Optimizing Soil Moisture in Subsurface Irrigation System Based on Porous Clay Capsule Technique

Water, a limited resource in most parts of the world, has always been a threat to arid and semi-arid zones. Using the sub-irrigation method with the clay capsule technique is one of the most practical strategies for conserving water and reducing water consumption in arid and semi-arid areas. A clay capsule is one of the porous pipes in a sub-irrigation system that can release water near the root zone. This paper has attempted to improve the physical and hydraulic properties of clay capsules based on changing the percentage of organic matter in the raw material (G0). The raw material used in making clay capsules is obtained from the calcareous soil of Nasr Abad village of Gorgan, Iran. The ratio of rice bran husk flour to G0 as improving hydraulic properties was 1:2, 1:5, 1: 10, 1:15, and 1:20 (kg of rice bran to kg of air-dried soil). The produced clay capsules were named G2, G5, G10, G15, and G20 respectively. The water discharge and soil water distribution of clay capsules were measured at 10, 25, 50, 80, and 100 kPa of hydrostatic pressures with a discharge-pressure automation instrument. The findings revealed a positive and significant relationship between increasing organic matter levels and the discharge of porous clay capsules. Unlike in G2 and G5, the relationship between discharges and hydrostatic pressure is linear in G10, G15, and G20. Meanwhile, the soil wetting shape followed a spherical trend due to the slow seepage of clay capsules. However, the soil-wetting shapes in G10, G15, and G20 were spherical and trended to vertical ellipsoids in G5 and G2. This method is of high importance for irrigating plants, especially in arid and semi-arid regions, and can efficiently manage the water shortage problem.


