Effect of Subsurface Drainpipe Parameters on Soil Water and Salt Distribution in a Localized Arid Zone: A Field-Scale Study

Abstract

The salt distribution characteristics in arid areas are directly related to the sustainable development of agriculture. We study the characteristics of spatial changes of soil water and salt in farmland under the full anniversary of different culvert pipe arrangements and optimize the salt drainage parameters of underground drains suitable for the local area so as to promote the management of saline and alkaline land in Xinjiang. A subsurface drainpipe salinity test was conducted in the Yanqi Basin (Bayingoleng Mongolian Autonomous Prefecture, Xinjiang Uygur Autonomous Region, China) to analyze changes in soil water and salt dynamics before and after irrigation-induced salt flushing, assessing the impact of drainpipe deployment parameters. It was found that at a 1.4 m depth of burial, the maximum desalination rates of soil in different soil layers from the subsurface drainpipes in 20, 30, and 40 m spacing plots were 78.28%, 50.91%, and 54.52%, respectively. At a 1.6 m depth of burial, the maximum desalination rates of soil in different soil layers from the subsurface drainpipes in 20, 30, and 40 m spacing plots were 70.94%, 61.27%, and 44.12%. Reasonable deployment of subsurface drainpipes can effectively reduce soil salinity, increase the desalination rate, and improve soil water salinity condition. This study reveals the influence of the laying parameters of subsurface drainpipes on soil water salinity distribution characteristics in arid zones, which provides theoretical support and practical guidance for the management of soil salinization in arid zones.

1. Introduction

Soil salinization in arid areas is a global environmental problem, especially in the Xinjiang region of China. Due to scarce precipitation and strong evaporation, the phenomenon of soil salinity accumulation is more serious, which seriously affects the local agricultural production and ecological environment. Optimizing the concealed pipe salt drainage technology can effectively control the groundwater level and reduce the return salinity hazards brought by the high groundwater level. At the same time, it improves salt drainage efficiency, reduces the accumulation of soil salts, improves the physical and chemical properties of soil, and thus improves the yield and quality of crops.

Studies have shown that concealed pipe drainage salt removal technology can effectively control the groundwater level. It has the ability to enhance the ability of precipitation to wash away salts and lower the groundwater level to inhibit salt return and is suitable for application in areas with shallow diving depths. Compared with the control treatment of open ditch drainage, the underground drainage pipe can reduce the flooding hazard by 70% in the rainy season when precipitation is relatively concentrated [1]. Culvert pipe drainage can reduce soil salinity in the tillage layer (0–20 cm) by 1.1% and can reduce soil salinity by an average of 1.8% during the critical period of cotton planting and seedling salt tolerance [2]. There are also studies to optimize the methods and systems through the layout parameters and drenching quota of field culverts. A more convenient and accurate optimization method is provided for the optimization of the layout parameters and drenching quota of inter-field concealed pipes in saline areas [3].

In the 1990s, concealed piping was introduced in Xinjiang, China, to try to use concealed piping salt drainage for salt washing, and it achieved remarkable results. The concealed pipe salt drainage technology follows the law of “salt comes with water and goes with water”. By installing a pipeline network with small holes under the soil, the salt in the soil is discharged into the underground pipeline with drenching water [4] by “washing salt” from the soil to achieve the purpose of controlling soil salinization. Currently, numerical simulations and field experiments are the two commonly used research methods. Some scholars use Hydrus-2D [5], COMSOL [6], and other software to simulate the process of water and salt distribution of salt drainage from underground drains. Considering the complex environmental factors such as weather, seasons, and groundwater variations, the simulation approach may be more desirable, so many scholars choose to conduct field experiments. The main research areas include northeastern Italy [7], the Hetao region of China [8], the Yinbei irrigation area of China [9], and the coastal saline area of China [10]. Studies have shown that the soil salinity profile characteristics change from surface aggregation to desalination after culvert pipe drainage [11], and the shallow culvert pipe drainage salinity reduction technology with drip irrigation can effectively manage saline soils and improve crop yields [12]. There are also studies on the average rate of decline in the depth of groundwater, the average rate of rebound, and the economic cost of laying underground drainpipes that are used to determine the optimal laying program of underground drainpipes [13,14,15]. This indicates that the optimal parameters of the underground drainpipe drainage and salt removal technology need to be adjusted according to the specific geographic environment and economic conditions.

