Water dynamics in two rockwool slab growing substrates of contrasting densities

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1 Scientia Horticulturae 107 (2006) Water dynamics in two rockwool slab growing substrates of contrasting densities S. Bougoul a, *, T. Boulard b a Département de Physique, Faculté des Sciences, Université de Batna, Rue Chahid Mohamed El Hadi Boukhlouf, Batna, Algeria b INRA-URIH, 400 route des Chappes, BP 167, Sophia Antipolis, France Received 7 January 2005; received in revised form 19 September 2005; accepted 30 November 2005 Abstract It is important to determine the dynamics of water and nutrients in the growing substrate used for soil-less cultivation because this allows better time and space management of the water and nutrient supply according to plant needs during each step of the crop cycle. In this study, we focus on the motion of water in rockwool slabs used as growing substrate for a sweet pepper crop. The transport equation is first quoted and root absorption described by a sink term for which the absorption law was experimentally determined. Control volume methods by means of commercial computer fluid dynamics (CFD) software were used to solve the water movement equations numerically in three dimensions. However, as Richards equation describing water movements in the substrate is quite different from the Navier Stokes equations used by the CFD, considerable modifications of the standard transport equations were required. The numerical results were then compared with experimental results and, once validated, the model was reused for a sensitivity analysis with respect to various physical parameters of the substrate. Based on this study, we propose an alternative substrate design more suitable for closed systems. # 2005 Elsevier B.V. All rights reserved. Keywords: Simulation; Rockwool slabs; Water movement; CFD; Sweet pepper; Irrigation 1. Introduction In the last few years, the physical and chemical properties of growing substrates have received considerable attention because they have a major influence on water and nutrient balances at root level and affect control of the recycling of the nutrient solution. They also facilitate the control of water and nutrient dynamics (Otten, 1994). Knowledge of the hydraulic and chemical properties of the growing medium is essential because it makes it possible to control the plant root environment (Da Silva et al., 1995). Growing substrates are porous media, often granular (e.g. pearlite), or fibrous (e.g. rockwool). As rockwool can provide water at low suction values and in a small volume, determination of its hydraulic properties is required. As it is also necessary to avoid ionic accumulation in the root zone by using high drainage rates (>30%), delivery of the recycled solution must be matched more closely with crop consumption, * Corresponding author. Fax: address: s_bougoul@hotmail.com (S. Bougoul). especially to avoid the drying and saturation episodes that hinder root absorption. To manage nutrient and water supply, it is therefore necessary to intervene quickly to adapt solution delivery to the plant needs. While the volume of water consumed by the plants is known almost instantaneously, moisture variations within the substrate particularly dry and saturated zones are not precisely located. For this reason, knowledge of moisture trends in the substrate depending on the successive watering and drainage cycles requires precise modelling of solution transfer. In this study, after first characterising the physical and hydraulic properties of rockwool slabs (Bougoul et al., 2005), we analysed solution motion in two types of rockwool slab (Floriculture 1 and Expert 1, produced by the Grodan company), used with a sweet pepper crop. The equation for water transport is presented and completed by a simplified model of water absorption by the plant roots. Water transfers were simulated by this model, and then compared with experimental results. Once validated, the model was used for a sensibility study in order to design a substrate type with geometry and physical properties better suited to recycling systems /$ see front matter # 2005 Elsevier B.V. All rights reserved. doi: /j.scienta

2 400 S. Bougoul, T. Boulard / Scientia Horticulturae 107 (2006) Materials and methods Rockwool slab is an artificial substrate, which offers a high degree of water permeability and high water content at low suction. A wide variety of substrates can be manufactured, differing mainly in the density and orientation of the fibres. The two rockwool slab types used in this study, Floriculture and Expert, produced by the Grodan Company (Denmark), were chosen for their contrasting hydraulic properties. Considering the known physical properties of these types of growing medium (Bougoul et al., 2005) in conjunction with series of measurements of root distribution (Brun et al., 2004) and water distribution (Longuenesse and Brun, 2004) withintheseslabsinproductionconditions,we set out to simulate numerically the movement of water within the slab. Our aim was to design a composite substrate that would allow