Spatiotemporal Variability in Water-carbon Flux and Water Use Efficiency Over an Agro-Ecosystem in the Changwu Tableland of the Loess Plateau, China

Abstract:

Study on the characteristics and coupling relationship of water and carbon fluxes in agro-ecosystem, will contribute to maintaining and improving agricultural productivity, and is also important for understanding the material circulation of terrestrial ecosystem. In this study, a 30 m-high tower was erected for mounting flux instruments in the Changwu Tableland of the Loess Plateau. Two sets of eddy covariance system on the tower representing for ecosystems with different spatial scales, one is the cropland ecosystem (2m height, the underlying is winter wheat mono-cropping cropland) and the other is agro-fruit ecosystem (30m height, including both cropland and apple orchard). Seasonal and interannual variations in evapotranspiration (ET), net ecosystem exchange (NEE) and ecosystem water use efficiency (WUE) for the two ecosystems were continuously measured from September 2004 to June 2010. Mean while, the relationship between actual ET and potential evapotranspiration (ET₀) was discussed. The results showed that, 1) Seasonally, ET of the cropland ecosystem was bimodal, peaked in May (Jointing and heading stage of winter wheat) and August (summer fallow period), respectively. The trend of NEE was consistent with the growth period of winter wheat, and the minimum occurred between April and May, the maximum WUE value also appeared that time. Seasonal ET of the agro-fruit ecosystem showed unimodal trend, which peaked in July. The lowest NEE occurred in July. The seasonal variation of WUE was smaller than that of the cropland ecosystem, and it got the highest in May. 2) Both of the annual ET in the cropland and the agro-fruit ecosystem fluctuated in these years, and the annual average ET were 437.12 and 417.41 mm, respectively. Because of the underestimated of latent heat flux caused by the energy imclosure, the observed ET was less than the results of the water balance calculation. The trend in NEE was similar, and the annual average NEE were -325.88 and -440.74 gC/m²/y, respectively. The annual average WUE of both ecosystems were 0.74 and 1.06 gC/Kg H₂O respectively. 3) On a daily scale, the seasonal trend of ET in the agro-fruit ecosystem was in accordance with ET₀, with a proportional relationship. However, on the annual scale, the trend of actual ET and ET₀ showed complementary relationship, to some extend.