Estimation of effective soil hydraulic properties and recharge flux by assimilating land surface soil moisture (SM) and soil temperature (LST)

Abstract:

Soil hydraulic properties (e.g. saturated hydraulic conductivity parameter) are key land surface variables of hydrologic and land surface models as they control the partitioning of soil moisture and regulate the movement of water within the soil column. However, in-situ measurements of these parameters are expensive and not applicable to large scale weather or climatic studies. The advancement of remote-sensing platform enables us to access different remotely sensed soil moisture (top 5cm) and land surface temperature (LST) products with good spatial and temporal coverage. These land surface state observations have implicit information on the partitioning of precipitation between evapotranspiration and recharge fluxes. The objective of this study is to develop a framework for coupled estimation of effective soil hydraulic properties and the subsequent recharge flux along with evapotranspiration flux using variational data assimilation (VDA) technique. In this study, an existing VDA framework for estimation of land surface heat and evaporative fluxes was modified to calculate soil effective saturated hydraulic conductivity as an additional parameter. The VDA framework assimilates LST and top SM as state observations into a coupled water energy balance model and provides estimates of key state and parameters (e.g., estimation control variables) of land surface evaporative and recharge fluxes. Uncertainties of fluxes and parameters will be calculated from the inverse of hessian matrix of cost function which provides a good approximation for the error covariance of parameter estimates. The success of the conceptual modelling framework to estimate the effective soil hydraulic parameters and the subsequent recharge flux was tested against synthetic soil moisture profile and recharge flux dataset generated using Hydrus 1D model under different combinations of climatic conditions and soil textures. Several numerical experiments (bottom boundary condition as free-drainage or Shallow ground water table at different depths) were conducted to test the applicability of the conceptual model.