Although some progress has been made in the management of soil salinization in arid zones [16,17], compared with other regions using concealed pipes, Xinjiang lacks mature experience and relevant theoretical guidance. In addition, the large-scale application of drip irrigation technology has raised new issues for the application of concealed pipe salt drainage technology. Under drip irrigation water conservation conditions, clarifying the water–salt distribution and spatial and temporal changes of salt drainage from concealed pipes in saline soils is the key to the application of this technology, which puts forward optimization needs for the spacing and depth layout parameters of concealed pipes. In summary, this study can more accurately assess the influence of concealed pipe layout parameters on soil water–salt distribution characteristics by analyzing the water content, salt content, desalination rate, and drainage mineralization of the soil before and after drenching. The differences in salt drainage of different concealed pipe layouts can be elucidated with a view to providing a scientific basis for the improvement of saline soils by concealed pipes in arid zones. It can also provide more effective technical support and strategies for soil health and sustainable agricultural development in arid zones.

2. Materials and Methods

2.1. Research Area

The experiment was conducted from April to November 2023 in Kerimuhar Village, Yanqi Basin, Bayin’guoleng Mongol Autonomous Prefecture, Xinjiang Uygur Autonomous Region (86°74′ E, 42°13′ N), China at an altitude of 1005.5 m. The area experiences an average annual temperature of 7.8 °C, with a large annual range (35.5 °C) and significant daily fluctuations (up to 20.2 °C), an annual precipitation of 64.5 mm, and an evaporation of 1853 mm. These climatic conditions cause soil moisture to evaporate and salt to easily accumulate. Through the preliminary soil sampling survey of the study area, soil texture was compared using standard color cards and soil condition manuals, soil bulk weight, field water holding capacity, and saturated water content, which were determined using the drying method. Soil particle size distribution was determined using a particle size analyzer in the laboratory, and soil pH was determined using a conductivity meter (FE38-Standard, ±0.5%). The results proved identical soil conditions in the six test plots included in the study. The soil salinity in the study area is high, the pH is alkaline, and the groundwater is shallowly buried. These parameters directly affect the efficiency of the drainage system and the dynamics of soil salinity, which are crucial for the study of subsurface drainage and salinity control. Soil types and physical indicators are shown in

Table 1. Soil types and physical parameters in the underground drainpipe test area.

Soil Depth/cm	Soil Composition/%			Soil Textural Type	Bulk Density /g·cm−3	Field Water Holding Capacity (vol %)	Saturated Water Content (vol %)	pH
	Sand	Slit	Clay
0–20	24.25	49.02	26.73	Loam	1.38	25.13	33.14	8.87
20–40	19.72	54.12	26.16	Powdery loam	1.52	22.51	29.36	8.85
40–60	20.81	56.76	22.43	Powdery loam	1.56	21.04	27.27	7.47
60–80	31.18	49.18	19.64	Loam	1.51	21.41	31.34	7.92
80–100	50.08	33.70	16.22	Loam	1.52	19.42	28.96	8.00
100–120	64.95	23.21	11.84	Sandy loam	1.65	13.31	23.59	8.12
120–140	77.99	15.24	6.77	Loamy sand	1.54	14.08	25.08	8.09
140–160	80.06	13.80	6.14	Loamy sand	1.48	15.68	27.48	8.25
160–180	60.59	19.31	20.10	Sandy clay loam	1.48	15.88	27.42	8.12

.

2.2. Experimental Design and Organization

The test construction of the subsurface drainpipe was carried out in April 2023 in the test area. One water collection well was set up at the end of each collection pipe with a depth of 2.5 m, and a solar pumping station was installed in the water collection well. The test area had different underground drainpipe spacing and burial depths for the test engineering parameters. The design of six test areas included two different depths (D = 1.4, 1.6 m) and three different spacings (S = 20, 30, 40 m) of the combination of underground drainpipes. There were a total of 24 subsurface drainpipes, each with a pipe length of 220 m. The test design program is shown in

Table 2. Subsurface drainage pipe layout parameters of each experimental treatment.

Treatments	D1S1	D1S2	D1S3	D2S1	D2S2	D2S3
Depth/m	1.4	1.4	1.4	1.6	1.6	1.6
Spacing/m	20	30	40	20	30	40

. Three replicates were set for each treatment combination to ensure the reliability and repeatability of the experimental results. The suction pipe adopts a PVC single-wall corrugated pipe with holes that are 90 mm in diameter, with an opening gap of ≤1 mm, an opening area of >250 cm²·m⁻², and a design slope drop of 0.1% (DB15/T 2567—2022). The collector pipe adopts a PVC hard plastic pipe with a diameter of 110 mm, with a design slope drop of 0.2%, and the subsurface drainpipe arrangement in the test area is shown in Figure 1.

