Porous media are commonly used for seepage irrigation, and have a long history of increasing the water use efficiency (WUE), as well as the crop yield. Subsurface irrigation with ceramic devices has significant water-saving effects; therefore, there is potential for using ceramics in subsurface irrigation. Khan et al. [1
] found that the water use efficiency reached approximately 94–97% when pots with different structural sizes were used for irrigation. Zhang et al. [2
] found that tomatoes irrigated by ceramic emitters under a working pressure head of 0 cm achieved the highest yield, approximately 1.17 kg pot−1
, but exhibited the lowest WUE, approximately 24.9 kg m−3
With the advancement of ceramic manufacturing technology, a new porous ceramic emitter based on pot irrigation has been manufactured, and its performance is a significant improvement compared with traditional clay pots and ceramic pipes [3
]. The working pressure head of a porous ceramic emitter was generally less than 100 cm [4
]. The outflow from the emitter could replenish the soil moisture in real-time, effectively irrigating the soil.
When porous ceramic emitters are used in field, a reasonable system design can reduce surface soil evaporation and deep percolation to avoid unnecessary water loss and increase the water use efficiency [6
]. In general, system design parameters must be determined by infiltration experiments under different working conditions in order to determine the infiltration characteristics of subsurface irrigation emitters (cumulative infiltration, wetting front, and soil water content distribution) [6
]. Emitter infiltration characteristics are affected by many factors, such as soil texture, bulk density, initial water content, emitter installation, buried depth, and emitter characteristics (structure size, design discharge, and working pressure head) [7
]. The discharge of clay pots in soil is greatly affected by the pot porosity [8
]. In practical applications, the pot porosity can be determined according to the water requirements for different crops [9
]. Das Gupta et al. [10
] used the VS2D model to simulate the infiltration characteristics of a clay ceramic pipe in a specific soil, and found that pipe discharge increased with increasing working pressure head, and soil capillary suction gradually decreased with increasing soil water content. The above research studied the influence of emitter characteristics on the discharge in the soil of clay pots and ceramic pipes by experiments and VS2D simulation.
In recent years, HYDRUS-2D/3D software has been used to study the infiltration characteristics of porous ceramic emitters, pots, and ceramic pipes in soil [11
]. Cai et al. [4
] found that the working pressure head and hydraulic conductivity of porous ceramic emitters had a great influence on the discharge and wetting front in loam. The greater the working pressure head and hydraulic conductivity, the larger the risk of deep percolation. Ren [13
] used HYDRUS-3D to simulate the infiltration characteristics for ceramic emitters under 0 cm working pressure head in clay loam. Emitter discharge gradually stabilized after 12 h, the working pressure head on the ceramic emitter increased, and the stabilized discharge also increased. Wang et al. [7
] simulated the infiltration characteristics of clay pipes under negative pressure conditions and found that the larger the negative working pressure head and hydraulic conductivity of the clay pipes, the greater the cumulative soil infiltration. HYDRUS-2D can accurately simulate the infiltration characteristics of ceramic pipes and pots under a working pressure head of 0–200 cm [5
]. When a porous ceramic emitter is buried in different soils, the infiltration characteristics will be different. Therefore, in order to choose reasonable design parameters (tape length, depth, work pressure head), it is important to study the influence of soil texture on the infiltration characteristics of porous ceramic emitters.
The purpose of this study is to: (1) compare experimental infiltration characteristics and HYDRUS-2D simulation results for porous ceramic emitters in clay loam and sandy loam; (2) study the influence of soil texture on the infiltration characteristics of porous ceramic emitters (cumulative infiltration, emitter discharge in soil, and wetting front); and (3) provide reasonable layout suggestions for porous ceramic emitters in sandy soil.
