Studies on water storage in forested catchments in Taiwan have primarily been aimed at longterm
and large-scale water storage estimations or economic assessments. However, there are few
studies on water conservation mechanisms or comparisons of evaluation methods. In particular,
investigation cases in head catchments where surface runoff originates are limited. This study used
the water balance method to estimate the catchment water storage, and used the monitored data of
5–55 cm soil moisture to estimate shallow soil water storage in a headwater catchment. Relationships
between catchment water storage and soil water storage were analyzed. Results showed that
the amount of catchment water storage continued to decrease during the study period while the
amount of soil water storage continued to increase or plateau. Amounts of catchment water storage
and soil water storage were negatively correlated. However, daily changes in catchment and soil
water storage values were positively correlated. Daily changes in soil water storage were approximately
11% of the daily changes in catchment water storage. These different trends in amounts and
daily changes between catchment water storage and soil water storage were attributed to the runoff
contributing area, hyporheic flow, and the calculation procedure of the water balance method.
When using the catchment surface area as the runoff contributing area, the runoff was overestimated,
and the amount of catchment water storage was underestimated. In addition, underestimation of
catchment water storage was cumulative when using the long-term hydrological balance method.
This resulted in a misinterpretation that the amount of catchment water storage continued to decrease.
For a headwater catchment featuring variable sources of runoff, daily changes in catchment
water storage could be appropriately evaluated by the short-term hydrological balance method.
Daily changes in soil water storage could be used as an indicator for assessing daily changes in
catchment water storage. Hydrological monitoring at different spatial scales would be helpful in
clarifying the water conservation function of a forested headwater catchment.