Seasonal source water and flow path insights from a year of sampling in the Chamkhar Chhu basin of Central Bhutan

Figures & data

Figure 1. The study site in Chamkhar Chhu basin including glacial areas, all sampling locations, meteorological station, and flow gauge location. Top left inset: regional locator. Top right inset: cryospheric sampling points at the toe of the Thanagang glacier. Bottom left inset: river sampling locations at the main stem–Riphug tributary confluence

Table 1. Sampling locations in the Chamkhar Chhu basin

Location name	ID	Sample type	Elevation (m)	Latitude	Longitude
Ice terminus 2	IT2	Ice	5,158	28.0103	90.6197
Ice terminus 1	IT1	Ice	5,140	28.0098	90.6203
Thana outflow	R1	Glacier	5,147	28.0124	90.6208
Churuthang	R2	River	4,556	27.9994	90.6670
Yarphu upstream	R3	River	4,095	27.9276	90.6878
Yarphu downstream	R4	River	4,008	27.9160	90.6816
Tsampa upstream	R5	River	3,713	27.8647	90.6632
Tsampa downstream	R6	River	3,641	27.8464	90.6647
Riphug upstream	R7	River	3,188	27.7748	90.7145
Riphug downstream	R8	River	3,173	27.7693	90.7187
Spring at Tsampawog	SP3	Highly reacted groundwater	3,617	27.8421	90.6655
Spring at Semthang	SP2	Moderately reacted groundwater	3,254	27.7884	90.7025
Spring at Riphug	SP1	Unreacted groundwater	3,160	27.7731	90.7147
Petsho	R9	River	3,027	27.7462	90.7288
Warubthang	R10	River	2,912	27.7169	90.7361
Chokhortoe	JP	Precipitation (rain)	2,772	27.6619	90.7485
Damphel	R11	River	2,725	27.6548	90.7534
Kharsa	R12	River	2,641	27.6268	90.7184
Kurje	R13	River	2,591	27.5934	90.7272
Chamkhar	R14	River	2,563	27.5498	90.7578
Gelikhar	R15	River	2,538	27.5263	90.7707

Figure 2. Hydroclimate data from 1996 to 2016 including 2016 water year indicating annual fluctuations for (top) temperature, (middle) precipitation, (bottom) and discharge data for the study region

Figure 3. The δ¹⁸O gradient of river water samples with elevation. Precipitation δ¹⁸O is available during four months and is shown at the elevation of the rain collector. Dashed lines represent isotopic gradient trendlines for river water below 4,095 m

Table 2. Summary statistics for isotopic relationships of seasonal river waters relative to the GMWL (slope = 8, intercept = 10) and RWL as shown in Figure 4

Season or reference	Slope	Intercept	Standard error, slope	Standard error, intercept	R^2	n
March	8.76	22.63	0.209	2.574	0.992	15
June	8.26	17.79	0.115	1.201	0.997	15
August	8.13	13.81	0.286	4.033	0.984	14
October	7.86	10.53	0.071	1.042	0.999	15

Figure 4. The isotopic relationship of seasonal river water and end-member samples relative to the GMWL. The RWL shown is derived from three bulk precipitation samples. The highly enriched June rain sample plots off the page (inset)

Figure 5. The evolution of solute concentrations in river water across the elevation gradient. Solutes shown are SO₄²⁻ (top) and Ca²⁺ (bottom)

Figure 6. The δ¹⁸O–SO₄²⁻ biplots with river, ice, and groundwater end-members for the months of March, June, August, and October. The Tsampa tributary inflow is also shown for reference. Dashed lines represent mixing lines and mixing triangles connecting likely river water end-members

Figure 7. Hydrograph separation utilizing the SO₄²⁻ tracer to identify flow path composition of river water over the elevation gradient. Variations in flow paths are shown seasonally: premonsoon (top left), early monsoon (top right), late monsoon (bottom left), and postmonsoon (bottom right). The error bars (red) represent the uncertainty (95 percent confidence) for each component

Figure 8. Hydrograph separation utilizing the δ¹⁸O tracer to identify source water composition of river water over the elevation gradient. Variations in flow paths are shown for early monsoon (left) and postmonsoon (right). The error bars (red) represent the uncertainty (95 percent confidence) for each component

Figure 9. Conceptual diagram indicating flow paths during (a) premonsoon, (b) monsoon, and (c) postmonsoon periods in the Chamkhar Chhu basin across elevations