Creating uniform and adequate soil moisture is one of the most challenging issues in irrigated lands. The use of irrigation systems with high water efficiency, such as a sub-irrigation system that provides the optimum water content of the soil, is recommended as a solution to reduce water losses (Adu et al., 2019;Cai et al., 2021) for many orchards at arid and semi-arid regions (Tesfay 2011; Bhople et al. 2014; Al-Mayahi et al. 2020). Also, information on soil moisture variation is an important factor for managing a sub-irrigation system design (Bhayo et al. 2018; Saefuddin et al. 2019). On the other hand, knowing that more than 90% of plants need moisture at the level of soil field capacity for optimal growth. Therefore, using new technology for controlling and monitoring of soil water content at about field capacity range plays a key role in developing soil and water management programs under unsaturated conditions (Ashrafi et al. 2002; Vasudevan et al, 2011; Ghorbani Vaghei et al., 2015; Gebru et al. 2018; Saefuddin et al. 2019).
Clay capsule nozzle is one of the new technologies in irrigation tools which is able slowly to seepage water into soil in the range of field capacity (Bainbridge 2001; Bainbridge 2002; Abu-Zreig et al. 2006; Siyal et al. 2009; Bahrami et al., 2010; Vasudevan et al., 2011; Ghorbani Vaghei et al. 2015; Abu-Zreig et al., 2018). Clay capsule is one of the porous pipes in the sub-irrigation system that can release water in the near root zone. Buried clay capsule irrigation system is known to be very effective in the management of water by supplying a low volume of water to the near root zone, based on the water requirement of crops. As well, it is one of the most important efficient traditional methods for small farms in many arid and semi-arid regions. This system has been used for sub-irrigation of fruits and vegetables during the last 1000 years in many small-scale drylands of the world (Bainbridge, 2001; Siyal et al. 2009; Bahrami et al., 2010; Ghorbani Vaghei et al., 2015) and also this method of irrigation is becoming increasingly common especially in the developed nations as a way of watering and fertilizing greenhouse crops (Das Gupta et al., 2009). Buried clay pot irrigation is still used limitedly in drylands of India, Iran, Pakistan, the Middle East, and Latin America and where the rainfall is less than 500 mm per year (Bainbridge, 2001; Ashrafi et al., 2002; Qiasheng et al., 2007: Bahrami et al. 2010; Siyal et al., 2011; Araya et al., 2014; Bhpole et al., 2014; Ghorbani Vaghei et al. 2015).
The water flow rate of buried clay capsules is an important factor for the designation, operation, and management of irrigation system (Jiusheng et al. 2004; Qiasheng et al. 2007; Bahrami et al. 2010; Ghorbani Vaghei et al. 2016; Bhayo et al. 2018; Saefuddin et al. 2019). However, the pattern of soil water distribution is affected by the physical and hydraulic properties of clay pots as well as the physical properties of soil (Qiasheng et al. 2007; Naik et al. 2008; Siyal and Sakaggs, 2009; Ghorbani vaghei et al., 2016). Once the water is released into the soil, its movement will be depended on the physical characteristics of the soil which with during time, its wetting pattern is completed.
The key parameters such as porosity and pore size distribution of clay capsules are used to predict the water flow rate of buried clay capsules (Cultrune et al. 2004; Naik et al. 2008; Freyburg and Schwarz, 2007; Bahrami et al. 2010). Also, the water flow rate of clay capsule is affected by some factors including the hydrostatic pressure, the saturated hydraulic conductivity of the clay capsule material, wall thickness, surface area, soil type, type of crop, and the rate of evapotranspiration (Abu-Zreig et al. 2006; Bahrami et al., 2010; Vasudevan et al., 2011). However, the hydraulic conductivity of clay capsules is one of the most important factors affecting outflow rate to provide enough water in the root zone (Bainbridge, 2001; Abu-Zreig et al. 2006; Siyal and Sakaggs, 2009; Ghorbani Vaghei et al. 2016). The scientific results showed that the relationship between the discharge of clay capsule and hydrostatic pressure is non-linear (Abu-Zreig et al. 2006; Qiasheng et al., 2007; Haijun et al. 2009; Das Gupta et al., 2009; Bahrami et al. 2010). Siyal and Skaggs (2009) reported that in sub-surface irrigation with porous clay pipe, the radius of wetted zone increased as a result of increased system water pressure (Siyal and Sakaggs, 2009). Also, Bahrami et al. (2010) which developed a fuzzy model to determine the soil wetted radius and depth based on using porous clay capsule irrigation method, reported that the wetted radius and vertical depth values at 25 kPa of hydrostatic pressure with the low discharge of clay capsules were about 13.5 and 22 centimeters, respectively. While, these parameters in clay capsules with high discharge rates were 14 and 45 centimeters, respectively. It means that increasing discharge rate increased the soil wetted depth that is a lot more than the changes of soil wetted radius (Bahrami et al. 2010).
For providing a good irrigation system, engineers must have a good understanding of the discharge ability of clay capsule types. Clay capsules irrigation technology is yet to be fully explored in Iran. Moreover, the effect of organic matter percent on the hydraulic properties of clay capsules has not been widely studied and reported. This material is also important to fabricate clay capsules with low weight and porous media with a good water discharge rate. Up to now, low research has been carried out on improving the discharge of clay capsules with added cheap organic matter in raw materials for seepage ability. Therefore, this research is aiming to develop and upgrade the seepage ability of clay capsules for providing soil moisture in the root zone by changing the material phase. As mentioned earlier, the achievements of this study will be useful for developing the use of buried clay capsule irrigation technology on small-scale land in Iran and also arid and semi-arid regions of the world.