roots to better exploit the whole slab volume and function more efficiently in a recirculation cropping system Theoretical background water movement Water movement in a porous medium satisfies the conservation equation of matter. For an incompressible fluid in a rigid porous medium, the combination of this equation and Darcy s equation is described by Richards equation for an unsaturated porous medium (Richards, S w where h is the suction, the vertical coordinate z is assumed to be positive downward, u the water content, S w the sink term corresponding to the water absorbed by the plants, K(h) the hydraulic conductivity of the medium and C(h) its differential water capacity, given by CðhÞ This formulation accounts for the capacity of the porous medium to release water or store it under the effect of a pressure gradient. To solve this equation, one must determine the system s boundary conditions, its initial conditions, and the values of the hydraulic conductivity, differential water capacity and sink terms. For the substrates studied, these parameters had already been experimentally determined (Bougoul and Boulard, 2004) Boundary conditions and initial condition for the movement of water Richards equation giving water movement was solved for the boundary and initial conditions of the growing medium, whose characteristics (geometry, position of the drippers and drainage slot) are recapitulated in Fig. 1. The base and sides of the substrate are impermeable, except for the drainage slot. At the top of the substrate, we impose flux conditions Fig. 1. Geometry of the growing media. corresponding to the watering flow rate in the water input zone; elsewhere, the flow rate is assumed to be nil Hydraulic properties Moisture variations inside the substrate were deduced from the resolution of Richards equation with appropriate values of the hydraulic properties (h,k,c), particularly with respect to its volumetric water content u. Several models describe the hydraulic properties of the growing medium, one of the most widely used being the model of van Genuchten (1980) and Mualem (1976). Conductivity versus suction for both steady state and transient regimes in evaporation and humidification were experimentally determined for Floriculture 1 and Expert 1 slabs and their corresponding parameter values were fitted on the Mualem van Genuchten model (Rathfelder and Abriola, 1994). Different results were experimentally determined for different regimes corresponding to various ranges of suction (Bougoul et al., 2005; Bougoul and Boulard, 2004). Water retention characteristics u(h) in the sorption and drying stages were also experimentally determined for both types of rockwool slab (Bougoul et al., 2005) and the results were adjusted on the van Genuchten model (van Genuchten, 1980). The differential water capacity was also deduced from the characteristic water retention curve (Heinen, 1997) Sink term Root absorption by sweet pepper was considered as a sink of nutrient solution with an absorption rate that depended on root density and consequently root distribution (Brun et al., 2004), and on the substrate s suction (Jinquan et al., 1999; Longuenesse and Brun, 2004). Knowing the potential absorption rate (S max ), the actual absorption rate (S) could be deduced by the following relation: S ¼ aðhþs max (3) where the relation a(h) depends on the substrate s suction (Fig. 2) and the potential absorption rate S max, depends on the relative density of roots in the substrate L nrdðz r Þ L r and on the atmospheric demand T p, which was experimentally determined (Longuenesse and Brun, 2004): S max ¼ T pl nrd ðz r Þ L r (4)

3 S. Bougoul, T. Boulard / Scientia Horticulturae 107 (2006) Fig. 2. Variation of the coefficient a(h) according to suction value. In this expression, L r represents the vertical depth of the substrate and the term L nrd ðz r Þ¼ L dðz r Þ R 1 the relative density 0 L dðz r Þ dz of roots in the substrate. The distribution of the roots in the substrates, as measured during the experiments performed in July 2002 (Brun et al., 2004), were reused to determine L nrd (z r ) Numerical analysis Richards equation is a non-linear equation whose solution requires a numerical method. It was implicitly solved using commercial Computer Dynamics software: CFD (1999), based on a control volumes method Customisation of the CFD 2000 software For a given species f, the standard fluid mechanics transport equation solved by the CFD software (CFD, 1999) is of the following ðrfþ I II ðg j III þ S f IV where term nos. I, II, III, and IV are respectively the nonstationary, convection, diffusion and source terms. r is the density, G f the diffusion coefficient and S f is the source term. The G f and S f values are specific to each variable f. For water movements described by a suction variable h (cm), considerable modifications of Richards equation (1) are needed to match it with the convection diffusion equation represented by relation (5) and used by the CFD software. Relation (1) was therefore rewritten @t S w (6) Note that compared to Eq. (5), the convective term is set to zero and the nonstationary, diffusion and source terms of the new representation (6) can easily be identified. 