The variety of sunflower for testing was “Dwarf Big Head 567DW”, which was sown on 24 May 2023, emerged on 5 June, and was harvested on 5 October 2023. Four rows of sunflowers were covered by a unit of mulch, and a one-drip irrigation belt was laid in the middle of each two rows of sunflowers. The width of the film was 1.45 m, the spacing of the narrow rows was 25 cm, the spacing of the wide rows was 60 cm, and the spacing of the drip irrigation belts was 85 cm (Figure 1). The irrigation water source was well water with a mineralization level of 0.8 g·L⁻¹. Spring irrigation was carried out on 10 May 2023, while irrigation-induced salt-flushing water quantities of 180 mm were applied to the soil. Drainage was continuously carried out for 16 days after the spring irrigation, and the mineralization level of the drainage water was monitored. Drip irrigation was applied once per fertility period after sowing. Fertilizer management was carried out during the fertility period of the sunflower, with a total of 150 kg·hm⁻² of urea (46% N), 120 kg·hm⁻² of potassium dihydrogen phosphate (18% N, 44% P₂O₅), and 90 kg·hm⁻² of potassium sulfate (50% K₂O).

2.3. Soil Sampling and Index Calculation

The soil sample sampling points were located on the intermediate subsurface drainpipes in each plot. In order to investigate the static spatial distribution of water and salt in the soil profile, soil samples were taken in the direction of the horizontal distance between culvert spacings and in the longitudinal vertical direction of the soil. The 20 m (S1) horizontal sampling points are located at 0, 5, and 10 m from the treatment center drain, the 30 m (S2) horizontal sampling points are located at 0, 7.5, and 15 m from the treatment center drain, and the 40 m (S3) horizontal sampling points are located at 0, 10, and 20 m from the treatment center drain, i.e., directly above the center drain, at 1/4B and 1/2B. Soil samples were collected along the vertical direction of the soil profile at the sampling points using the soil auger method. The sampling depth was determined according to the buried depth of the subsurface drainpipe, which was 0–20 cm, 20–40 cm, 40–60 cm, 60–80 cm, 80–100 cm, 100–120 cm, 120–140 cm, and 140–160 cm (when the buried depth of the subsurface drainpipe was 1.6 m), and the samples were randomly taken 3 times from each layer.

Soil samples were collected after spring irrigation. Soil samples were collected randomly from three sample points in each microzone (0–20 cm, 20–40 cm, 40–60 cm, 60–80 cm, 80–100 cm, 100–120 cm, 120–140 cm, 140–160 cm). Soil from all sampling points in each microzone was homogenized to form a mixed sample, which was representative of a microzone. Finally, a total of 378 soil samples of approximately 500 g each were collected from 6 treatments × 3 sampling points × 3 replicates. The soil samples were packed into self-sealing bags and aluminum boxes, respectively. All soil samples were removed from large soil fauna visible to the naked eye, stones, and plant debris (e.g., straw and roots).

(1) Soil water content. Soil samples were collected with a ring knife at critical points. The soil mass water content was determined and converted to volumetric water content after drying the soil to a constant weight at 105 °C using the drying method and calculating soil bulk density (BD) as follows:

BD = DM/V

(1)

where BD is bulk density, g·cm⁻³. DM is soil dry mass, g. V is soil volume, cm³.

Soil water content (θ) is usually expressed as a mass percentage (%), and in order to convert this to a volume percentage, the following formula can be used:

θ = (M − DM/DM) × 100%

(2)

θ_v = θ × BD

(3)

where M is soil wet mass, g. BD is bulk density, g·cm⁻³. θ_v is soil water content (by volume), %. θ is soil water content (by mass), %.

(2) Soil salinity. The soil samples taken were air dried, ground, sieved through a 2 mm sieve (Shaoxing Shangyu Huafeng Hardware Instrument Co., Ltd., Shaoxing, China), and mixed with a water–soil ratio of 5:1, and the conductivity of soil standard liquid was determined using a conductivity meter (FE38-Standard, ±0.5%). The formula for calculating total soil salinity was based on soil conductivity as follows:

S = EC_1:5 × 0.4017 + 0.0353

(4)

where S is total salt, g·kg⁻¹. EC_1:5 is soil conductivity, μS·cm⁻¹.

