3.2. Temporal dynamics of soil water storage
The time series for soil water storage (SWS) during the study period under different treatments are shown in Figure 4. It is clear that both the extreme drought in 2015 (dry spell I) and the moderate drought in the 2016 (dry spell II) greatly decreased the SWS at all soil depths under the different treatments, with a clear lag in the 180-280 cm layer for dry spell I. In particular, the soil water content in the 0-20 cm layer under the SHT and control fell below the permanent wilting point (Table 1). Generally, the treatments that combined terraces and mulches (JBT and MST) exhibited higher SWS values than terraces alone (SHT) or the control. Furthermore, the differences between the combined treatments and the others seemed to be greater during the two dry spells.
/Figure 4/
The annual average SWS was clearly lower in 2015 and 2016 than in 2014 in the 0-20 cm layer but not in the deeper layers, indicating the effects of seasonal droughts on SWS at the annual scale are probably only apparent in the surface layer. In the 0-20 cm layer, the combined treatments had, on average, 14.6% and 30.4% higher SWS than the SHT and control, respectively, and the difference between these treatments and the control was statistically significant (P <0.05) in 2014 and 2015. In the 20-100 cm layer, the combined treatments had, on average, 20.7% and 36.0% higher SWS than the SHT and control, respectively, for the different years. The difference was highly significant (P <0.01) for all years, highlighting the advantage of combining terracing and mulching in order to retain soil water.