3. Results After rearrangement of the basic equation, water movement in the substrate was determined by numerical resolution of Richards equation using the CFD software together with hydraulic properties (K(h), u(h)) experimentally deduced from a previous study (Bougoul et al., 2005). The irrigation rate was set, as in the experiments, to 2l/h (dripper s value) on an area equivalent to the base of the planting cube (100 cm 2 ) Validation Numerical calculations were compared with experimental measurements for both substrate types (Floriculture 1 and Expert 1 ) and for two irrigation frequencies: a high frequency one (irrigation every 0.3 mm of atmospheric demand) and a lower one (every 0.6 mm). We will examine simulated water dynamics in the substrate throughout 1 day in July 2002, characterised by 12 irrigations for the high frequency regime and 7 irrigations for the lower one. Iso-suction areas in the slabs (Fig. 3) are represented with the same colours for both the numerical and experimental results, in accordance with the scale in the legend (saturated zones in red and dry zones in blue). From the qualitative standpoint, a comparison of the computational and experimental results shows similar distributions of humidity in the slab. It also shows the same dynamics after watering for both substrate types and both irrigation regimes. In particular, one can observe similar periodic moisture variations in the substrate. Focusing on water distribution at the end of irrigation (Fig. 4), the following remarks can be made for all treatments studied (high and low irrigation frequencies, Floriculture 1 and Expert 1 slabs): for all treatments, the area just above the drainage slot is always very dry, with suction values of over 10 cm. Despite the obvious difference in hydraulic conductivity between the two materials, we observe the same water distribution for both slabs and both irrigation protocols. This dry zone therefore seems to depend mainly on the position of the drippers, irrespective of the substrate s physical properties; the saturated zones (yellow and red in Fig. 4) are always located just below the drippers and at the base of the substrate. The area of these zones depends on the type of the substrate, especially its hydraulic conductivity; comparing the two rockwool slab types (Fig. 4), one can see that the saturated zones are more extensive in the highdensity rockwool (Floriculture 1 ) than in the lower-density slab (Expert 1 ). the high-frequency irrigation rate also generates more extensive saturated zones (see Expert 1 results in Fig. 4).

4 402 S. Bougoul, T. Boulard / Scientia Horticulturae 107 (2006) Fig. 3. Scenarios of water distribution after irrigation. (For interpretation of the references to colour in this figure, the reader is referred to the web version of the article.) It is hard to compare the computed and measured values for water content in the slabs because the measured values are averaged across the whole width of the slab (TDR measurement) whereas the computed ones correspond to (almost) punctual values distributed along a vertical section passing through the drainage slot. However, the slabs hydraulic behaviour can be seen to have similar dynamic tendencies: Fig. 4. Simulation scenarios for wetting and re-wetting conditions: suction (m). (For interpretation of the references to colour in this figure, the reader is referred to the web version of the article.)

5 S. Bougoul, T. Boulard / Scientia Horticulturae 107 (2006) Fig. 5. Moisture variations in both types of substrate at the end of the night. larger saturated areas for the high-density rockwool (Floriculture 1 ) than for the lower-density one (Expert 1 ); larger saturated areas for the treatment with the higher irrigation frequency rate Substrate optimisation based on simulation studies Simulated and experimental results both highlight the considerable extent of the areas with very high humidity (>90%) for the Floriculture 1 slab (about half of the slab volume) whereas these areas are much less extensive for the Expert 1 slab (less than the one-third of the slab volume). In order to design a more optimal substrate, we have therefore proposed a composite slab, made up of two rockwool types, the density of each allowing for closer control of the slab s hydraulic conductivity and hence on its humidity. Part of this optimal slab (the top one-fourth) was composed of a high-density rockwool (Floriculture 1 type) and the remaining part (the bottom three-fourth) was composed of a lower-density rockwool (Expert 1 type). After simulating the hydraulic behaviour of this optimal slab for the same boundary conditions and irrigation regimes as with the Expert 1 slab type, we note (Fig. 4), that the dry zones are far smaller, the wet zones (>90%) now occupying about two-third of the entire volume of the substrate (compared to less than 25% for Expert 1 ). Fig. 4 also shows that the enlargement of the wet zones, allowing for much better exploitation of the slab volume by the plant roots, is especially