Introduction
Global population growth and improved lifestyles have increased the demand for food and nutrition security. Meeting these demands requires water, which is a limited resource in most parts of the world and a threat to arid and semi-arid zones. Several attempts have been made to introduce novel methods for controlling and optimizing soil water content considering the rise in water demand for agricultural needs and the fast deterioration of irrigation water (Elmaloglou et al. 2013;Colombani et al. 2016;González Perea et al. 2016;Rezazadeh et al. 2019;Cai et al. 2021;Jafari et al. 2022;Moazenzadeh et al. 2022).
Maintaining uniform and adequate soil moisture is a challenge in irrigated fields. One of the most promising strategies for conserving water and reducing water consumption is the use of a sub-irrigation system. They maximize water availability in the field capacity state, releasing water only when the moisture in the near root zone decreases. Soil moisture at the field capacity improves soil and plant root oxygenation. This system is also recommended as one of the solutions to reduce water losses (Adu et al. 2019;Cai et al. 2021) in arid and semi-arid regions' orchards and crop cultivation (Tesfaye et al. 2011;Siyal et al. 2013;Al-Mayahi et al. 2020). Soil moisture variability is also one of the most critical aspects of planning and managing a sub-surface irrigation system (Bhayo et al. 2018;Saefuddin et al. 2019;Moazenzadeh et al. 2022). It is now clear that more than 90% of plants require moisture equivalent to the soil's field capacity for optimal growth. Therefore, using new techniques to control and monitor soil moisture content in the field capacity range will play a vital role in the development of soil and water management programs under unsaturated conditions (Ashrafi et al. 2002;Vasudevan et al. 2011;Gebru et al. 2018;Saefuddin et al. 2019). In the unsaturated phase, moisture is a function of the soil matric potential, which equals -33 kPa in the field capacity.
The clay capsule nozzle is one of the new technologies in watering tools. This nozzle can slowly let water seep into the soil (Bainbridge 2001(Bainbridge , 2002Abu-Zreig et al. 2006Bahrami et al. 2010;Vasudevan et al. 2011). The clay capsule is one of the porous pipes in the sub-irrigation system that can release water into the near root zone. Buried clay capsule irrigation systems are known to be very efficient in water management since they supply a low volume of water based on the water requirements of crops near the root zone. Also, it is an effective traditional method for small farms in many arid and semi-arid regions. This system was invented in Iran and northern Africa  and has been used for over 2000 years in many small-scale dry lands around the world where water is insufficient or unsuitable for surface irrigation systems (Bainbridge 2001;Bahrami et al. 2010;Vasudevan et al. 2014). In Some arid and semi-arid regions of India, Iran, Pakistan, the Middle East, and Latin America where the annual rainfall is less than 500 mm, clay pot irrigation is occasionally used to provide the water requirement of plants (Bainbridge 2001;Ashrafi et al. 2002;Qiasheng et al. 2007;Bahrami et al. 2010;Siyal et al. 2011Siyal et al. , 2013. Nowadays, this irrigation method is becoming more common, especially in developed nations, due to its simplicity and auto-regulative capabilities (Abu-Zreig et al. 2006;Das Gupta et al. 2009).
The water flow rate of clay capsules is an important factor in the designation, operation, and management of the irrigation system (Qiasheng et al. 2007;Bahrami et al. 2010;Ghorbani Vaghei et al. 2016;Bhayo et al. 2018;Saefuddin et al. 2019). However, the physical and hydraulic properties of clay pots, as well as the physical properties of soil, can influence the soil water distribution pattern (Qiasheng et al. 2007;Naik et al. 2008;Ghorbani Vaghei et al. 2016). Once the water is released into the soil, its movement depends on the physical characteristics of the soil for some time, until its wetting pattern becomes completed (Elmaloglou and Malamos 2007;Elmaloglou et al. 2013).
The key parameters such as porosity and pore size distribution of clay capsules are used to predict the water flow rate in buried clay capsules (Cultrone et al. 2004;Naik et al. 2008;Freyburg and Schwarz 2007;Bahrami et al. 2010). Several factors, including hydrostatic pressure, saturated hydraulic conductivity of the clay capsule material, wall thickness, surface area, soil type, crop type, and evapotranspiration rate, influence the water flow rate of clay capsules (Abu-Zreig et al. 2006;Bahrami et al. 2010;Vasudevan et al. 2011). According to scientific evidence, the hydraulic conductivity and hydrostatic pressure of clay capsules are the most important factors in providing sufficient water to the root zone (Bainbridge 2001;Abu-Zreig et al. 2006;Ghorbani Vaghei et al. 2016). A key important aspect of hydrostatic pressure is that the relationship between the discharge of the clay capsule and hydrostatic pressure is non-linear (Abu-Zreig et al. 2006;Qiasheng et al. 2007;Liang et al. 2009;Das Gupta et al. 2009;Bahrami et al. 2010).  reported that in sub-irrigation with porous clay pipe, the radius of the wetted zone increased as a result of increased system water pressure. Bahrami et al. (2010) also developed a fuzzy model to determine the soil wetted radius and depth using the porous clay capsule irrigation method. They reported that in low discharge clay capsules, the wetted radius and vertical depth values were about 13.5 and 22 cm at a hydrostatic pressure of 25 kPa, respectively. The values of these parameters were 14 and 45 cm in clay capsules with high discharge rates, respectively. Contrary to drip irrigation, where vertical water movement is more than horizontal, the soil-wetted depth and radius appear to have increased with increasing discharge rate, resulting in a nearly spherical soil-wetting front (Bahrami et al. 2010).
The engineers should have a good understanding of the discharge rates of different types of clay capsules to achieve a high-performance sub-irrigation system. Clay capsule irrigation technology is yet to be fully studied in Iran. Moreover, the effect of organic matter percent on the hydraulic properties of clay capsules has not been widely probed and reported. This material is also important to fabricate clay capsules with light weight and porous media with an acceptable water discharge rate. Therefore, this study aims to develop and improve the seepage ability of clay capsules to provide soil moisture in the root zone by changing the material phase. As mentioned earlier, the results of this study will help in the deployment of buried clay capsule irrigation technology not only on small-scale land in Iran but also in arid and semi-arid regions of the world.