4. HYDRUS-2D Model Application
Emitter discharge is primarily affected by soil texture when the working pressure head and hydraulic conductivity of the ceramic emitter are constant. Twelve representative soils in HYDRUS-2D were selected for analysis (Table 3
]. This soil database is used to obtain the bulk density, as well as the clay and sand contents, for the 12 textural classifications. Saturated water content (θs
) is inferred from bulk density. Meanwhile, saturated hydraulic conductivity (Ks
) and water retention parameters (θr
, and l
) are computed using the saturated water content, as well as the clay and sand contents [20
]. Field capacity is calculated by the method described by Twarakavi et al. [21
4.1. Infiltration Characteristics
4.1.1. Cumulative Infiltration, Discharge, and Matrix Potential
shows data for the cumulative infiltration of the porous ceramic emitter at 12 h under different soil textures. The cumulative infiltration of the emitter in silty clay, sandy clay, and clay is small, between 0.07 L (dm3
), and 0.28 L, respectively (Table 4
). This is primarily because the clay content in these soils is too high, so the contact area between the soil particles is large and the pores are small, reducing the saturated water conductivity, increasing the difficulty of soil water diffusion, and decreasing the cumulative infiltration. The cumulative infiltration of loam is 1.28 L. Although the saturated hydraulic conductivity of loam is smaller than that of sand, the cumulative infiltration is the largest.
shows the emitter discharge in loam and sand and the matric potential around the emitter with time. As irrigation time increases, emitter discharge gradually decreases to a stable value of 0.08 L/h and 0.10 L/h for sand and loam, respectively. At this time, the matric potential around the emitter is about −10 cm in sand and is −14 cm in loam. Under zero working pressure head, the emitter effluent primarily depends on the matrix suction of the soil [22
]. During irrigation, soil matrix potential around the emitter gradually increases (the absolute value gradually decreases), so the water potential difference inside and outside the emitter gradually decreases, which in turn reduces the emitter discharge. Because the sandy soils have a small saturated water content and field capacity, the soil around the emitter will be saturated quickly; therefore, the soil matrix potential around the emitter will be smaller. Since the internal working water pressure of the emitter is 0 cm, the water potential difference between the inside and the outside of the emitter is 10 cm and 14 cm for sand and loam, respectively, so the emitter discharge in loam is also 1.4 times larger than in sand.
The simulated cumulative infiltrations are all smaller than the measured cumulative from the experiment (Table 3
). This is mainly because the saturated water content of the two soils used in the experiment is greater than the saturated water content of the soils used in simulations, but the saturated water contents are all 0.46 cm3
for the 12 kinds of soils, which is provided in HYDRUS. Therefore, for the same soil texture, such as L-soil (silty loam) and silty loam, the measured saturated water content is higher, resulting in a larger cumulative infiltration.
4.1.2. Wetting Front
shows the wetting front for different soil textures after 12 h. For sandy soil with a high permeability and low saturated water content, the wetting front mainly appears as an ellipse, the downward vertical wetting front is larger than the horizontal wetting front, and the horizontal wetting front is larger than the upward vertical wetting front. However, in clay soils with a smaller permeability and higher saturated water content, the wetting front appears to be closer to a circle. The ratio of the downward vertical wetting front to the horizontal wetting front (aspect ratio) is shown in Table 4
. Aspect ratios are between 83.34%–148.96%. For clay soils, the aspect ratio is closer to 1. For sandy soils, the aspect ratio is significantly greater than 1, and water is more likely to move to the lower part of the soil layers. For silty soils, the aspect ratio is less than 1. This is mainly due to the fact that the porous ceramic emitter is installed upwards in this study, and the bottom of the emitter is a 5 mm thick impervious bottom cover, limiting water movement in the lower soil layers. The change in aspect ratio is mainly determined by the mass fraction of clay particles in the soil [24
]. Sandy soil has less clay content and a high sand content, creating much larger pores and a small specific surface area, resulting in a strong water permeability. However, for sticky soil, the clay content is higher and specific surface area is larger, so the soil’s matrix suction is larger, but water permeability is weak. Therefore, as clay content increases, the soil matrix suction increases but water conductivity decreases, increasing water retention. During infiltration, water transport within soil is affected by both matrix potential (matrix suction) and gravitational potential [25
]. Under the same experimental conditions, the effect of the matrix potential in sandy soil is smaller than gravitational potential, while the effect of the matrix potential in clay soil is much greater than gravitational potential. Therefore, during infiltration, the effect of matrix potential increases as clay content increases, and the effect of gravity potential decreases; thus, the aspect ratio is gradually reduced, and the wetting front gradually changes from an ellipse to a circle. When ceramic emitters are buried in sandy soil, the risk of deep percolation is high due to poor water retention; therefore, some engineering measures must be taken to reduce the risk of deep percolation and improve the crop water use efficiency in sandy soil.
