H222-12
Stable isotope-based approach to validate effects of forest thinning on the amount and mechanism of groundwater recharge in Japanese forest plantation

Thursday, 17 December 2020: 04:33
Virtual
Saki Omomo1, Yuichi Onda2, Mohamed Racim Boutefnouchet2, Chen-Wei Chiu3, Takashi Gomi4, Sean Hudson5, Yupan Zhang1 and Janice Elaine Hudson5, (1)University of Tsukuba, Tsukuba, Japan, (2)Univ Tsukuba, Tsukuba Ibaraki, Japan, (3)Tokyo University of Agriculture and Technology, Tokyo, Japan, (4)Institute of Global Innovation Research, Tokyo University of Agriculture and Technology, Tokyo, Japan, (5)University of Delaware, Newark, DE, United States
Abstract:
Rainfall partitioning causes a large amount of precipitation to be lost to evapotranspiration in forests, and it is important to optimize that amount to maximize the water recharge function of forests. Analysis of δ18O has shown that soil water that recharges groundwater is different from soil water used by plants in Mediterranean climate (Brooks et al., 2010).

Previous studies have shown that thinning increases throughfall (Sun et al., 2018) while groundwater recharge can change depending on the rate of thinning because thinning improves the light environment and increases evaporation (Sun et al., 2017). The water that recharges groundwater is turning out to be isotopically different from the water used by plants (Evaristo & McDonnell, 2019). However, since the contribution of preferential flow is important in monsoon such as Japan (Uchida et al., 1999), it is necessary to identify whether it is primarily matrix or priority flows that recharge forest groundwater. In this study, we collected water samples using suction lysimeters and zero-tension lysimeters in Japanese cedar and cypress forests and reproduced before and after thinning with different upper tree density and understory condition.

The temporal variation of δ18O values of water sampled by the zero-tension lysimeter was like that of throughfall. Therefore, it is likely that the water collected by the zero-tension lysimeter is preferential flow. The δ18O values of mobile water at 80 cm, which is assumed to be soil water recharging groundwater, is very close to the value of preferential flow. This result suggests that the water recharging the groundwater may be strongly influenced by the preferential flow. Mobile water at 80 cm was isotopically heavier in the open canopy area than in the closed canopy area. Therefore, isotopically heavier water is found to be recharged the groundwater after thinning.

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