Water Uptake Response of Plant. in Subsurface Precision Irrigation System *

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1 Research Paper EAEF 6(3) : , 13 Water Uptake Response of Plant in Subsurface Precision Irrigation System * Mohamad SHUKRI Bin Zainal Abidin *1, Sakae SHIBUSAWA *, Motoyoshi OHABA *, Qichen LI *, MARZUKI Bin Khalid *3 Abstract The spatial and temporal variability of root-water uptake from the soil remain unclear due to the limitations of the measuring method and subsequently difficult in attempting to accurately model the soil-plant-atmosphere system. In this study, an experimental setup was designed to collect the data needed to develop a more realistic plant water uptake model using a modified subsurface irrigation system that allows the change of soil moisture content. The plant uptake response was determined from evapotranspiration, water consumption and soil moisture content at different soil depths. A plant water uptake model was reviewed and an extension of this model was proposed for the modified subsurface irrigation system. Preliminary investigation was performed using the experimental data and the results were essential. [Keywords] phytotechnology, soil-plant system, plant response, subsurface moisture control, site-specific irrigation I Introduction Recent climate change has caused heavy droughts and severe floods around the world, leading to increased concerns about water shortages not only for agriculture but also for industry and daily life. The March 11 Great East Japan Earthquake, with the ensuing tsunami and nuclear plant disaster, caused shortages of fresh water for both human consumption and agricultural use. Water-saving management is a key technology not only for arid and drought-prone areas but also for disaster areas (Smith and Baillie, 9). This was the motivation for our project to develop a site-specific irrigation system to meet the water demand for plant growth by applying precise control. Various methods have been developed to supply a sufficient amount of water to the root zone of plants. Subsurface drip irrigation has been proven to generate a higher yield and quality of crops (Camp, 1998). This approach relies on the concept of irrigating only the root zone of a crop while maintaining the soil moisture content at the optimum level (James, 1988). Higher water-use efficiency is achieved in irrigation management by manipulating the irrigation frequency and emitter arrangement of the subsurface drip irrigation system (Camp, 1998). Since the irrigation management deals with complex soil-plant-atmosphere continuum (SPAC), the accuracy of irrigation management model primarily depends on how accurately the system is modeled. Water uptake through evapotranspiration is the major component of an irrigation management model and represents the amount of soil moisture that is utilized through consumptive use by the crop. It is generally based on the soil water deficit determined by using a soil-water balance model, and the crop-water requirement estimated by using the energy balance method (Ayars et al., 1999; Jones, 4). However, it is difficult to model the components of a soil-water balance model because of the spatially variable and time-variant nature of the root system. Nevertheless, investigation has been extensive and different models have been developed to describe the moisture uptake by the plant root system. Efforts have been made to simulate the soil water movement with water uptake by roots using both microscopic (Novak et al., 5) and macroscopic (Mathur and Rao, 1999; Azhar and Perera, 6) approaches. Most models adopt the macroscopic approach because of the difficulty measuring geometrical root data in the microscopic approach. In the macroscopic approach, water extraction by plant roots is treated as a sink term distributed in the root zone. The sink term is incorporated into Richards equation, which describes the water movement in variable unsaturated soil. In * Partly presented at the International Symposium on Machinery and Mechatronics for Agriculture and Biosystems Engineering at Jeonju, Korea in 18- June 1. *1 JSAM Member, Corresponding author, United Graduate School of Agricultural Science, Tokyo University of Agriculture and Technology, Saiwai-cho, Fuchu, Tokyo , Japan; shukri75@yahoo.com * JSAM Member, Environment and Agricultural and Engineering System Laboratory, Tokyo University of Agriculture and Technology, Japan *3 Centre for Artificial Intelligence and Robotics, University of Tech. Malaysia, Jalan Semarak, 541 Kuala Lumpur, Malaysia

