3.3.1 Urban Environmental Conditions
Table 4.2 summarizes the daily variation in environmental parameters during 2008 and 2009. There was no significant difference between the air temperature (Ta), wind speed (w), soil temperature (Ts10), or photosynthetically active radiation (PAR) in 2008 and 2009. The annual average Ta, w, Ts10, and PAR were approximately 13.6 °C, 1.05 m/s, 14.2 °C, and 231.7 mol/m2/s, respectively. The vapor pressure deficit (D) varied from 0.11 to 3.34 kPa and showed a seasonal trend (Fig. 4.1). D increased from around 0.57 kPa in March to a maximum of around
3.3 kPa in June before falling to less than 0.5 kPa at the end of the season in November (Fig. 4.1). The atmosphere was very dry in spring, and D often exceeded 2.0 kPa. Moreover, D during May and June in 2008 (1.37 and 1.02 kPa) was
Table 4.2 Average daily values of urban environmental factors and irrigation times in Beijing in 2008 and 2009
significantly lower than in 2009 (1.83 and 2.10 kPa), which was the result of the lower air relative humidity (RH) and higher Ta during that period in 2009.
The year 2008 was the wettest year since records began to be kept in 1999, whereas 2009 was a dry year in which rainfall was below average (Statistical Bureau, 1999-2009). Rainfall in 2008 was 67.5 % higher than in 2009 (724.8 vs. 432.8 mm), but the time of local irrigation was only one-sixth of that in 2009 (2 vs. 12 times). As a consequence, the soil layer was 1.4 % wetter in 2009. In correspondence with the precipitation and irrigation, the soil moisture conditions varied greatly (Fig. 4.1). Moreover, soil water content at a depth of 10 cm was always lower and fluctuated more than that at a depth of 30 cm.