4.2. Optimization Layout of Porous Ceramic Emitters in Sandy Soil
Poor water retention is not conducive to crop growth. In sandy soil, the field capacity is lower, and irrigation water is easily transported to the lower soil layers. In order to improve the crop water use efficiency in sandy soil, there are two feasible ways to improve soil water retention or prevent soil moisture from migrating to a deeper layer [26
]. If a weak aquifer is placed 25 cm below the emitter, similar to adding a layer of clay during an experiment, it may prevent the soil moisture from migrating downward. Figure 6
shows the change in wetting front at different times in the sand when the clay aquifer (Figure 1
) is placed under the emitter. As irrigation time increases, irrigation water in the sand gradually migrates downward and reaches the clay layer at around 20 h. The irrigation water is then transferred to the horizontal direction by the clay layer barrier, and the infiltrated water accumulates in the clay layer. The soil water content in the clay layer gradually reaches saturation at 0.38 cm3
). After 60 h, the clay layer was completely saturated. At this time, water began to migrate downward through the clay layer. The soil water content below the clay layer was already around 0.11 cm3
, and deep percolation began to occur at 120 h. The deep percolation rate (deep percolation rate = deep percolation discharge/irrigation amount ×100%) was 5.0%. Without the treatment of the clay layer, the deep percolation rate was as high as 17.8%. Therefore, in the area where the soil texture is sandy and a ceramic emitter is used for irrigation, the use of a clay layer can reduce deep percolation and improve the water use efficiency.
The process of installing a clay layer requires a large workload and is suitable for some areas where desertification control is required. For other sandy soil areas, a water retaining agent may be added to the sandy soil to improve its texture and water retention [27
]. Table 5
shows the hydraulic parameters of sandy loam and polyacrylamide-mixed (PAM-mixed) sandy loam [29
shows the wetting front for a ceramic emitter buried in sandy loam and PAM-mixed sandy loam after 120 h of irrigation. The wetting front changes significantly after the addition of the water retaining agent. At 120 h, the water content in sandy loam soil is smaller than that of PAM-mixed sandy loam. The highest water contents are 0.29 cm3
and 0.33 cm3
, respectively; however, the cumulative infiltration of the emitter in the sandy loam is significantly higher than in the PAM-mixed sandy loam, which is 36.1 L and 19.5 L, respectively, indicating that more irrigation water in the sandy loam migrates to the deeper layers, resulting in deep percolation. Deep percolation rates in the two soils are 42.7% and 8.2%, respectively. Therefore, the use of a water retention agent can significantly improve soil water retention and reduce the risk of deep percolation.
Overall, there are two effective ways to reduce deep water percolation: placing a clay interlayer and adding water retention agent. In a subsurface irrigation system, the deep percolation rate needs to be less than 5% to improve benefits for crops. Therefore, the method of placing a clay interlayer may be better than adding water retention agent in sandy soil because of the high water use efficiency. In the areas where land levelling and desertification control are required, this method could be a good choice. On the other hand, this method could cost a large amount manpower and material resources, and not be effective. Therefore, in areas where there is a need for deep tillage, adding water retention agent would be simple and useful.
This study compared the simulated and measured infiltration characteristics of porous ceramic emitters in L-soil and H-soil. The HYDRUS-2D model was used to simulate water movement with ceramic emitters in different soils.
For 12 different soil texture conditions, the aspect ratio of the wetting front is basically between 0.84–1.49. In sandy soil, the wetting front appears as an ellipse; but in clay soil, the wetting front is closer to a circle. As irrigation time increases, emitter discharge gradually decreases to a stable value; however, emitter discharge for different texture soils is quite different. The emitter discharge in silty clay, sandy clay, and clay is small. The clay content in these soils is too high, increasing the difficulty of soil water diffusion, and decreasing the emitter discharge. The emitter discharge in loam is 1.4 times that in sand, because the soil matrix potential around the emitter is smaller due to the lower saturated soil moisture content and field capacity.
In order to improve the crop water use efficiency in sandy soil, water retention can be improved by adding a clay interlayer or water retention agent to reduce the risk of deep percolation. The method of placing a clay interlayer is better than adding water retention agent in sandy soil because of the high water use efficiency. However, in areas where there is a need for deep tillage, adding water retention agent could be a good choice.