(3) Soil desalination rate. The proportion of the reduced value of soil salinity in the study area to the initial value was calculated as follows:

N = (EC₁ − EC₂)/EC₁ × 100%

(5)

where N is the desalination rate, %. EC₁ is the soil conductivity before irrigation, μS/cm. EC₂ is the soil conductivity after irrigation, μS·cm⁻¹. When N > 0, it means soil desalination. When N < 0, it means soil accumulation of salts. And, when N = 0, it means salt equilibrium.

(4) Drainage mineralization. The mineralization data were measured by the gravimetric method based on the Chinese standard specification (SL 79-1994) [18].

(5) Data analysis and statistics. Excel was used to organize the data and obtain the graph, and the descriptive analysis of the samples was carried out using SPSS 27.0 software. The ANOVA and post hoc tests were performed for burial spacing and sampling points separately from the desalination rate.

3. Results and Analysis

3.1. Water Content

Soil volumetric water content before and after irrigation is shown in Figure 2, respectively, where the water content is the average of the replicate treatments. Before spring irrigation, the soil water content was at a higher value due to the beginning of permafrost thawing. The soil profile water content of each treatment before spring irrigation showed a trend of gradual increase from top to bottom. As the air and ground temperatures increased, strong evaporation and crop growth began to consume water, resulting in a lower water content in the top soil layer (0–40 cm). When the depth of the subsurface drainpipe is 1.4 m, the water content of the soil layer from 0 to 60 cm is about 17.58–23.88%, and the water content of the soil layer below 100 cm is about 21.07–27.26%, which is close to the saturated water content. When the buried depth is 1.6 m, the water content of the 0–60 cm depth soil layer is about 20.25–25.77%, and the water content of the soil layer below 100 cm is about 20.58–24.57%. When the subsurface drainpipe is buried deeper, the soil close to the top of each culvert has better drainage properties due to the better drainage properties of the deeper soil. The water content of the soil after irrigation was greater at a spacing of 30 m compared to both 20 m and 40 m. The water content of the soil after irrigation was greater at a spacing of 30 m compared to both 20 m and 40 m. Before irrigation, the soil volumetric water content of each soil layer was basically between 10.22 and 27.27%. After irrigation, the volumetric water content of each soil layer was in the range of 18.65–30.79% at a spacing of 20 m and in the range of 23.76–30.85% at a spacing of 40 m, while at a spacing of 30 m, the volumetric water content of each soil layer was basically between 25.30 and 36.97%. This indicates that the moisture retention of the soil and the spacing of the subsurface drainpipes are not in a single linear relationship (Figure 2).

The higher soil water retention under 30 m drainpipe spacing (Figure 2) likely results from an optimal balance between drainage efficiency and lateral water redistribution. Narrower spacings (e.g., 20 m) may enhance drainage capacity, leading to faster water removal and reduced retention in the root zone. Conversely, wider spacings (40 m) could limit drainage effectiveness, allowing waterlogging in localized areas but failing to sustain uniform soil moisture. The intermediate 30 m spacing appears to promote capillary fringe connectivity between adjacent drainpipes, facilitating gradual water release while minimizing excessive drainage—a phenomenon observed in similar loamy soils under arid conditions. Additionally, soil permeability heterogeneity may play a role, possibly due to reduced mechanical compaction during pipe installation. This could enhance infiltration rates and temporary water storage in the soil matrix.

3.2. Salinity

The soil profile salinity before and after the test at different subsurface drainpipe burial depths is shown in Figure 3, respectively, where the salinity is the average value of its sampling points at the same depth. Before irrigation, the salinity of the soil profile of each treatment showed an upward and downward trend, and the surface layer of the soil showed an obvious salt aggregation phenomenon. Before irrigation, the salt content of 0–40 cm surface soil fluctuated at the average level of local salt content (13.48 g·kg⁻¹). The salt content of the soil layer below 40 cm varied in the range of 4.48–17.84 g·kg⁻¹. After irrigation, due to the leaching effect, the salt content was reduced as a whole. Soil salinity varied in the range of 2.06–25.98 g·kg⁻¹. The phenomenon of salinity aggregation in the surface layer of the soil was weakened, and the tendency of salinity in the soil profile was weakened. In some treatments, salt accumulation is significantly reduced, which may be related to soil permeability and leaching efficiency. In these treatments, soil permeability is better, and salt can be more effectively removed from the soil through leaching. This reduction in salinity helps to improve soil chemistry and reduce the negative effects of salt stress on crop growth, thereby increasing crop yield and quality. At the same time, lower salinity also helps maintain the activity of soil microorganisms and promotes the health of soil ecosystems.