Experimental Procedures
The raw material used for the fabrication of porous clay capsules was obtained from NasrAbad village of Gorgan city in Golestan province, Iran. The mineralogy of the raw material and the baked material, as well as its corresponding less than 2 mm fraction, were determined by X-ray diffraction (XRD), and X-ray fluorescence (XRF) analysis. The particle size distribution of raw material was achieved by the hydrometer and dry sieving method (Gee and Bauder 1986).

Fabrication of Clay Capsules
To fabricate the clay capsules, rice bran husk flour obtained from rice crop residuals is added to the raw material. The raw material used for making clay capsules is obtained from the calcareous soil of Nasr Abad village (Golestan province), northeastern of Iran. To make the clay capsules, the ratio of rice bran husk flour to the soil amount was 1:2, 1:5, 1: 10, 1:15, and 1:20 (Kg of rice bran husk flour to Kg of air-dried soil), and the produced clay capsules were named G2, G5, G10, G15, and G20 respectively. This study was carried out in the Soil Science Department of Tarbiat Modares University (T.M.U) and the clay capsules were produced in clay capsule cells manufactured by T.M.U. The porous clay capsules with cylindrical shapes were fabricated using a clay capsules automation machine (Fig. 1). The machine delivers 20 raw clay capsules at a certain time. After preparing the raw clay capsules, they were released under atmospheric conditions for 48 h (air-dried) and were then placed in an electric dryer at 60 °C for 24 h. The clay capsules were thereafter transferred to a kiln and fired at a temperature of 980 °C for 8 h at a rate of 3 °C per minute (Fig. 2). The porous clay capsule dimensions were 20 cm long and 10 mm thick with regular inner and outer diameters of 1.5 cm and 3.5 cm, respectively (Fig. 3). The wall thickness and outer diameter of the clay capsules were measured with a vernier caliper and found to be achieved 1.5 ± 0.11 cm and 3.5 ± 0.20 cm, respectively. The open end of each clay capsule was covered with a plastic female coupling head to fit into an irrigation pipe.

Measurement of Clay Capsule Discharge in the Laboratory
The clay capsule flow rate plays an important role in providing water volume to the root zone while implementing a sub-irrigation system (Bahrami et al. 2010). The discharge of clay capsules was measured by a Discharge-Pressure Automation Device (DPAD). The DPAD automatically measures the discharge of capsules and has double cylindrical structures that provide a test for a long time (Fig. 4). Also, it is capable of measuring accurately the discharge of the clay capsule in the range of 45-2000 ml per hour with an error of about 5 ml per hour.
A porous clay capsule was immersed in a volumetric flux to determine the average discharge (Q pcc ). This experiment was repeated 35 times to measure the discharge of each type of clay capsule. The saturated hydraulic conductivity ( K S PCC ) of the porous clay capsules (PCC) was also measured based on the steady head method (Eq. 1) described by Bahrami et al. (2010): Q pcc is the discharge of the clay capsule, K S PCC is the saturated hydraulic conductivity of the clay capsule, and ΔH, ΔL , and A are hydrostatic pressure head, wall thickness, and surface area of the clay capsule, respectively. The wall thickness of the clay capsule was measured with a Vernier Caliper. The clay capsule surface area was calculated using the following equation; A is the surface area of the clay capsule. D and h are the outer diameter and length of the clay capsule, respectively. (1) Electric kiln in range of 0-1200 °C 1 3

Distribution Pattern of Soil Moisture Due to the Seepage of Porous Clay Capsule
The Spatiotemporal pattern of soil moisture distribution is a necessary factor in the designation, operation, and management of the clay capsule irrigation system. Therefore, this study aims to recognize the ability of different clay capsules to generate various forms of soil moisture patterns during irrigation time at 25, 50, 80, and 100 kPa hydrostatic pressures. It occurred in a container (60 cm in diameter and 100 cm in height) filled with air-dried soil. The results help in determining the appropriate soil depth to insert the porous clay capsules to minimize water evaporation from the soil surface. Therefore, each clay capsule was placed vertically at a depth of 20 cm from the soil surface in clay loam texture, and their wetness shape was recorded after 24 and 48 h. Then the wetted radius (r) and wetted depth (d) in the soil profile were determined for each of the 5 types of porous clay capsules at 25, 50, 80, and 100 kPa hydrostatic pressures. Table 1 displays the results of the XRF chemical analysis of the raw materials utilized to make the clay capsule. According to Table 1, the earth-alkaline oxides [Calcium oxide (CaO) and Magnesium oxide (MgO)] are significant in raw materials. Iron oxides and earth-alkaline oxides (CaO and MgO) play an important role in improving the physical and chemical properties of soil. According to Cultrone et al. (2004), these substances have an impact on the porosity of bricks. Earth-alkaline elements, as opposed to iron oxides, increase porosity Vieira 2004, 2014).

