Optimization of Subsurface Drip Irrigation Schedule of Alfalfa in Northwest China

Abstract

Determination of an optimum irrigation amount and buried depth of subsurface drip irrigation laterals is significantly important in improving crop yield and irrigation water using efficiency in arid regions. In this study, the effects of three irrigation volumes (15.0, 22.5 and 30.0 mm) and three burial depths (10, 20 and 30 cm) of drip laterals on water consumption, yield and water productivity of alfalfa were investigated in a field trial. The water balance equation and FAO-56 crop coefficient methods were applied to determine the evapotranspiration of alfalfa. The results showed that the alfalfa evapotranspiration estimated by the dual crop coefficient method was closer to the actual measured values. The alfalfa water consumption, yield and water productivity increased significantly (p < 0.01) with the increase in irrigation amount, but the increasing trend turned to decrease (p < 0.05) with the increase of the buried depth of the drip irrigation laterals. During the whole growing period of alfalfa, the total water consumption ranged from 400 to 500 mm, the total yield ranged from 7500 to 12,000 kg/hm² and the water productivity was 1.80 to 2.50 kg/m³. An optimized irrigation amount of 22.5 to 30.0 mm for an irrigation event with a frequency of 5 to 7 days using buried drip irrigation with an irrigation lateral depth of 20 cm was recommended for alfalfa in the study area.

1. Introduction

China is a country with a large area of arid and semi-arid areas in the world. The area of dry land accounts for 52.5% of the total land area of the country, which is mainly distributed in 15 provinces and regions in the northeast, north and northwest of China [1]. The shortage of water resources is the main limiting factor for agricultural production in arid and semi-arid areas in northern China [2]. Water-saving irrigation is an important means to save water resources and improve agricultural water use efficiency [1,3,4].

In recent years, alfalfa has been planted in large areas in arid and semi-arid water shortage areas in China. As a traditional typical forage grass, alfalfa has the characteristics of drought resistance, high yield, strong adaptability, high protein content and so on [5]. It is very popular among herders, and its planting area is very wide in animal husbandry and semi-animal husbandry areas [6]. However, due to the physiological characteristics of high water consumption in alfalfa production, up to 190.81 mm, the contradiction between the rapid expansion of the alfalfa planting area in northwest China and the serious shortage of water resources in regional agricultural production is increasingly acute [7]. It is imperative to improve the yield and quality of alfalfa to relieve the pressure of natural grassland and maintain the sustainable development of animal husbandry in those areas. According to Sun et al. [8], the water consumption of alfalfa during the whole growing season can reach 2250 mm, which is extremely large compared with other crops. In addition, the shortage of water resources has restricted the development of local animal husbandry, and water-saving irrigation planting alfalfa has become the way to develop artificial irrigation forage land. In light of the multiple cutting and growth characteristics of alfalfa, it is not suitable for surface drip irrigation. In recent years, buried drip irrigation for alfalfa has been greatly promoted in pastoral areas, which is also an important means to save water resources and improve agricultural water use efficiency [9,10]. Compared with surface irrigation technology, buried drip irrigation has many advantages, such as water saving, yield increase, quality improvement, reduction of ineffective evaporation on the soil surface, improvement of soil conditions in the crop root zone, the convenience of field management and operation and prevention of capillary aging [11,12,13]. Buried drip irrigation technology also has disadvantages, such as easy blockage of capillary pipes, difficulty in measuring various indicators and high management requirements during equipment operation [14]. Previous research showed that compared with furrow irrigation, the water use efficiency of alfalfa under subsurface drip irrigation can be increased by 20% [15]; compared to field flooding irrigation, buried drip irrigation can increase alfalfa yield by about 20% while saving 40% of irrigation water [10]; compared with sprinkler irrigation, a well-designed subsurface drip irrigation system can increase alfalfa yield by about 7% while saving 22% of irrigation water [15]. Because of its technical advantages of saving water and increasing production, subsurface drip irrigation has provided a path choice for the water-saving and efficient production of alfalfa in this region and has been gradually popularized and applied [16]. Xia et al. [17] conducted a preliminary study on the burial depth of the subsurface drip irrigation laterals of alfalfa in the windy sand beach area of northern Shaanxi, pointing out that treatment with a burial depth of 10 cm was beneficial to the growth of alfalfa in the seedling stage; in the whole growing period, the comprehensive evaluation result of the influence of different burial depths on the growth characteristics of alfalfa was that a burial depth at 30 cm > burial depth at 20 cm > burial depth at 10 cm. Tong et al. [18] studied the water requirement and irrigation schedule of alfalfa under surface drip irrigation in the Xilin River basin and pointed out that the peak of water demand for the first growing season of alfalfa appeared from late June to early July; the maximal water demand strength could reach 5.09 mm/d; the water requirement of the second growing season of alfalfa appeared from early August to middle August; the maximal water demand strength could reach 5.23 mm/d. Kou et al. [19] studied the effects of subsurface deficit irrigation on the water consumption, yield and quality of alfalfa and pointed out that the yield of alfalfa decreased, water consumption decreased and water use efficiency increased with the increase in water deficit. Zhao et al. [20] conducted a preliminary study on the response of water consumption law and the yield of alfalfa in shallow embedded drip irrigation to different irrigation quotas, pointing out that the irrigation quota was greater than 45.0 mm; the increasing trend of water consumption intensity was unclear and similar to that of surface irrigation. Guo et al. [21] conducted a preliminary study on the response of emergence and the growth of spring maize under subsurface drip irrigation to trenching depth in North China, pointing out that when the soil texture was silty loam and the buried depth of drip irrigation laterals was 30 cm. Dou et al. [22] studied the field application of subsurface drip irrigation technique for maize in aeolian sandy soil and pointed out that the soil moisture content below the 20 cm soil layer was kept above 6% for a long time under subsurface drip irrigation, and the soil moisture content dropped rapidly when the soil moisture at the 20 cm soil layer reached the surface.