3.3. Desalination Rate

After the test of significance (F-test), the results are shown in

Table 3. Significance test (F value).

Treatments	0–20 cm	20–40 cm	40–60 cm	60–80 cm	80–100 cm	100–120 cm	120–140 cm	Average Value
Burial depth	18.81 *	4.92	634.22	97.70 *	835.60	495.92	22.00	189.90
Spacing	6.51 *	969.01 *	372.99 *	388.40 *	983.19 *	1255.36 *	82.60 *	918.09 *
Burial depth × spacing	27.29 *	1519.36 *	348.45 *	12.17 *	24.41 *	246.88 *	123.93 *	194.67 *

* Indicates significant differences between treatments at the 0.05 level.

. The difference between the treatment groups was significant (p < 0.05), so further ANOVA and multiple comparisons (Duncan) were performed, as shown in Table 3.

The desalination rate of soil under different subsurface drainpipe layouts was analyzed, as shown in

Table 4. Soil desalting rate under different distances and buried depth tube treatment.

Sampling Point 0
Treatments	D1S1	D1S2	D1S3	D2S1	D2S2	D2S3
0–20 cm	30.58 ± 1.66 b	47.25 ± 1.78 a	15.22 ± 0.86 c	52.96 ± 3.20 a	38.67 ± 13.69 b	44.12 ± 2.42 a
20–40 cm	42.07 ± 1.59 b	50.91 ± 1.28 a	−12.88 ± 0.97 e	52.51 ± 2.13 a	−5.23 ± 0.29 d	27.35 ± 2.21 c
40–60 cm	42.26 ± 2.64 a	34.77 ± 0.70 b	27.03 ± 2.04 c	31.62 ± 1.90 b	−23.64 ± 2.13 d	24.97 ± 2.00 c
60–80 cm	74.09 ± 5.20 a	26.93 ± 2.29 b	−32.49 ± 0.21 c	58.81 ± 5.61 c	19.05 ± 1.81 b	−30.63 ± 3.53 a
80–100 cm	77.72 ± 4.85 a	22.38 ± 1.23 b	5.09 ± 0.54 c	31.13 ± 3.74 d	−4.00 ± 0.31 b	−33.89 ± 2.54 c
100–120 cm	78.28 ± 7.92 a	38.18 ± 2.91 c	14.19 ± 2.71 d	59.58 ± 2.98 b	−15.71 ± 1.79 c	16.77 ± 3.84 c
120–140 cm	28.75 ± 1.44 b	29.25 ± 2.46 b	31.4 ± 3.57 b	39.63 ± 2.98 a	24.04 ± 2.2 c	16.09 ± 0.80 d
Average value	53.39 ± 5.67 a	35.67 ± 3.78 c	6.8 ± 1.34 e	45.03 ± 2.25 b	5.41 ± 0.27 e	13.57 ± 1.68 d
Sampling point B/4
Treatments	D1S1	D1S2	D1S3	D2S1	D2S2	D2S3
0–20 cm	46.66 ± 8.24 a	35.00 ± 8.32 a	24.83 ± 0.76 b	46.75 ± 2.43 a	0.51 ± 0.03 c	−10.62 ± 0.53 d
20–40 cm	32.23 ± 1.22 c	22.30 ± 0.56 d	29.44 ± 0.64 e	55.57 ± 2.24 b	61.27 ± 3.06 a	−7.50 ± 0.38 e
40–60 cm	13.40 ± 1.61 c	23.42 ± 2.47 b	16.43 ± 1.35 b	68.62 ± 6.14 a	59.10 ± 6.84 c	−4.75 ± 0.45 d
60–80 cm	41.64 ± 2.91 b	25.86 ± 3.10 c	−7.53 ± 0.21 d	70.82 ± 9.43 a	53.20 ± 5.45 c	−26.48 ± 3.18 e
80–100 cm	42.45 ± 2.65 b	−4.02 ± 0.22 e	−31.28 ± 0.25 d	70.94 ± 8.52 a	−14.21 ± 0.2 e	14.40 ± 1.08 c
100–120 cm	15.99 ± 0.8 b	11.51 ± 1.58 c	0.48 ± 0.71 b	58.38 ± 2.92 a	−51.58 ± 1.79 d	−26.04 ± 3.3 e
120–140 cm	18.51 ± 2.93 d	20.81 ± 1.04 cd	18.27 ± 2.57 b	49.03 ± 5.45 a	−69.29 ± 6.68 c	−82.39 ± 8.13 e
Average value	30.13 ± 1.51 b	19.27 ± 0.96 c	7.23 ± 0.36 d	59.55 ± 2.98 a	−7.67 ± 1.35 c	−27.44 ± 4.37 e
Sampling point B/2
Treatments	D1S1	D1S2	D1S3	D2S1	D2S2	D2S3
0–20 cm	30.09 ± 2.18 b	−8.24 ± 1.45 e	38.43 ± 8.76 c	6.51 ± 1.26 d	14.73 ± 2.03 c	26.24 ± 3.53 b
20–40 cm	41.62 ± 5.62 a	−13.42 ± 1.59 e	45.76 ± 5.28 a	−1.48 ± 0.07 d	18.02 ± 1.9 c	31.80 ± 5.59 b
40–60 cm	37.15 ± 4.09 a	−33.10 ± 4.11 d	10.22 ± 1.52 c	−41.7 ± 6.32 a	6.26 ± 1.31 c	18.14 ± 0.91 d
60–80 cm	22.28 ± 1.31 b	−10.00 ± 1.21 c	74.11 ± 15.72 a	−46.78 ± 0.47 d	13.13 ± 0.66 b	−0.89 ± 0.04 c
80–100 cm	−3.11 ± 0.14 c	45.39 ± 5.00 a	48.38 ± 3.48 a	−10.89 ± 2.60 d	17.13 ± 0.86 b	−39.13 ± 6.96 e
100–120 cm	10.51 ± 1.25 c	43.85 ± 6.82 b	58.51 ± 4.23 a	−40.16 ± 2.64 e	−1.40 ± 0.09 d	14.56 ± 1.73 c
120–140 cm	8.78 ± 2.91 b	54.52 ± 8.21 a	50.89 ± 6.37 a	−35.36 ± 4.51 d	−5.69 ± 0.28 c	−49.94 ± 7.50 e
Average value	21.05 ± 3.05 b	11.29 ± 2.56 c	46.39 ± 6.32 a	−24.03 ± 2.20 e	6.61 ± 1.33 d	0.11 ± 0.05 d