Mineralogy of Clay Capsules Before and After Firing
The raw materials contained small amounts of coloring oxides. The earth-alkaline oxides (CaO and MgO) are more prominent than coloring oxides in the raw materials, as seen in Table 1. Therefore, this phenomenon may have contributed to the baked porous clay capsules' buff light color (Fig. 5). It was reported that the buff color of baked ceramic was related to the earth-alkaline oxides content, sand, clay types, and small amounts of coloring oxides such as Iron (III) oxide (Fe 2 O 3 ), Titanium dioxide (TiO 2 ) and Chromium (III) oxide)Cr 2 O 3 ( of the raw material. (Monteiro and Vieira 2004;Wiśniewska et al. 2021).
Several studies have been conducted to show how temperature affects the porosity of clay bricks (Johari et al. 2010;Bories et al. 2014;Aouba et al. 2016;Srisuwan and Phonphuak 2020;Martínez-Martínez et al. 2022). Johari et al. (2010) reported that the percentage of clay bricks' porosity significantly reduced from 39.33% to 5.87% when sintered from 1000 °C to 1250 °C. Martínez-Martínez et al. (2022) also showed that ceramic bodies fired at high temperatures (1100-1200 °C) had less open porosity. Therefore, in this research, the optimum firing temperature for baking clay capsules was selected below 1000 °C. The research findings demonstrated that phyllosilicates' chemical structure changed dramatically as kiln temperature increased (Newman 1987;Aouba et al. 2016). The chemical composition of clay is converted to a new arrangement in the firing process of clay capsules at a high temperature (around 1000 °C). XRD analysis revealed that chlorite converted to microclinic within the raw clay capsules at 980 °C (Table 2). After the firing process, the microcline joined and created large sand-like particles.

Physical Properties of Porous Clay Capsules
The raw material in G2, G5, G10, G15, and G20 had a silty clay loam soil texture. After baking at 980 °C, the soil texture changed to loamy sand. High firing temperatures do not convert the clay structure to sand during the sintering process, but it is because of the mechanism that causes soil particles smaller than 0.002 mm to join and form larger particles. A larger soil particle can play the role of sand in sedimentation analysis (hydrometer method). In this research, the results showed that by increasing the firing temperature up to 980 °C, the size distribution of soil particles shifted from 0.002 to 2 mm and act as a sand particle role in sedimentation analysis. The physical properties of the porous clay capsules used in this study are summarized in Table 3. The bulk density and total porosity of the clay capsule were significantly affected by the firing temperature in the kiln. The increase in temperature during the firing phase results in a lower bulk density and a higher discharge rate due to   Table 3 is that the rice bran husk flour in the G2 generates a greater pore volume than the G20 after being fired at a high temperature. In conclusion, G2 and G20 baked clay capsules have the highest and lowest porosities, respectively. The percentage of total porosity of G2, G5, G10, G15, and G20 (baked porous clay capsules) was 25.5, 22.7, 20, 15.6, and 13, respectively. The hydraulic conductivity and water seepage of clay capsules are influenced by their porosity (Abu-Zreig et al. 2006;Bahrami et al. 2010;Ghorbani Vaghei et al. 2016;Abu-Zreig et al. 2018). They discovered that higher hydraulic conductivity in clay capsules is related to higher porosity. Therefore, the seepage of the G20 is expected to be less than that of the G2.

