# Water-Use Characteristics and Physiological Response of Moso Bamboo to Flash Droughs
## 4 Discussion
### Water Budgets and Partitioning of the Moso Bamboo Forest  
The variation in precipitation quantity is significant in terms of altering the other water budget components: ET and soil moisture [61]. Maintaining a dynamic balance between precipitation and water consumption is the basis of ensuring ecosystem health and sustainable development [12]. Therefore, understanding the water-use dynamics of plants and the interactions between forests and their hydroclimate, as well as the role of forests in water partitioning, is essential for water resource and forest management [14]. To evaluate the effect of flash drought conditions on water budgets of Moso bamboo, we determined precipitation, ET, PET, I\({}_{\text{P}}\), and soil water storage from 2012-2014 (Figure 7a). Total precipitation (\(P\)) during the period was 1625 mm, 1314 mm, and 1349 mm in the study site in 2012-2014, respectively. Annual PET was 865 mm, 1017 mm, and 814 mm in 2012-2014, respectively (Figure 7a). Evapotranspiration is controlled by relative proportion and timing of available water and available energy [62, 63], and the annual ET was 677 mm, 730 mm, and 654 mm in 2012-2014, respectively (Figure 7a). To understand the water sources for forest growth, we calculated the monthly water balance between rainfall and transpiration. We hypothesized that all precipitation-deducted ET (P-ET) is stored in the soil. The soil water storage was 760 mm, 297 mm, and 533 mm from 2012 to 2014, respectively (Figure 7a). The proportion of average annual water storage was about 51.4%. Over the three years, there was a surplus of precipitation, which does not seem to cause a shortage of water in the ecosystem. However, in a few months of the year, total transpiration exceeded rainfall, and the distribution of monthly P-ET was negative. Although summer is an important season for rainfall, the huge ET caused by the flash drought can cause net loss of water in some months. Especially in the flash drought period of 2013 when P-ET was \(-\)168 mm (Figure 7b). Dryness index reached 14.34 in July, which was significantly higher than the annual average of 2.17 in 2013. It was also higher than the average of 0.73 and 0.95 in 2012 and 2014, respectively (Figure 7b). [PERSON] et al. [64] also reported that the decrease of precipitation and the increase of evapotranspiration will lead to the decrease of surface run-off and available water. This extra loss comes from the deeper soil layers, which leads to SWC_0-50 cm sharply decreasing during the flash drought period. The SWC_0-50 cm decreased rapidly (Figure 2f), and persistent drought conditions even caused physiological dehydration of Moso bamboo to deciduous to resistant flash drought. The I\({}_{\rm p}\) of Moso bamboo was small (5.3-5.4%) and there was no significant difference in the three-year investigation at this study site. This was lower than the I\({}_{\rm p}\) of 11.9% observed for Moso bamboo reported by [PERSON] and [PERSON] [41] in Jiangxi Province in China, and the I\({}_{\rm p}\) of 9.2-12.2% reported by [PERSON] et al. in Japan [65]. These inconsistencies may due to the methodological differences. According to the calculation of water budget over three years, about 50.0-66.1% of precipitation in the study area returned to the atmosphere through ET. This was basically consistent with the estimation of ET using the Eddy covariance method in 2011 (745/1543) as reported by [PERSON] et al. [27]. Forest ET is one of the main driving factors of precipitation [66; 12]. The phenomenon of physiological dehydration and defoliation appeared in Moso bamboo forests after about 30 days of extreme drought conditions. Soil volumetric water content at 50 cm depth decreased from 0.35 m\({}^{3}\)-m\({}^{-3}\) to 0.17 m\({}^{3}\)-m\({}^{-3}\) in the initial stage of the flash drought. Mean stand density was about 4500 trees\(\cdot\)ha\({}^{-1}\) during the period. We hypothesized that the water consumption of Moso bamboo was linearly related to the density in this study site.