D1 and D2 represent burial depths of 1.4 and 1.6 m, respectively; S1, S2, and S3 represent spacings of 20, 30, and 40 m, respectively. Lowercase letters indicate significant differences between treatments at the 0.05 level.

. Except for individual treatments, the layout of subsurface drainpipes played an important role in controlling and discharging salts from the soil. The soil desalination rate gradually decreased from top to bottom in the soil body. In the 0–140 cm soil layer at a burial depth of 1.4 m, the subsurface drain in the 20 m spaced plots had the greatest desalination rate of the soil, which was 78.28%. It was 27.37 and 23.76 percentage points higher than that of 30 m and 40 m treatments, respectively. At a 1.6 m burial depth, it was 70.94% for the 20 m treatment. At the depth of 1.6 m, the 20 m treatment was 70.94%, which was 9.67 and 26.82 percentage points higher than the 30 m and 40 m treatments, respectively.

From the two directions of subsurface drainpipe layout spacing and soil depth, the soil showed a trend from large to small. This indicates that the soil desalination rate becomes larger with the decrease in soil spacing, and the closer the horizontal distance to the subsurface drainpipe, the higher the desalination rate, and the spacing of the drainage culvert has a greater influence on the desalination effect of soil in different profiles. In addition, negative values of the desalination rate appeared in the test plots with 30 m and 40 m spacings at a 1.6 m burial depth. Negative values mainly appeared in the 60–160 cm soil layer, which indicated the phenomenon of salt accumulation in the soil. At deeper depths, a negative desalting rate occurs, which may be caused by an increase in capillarity or a decrease in leaching efficiency at a certain spacing. Water rises through capillary action, causing salt to accumulate in the surface layer, resulting in a negative desalination rate. The effect of interval on the desalting rate may be related to flow dynamics or leaching mode. Smaller intervals can more effectively promote water penetration and salt leaching, thereby improving the desalination rate. Therefore, when designing the underground drainage system, a reasonable interval should be considered to improve the desalting effect.