2 SHUKRI, SHIBUSAWA, OHABA, LI, MARZUKI: Water Uptake Response of Plant in Subsurface Precision Irrigation System 19 this sink term approach, root uptake is calculated from the crop transpiration rate, rooting depth and soil water potential, which is relatively easy to implement (Belmans et al., 1979; Mathur and Rao, 1999). Crop-based method has being used as a new measurement paradigm in irrigation management system. This method allows the system to adapt to the variability in crop-water demand according to the individual crop-water responses (Jones, 4). A recent trial using capillary flow from a water interface medium into the soil has resulted in higher quality and higher water-use efficiency of greenhouse peppers (Nalliah and Ranjan, 1). Further advancement of irrigation management strategy along this new paradigm is our main focus. Our project was developed based on the phytotechnology platform (Shibusawa, 1989; 1995) for a plant-based system that enables the measurement of root water uptake and upward capillary water flow in real time. This paper reports our attempt to analyze the water uptake response of a plant in a modified subsurface irrigation system. This plant-based, water-sensing, continuous self-irrigation system allows real-time measurement of the spatial and temporal variability of water uptake and upward water flow. Comparison was performed based on a macroscopic model to confirm the operational of the system by managing the soil moisture and the crop evapotranspiration. II Materials and Methods 1. Root water uptake model The root water uptake and one-dimensional upward flow governed by the law of soil water conservation is described by Richard s equation (Eq. (1)) (Jury and Horton, 4). It has been used in numerous studies on root water uptake model using the macroscopic approach (Marino and Tracy, 1988; Antonopoulos, 1997; Mathur and Rao, 1999; Azhar and Perera, 6). In the equation, the root water uptake is denoted by a sink term S as the volume of water per unit volume of soil per unit time. h h C( h) K h S t z ( ) 1 (1) z The sink term in Eq. (1) is defined by Prasad (1998) by the pressure head dependant reduction factor, rooting depth and the transpiration rate. The reduction factor is determined by the wilting point and soil saturation as a function of the pressure head (Feddes, 1978). Based on Feddes (1978), at the optimal soil moisture, the root water extraction will be equal to the potential transpiration rate. The root water extraction will reduced by that reduction factor when the soil moisture is limited. The plant transpiration is estimated by Antonopulus, (1997) based on crop coefficient (Allen, 1998) and leaf area index. The potential evapotranspiration ET o is determined by Eq. () based on the FAO Penman-Monteith equation for the reference crop evapotranspiration (Allen, 1998) as ET O 37.48( Rn G) u T 73 (1.34u ) e e where R n is net radiation at the crop surface, G soil heat flux density, T mean air temperature at a height of m, u wind speed at a height of m, e s saturated vapor pressure, e a actual vapor pressure, e s - e a saturated vapor pressure deficit, Δ slope of vapor pressure curve, and γ psychrometric constant. The upper boundary condition in Eq. (1) is defined by the potential evaporation. h, t h ( t) b s a () (3) For the bottom boundary (Eq. (3)), the modified subsurface irrigation system can provide a controllable pressure head at the soil bottom, by regulating the water supply depth Δh as described by Eq. (4) : d m ( z) q( t) K( z) g at z h (4) dz where q(t) is water consumption per unit cross-sectional area per unit time, K(z) hydraulic conductivity of the fibrous interface, ρ mass density of water, g gravitational acceleration, and Ψ m (z) matrix potential of vertical fibrous interface. The pressure head applied to the soil bottom can be changed by manipulating the water supply depth Δh in the reservoir.. Experimental setup Fig. 1 shows the experimental setup. A modified subsurface irrigation system was built for a plant pot containing commercial organic soil (1) (RF-7, Masaki, Japan). The soil compositions was 18.8 % sand, 1.5 % silt, 1.5 % clay and 56. % organic compost. Soil was added to the pot at increments of 1 cm and each addition was manually compressed with a steel plate, giving a packed dry bulk density was.18 g cm -3. The soil moisture at the packed bulk density was.4 m 3 m -3. A small reservoir () is positioned under the pot and water rises up to a fibrous interface (3) through a fibrous string (4). The characteristics of the fibrous materials are shown in Table 1. Infiltration of water to the soil above the fibrous interface is achieved by capillary rise. The soil moisture was determined by setting a suitable water supply depth Δh (5) using a manual jack (6) below a level regulator water tank (7). A test pot with a fibrous interface of cm in diameter was used The water used by the soil and the plant was determined

