Seasonal and inter-annual variability of the Bowen smith ratio over a semi-arid grassland in the Chinese Loess Plateau

Energy partitioning between grassland sensible (H) and latent (LE) heat fluxes is critical in determining boundary layer development, the hydrological cycle, weather and climate. In this report, we describe the seasonal and inter-annual variability of the Bowen ratio (β) over semi-arid grassland of the Loess Plateau in the north boundary belt of the summer monsoon of China. β was directly measured by an eddy covariance system over a 6-year period (2007-2012), during which the studied grassland experienced a large range in precipitation, extending from 555.5 mm (2007) to 262.8 mm (2011). The annual Bowen ratio (βa) based on annual energy budgets ranged from 0.90 (2007) to 1.93 (2011) with an annual mean value of 1.32, while the annual midday (10:00-16:00 GMT + 8) β ranged from 2.07 (2007) to 4.19 (2011) with an annual mean value of 3.02. The mean midday β during the dry years (3.66, 2009, 2010 and 2011) was higher than that observed during the wet years (2.39, 2007, 2008 and 2012). The 6-year annual mean values of the bulk surface resistance (Rs), aerodynamic resistance (Ra) and climatological resistance (Ri) varied from 272.27 to 410.10 s m−1, 31.62-34.89 s m−1 and from 64.5 to 87.13 s m−1, respectively, with annual mean values of 337.08, 32.83 and 73.05 s m−1, respectively. The normalized difference vegetable index (NDVI) had a direct influence on β as mediated by Rs, and there was a logarithmic function between ET/ETeq and Rs in the semi-arid grassland. A positive trend between β and the surface resistance (Rs*) normalized by the climatological and aerodynamic resistance was observed at this site. Moreover, the annual effective precipitation frequency was significantly related to the midday β during the growing and non-growing seasons, which indicated that the annual effective precipitation, not the annual precipitation, indirectly affected the annual β through its effect on the shallow soil water content (SWC). The results suggested that inter-annual variations in β was primarily controlled by an annual effective precipitation frequency and seasonal variations in β was directly affected by NDVI as mediated by Rs over the semi-arid grassland in the Chinese Loess Plateau.