great towards the end of the day. However, these wet and almost saturated conditions must be periodic, in order to establish a cycle that keeps plant roots free of anoxic conditions. Periodic drying can be routinely achieved at night, when irrigation is stopped and the substrate dries out until the first irrigation the next day. Substrate humidity in the composite substrate (with low- and high-frequency irrigation rates) was therefore investigated at the end of the night, just before the first irrigation, and compared with Floriculture 1 slabs and Expert 1 slabs irrigated with low-frequency irrigation rates (Fig. 5). Simulation results for the four trials generally show an absence of water stagnation and of the consequent risk of anoxia in the lower parts of the substrate. However, with the optimised composite substrate, thanks to the different hydraulic properties of the two rockwools used, larger wet zones are maintained in the slab without generating anoxic conditions, so providing better conditions for root absorption. 4. Conclusion Solution transfers in a rockwool slab partially occupied by plant roots can be satisfactorily simulated using the numerical schemes of CFD software. Experimental and simulation studies show that physical substrate characteristics (particularly hydraulic conductivity) and watering frequency are both crucial parameters. However, root distribution is also a key factor; the determinants of this factor are still little known and need further research. Both experimental and modeling studies show that saturated areas are located below the drippers and along the base of the rockwool slab and that the dry zones extend along the surface of both substrates, particularly near the two ends. Both experimental and numerical results also show larger saturated areas in the high-density slabs (Floriculture 1 ) than in the lower-density ones (Expert 1 ). Watering protocol, particularly frequency, has important effects on humidity in the slab: the Floriculture 1 substrate can retain a high percentage of water, which in turn can generate anoxic conditions in the lower parts of the medium when irrigation frequency is high. By contrast, with the lower-density

6 404 S. Bougoul, T. Boulard / Scientia Horticulturae 107 (2006) slabs (Expert 1 ) and low the irrigation frequency, at the top of the slab there are larger dry zones where roots cannot develop easily. Once qualitatively validated, the numerical model enabled us to perform sensitivity studies and test an optimised substrate suitable for water recycling conditions. We designed a composite slab that can keep higher water content in its upper part and higher hydraulic conductivity in its lower part, so maintaining a more even distribution of humidity from top to bottom of the slab while decreasing the drainage inertia of the whole slab. Acknowledgements This project was carried out with financial support from the European Commission under the RTD programme Quality of life and management of living resources (project QLRT ). It does not necessarily reflect the Commission s views and in no way anticipates its future policy in this area. References Bougoul, S., Ruy, S., de Groot, F., Boulard, T., Hydraulic and physical properties of stonewool substrates in horticulture. Scientia Horti. 104, Bougoul, S., Boulard, T., Simulation of solution movements in rockwool slabs used as growing media for a sweet pepper crop. In: Greensys Congress, AGENG2004, Louvain, September 13 17, Brun, R., Longuenesse, J.J., Reich, P., Barthelemy, L., Distribution water in rockwool slabs. In: Greensys Congress, AGENG2004, Louvain, September 13 17, CFD2000/STORMv 3.45, CFD systems. Pacific Sierra Corp., USA. Da Silva, F., Wallach, R., Chen, Y., Hydraulic properties of rockwool slabs used as substrates in horticulture. Acta Horti. 401, Heinen, M., Dynamics of water and nutrients in closed, recirculating cropping systems in glasshouse horticulture with special attention to lettuce grown in irrigated sand beds. PhD Thesis. Wageningen University, Wageningen, The Netherlands, 270 p. Jinquan, W., Renduo, Z., Shengxiang, G., Modeling soil water movement with water upatke by roots. Plant Soil 215, Longuenesse, J.J., Brun, R., Distribution of root density and root activity within rockwool slabs. In: Greensys Congress, AGENG2004, Louvain, September Mualem, Y., A new model for predicting the hydraulic conductivity of unsaturated porous media. Water Resour. Res. 12, Otten, W., Dynamics of water and nutrients for potted plants induced by flooded bench fertigation: experiments and simulation. PhD Thesis. Wageningen University, Wageningen, The Netherlands, 115 p. Rathfelder, K., Abriola, L.M., Mass conservative numerical solutions of the head-based Richards equation. Water Resour. Res. 30 (9), Richards, L.A., Capillary conduction of liquids through porous mediums. Physics 1, Van Genuchten, M.TH., A closed-form equation for predicting the hydraulic conductivity of unsatured soils. Soil Sci. Soc. Am. J. 44, All in-text references underlined in blue are linked to publications on ResearchGate, letting you access and read them immediately.