3.4. Mineralization Degree

Figure 4 shows the dynamic change curve of the mineralization degree of different spacing plots during spring irrigation irrigation-induced salt flushing of the subsurface drainpipe, and the mineralization degree indicates the total amount of inorganic mineral components contained in the water. The degree of mineralization of the drainage water of each treatment was at a high level in the drainage stage and generally showed a trend of increasing and then decreasing. At a 1.6 m burial depth, the mineralization degree of the water discharged from the subsurface drainpipe was higher in general, which was due to the accumulation of salts in the multi-soil layer. The 20 m spacing pipe drainage stage mineralization degree was at 7.46–17.93 g·L⁻¹ and peaked on the 8th−10th day of drainage and then gradually decreased. The mineralization was in the range of 8.1–16.4 g·L⁻¹ in the 30 m spacing pipe drainage stage, while it was in the range of 6.93–15.14 g·L⁻¹ in the 40 m spacing treatment. The results showed that a smaller subsurface drainpipe spacing increases the drainage flow rate, leading to higher drainage mineralization and conductivity. After irrigation, the water carries away a large amount of salt from the soil, and the water discharged at this time is highly mineralized. The salts are continually taken away as the drenching time increases. The discharged water carries less and less salt and mineralization decreases.

The change in the mineralized water level may be related to the increase in salt leaching. In some treatments, the mineralized water level is higher, which may be due to the salt being removed from the soil by leaching, causing the mineralized water level to rise. High mineralization rates can have short- and long-term effects on soil health and the efficiency of underground drainage systems. In the short term, high mineralization rates may improve soil chemistry and increase soil fertility. While subsurface drainage effectively reduces root-zone salinity, the discharge of saline effluent poses environmental risks. In the long term, high mineralization rates can lead to soil acidification and affect the activity of soil microorganisms, which can negatively impact soil health. High salt loads in drainage water can degrade receiving water bodies, impairing aquatic ecosystems and limiting agricultural reuse. To mitigate these impacts, future drainage systems in arid zones should integrate salt interception schemes (e.g., evaporation ponds, phytoremediation basins) or adopt cyclic irrigation strategies that blend drainage water with freshwater sources.

4. Discussion

The spacing of culvert pipes had a significant effect on soil moisture distribution. The distribution of relative soil moisture content in the soil profile before and after irrigation under different drainage treatments in the area 1/2 from the spacing of the culvert pipe showed that the relative soil moisture content decreased first and then increased after irrigation under controlled drainage conditions. Underground pipe drainage had a significant effect on soil water distribution, especially at 0–40 cm, which is the main rhizosphere soil of the crop, and its salt content is an important factor restricting the growth of the crop. The results of this study showed that under the condition of buried culvert pipes in each treatment, the overall salinity of the soil in each treatment in the area of 1/2 spacing of culvert pipes leached to the lower layer with water. The overall salt content of the soil was reduced, and the water content was more evenly distributed. The salt content of the soil in the 0–60 cm layer was significantly reduced in each treatment in the area of 1/2 of the distance of the culvert pipe, and the variation of water content was larger, which ensured the seeding and growth of crops. In the horizontal distance of the culvert pipe, the salinity before and after irrigation showed a trend of increasing and then decreasing along the horizontal distance of the culvert pipe, which was consistent with the results of Mao et al. [14].

The observed salt accumulation in soil may be partially attributed to capillary rise during dry periods, which transports dissolved salts from deeper groundwater toward the surface. Although this study focused on subsurface drainage effects, future work should integrate capillary dynamics into soil water–salt models to better predict long-term salinity management strategies. This study assumes negligible capillary rise effects during the experimental period due to the deep groundwater table (1.2~3.6 m) reported in the study area. However, localized capillary action in fine-textured soil microzones cannot be entirely ruled out.

The soil desalination rate under salinity removal conditions is affected by the layout parameters of the culverts. The section at 1/2 of the spacing of the concealed pipe is the farthest end of the control area of the concealed pipe, i.e., the section with the worst desalination effect. And, the control area of the underground drainpipe depends on the underground drainpipe spacing. The larger the underground drainpipe spacing is, the farther the control boundary is from the underground drainpipe. The desalination effect of the soil above 80 cm is also worse, and the desalination rate of the soil above 80 cm is significantly negatively correlated with the spacing of the culvert pipes [11,19]. In this study, the desalination rates of D1S1, D1S2, D2S1, and D2S2 were lower at 1/2 of the culvert spacing, while the desalination rates of D1S3 and D2S3 were lower at 1/4 of the culvert spacing. This may be due to the larger spacing of the underground drainpipes under the S3 treatment, and the salts at a greater distance from the underground drainpipes move with the water toward the underground drainpipes and accumulate. The salts converged in the middle of the distance from the underground drainpipe and the control boundary of the underground drainpipe (1/4 of the underground drainpipe spacing) and were not discharged into the underground drainpipe with the water in time. Qian et al. [20] concluded that different depths of culverts with surface irrigation could effectively reduce soil salinity, and soil salinity content in the horizontal direction of different depths of culverts decreased, but the difference in the reduction rate of the culverts at each distance from the culverts was relatively small. The results of this study showed that at the same spacing, the fertility and non-fertility water content, salinity, and desalination rate of the D2 treatment were different from those of the D1 treatment, but the overall difference was not significant. In this study, the average desalination rate of soil in the 0–60 cm soil layer under the D2S1 and D2S3 treatments was greater than that of D1S1 and D1S3, respectively, but the average desalination rate of soil in the 0–60 cm soil layer under the D2S2 treatment was less than that of the D1S2 treatment. This is slightly different from the significant positive correlation between the desalination rate of soil above 80 cm and the buried depth of the culvert pipe obtained by Qian et al. [20]. It may be caused by the high initial salt content of D2S2.

