Utilizing the HYDRUS model as a tool for understanding soilless substrate water dynamics

J.S. Fields, J.S. Owen, R.D. Stewart, J.L. Heitman
Water is a finite resource that is essential for production of containerized crops. Understanding water dynamics within soilless substrates is an essential step in maximizing crop water use efficiency. How water is transported through or within the substrate profile and interacts with the void spaces as shaped by the individual substrate component particles is poorly understood. In an effort to bridge this knowledge gap, computer models were implemented to predict water dynamics within substrates comprised of different components. The soil water model HYDRUS can predict water movement through a porous media during transient (irrigation) and steady state (between irrigations) conditions. Incorporating defined initial and boundary conditions as well as container height and substrate hydraulic properties into the model enables HYDRUS to predict movement and location of water in unsaturated porous media by solving the Richards equation. In our initial efforts, water dynamics in both Sphagnum peat and pine bark (Pinus taeda L.) based substrates were modeled while progressing towards equilibrium and during a simulated irrigation event (pulse of water) resulting in predictions with expected and realistic outcomes that coincide with observations from prior experiments. The model results showed that the pine bark substrate achieved equilibrium in less time and exhibited a steeper moisture gradient during steady state conditions than did the peat substrate. This is likely a result of larger saturated hydraulic conductivity and lower water holding capacity in the bark-based substrate. During transient conditions, water distribution in the peat mix was predicted to be uniform, as opposed to rapid preferential movement (channeling) predicted in the pine bark mix. Though still requiring further validation, these results suggest that it may be possible to engineer more water efficient soilless substrate and also to enact more informed irrigation scheduling, which when combined can increase water retention and subsequent crop water use efficiency. Future applications of HYDRUS could allow researchers to evaluate new substrates before use in production, and improve understanding of how water interacts with substrate particles and pores.
Fields, J.S., Owen, J.S., Stewart, R.D. and Heitman, J.L. 2017. Utilizing the HYDRUS model as a tool for understanding soilless substrate water dynamics. Acta Hort. (ISHS) 1168:317-324
pine bark, Sphagnum peat, irrigation, water distribution, nursery, greenhouse

Acta Horticulturae