A frequency-domain model to predict surface soil moisture, root zone soil moisture and aquifer recharge

This study aims to develop and evaluate a new simple unsaturated zone model in frequency domain interconnecting different types of ground or satellite-based observations (effective rainfall, surface soil moisture, root zone soil moisture, groundwater recharge) of a single dynamic system. The proposed formulation based on 5 coherent transfer functions (Tfs) linking observations 2 by 2, with a limited number of parameters, is rooted in the Nash linear reservoir model. The curve shape of the TFs can be adjusted by key parameters such as flow complexity, response time and the share of surface runoff  each of which carries distinct and well-defined physical interpretations.  The model's efficacy was assessed using surface soil moisture, root zone soil moisture and and recharge data in a fractured crystalline-rock aquifer situated in Ploemeur, South Brittany, France. Each TF's was initially independently optimized, with parameters assigned through a detailed review of the literature and consideration of the physical characteristics of the site. The outcomes highlighted the methodology's potential, offering a comprehensive depiction of root-zone soil moisture and aquifer recharge dynamics. In the subsequent phase, the model parameterized via a joint optimization of TFs. we find out that the joint approach has the ability to elevate the accuracy and reliability of the model, ensuring its stable and robust behavior. The simplicity of the procedure, with a small number of easily interpretable parameters, makes it suitable for broader applications in different regions.