Differential behaviour of a Lesser Himalayan watershed in extreme rainfall regimes

Abstract

Climatic extremes including precipitation are bound to intensify in the global warming environment. The present study intends to understand the response of the Tons sub-watershed in Lesser Himalaya, in 3 years with entirely different hydrological conditions (July 2008–June 2011) in terms of discharge, sediment flux and denudation rates. Within an uncertainty limit of ±20%, the mean interannual discharge (5.74 ± 1.44 m ³ s ⁻¹) (±SE), was found highly variable (CV: 151%; 0.8–38 m ³ s ⁻¹). In a normal rainfall year (2008–2009; ∼1550 mm), the discharge was 5.12 ± 1.75 m ³ s ⁻¹, whereas in a drought year (2009–2010), it reduced by 30% with the reduction in ∼23% rainfall (CV: 85%). In an excessive rainfall year (once-in-a-century event) (2010–2011; ∼3050 mm), discharge as well as total solid load was ∼200% higher. Monsoon months (July–September) accounted for more than 90% of the annual solid load transport. The ratio of dissolved to suspended solid (C/P ratio) was consistently low (<1) during monsoon months and higher (1–7) during the rest of the dry period. C/P ratio was inversely (R ²=0.49), but significantly (P <0.001) related to the rainfall. The average mechanical erosion rate in the three different rainfall years was 0.24, 0.19 and 1.03 mmyr ⁻¹, whereas the chemical erosion was estimated at 0.12, 0.11 and 0.46 mmyr ⁻¹, respectively. Thus, the average denudation rate of the Tons sub-watershed has been estimated at 0.33 mmyr ⁻¹ (excluding extreme rainfall year: 1.5 mmyr ⁻¹). Our results have implications to understand the hydrological behaviour of the Lesser Himalayan watersheds and will be valuable for the proposed and several upcoming small hydropower plants in the region in the context of regional ecology and natural resource management.