Old and New Water in Subsurface Flow from a Forest Soil Block

In order to understand how infiltrating water mixes with water and solutes already present in the soil, percentages of old and new water in subsurface flow from a forest soil block subjected to simulated rainfall were studied. A rainfall simulator was used to apply simulated rainfalls of four intensities 6.3, 4.3, 2.8, and 1.3 cm h−1. Each intensity was repeated four times. Sodium bromide was added to the first replicate, but not the two following replicates to insure that old and new water had different chemical signatures. Subsurface flow was measured at four depths, 14, 26, 44, and 64 cm using a system of collection troughs and tipping buckets. Discrete water samples for bromide analysis were taken at 1‐ to 10‐min intervals throughout each simulation. Old water percentages declined as the block was wetted and reached a minimum when peak subsurface flow was achieved. Minimum old water percentages during the simulations ranged from 0% for the 6.3 cm h−1 simulation to 28% for the 1.3 cm h−1 simulation. After rainfall stopped subsurface flow declined rapidly and old water percentages increased. Total subsurface flow was composed of 8.0, 3.7, 3.9, and 31% old water for simulated intensities of 6.3, 4.3, 2.8, and 1.3 cm h−1, respectively, indicating a trend of increasing old water with decreasing rainfall intensity. Rapid subsurface flow through soil macropores was the most likely mechanism of new water release.