A simplified parameterization of the monthly topsoil water budget

In this paper we propose simple equations for the different components of the topsoil water budget (runoff, infiltration, evapotranspiration, and soil moisture). These equations are derived by interpretation of the trends of average monthly soil moisture, and monthly aggregated infiltration, runoff, and evapotranspiration obtained from the output of a daily hydrologic model of topsoil simulating representative climate and soil conditions across Europe. The equations we propose imply calculations of the same level of complexity as Thornthwaite's method, while retaining consistency with the daily step model and capturing the nonlinearities of some processes. Therefore they can be proposed as a screening level tool for the analysis of soil water budget in practical applications. The method only requires input on monthly precipitation, potential evapotranspiration, and basic soil properties related to physically meaningful quantities. The proposed method provides results at monthly steps, which are fairly consistent with other, computationally more demanding models.