Chapter 3 − Water fluxes and quality of a forested catchment over
3.3.1. Water fluxes and hydrological conditions
The 2013 summer monsoon yielded 826 mm of precipitation of which 95.4% fell as throughfall, and the 2014 summer monsoon yielded 94 mm of precipitation of which 83.0%
fell as throughfall (Table 3.1). During the former, sixteen individual rainfall events occurred and ranged from 8 to 127 mm in precipitation, while only three events which ranged from 15 to 45 mm in precipitation occurred during the latter. The three most important events in terms of precipitation occurred in 2013 and had return periods of 3.41, 2.17 and 1.25 years, while the other events had return periods below one year. Although most of the events of the 2013 summer monsoon occurred within a period of only 16 days (Fig. 3.2, from day of year (DOY) 189 to 204), the events were more evenly distributed in time than in 2014. As a matter of fact, a period of 15 days elapsed between the 2nd and the 3rd rainfall event of the 2014 summer monsoon (Fig. 3.2, from DOY 191 to 205).
Chapter 3 − Water Fluxes and Quality of a Forested Catchment
89 Table 3.1. Selected characteristics of the 2013 and the 2014 summer monsoon, and major hydrological fluxes and conditions at the study site.
2013 summer monsoon 2014 summer monsoon
Period (Julian day)a 168–216 183–210
Period length (day) 49 28
Precipitation (mm) 826 94
Throughfall (mm) 788 78
Ratio of throughfall to precipitation (%) 95.4 83.0
Runoff (mm) 617 11
Runoff coefficient (%) 74.7 11.7
Flow regime coefficientb 444.0 6.4
Evapotranspiration (mm) 161 104
Water balance (mm) 48 -21
Number of individual rainfall events 16 3
Stream water temperature mean (°C)c 10.8 14.3
Maximum extent of the stream (m)d 226 65
a GRMA (2013, 2014).
b Ratio of maximum discharge to minimum discharge.
c At the catchment outflow.
d From the catchment outflow.
3.3.1.2. Runoff
Total runoff for the summer monsoon was over one order of magnitude greater in 2013 than in 2014 (Table 3.1). During the 2013 summer monsoon, hourly specific discharge ranged from 0.02 to 8.88 mm hr-1 and averaged 0.50 ± 0.86 mm hr-1 (Fig. 3.2). It reached its lowest value shortly following the onset of the summer monsoon, and subsequently varied with the occurrence of rainfall events. Both intermittent springs were responsive to major rainfall events and flowed for periods ranging from hours to several days following the end of an event. During the 2014 summer monsoon, hourly specific discharge ranged from 0.008 to 0.051 mm hr-1 and averaged 0.016 ± 0.008 mm hr-1. It was relatively low until shortly before the end of the summer monsoon, when it substantially increased in response to the 3rd rainfall event (Fig. 3.2, DOY 206). The most upstream spring was first activated on the 3rd event and flowed for only a few hours following the end of the event, while the other spring was never
activated. At the onset of the 2013 and the 2014 summer monsoon, daily specific discharge was 0.56 and 0.38 mm d-1, respectively.
The 2013 and the 2014 summer monsoon greatly differed in terms of runoff coefficient, which was more than six times higher in 2013 than in 2014 (Table 3.1). In 2013, the runoff coefficients of individual rainfall events of the summer monsoon ranged from 9.4 to 124.7% and averaged 60.5 ± 33.5%, while in 2014 they ranged from 6.0 to 14.7% and averaged 10.7 ± 3.6%. Three clusters were derived from the entire set of runoff coefficients (5 to 15%, 25 to 40%, 65% and above).
160 170 180 190 200 210 220 230 240 0.01
0.1 1 10
Specific discharge(mm h-1 ) 2013
0 5 10 15 20 25 30 35 -1 Throughfall (mm h)
160 170 180 190 200 210 220 230 240
Day of year (DOY)
2014
Figure 3.2. Specific discharge (hourly mean) and throughfall (hourly) as a function of time for the period from June 1 to August 31 of 2013 and 2014. The shaded areas correspond to the summer monsoon.
3.3.1.3. Evapotranspiration
Total actual evapotranspiration for the 2013 and the 2014 summer monsoon was 161 and 104 mm, respectively (Table 3.1). Daily values (ETd) ranged from 0.5 to 4.8 mm and averaged 3.3 ± 1.4 mm for the former and 3.7 ± 1.0 mm for the latter. At clear-sky conditions, ETd averaged 4.5 ± 0.2 mm d-1. The values fell within the ranges reported for the same period of the year for analogous sites in South Korea (Khatun et al., 2011b; Shin et al., 2012; Yeom et al., 2015).
Chapter 3 − Water Fluxes and Quality of a Forested Catchment
91
3.3.1.4. Soil moisture and water table
The ranges in daily average soil moisture (θv) for the 2014 summer monsoon did not overlap those for the 2013 summer monsoon, except for one value at 10-cm depth (Fig. 3.3).
The greatest range in values was observed in 2013, at 50-cm depth (15.7 to 26.8%), and the smallest range in values was observed at the same depth in 2014 (9.4 to 12.6%). θv did not exceed the value of field capacity proposed by Saxton and Rawls (2006) for a soil of loam texture. However, we believe that the average value of field capacity was much lower at the site, due to the high fraction of coarse gravel and stones of the soil (Payeur-Poirier et al., in preparation). While θv at 50-cm depth was lower than at shallower depths at the onset of the 2013 summer monsoon, it substantially increased during the first half of the summer monsoon to finally exceed the values at shallower depths. During the second half of the summer monsoon, relatively high values of soil moisture were sustained at all depths. In 2014, soil moisture at the onset of the summer monsoon was lower than at the onset of the 2013 summer monsoon at all depths (Fig. 3.3). It slightly increased in response to the 1st and 2nd rainfall events, but thereafter decreased until it reached its lowest values before the occurrence of the 3rd event. This event induced a substantial increase in soil moisture and values slightly exceeded the respective value at the onset of the summer monsoon. The ASMI of individual rainfall events ranged between 96 and 138 mm in 2013 and between 67 and 81 mm in 2014.
Over both years of the summer monsoon, a water table was observed only once for a period of 12 hours, in 2013. It occurred in the lower part of the riparian area and was associated with the flow of an intermittent spring. The water table had a maximum depth of 38 cm and did not intersect the soil surface.
150 160 170 180 190 200 210 220 230 0.08
150 160 170 180 190 200 210 220 230
Day of year (DOY)
150 160 170 180 190 200 210 220 230 0.08
Figure 3.3. Soil moisture (θv, daily mean) at 10-, 30- and 50-cm depth at the catchment scale as a function of time for the period from June 1 to August 24 of 2013 and 2014. The light grey areas correspond to the 2013 summer monsoon; the dark grey areas correspond to the 2014 summer monsoon.