasingly important in understanding climate, plant community dynamics, carbon and nutrient biogeochemistry, and the structure and function of aquatic ecosystems. The necessity of understanding terrestrial water cycles has been accelerated by climate change, particularly due to CO2-induced greenhouse warming (Houghton et al. 1990; GCIP-GEWEX 1993; IGP-BAHC 1993; Watson 1995; Kaczmarek et al. 1996). Global change is of direct relevance to human society and has begun to play a role in the overall environmental policy-making process. Evapotranspiration, as an important component of the terrestrial water cycle, represents more than 60% of precipitation inputs at the global scale (Korzoun et al. 1978; L'vovich and White 1990). Through links between stomatal conductance, carbon exchange, and water use efficiency in plant canopies (e.g. Raich et al. 1991; McGuire et al. 1992; Woodward and Smith 1994; Sellers et al. 1996) evapotranspiration serves as a regulator of key ecosystem processes. The reduction of evapotranspiration through widespread land cover change may lead to an overall diminishing of the water cycle, including the recycling of precipitation and generation of runoff (Shukla et al. 1990; Durbridge and Henderson-Sellers 1993; Lean et al. 1995). Differences in the treatment of evapotranspiration are prominent among both climate and terrestrial ecosystem models (Shuttleworth 1991; VEMAP Members 1995). Many water-balance models in the literature lack a sound evapotranspiration technique, and they often account for evapotranspiration with little biophysical justification. The reason for this gap is due to the fact that evapotranspiration has always been difficult to measure, especially on an ecosystem spatial scale. The method by which evapotranspiration is measured requires a tower above the canopy to record water fluxes out of the forest ecosystem. Inasmuch as the majority of moisture supplied by precipitation returns to the atmosphere as evapotranspiration, and since evapotranspiration is one of the most difficult processes to evaluate in hydrologic analysis, estimates are generally considered to be a significant source of error in streamflow simulation. Several methods for estimating evapotranspiration have been introduced in the literature. Vörösmarty et al. (1998) tested and compared 11 methods on various watersheds in a global-scale water balance model. The primary objective of my study is to compare a similar set of potential evapotranspiration methods that are commonly employed in global-scale water balanceand terrestrial net primary production models. The methods include surface-dependent methods developed by Shuttleworth and Wallace (1985), Monteith (1965), Priestley and Taylor (1972), McNaughton and Black (1973), and the reference-surface method by Penman (1948). While Vörösmarty et al. worked on a global scale, I assess these methods at a forest ecosystem-scale using input data from a tower at the Blodgett Forest Research Station in California. Methods Evapotranspiration methods Five potential evapotranspiration models of increasing complexity were tested under two classes of land surface speciation (Shuttleworth 1991; Federer et al. 1996). Reference-surface evapotranspiration is defined as evaporation that would result from a specific land surface, referred to as a “reference crop.” Surface-dependent evapotranspiration is defined as the evaporation that would occur from any of a variety of designated land surfaces. For the Priestley-Taylor model, the simplest of the five, cover dependency is defined solely by albedo: Total evapotranspiration = 1.26∆A/(∆ + γ) where ∆ is the differential of saturated vapor pressure versus temperature, A is total available energy, and γ is the psychrometric constant. The “α value” of 1.26 is given as a constant by Priestley and Taylor, but this value has been determined to be a function of soil moisture (Flint and Childs 1991). The McNaughton-Black model is defined as follows: Total evapotranspiration = cpρD/γrcs where cp is specific heat at constant pressure, ρ is air density, D is vapor pressure deficit, and rcs is bulk stomatal resistance of the canopy. The Penman model is defined as follows:Total evapotranspiration = (∆A + 73.64ρwγ(1 + 0.54u)D)/( ∆ + ρ) where ρw is water density, and u is wind speed. For the Shuttleworth-Wallace model, the evaporation from the soil, λEs, and the transpiration from the canopy, λEc, are derived from the Penman-Monteith combination equations: λEs = (∆As + ρcpD0/rsa) / (∆ + γ(1 + rss/rsa)) λEc = (∆(A - As)+ ρcpD0/rca) / (∆ + γ(1 + rcs/rca)) where As is available soil energy, and D0 is vapor pressure deficit in the canopy; rsa, rss, and rca are all aerodynamic resistances. D0 is derived from the Ohm's law electrical analog for the vapor pressure and temperature difference between the canopy and the reference height above the canopy where fluxes out of the vegetation are measured. D0 is a function of the measurable vapor pressure deficit at the reference height, D: D0 =
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