Also, the study showed that a 30 m spacing reduced pipe density by 33% compared to a 20 m spacing, significantly lowering material costs (e.g., 1000 m of drainage requires ~33 pipes at 30 m vs. 50 pipes at 20 m). Field trials in Xinjiang indicated a 15–20% reduction in total installation costs for 30 m systems [2]. Despite higher initial costs for the 20 m spacing, the 30 m system achieved comparable salinity control (Table 3) with lower operational energy demands for drainage pumping [3]. The 30 m spacing may also mitigate maintenance risks. Wider spacings (e.g., 40 m) are prone to uneven drainage and localized waterlogging, increasing sediment accumulation in pipes. Conversely, 30 m spacing balances hydraulic efficiency with reduced clogging frequency, as observed in a 5-year monitoring study of arid-zone drainage systems [12]. Nevertheless, periodic flushing (e.g., biennial high-pressure cleaning) remains essential to prevent long-term perforation blockage. A comprehensive cost–benefit analysis accounting for labor, energy, and maintenance over the system’s lifespan (e.g., 20 years) is needed to fully validate the 30 m spacing’s economic optimality. Future research should integrate hydrological models with lifecycle cost frameworks to guide region-specific drainage designs.

Previously, in the underground drainpipe desalination test, it was found that the underground drainpipe spacing (p < 0.05) and burial depth (p < 0.01) were significant factors affecting the desalination rate, while the pipe diameter had no significant effect on the desalination rate [21,22]. The results of this paper do not reflect the significance of the buried depth of the underground drainpipe, but this may be because the buried depth of the underground drainpipe was only set at two levels of 1.4 m and 1.6 m. More and more comprehensive combinations of culvert parameters are needed to better select the appropriate culvert parameter combinations in this region to more clearly and comprehensively describe the effects of culvert action on water and salt transport in this region.

This study focused on a single growing season (April–November 2023) to evaluate the short-term effects of subsurface drainpipe configurations on soil water and salt redistribution. While this period captures critical phases of crop growth and irrigation management in arid zones, the findings may not fully represent long-term salinity evolution, which requires multi-year monitoring to account for interannual climatic variability and cumulative salt accumulation/leaching processes. In addition, seasonal changes in irrigation practices and precipitation patterns may affect observed soil salinity dynamics. Future studies should extend monitoring over multiple years to separate the long-term effects of subsurface drainage systems from seasonal effects, especially under climate change scenarios that may alter regional hydrological regimes.

5. Conclusions

In this study, the effects of the placement parameters of subsurface drainpipes on the soil water and salt distribution characteristics in the arid zone were thoroughly investigated, and the results showed that the moisture retention of the soil was not in a single linear relationship with the spacing of the culvert placement. Compared with 20 m and 40 m, the water content of the irrigated soil was larger at a spacing of 30 m. Before irrigation, the volumetric water content of each soil layer was basically between 10.22 and 27.27%. After irrigation, the volumetric water content of each soil layer was between 18.65 and 36.97%. After irrigation, there was an overall reduction in salt content due to leaching. Soil salinity content varied in the range of 2.06–25.98 g·kg⁻¹. The phenomenon of salt aggregation in the soil surface layer was weakened, the tendency of soil profile salinity to be larger up and smaller down was weakened, and the difference in soil salinity content in different depths was reduced. In the drainage stage, the mineralization of the drainage water of each treatment was at a high level, and the overall trend of first increasing and then decreasing was shown. As the main factor affecting water salinity redistribution, the choice of burial spacing is particularly important. The results emphasize the importance of subsurface drainpipe placement parameters in soil water–salt management in arid zones and provide specific parameter suggestions for actual soil salinization improvement projects.