Seasonal and interannual variability in evapotranspiration of native tallgrass prairie and cultivated wheat ecosystems

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Abstract

Year-round measurements of evapotranspiration (ET) were made, using the eddy covariance technique, in tallgrass prairie and winter wheat ecosystems in north-central Oklahoma during 1996–2000. Seasonal and interannual variability in water vapor flux was examined in terms of relevant controlling variables. During the growing season, the daily ET was 3.5–5.0 mm day−1 for the prairie and 2.5–7.0 mm day−1 for wheat. Annual ET ranged 640–810 mm for the prairie, and 710–750 mm for wheat. “Non-growing” season ET was about 25% of the annual ET in the prairie (during November–April) and about 50% of the annual ET in wheat (during July–February). Differences in ET between the two ecosystems and the corresponding interannual variability were related mostly to the effects of soil moisture stress and variations in green foliage area, while the weather parameters had a smaller impact. The ET model of Priestley and Taylor (1972) [Priestley, C.H.B., Taylor, R.J., 1972. On the assessment of surface heat flux and evaporation using large-scale parameters. Mon. Weather Rev. 100, pp. 81–92.] was modified to incorporate the effects of soil moisture and foliage area, and tested against field measurement. The modification improved the prediction of ET significantly as compared to the original model. On average, the overestimation of the actual ET by the original model reduced from 47 to 9% at the prairie site, and from 20 to 4% at the wheat site. Improved performance of the modified Priestley–Taylor model over a wide range of environmental conditions makes it potentially a practical tool for predicting ET in grasslands and agricultural systems.

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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