Hydraulic Properties of Porous Clay Capsules
The saturated hydraulic conductivity is influenced by the total porosity and wall thickness of clay capsules. Although there was a slight difference in size and wall thickness, the clay capsules had different porosities. Table 4 represents the saturated hydraulic conductivity and seepage of various clay capsules (G0, G2, G5, G10, G15, and G20). The clay capsule porosity was positively correlated with the amount of rice bran husk flour. Rice bran husk flour was used as organic matter in the raw material composition when fabricating mentioned clay capsules. The increase in organic matter content caused the porosity of clay capsules to increase from G20 to G2. The saturated hydraulic conductivity of G15 and G20 was lower than those of G2 and G5, due to their lower porosity (see Table 4). The results showed that increasing the pressure head resulted in a very small or non-significant increase in saturated hydraulic conductivity. So, ignoring this increase, it can be considered that the hydraulic conductivity of clay capsules was constant with increasing pressure head. The saturated hydraulic conductivity of G20 increased from 10 × 10 −3 to about 17 × 10 −3 17 (cm.h −1 ) with increasing hydrostatic pressure head, which was ignorable. The increase in organic matter content caused the porosity of clay capsules to increase from G20 to G2. The saturated hydraulic conductivities of G15 and G20 were lower than those of G2 and G5 due to their lower porosity (see Table 4). The results showed that increasing the pressure head resulted in a very small or non-significant increase in saturated hydraulic conductivity. To sum up, the hydraulic conductivity of clay capsules was constant with increasing pressure. The saturated hydraulic conductivity of G20 had an ignorable change and increased from 10 × 10 −3 to about 17 × 10 −3 (cm.h −1 ) as the hydrostatic pressure head increased. The hydrostatic pressure head and the saturated hydraulic conductivity of clay capsules affected the discharge of clay capsules. The results in Table 4 demonstrate that the relationship between the discharge of G10, G15, and G20 with hydrostatic pressures is linear (Fig. 6), and by increasing the amount of fine sand in G5, and G2 this relation becomes non-linear. The water discharge values of G15 and G20 were low and varied from 600 to 3500 cm 3 .hr −1 , while those values for G10, G5, and G2 were 3900-15100, 13700-37500, and 20500-52900 cm 3 .hr −1 , respectively. Adding 5-6.6% rice bran husk flour to G0 increased the hydraulic conductivity and water discharge of G15 and G20 by about ten times, and adding 20% rice bran husk flour to G0 improved both the seepage and hydraulic conductivity of G5 by about 228 times. Hence, it is clear that adding rice bran husk, a practical method for producing various clay capsule types with different water discharge rates has positively affected the porosity of clay capsules.
In the buried clay capsule irrigation system, the clay capsule nozzles must seep water gradually into the soil, which results in the soil water content in the range of field capacity. Therefore, water seepage is decisive in choosing nozzles for sub-irrigation systems, and clay capsule discharges of less than 2000 (cm 3 .hr −1 ) are approved. According to Table 4, the discharges of G2 and G5 nozzles should not be used in sub-irrigation systems. However, the G5 and G2 nozzles are suitable for installation in surface irrigation systems. It should be noted that the relationship between the discharge of G10, G15, and G20 with hydrostatic pressures is already linear (Fig. 5). But, in G2 and G5, this relationship becomes non-linear. According to Fig. 5, a strong positive linear relationship was observed between G10, G15, and G20 at low hydrostatic pressure conditions, from 25 to 50 kPa. On the other hand, most farmers can supply water pressure of up to 50 kPa on the farm. Therefore, G10, G15, and G20 can be good choices for installation in arid and semi-arid areas.