3 13 Engineering in Agriculture, Environment and Food Vol. 6, No. 3 (13) from the change in water level in the water supply tank (8), measured with a magnetostrictive-type water level sensor (9) (HL-G1--R-S, Watty, Japan). Hereafter, this is denoted as water consumption. The microclimate parameters inside the phytotron were measured using an air temperature-humidity sensor (1) (HMP155, Vaisala, Finland) and a solar radiation sensor (11) (LI-19, Li Cor, USA). A set of capacitance-type soil moisture sensor (1) (EC-5, Decagon, USA) calibrated for the organic soil were vertically positioned in order at four different soil heights at 5-cm intervals above the fibrous interface. An electronic balance (13) (GP3KS, A&D, Japan) was used to measure changes in the soil and plant mass. Evapotranspiration rate was determined from the mass change (Blizzard and Boyer, 198) and the water consumption data based on simplified water balance model (Yuan et al. 1). Hereafter, this is denoted as crop evapotranspiration. A data logger (GL8, Graphtech, Japan) was used to store the data from the sensors and the sampling time was 5 min. Z (1) (11) (9) entry of direct sunlight, except for the back wall which is a metal sheet. The air temperature in the phytotron was set at 5 C from 6: to 18: and 15 C from 18: to 6:. The air humidity was set at 7 % and air flow from the floor was continuous at.5 ms -1. The phytotron is located at the Faculty of Agriculture, Tokyo University of Agriculture and Technology in Fuchu, Tokyo. The experiment was conducted from July to September 11 and tomato was used as the test plant. The tomato was prepared by a commercial germinator (YB-38, Sakata Seed, Japan) and was transplanted into the pot at age of 3 days. Nutrients were supplied by adding liquid fertilizer of (Hyponex, Japan) in the water supply tank at a ratio of 1:1. The experimental schedule is shown in Table. The experiment was conducted as denoted by the number of days after transplanting. During the early growth period, from Day 1 to Day 3, water supply depth Δh was set at cm to provide the maximum water consumption for the plant Δh was -11cm from days 31 to 55, and was changed to -3 cm from days 57 to 9 when the plant showed symptom of water-stress. The following discussion is based on the measurement made on two periods from days and after transplanting. Data at days 55 and 65 is emphasized for the soil moisture response analysis. P4 (1) (8) (1) P3 3 cm P P1 (3) O (7) Δh (5) () (6) (4) (1 (13) Fig. 1 Experimental setup for the modified subsurface irrigation system The experiment was conducted in a phytotron with dimensions of 1.8 m in height, 1.75 m in width and 1.75 m in length. The roof and walls of the phytotron are glass to allow Table Days after transplanting and corresponding water height Days after transplant Water supply depth, Δh (cm) III Results and Discussion 1. Evapotranspiration and water consumption Fig. shows the water consumption (Q), the potential evapotranspiration (ET O ), and the crop evapotranspiration (ET C ) for days and after transplanting. The observed values of ET O at days and were higher than that for ET C and Q. Further, ET C was larger than Q at days However, the values of ET C and Q were Table 1 Fibrous interface characteristics in the experiment Interface type Horizontal interface (Toyobo 71s) Vertical interface (Toyobo A-1) Absorption rate (cm min -1 ) 5.5 (horizontal). (vertical) Absorption quantity (%) Porosity (%) 9 8 Thickness (mm)..8 Size (cm). (diameter) 15 (width length) Weight (g m - ) 1 7

4 SHUKRI, SHIBUSAWA, OHABA, LI, MARZUKI: Water Uptake Response of Plant in Subsurface Precision Irrigation System 131 smaller than that at days A similar response of ET O, ET C and Q was observed at day 65 when ET O was lower than.5mm/min. Q, ETc & ETo (mm/min) Q, ETc, ETo (mm/min) Fig. Water consumption, crop and potential evapotranspiration at Days (a) and (b) after transplanting (a) Days 54-56, Δh = -11 cm (b) Days 64-66, Δh = -3 cm The relationship between ET C and ET O can be described by using crop coefficient. The crop coefficient usually depends on the soil moisture and plant physiology (Allen, 1998). The increase in ET C at the same value of ET O from day to day may indicate the increasing of the crop coefficient probably as the result of the soil moisture increase when water supply depth was change from -11 to -3cm. Water