Major hydrological fluxes and conditions

The summer monsoon precipitation measured in close proximity to the study site was 826 mm, which was similar to the regional value and accounted for 84.7% of the precipitation

from June through August. During the summer monsoon, sixteen individual rainfall events occurred and yielded 95.8% of the total precipitation for this period. The events were unevenly distributed over time, as nine events occurred over a period of only 16 days and accounted for 72.5% of the summer monsoon precipitation (Fig. 2.2, from DOY 189 to 204).

The individual events varied in characteristics, as precipitation ranged from 8 to 127 mm, duration ranged from 1.75 to 33.9 h, and average precipitation intensity ranged from 1.8 to 10.4 mm h^-1. Throughfall for the summer monsoon was 788 mm, accounting for 95.4% of the precipitation. Average throughfall intensity for individual events ranged from 1.7 to 9.4 mm h^-1. Throughfall intensity reached a maximum of 113 mm h^-1 over a 5-min period. Further characteristics of six major rainfall events selected to cover the most intense period of the summer monsoon in terms of events are presented in Table 2.2 (see also Fig. 2.2).

2.3.1.2. Runoff

Catchment runoff for the summer monsoon totaled 617 mm and, from June through August, 684 mm. Over the former, hourly specific discharge ranged from 0.02 to 8.88 mm hr

-1 and averaged 0.50 ± 0.86 mm hr^-1 (Fig. 2.2). A diurnal variation in discharge was observed at low-flow conditions, of which the maximum value was 0.017 mm hr^-1. Before the summer monsoon, discharge followed a decreasing trend and did not reach a steady baseflow value (Fig. 2.2). From the onset of the summer monsoon onward, discharge was very responsive to rainfall events as its initial increase usually occurred between 5 and 30 min following the start of an event. Following the 4^th event of the summer monsoon, discharge did not return to low-flow conditions for the rest of the study period. The recession analysis revealed that following the summer monsoon, discharge would have receded for ~18 days before reaching its value at the onset of the summer monsoon. The constants derived from the recession rate equations ranged from 0.02 to 0.15. The two intermittent springs were first activated on the 4^th and 7^th rainfall events (DOY 183 and 192), respectively, and following the end of an event flowed for periods ranging from hours to several days.

The runoff coefficient of individual rainfall events ranged from 9.4 to 124.7%, with a median of 67.3%. Throughout the summer monsoon, the coefficient followed an initial increasing trend, stabilized for the most intense period in terms of events, and remained high or higher thereafter. The runoff coefficients of six selected major rainfall events are presented in Table 2.2.

Chapter 2 − A Sudden Shift in Runoff Generation Processes at a Forested Catchment

49

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160 170 180 190 200 210 220 230 -75 conductivity (EC, hourly mean), Ca²⁺ and Mg²⁺ concentration and (c) relative abundance of deuterium (δ²H) of stream water as a function of time from June 1 to August 31, 2013. Ca²⁺, Mg²⁺, EC and δ²H were the only tracers considered as conservative. The shaded areas correspond to the summer monsoon; the white lines separate the initial and the major period.

Selected major rainfall events are labeled by order.

Table 2.2. Characteristics of selected major rainfall events of the 2013 summer monsoon.

Event order 4^th 5^th 9^th 11^th 13^th 15^th

Start (day of year) 183.4 189.1 194.9 199.5 203.5 212.0

Duration (h) 17 32 12 13 31 6

Precipitation (mm) 41 64 127 106 81 48

Throughfall (mm) 45 60 115 97 90 48

Throughfall intensity mean (mm h^-1)^a 2.6 1.9 9.4 7.7 2.8 8.0 Throughfall intensity max (mm h^-1)^b 41.2 49.1 112.8 58.4 67.2 50.4

Runoff (mm)^c 13 42 104 91 101 41

Runoff coefficient (%) 31.7 65.6 81.9 85.8 124.7 85.4

a Excluding intra-event rainless periods.

b Based on values measured at a 5-min frequency.

c From the start of the event to when discharge on the receding limb was measured or calculated as equal to its initial value.

2.3.1.3. Soil moisture and water table

Soil moisture (θv) measured at a 5-min frequency ranged from 14.5 to 40.1%. Its temporal dynamics were mainly governed by the sequence of rainfall events. θv generally increased with depth, except at some periods and at the hillslope area (Fig. 2.3). Temporal variation patterns in θv differed between areas and depths, although differences in values were at some periods insignificant. At each depth, the largest range in daily θv over the study period was observed at the riparian area, followed by the hillslope and toeslope areas. During the summer monsoon, lag times in increase following the start of an event varied between 5 min and ~1 day and recession periods were usually in the order of a few days. θv reached its lowest values at around the onset of the summer monsoon. It then rapidly increased with the occurrence of rainfall events until the 5^th event, and thereafter varied at a lesser extent with the occurrence of events until after the summer monsoon. θv then receded and, if not for a typhoon-related event that occurred at the end of the study period, it would have likely receded near or to the values at the onset of the summer monsoon within the study period (Fig. 2.3). The values of θv fell within the ranges reported for the same period of the year for analogous sites in South Korea (Jeong et al., 2012; Jung et al., 2010; Jung et al., 2012; Kwon et al., 2009). Unsaturated soil hydraulic conductivity (K(θ_v)) displayed greater absolute ranges and differences between some depths than θv (Fig. 2.3). At the hillslope area, K(θv) at 50-cm

Chapter 2 − A Sudden Shift in Runoff Generation Processes at a Forested Catchment

51 depth was much more responsive to events than at shallower depths, especially from the 5^th rainfall event onward. At the riparian area, K(θv) at 30 and 50-cm depth was generally similar and generally much higher than at 10-cm depth, also especially from the 5^th rainfall event onward.

Despite the relatively shallow soil, a water table was measured only once for a period of 12 hours in the piezometers installed in the lower part of the riparian area, starting 2 hours before the maximum throughfall intensity of the summer monsoon. It lasted 1 hour longer at the piezometer located closer to the intermittent spring flowing at this location and further away from the stream than at the other piezometer. The water table had a maximum depth of 38 cm and did not intersect the soil surface. A water table was not measured at any other time or any other piezometer over the study period. However, at the location where the stream reached its maximum extent, the emergence of water at all times indicated the presence of a saturated zone. Water depth at this location was a maximum of ~40 cm higher during and

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Day of year (DOY) 10-cm depth 30-cm depth 50-cm depth

160 170 180 190 200 210 220 230 240 0.00 three depths at the (a, d) hillslope, (b, e) toeslope and (c, f) riparian areas of the study site as a function of time from June 1 to August 31, 2013. The shaded areas correspond to the summer monsoon; the white lines separate the initial and the major period.