Seasonal and interannual variability in evapotranspiration of native tallgrass prairie and cultivated wheat ecosystems
Introduction
Evapotranspiration (latent heat flux) is an essential component of the energy and water budgets in grassland and agricultural ecosystems (e.g. Knapp, 1985, Verma et al., 1989, Hunsaker et al., 2000). Understanding the processes that affect evapotranspiration in these and other communities at different temporal scales and under a variety of environmental conditions is important in modeling ecosystem production (Williams et al., 2004), the water balance of terrestrial ecosystems (Yunusa et al., 2004), and atmospheric circulation (Heijmans et al., 2004).
Many micrometeorological studies have examined the variability in evapotranspiration in relation to weather parameters, and have provided a number of relevant models and empirical equations (e.g. Penman, 1948, van Bavel, 1966, Priestley and Taylor, 1972). In earlier research, the use of soil moisture and canopy conditions was limited, which may have led to the poor performance of some weather-based evapotranspiration models, especially when used on a long-term basis. Recent studies have begun to recognize the role of the soil moisture and canopy characteristics (e.g. phenology, leaf area index) in accurate prediction of evapotranspiration (e.g. Shuttleworth and Wallace, 1985, Massman, 1992, Stannard, 1993). Most investigations have, however, been limited to growing seasons. Furthermore, very few studies have examined multi-year variability in ET of grassland and agricultural ecosystems. Such information is critical for an accurate assessment of the overall water balance of these systems. Here we report results on evapotranspiration and energy fluxes from a study conducted during 1996–2000 in tallgrass prairie and winter wheat ecosystems. Our main objective is to examine the seasonal and interannual variability in evapotranspiration in terms of relevant controlling factors (weather variables, soil moisture, and foliage area). We also take advantage of our soil moisture and foliage area information to modify the evapotranspiration model of Priestley and Taylor (1972), and test the modification against our measurements in a wide range of environmental conditions.
Section snippets
Study sites
Our study was conducted at a native tallgrass prairie site (36°56′N, 96°41′W, elevation 350 m) and a cultivated wheat site (36°45′N, 97°05′W, elevation 310 m) in north-central Oklahoma during the period from early August 1996 through early April 2000. The prairie site (about 500 m × 500 m), located 69 km north-east of Ponca City, OK is surrounded by rolling hills occupied by stretches of grazed and ungrazed tallgrass prairie 1–6 km long. To improve pasture quality, the tallgrass prairie was burned
General weather conditions
Relevant information on weather parameters at the prairie and wheat sites are given in Table 1. Mean annual values of incoming shortwave radiation (Rs) and net radiation (Rn) were similar in all years (year-to-year differences did not exceed 5%). The year 1997 was slightly cooler and more humid than 1998 and 1999. The total annual precipitation in 1997 was similar to that in 1998, but slightly smaller than in 1999.
Soil moisture and foliage area
The annually averaged soil moisture content was similar at both sites in all
Summary and conclusions
A micrometeorological study was conducted in two ecosystems (tallgrass prairie and winter wheat) in the north-central Oklahoma during 1996–2000. The annual ET of the tallgrass prairie ranged between 637 and 807 mm. The distribution of precipitation (not the total annual precipitation) played a crucial role in determining the annual ET. The ratio of annual ET to annual precipitation ranged from 0.53 to 0.73.
The annual ET of winter wheat was between 714 and 750 mm. The annual wheat ET was lower
Acknowledgments
This research was supported by the Office of Science, Biological and Environmental Research Program (BER), US Department of Energy, through the Great Plains Regional Center of the National Institute for Global Environmental Change (NIGEC). We express our special thanks and most sincere appreciation to Dr. Andy Suyker for his hard work and assistance in data collection and analysis. We also acknowledge the technical assistance of Rob Clement, James Hines, John Martin, Martha Martin, Todd
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