Interannual and seasonal variability in evapotranspiration and energy partitioning over an irrigated cropland in the North China Plain
Introduction
Evapotranspiration is a major component of the energy and water balances in agricultural ecosystems (Burba and Verma, 2005, Steduto and Hsiao, 1998). Understanding the seasonal and interannual variability in evapotranspiration and energy partitioning, based on the long-term measurements of fluxes, is important for identifying weather, soil water, and crop factors which regulate fluxes at different temporal scales, and is vital for modeling crop production and water balance (Burba and Verma, 2005). There is a particular need for in-depth study of evapotranspiration for the improvement of water resource use and management in water-limited agricultural areas.
The North China Plain, the largest agricultural production area in China, is a water-limited region. More than 50% of the nation's wheat and 33% of its maize production is grown on this plain, through a wheat/maize double cropping system (Wang et al., 2008). However, the area is experiencing serious water shortages (Cai, 2008), primarily due to the high water consumption by the agriculture sector through crop transpiration and soil evaporation. The diversion of large amounts of irrigation water diverted from the rivers or pumped from the groundwater have caused serious environmental problems, such as drying up of the river and formation of groundwater funnels (Yang et al., 2004, Kendy et al., 2003). To protect limited water resources, accurate quantification of the water requirements for the two predominant crops is essential.
Irrigation of large areas of cropland clearly alters the regional energy and water cycles, creating a unique regional environment (Prueger et al., 1996). Extensive irrigation not only increases evapotranspiration and reduces runoff, but also decreases sensible heat flux. For example, modeling study in the Colorado river basin has demonstrated that irrigation water requirements of 15.4 mm year−1 could result in a decrease of 37% in streamflow and a decrease in surface temperature of 0.04 °C (Haddeland et al., 2006). Evaluation of the basic characteristics of evapotranspiration and energy partitioning in response to irrigation is therefore helpful in identifying the effect of irrigation on regional water and energy balances in the North China Plain.
The eddy covariance technique (EC) is generally considered to be a highly suitable method for providing direct and trustworthy flux measurements (Baldocchi, 2003). A large number of studies have used this technique to characterize the temporal and spatial variability of canopy-scale carbon dioxide, water vapor, and energy fluxes across diverse ecosystems (Baldocchi et al., 2001). However, there have been few examinations of the variations in evapotranspiration and energy fluxes in the irrigated croplands of the North China Plain using this technique (Wang et al., 2006, Qin et al., 2008). Since March 2005, long-term observations of energy and water budgets, in association with hydrometeorological elements, have been carried out over an irrigated wheat/maize rotation cropland in the central North China Plain, as one of the reference sites for the Coordinated Energy and water cycle Observations Project (CEOP, http://monsoon.t.u-tokyo.ac.jp/ceop2/index.html). In the present study, we used three years of consecutive measurement data, covering the period from October 2005 to September 2008, to analyze variations in evapotranspiration and energy partitioning in this region. This included three full winter wheat seasons and three full summer maize seasons. Interannual variability was determined using a crop year defined from October to September of the following year. The main objectives were to examine the seasonal and interannual variations in evapotranspiration in terms of relevant controlling factors (weather, soil moisture, canopy conductance), and to quantify the seasonal and interannual distributions of evapotranspiration (latent heat flux) and energy partitioning.
Section snippets
Site description
The Weishan flux site (N36°39′, E116°03′) is located near the center of the Weishan Irrigation District (area: 4448 km2, approximately 36.14°–37.01°N and 115.43°–116.51°E) along the downstream of the Yellow River in the North China Plain (Fig. 1). Its regional climate is temperate and semi-humid with 532 mm (average: 1984–2007) of mean annual precipitation and 1950 mm (average period: 1961–2005) of mean annual pan evaporation (20 cm diameter evaporation pan). Approximately 70% of its annual
Energy balance closure
One measure for testing data quality is to test for closure of surface energy balance (Wilson et al., 2002). Using half-hourly data from June 2005 to December 2008, the slope between available energy flux (Rn–G) and the sum of sensible and latent heat fluxes for this site was 0.74, the intercept was 11.87 W m−2, and the coefficient of determination (R2) was 0.83. The regression statistics of LE + Hs on Rn–G for half-hourly data are shown in Table 1. For the three-year period, the slope of the
Conclusions
Using the EC technique, evapotranspiration and energy fluxes were measured over a typical irrigated wheat/maize rotation cropping field in the North China Plain for a three-year period. Although annual precipitation was significantly below the normal, the soil water content rarely fell below the threshold for moisture stress because of the sufficient irrigation during the wheat seasons and sufficient precipitation during the maize seasons. The annual ET ranged from 595 to 609 mm, and the ET
Acknowledgements
This research was supported by the National 973 Project of China (Project No. 2006CB403405), the National Natural Science Foundation of China (Project No. 50939004), and the Doctoral Program Foundation of Institutions of Higher Education of China (Project No. 2007000307). We thank Dr. Matthias Mauder at University of Bayreuth for the support for the TK2 software package. We would like to express our appreciation to one anonymous reviewer, whose comments and suggestions led to significant
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