con. (Q) Crop ET (ETc) Pot. ET (ETo) Water con. (Q) Crop ET (ETc) Pot. ET (ETo) On the other hand, the relationship between ET C and Q can be described by using soil-plant-atmosphere continuum (SPAC) approach (Blizzard and Boyer, 198). Water lost by transpiration of a plant is replenished by extracting water from the soil. As the soil was connected to the reservoir through the fibrous interface, the water flow continued. The volume of the water flow was indicated by the Q. Thus the change of the ET C value will also affect the Q. The increased of ET C at larger soil moisture content also increases the Q. The results show that the crop evapotranspiration and water consumption are affected by the potential evapotranspiration (Allen, 1998). Furthermore crop evapotranspiration and water consumption are depended on the soil moisture content (Prasad, 1998; Feddes, 1978) which was determined by the water supply depth for the fibrous interface in the reservoir.. Soil moisture content Fig. 3 shows the time variation of the soil moisture content at different soil height at days and The results indicate a different level of soil moisture at various soil heights at the two periods. At day the maximum soil moisture was 3 % while at day was 66 % at the same soil height. The smaller soil moisture at day may be caused by high root water uptake. Water supply to the root area may be insufficient thus decreases the soil moisture. The increased of soil moisture was observed at day when the water supply depth was changed from -11 to -3 cm. The larger soil moisture is caused by the increase of water flow from the reservoir into the soil area (Fig. ). The water supply to the root area may be sufficient for the root water uptake thus increases the soil moisture. VWC (%) VWC (%) 8 (a) Days 54-56, Δh = -11 cm P1 (5 cm) P (1 cm) P3 (15 cm) P4 ( cm) (b) Days 64-66, Δh = -3 cm P1 (5 cm) P (1 cm) P3 (15 cm) P4 ( cm) Time (hous) Fig. 3 Soil moisture content at various soil heights at days and after transplanting The result in each period shows that the soil moisture decreases with increasing soil height. In particular, at day 64-66, the highest soil moisture observed was 66 % at P1 followed by 55, 39 and 1 % at P, P3 and P4 respectively. The difference in soil moisture at various soil heights is caused by potential gradient. This can be explained by the

5 Soil height (cm) 13 Engineering in Agriculture, Environment and Food Vol. 6, No. 3 (13) Buckingham-Darcy law (Jury and Horton, 4). Based on the law, the water flow from the reservoir to fibrous interface and to the soil surface is driven by the matrix potential gradient. The matrix potential value can be determined from the soil moisture data by using a soil water retention curve (Fig. 4). A slight change in soil moisture was observed during 6: to 18: at Day This may be caused by the root water uptake during transpiration and recovery of the soil moisture when the transpiration stops. 3 (a) Day 55, Δh = -11 cm Series1 Upward flow 5 Series Water uptake Volumetric water content (m³/m³) Matrix potential (-kpa) Fig. 4 Apparent soil water retention curve for the organic soil.6 (a) Day 55, Δh = -11 cm P1 (5 cm) P (1 cm) -.6 P3 (15 cm) P4 ( cm) (b) Day 65, Δh = -3 cm P1 (5 cm) P (1 cm) -.6 P3 (15 cm) P4 ( cm) Fig. 5 Soil moisture change at various soil heights at days 55 and 65 after transplanting Soil height (cm) (b) Day 65, Δh = -3 cm Series1 Upward flow Series Water uptake Fig. 6 The water uptake and upward flow responses at various soil heights at days 55 and 65 after transplanting 3. Soil moisture change and water uptake Fig. 5 shows the soil moisture change rate at different soil heights at days 55 and 65. In both days, the moisture changes show the negative and positive responses. In particular, at day 65, these variations are so distinguished characteristics. The largest values can be observed at P1 that the data distribute within -.6 to.5 % min -1. Smaller values were observed at the higher soil heights. These values of changes are depended on the soil moisture (Fig. 3 (b)). At day 55, the variations were smaller than that at day 65. The largest value can only be observed at P1 and P4 that distribute within -. to. % min -1. These small values of changes are also depended on the soil moisture which was lower at day 55 than that at day 65 (Fig. 3) The maximum positive response observed at P4, during 6: to 1:, at both days may have been caused by the condensation of vapor in the upper soil in the morning. The soil moisture change can be related to the water conservation law which is defined by Eq. (1). Based on our experimental system, water flowing into the soil from irrigation depends mainly on the soil water capacity and root water uptake. Thus, the change in soil moisture may indicate the water uptake and upward flow status concurrently based on magnitude and sign. At balance, the value may indicate zero moisture change. Based on the results, our consideration

