SWBCM: a soil water balance capacity model for environmental applications in the UK

Abstract

The paper presents a daily-time step, multi-horizon capacity model of soil-water balance (SWBCM—Soil Water Balance Capacity Model) suitable for ecological and environmental applications investigating the spatial and temporal variability of soil water content determined by changes in soil hydraulic conductivity, soil water storage capacity and the pathways of water movement through the soil and across soil types. SWBCM simulates soil water content at horizon level and encompasses limits on the amount of drainage from one horizon to the next to allow the formation of temporary perched water tables, lateral drainage, matric potential and surface runoff. The model incorporates a dynamic sub-model of grass growth (SWARD—Dowle, K., Armstrong, A.C., 1990. A model for investment appraisal of grassland drainage schemes on farms in the UK. Agric. Water Manage. 18, 101–120; Armstrong, A.C., Castel, D.A., Tyson, K.C., 1995. SWARD: a model of grass growth and the economic utilisation of grassland. In: Pereira L.S., van den Broek B.J., Kabat P., Allen R.G. (Eds.), Crop-Water Simulation Models in Practice. Wageningen Press, Wageningen, The Netherlands, pp. 189–197). SWBCM’s predictive ability is tested across a range of soil types under permanent grass in the UK and outputs are compared with predictions made by MACRO (Jarvis, N.J., 1994. The MACRO Model Version 3.1. Technical Description and Sample Simulations. Swedish University of Agricultural Sciences, Department of Soil Sciences, Reports and Dissertations 19, Uppsala, Sweden, 51 pp.), a mechanistic solute transport model which incorporates a physically-based preferential flow model in which total soil porosity is divided into two flow domains (macro-pores and micro-pores), each characterised by a flow rate; soil water flow in the micro-pore domain is modelled using Richards’ equation. In the modelling experiment, SWBCM simulations have been shown to provide good approximations of point-scale experimental data under a range of soil, climate and drainage management conditions in the UK. SWBCM simulations are close to those developed by the mechanistic MACRO model, suggesting that the capacity model can be applied to describe the water balance of multi-horizon UK soil profiles. The modelling approach used is considered to be applicable to the wide range of soil lower boundary conditions, ranging from free-draining to impermeable, occurring in the field and to simulate transient perched water tables that commonly occur in most temperate, high latitude countries such as the UK.