Soil Wetness by Clay Capsules
Soil wetting patterns and wetting rates depend on many factors, including the physical properties of the soil and the hydraulic properties of the clay capsules. Knowing wet zone patterns around clay capsules is a prerequisite for designing efficient irrigation systems and managing water resources.
In this study, the hydraulic properties of six clay capsule types (G0, G2, G5, G10, G15, and G20) with significantly different raw material properties were investigated. The soil Fig. 6 The relationship between hydrostatic pressure head and discharge of clay capsules wet ability of these clay capsules placed vertically in a clay loam soil texture at a depth of 20 cm from the soil surface is shown in Table 5. The hydrostatic pressure effect was tested to assess soil moisture distribution patterns. The results showed that the soil wetting shape followed a spherical trend due to the low discharge of clay capsules in G10, G15, and G20 at 24 h irrigation times, while in G5 and G2 the spherical wetting shape was observed at 4 h of irrigation. The pattern of soil moisture distribution changed from spherical to elliptical in G5 and G2 as time passed. Meanwhile, results revealed that the spherical soil moisture distribution shape remained constant for G10, G15, and G20 during the irrigation time of up to 48 h. In general, the soil wetting shape in G10, G15, and G20 were spherical, and in G5 and G2 they trended toward vertical ellipsoids. Table 5 confirms that the shape and size of soil wetness are related to the water pressure of the irrigation system. The size of wetness patterns was smaller in conditions with low hydrostatic pressure (10-50 kPa) than in conditions with high-pressure head (80-100 kPa), which confirms other findings in scientific research Bahrami et al. 2010).
Evaluation of wetting radius and depth for G2 and G5 indicated that soil water content increased significantly after 4 h of watering and was approaching soil saturation. These results showed that G2 and G5 had no auto-regulative seepage ability during the irrigation period. Therefore, suitable watering times for G2 and G5 are less than 4 and 8 h, respectively, while elapsed irrigation times were 24, 48, and 48 h for G10, G15, and G20, respectively. The wetted radius and depth of G10 were greater than those of G15 and G20 at the same watering time. The wetting radius and depth of G10 with the low pressure (25 kPa) head and the high pressure (100 kPa) head were 100 cm and 109 cm, respectively, for a 24 h watering time (Table 5).
To produce soil water content within the field's capacity, the seepage of the clay capsule into the soil must be done gradually. In this regard, the soil wetted radius and depth showed

Conclusion
This paper proposes a sustainable solution for optimizing soil moisture in the subsurface irrigation system based on the porous clay capsule technique. In the current research, different porous clay capsules were designed as novel nozzles for subsurface irrigation with extraordinarily reduced water consumption in agriculture. The impact of modified soil containing rice bran husk flour on the physical and hydraulic properties of porous clay capsules was probed in this study. The ratio of the rice bran husk flour used to make the porous media influenced the clay capsule discharge and saturated hydraulic conductivity. The saturated hydraulic conductivity and discharge of the clay capsules were increased with an increase in the rice bran husk flour. The saturated hydraulic conductivity of different clay capsule types was constant when the working pressure head increased from 25 to 100 kPa. At the same time, the discharge of clay capsules increased by about 63-80% under these conditions. It seems that other organic pore-maker, such as wheat husk flour, processed tea waste, sucrose, sewage sludge, oily waste, and animal excreta can be used to improve the hydraulic properties of clay capsule nozzles. The results confirmed the G2 and G5 auto-regulative seepage disabilities during irrigation time; therefore, it is recommended to evaluate their irrigation performance as ceramic emitters in surface irrigation systems in future studies. Soil wetting shapes followed a spherical trend in G10, G15, and G20 and were already constant during irrigation time up to 48 h. A suitable clay capsule must be able to create a field capacity moisture in the soil. While in G2 and G5, the soil is saturated and many crops cannot withstand saturated soil conditions. Because, wet soils prevent roots from absorbing enough oxygen, causing them to die. Rice is one of the exceptions to this rule, it is recommended to evaluate the performance of G2 and G5 in rice culture based on the near-saturated soil matric potential technique. Furthermore, the permeability of porous pipes varies with irrigation time and tends to decrease with each non-potable water application. Therefore, analyzing the clay capsules' discharge rate when using non-potable water would be helpful in future studies. Based on the results of this research, G10 and G15 can play a significant role in providing soil moisture due to their auto-regulative seepage. Hence, it is advised to investigate their efficiency and performance in orchard lands.