Estimation of apparent thermal diffusivity of soil at lesser-Himalayan experimental catchment, Uttarakhand, India, for analytical subsoil temperature modelling

Abstract

Understanding the linkages between climate—land surface—hydrology and Himalayan ecosystem is challenging due to large spatial heterogeneity and data scarcity. Soil temperature is a key variable affecting this process. However, apparent thermal diffusivity values and its seasonal variation representative of the Himalayan region are not known for improving the soil temperature modelling. Data generated from an experimental watershed in the lesser Himalayan region near Chamba, Uttarakhand, is used for this purpose. Daily thermal diffusivity of the selected soil reach of 2–30 cm depth is modelled using the equation based on diurnal soil temperature amplitudes, which further yields daily damping depth. Periodic subsoil thermal profiles for four different depths were reproduced analytically using sinusoidal (harmonic) function by optimizing the phase constants that show significant seasonal variation ranging from 1.76 to 2.28, 2.06 to 2.82, 1.77 to 2.70 and 1.02 to 2.45 at a depth of 2, 6, 15 and 30 cm, respectively. The average Nash-Sutcliffe efficiency ηNS = 96.29% of four soil depths justifies the adopted model and soil thermal properties. Study shows that the thermal diffusivity of lesser Himalayan region range between 0.0022 m2/ h (monsoon) and 0.0019 m2/h (winter) with an annual mean of 0.0020 m2/h. These variations in thermal diffusivity and phase constants reflect the distinct seasonality of the Himalayan land surface forcing.