Soil–Litter–Iso: A one-dimensional model for coupled transport of heat, water and stable isotopes in soil with a litter layer and root extraction

We present a new isotopically enabled hydrologic scheme, “Soil–Litter–Iso”, suitable for use as part of an isotopically enabled land surface model. Soil–Litter–Iso is a one-dimensional model for coupled transport of heat, water and stable isotopes (HDO and H218O) in soil and litter. It is sufficiently efficient for use at regional scale, yet includes the complexity of coupled heat and water transport enabling decomposition of the total moisture flux into liquid and vapour components. The numerical implementation is based on Ross’ fast solution to the Richards equation (). This, combined with the explicit solution of the energy and moisture equations at the soil/air interface, permit the isotopic calculations to be performed with thick soil layers and large times steps, resulting in significantly improved computational efficiency compared with existing isotopically-enabled soil models of similar complexity. We demonstrate the model’s numerical accuracy by conducting a series of established test-cases and comparing predictions of steady-state isotopic concentration profiles with corresponding analytical solutions. We also demonstrate the model’s operation within a land surface model by performing simulations for the forested flux site at Tumbarumba in south-eastern Australia. These simulations show that the total evapotranspiration (ET) flux, its components and their isotopic signatures are very sensitive to the inclusion of litter, and that the model is a useful tool for assessing when the isotopic signatures of the ET components are sufficiently distinct to be useful for flux partitioning.