6 Cumulative VWC change (%) SHUKRI, SHIBUSAWA, OHABA, LI, MARZUKI: Water Uptake Response of Plant in Subsurface Precision Irrigation System 133 is that the negative moisture change was dominated by the water uptake and the positive moisture change by the upward flow. Thus the water uptake can be characterized by the negative volumetric water content (VWC) change unit and upward the flow by the positive VWC change unit. Fig. 6 shows the maximum root water uptake and upward water flow corresponding to the soil heights at days 55 and 65. The data were determined based on the maximum and minimum responses of soil moisture change at each height from the result in Fig. 5. At day 55, the range between water uptake and upward flow at various heights were from. to.4 % min -1 while at day 65, from.35 to.11 % min -1. The increased range at lower heights in the two days indicated that the upward flow was depended on the water uptake. The upward flow may be affected by the lower matrix potential generated from the water uptake at each soil height. The results also indicate the dependence of root uptake and upward flow on the soil moisture (Prasad, 1998; Feddes, 1978) which is also characterized by the soil height (Fig. 3). Cumulative VWC change (%) 3 (a) Day 55, Δh = -11 cm P1 (5 cm) P (1 cm) P3 (15 cm) P4 ( cm) Time (Hours) 3 (b) Day 65, Δh = -3 cm P1 (5 cm) P (1 cm) P3 (15 cm) P4 ( cm) Time (Hours) Fig. 7 Cumulative of soil moisture change at various soil heights at days 55 and 65 after transplanting 4. Self irrigation effect Fig. 7 shows the cumulative VWC change at different soil heights at days 55 and 65. At day 55, the cumulative VWC decreased significantly at P1 to -3 %. P and P3 were unchanged while P4 increased to.5 % during the daytime. At day 65, the cumulative VWC at P1, P and P3 increased slightly, then decreased during the daytime to - %, -.5 % and.1 %, and increased after midday to %,.8 % and 1. % respectively. P4 also increased to.5 % during the daytime similar to day 55. The decreasing of cumulative VWC at P1 at day 55 indicates that the upward flow was unable to compensate the water uptake loss, thus causing a water deficit. At day 65, the slight increase in cumulative VWC indicates small upward flow before evapotranspiration (Fig. ) while the substantial decrease in the daytime indicates a large water uptake. On the contrary, the significant increase after midday indicates that the large upward flow was able to compensate the water uptake loss and then recover the water deficit at various soil heights. The increase of P4 at the two days, probably caused by the thermal effect as discussed earlier. The results may suggest an efficient method of recovering and managing the water deficit by manipulating the pressure head using the function of the water supply depth Δh as proposed by Eq. (4). IV Summary and Conclusions An attempt to analyze the spatial and temporal variability of root water uptake by using the modified subsurface irrigation system has been evaluated. Our results show the real dynamics of water uptake in relation to the upward flow from the continuous supply via the modified subsurface irrigation system. We found that the crop evapotranspiration, water consumption, water uptake and upward flow were affected by the soil moisture content, which can be changed by selecting a different water supply depth Δh for the fibrous interface in the reservoir of the modified subsurface irrigation system. An extension of the previous macroscopic root water uptake model has been considered to accommodate the functionality of the modified subsurface irrigation system. Our findings can be used to control the water deficit in the soil-plant for subsurface precision irrigation schemes. In future, we will focus on developing the irrigation management strategy for our subsurface irrigation system based on the change of water supply depth for the fibrous interface in the reservoir. References Allen, R. G., L. S. Pereira, D. Raes and M. Smith Crop Evapotranspiration: Guidelines for Computing Crop Requirements. FAO Irrigation and Drainag. Paper No. 56. Rome, Italy: FAO. Antonopoulos, V. Z Simulation of soil moisture dynamics on irrigated cotton in semi-arid climates. Agricultural Water Management 34:33-46.

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