3.1. Precipitation and fine root distribution
The cumulative frequency of annual total precipitation (PT) and growing-season precipitation (PG) are shown in Figure S3, from 1960 to 2012 at the study site. The medians of PT and PGwere 479.7 and , respectively, which are similar to the annual means of PT () and PG () during this period. PT in 2014 (), 2015 () and 2016 () corresponded to 55.6%, 84.1% and 59.0%, respectively, in terms of cumulative frequency. PG amounted to 56.6%, 98.7% and 67.0% in 2014 (), 2015 () and 2016 (), respectively. Thus, 2014 was considered to be a normal year, whereas 2015 and 2016 were classified as a severe and moderate drought year, respectively. It is noteworthy that the percentages for PG in 2015 and 2016 were clearly lower than for PT, indicating that less precipitation occurred than in the 2014 growing season.
Figure 2 shows temporal patterns of daily and monthly precipitation for different water years. It was clear that there was a severe spring (April and May) and summer (June and July) drought in 2015. The total precipitation for spring and summer 2015 accounted for only 60.1% and 21.1%, respectively, of that in 2014. In July 2015, the total precipitation amounted to only and no water was made available to the jujube trees as a result of almost all of the eight rainfall events (precipitation< ). There was a moderate autumn drought in 2016. The total precipitation in August and September 2016 accounted for only 54.8% and 39.9% of that in 2014 and 2015, respectively.
/Figure 2/
The distribution of fine root length density (FRLD) down the soil profiles from 0 to 500 cm is shown in Figure 3. In general, the FRLD decreased exponentially with soil depth. The top 300 cm accounted for 89.9% of the total fine root length down the entire profile (0-500 cm), indicating that this is the main fine root layer. The top 100 cm contained 52.5% of fine root length. Therefore, we studied soil water storage dynamics and soil water use within the top 300 cm, with the top 100 cm defined as the shallow layer and below this was the deep layer.
/Figure 3/