Water balance observations reveal significant subsurface water seepage from Lake Nam Co, south-central Tibetan Plateau

• First detailed long-term water balance observations on a large Tibetan lake.
• Two catchment runoff coefficients and two glacial runoff depths were obtained.
• First solid result demonstrating the existence of Tibetan lake water seepage.
• Results significantly affect environmental studies on Tibet and the surrounding areas.

SummaryHydrological observations including precipitation, runoff, evaporation and lake level were performed in detail in the basin of the Lake Nam Co in the southern-central Tibetan Plateau from 2007 to 2011. Two runoff coefficients at differing catchment scales were obtained and applied to derive the surface water inflow to the lake from the non-glaciated areas. Total runoff was partitioned into contributions from glaciated and non-glaciated areas at two additional catchment scales. The runoff from the non-glaciated areas amounted to 1400 mm lake depth during the observation period from May 18 to October 17, 2008, during which the most complete set of observations was obtained. Neglecting groundwater contributions, this amount accounted for 55–65% of the total water input to the Lake Nam Co. The precipitation contribution and the meltwater inflow from the glaciers accounted for 23–28% and 7–22% of the total water input to the lake, respectively. The water input was found to be 810–1220 mm lake depth (1.63 × 109–2.47 × 109 m3) more than the output of the lake during a 5-month period in 2008. Similar results were obtained for the years 2007 and 2009–2011. As there is no surface outflow from the lake, the large water imbalance can only be explained by water seepage, corresponding to an average outflow of 120–190 m3 s−1 or 5–8 mm lake water depth per day. It is likely that the main pathway for lake water seepage is through the sub-surface fault system, which is known to exist in the area.