At present, subsurface drip irrigation technology is considered one of the most water-saving irrigation technologies [23]. Previous studies have investigated the growth characteristics of various crops under different irrigation amounts or burial depths of drip irrigation laterals. Due to the difference in crop types and regional climate characteristics, there still are some divergences among their results that exist. In addition, the detailed quantification of optimized subsurface irrigation schedules for certain crops has not been clearly presented for wide application. Therefore, it is very necessary to systematically analyze the comprehensive impact of the amount of subsurface drip irrigation and the burial depth of laterals on the water consumption, yield and water productivity of alfalfa.

Hence, an experiment was conducted in this research to investigate the effect of different irrigation amounts and burial depths of subsurface drip irrigation laterals on alfalfa growth, water consumption, yield and irrigation water productivity. FAO-56 crop coefficient methods were used to calculate the evapotranspiration of alfalfa, and the model was verified based on the measured values with the water balance equation. By comparing the response of alfalfa growth under different treatments, an optimized subsurface drip irrigation schedule and lateral burial depth for local alfalfa were proposed. The results of this research are important in improving alfalfa yield, relieving the pressure of natural grassland and maintaining sustainable development of animal husbandry in arid regions of China.

2. Material and Method

2.1. Experimental Site

The experiment was carried out in Angsu Town, Etuokeqian Banner, Ordos City, Inner Mongolia, China. Etuokeqian Banner is located in the southwest of Ordos City, Inner Mongolia, in the hinterland of Maowusu Sandy Land at the junction of Inner Mongolia, Shaanxi and Ningxia. It is 1300–1400 m above sea level, 106°30′–108°30′ E longitude and 37°38′–38°45′ N latitude. It has a semi-arid continental climate with an average annual temperature of 7.9 °C, an average annual precipitation of 261 mm, an average annual evaporation of 2498 mm, an average annual wind speed of 2.6 m/s and an average annual sunshine time of 2958 h. The annual average frost-free period is 171 d, and the maximum depth of frozen soil layer is 1.54 m.

Indoor particle analysis was conducted for the soil samples at 0–40 cm depth in the experimental area. The results showed that the soil bulk density was 1.62 g/cm³, and the specific gravity was 2.71. The soil type was sandy soil. The undisturbed soil was taken from the study area with ring cutters, and the field water capacity was measured in the laboratory and compared with the field measurement. It was determined that the field water capacity of the soil layer at 0–40 cm in the test area was 22.86%. The change in groundwater level in the test area was measured with a HOBO automatic groundwater level meter (U23-002, Bourne, MA, USA), and the groundwater burial depth in the test area was 1.2–2.0 m.

2.2. Experimental Design

Three-year-old artificial drilling alfalfa was used as experimental material in this experiment, and the variety was Grassland No. 2. Two factors and three levels of orthogonal combination design were adopted in the experiment, and two factors were set, namely, irrigation level (W) and burial depth of laterals (D). Three different irrigation levels were set according to each irrigation amount, which was, respectively, called low-water treatment (W1: 15.0 mm), medium-water treatment (W2: 22.5 mm) and high-water treatment (W3: 30.0 mm). Each irrigation level was provided with 3 types of burial depths of laterals, which were 10 cm (D1), 20 cm (D2) and 30 cm (D3), respectively. There were 9 treatments in total; each treatment was repeated 3 times, and there were 27 test plots in total. The length of each treatment was 20.0 m, the width was 5.0 m, the area was 100 m² and the total area of the test plot was 900 m². In order to avoid the influence of each treatment on each other, a 2 m wide isolation belt was set between each treatment, and the distance between different repeats of the same treatment was 1 m.

Patch-type drip irrigation laterals were used in experiment with wall thickness of 0.4 mm, emitter flow of 2.0 L/h and emitter spacing of 0.3 m. The row spacing of alfalfa plants was 15 cm. Each drip irrigation lateral controlled 4 rows of alfalfa, and the spacing of drip irrigation lateral was 60 cm. The irrigation date and times of each treatment were the same. The irrigation date was determined according to the lower limit of the appropriate water content for W2D2 treatment, and the irrigation amount was measured with water meter.

Table 1. Effective rainfall during the whole growing season of alfalfa.

First batch	Date (m-d)	4/18	5/1	5/10	5/21	6/3				Total
	Effective precipitation/mm	11.6	5.8	6.4	6.9	5.6				36.3
Second batch	Date (m-d)	6/24	6/28	7/6	7/16	7/20
	Effective precipitation/mm	2.7	3.7	3.2	4.1	12.4				26.1
Third batch	Date (m-d)	8/3	8/8	8/11	9/3	9/8	9/10	9/17	9/27
	Effective precipitation/mm	13.3	11.5	8.9	11.3	19.5	4.1	13.8	20.8	103.2

shows the effective rainfall during the whole growing season of alfalfa. According to the rainfall date and irrigation cycle, each treatment of the first, second and third batch of alfalfa was irrigated 4, 6 and 3 times, respectively. The irrigation date and irrigation quota of each test treatment of drip irrigation alfalfa are shown in

Table 2. Irrigation time and quota of subsurface drip irrigation of alfalfa at each experimental treatment.

	Treatments	Irrigation Quota/mm						Total/mm
		4/24	5/6	5/15	5/27
	W1	15.0	15.0	15.0	15.0			60
First batch	W2	22.5	22.5	22.5	22.5			90
	W3	30.0	30.0	30.0	30.0			120
		6/8	6/13	6/18	6/25	7/1	7/13
	W1	15.0	15.0	15.0	15.0	15.0	15.0	90
Second batch	W2	22.5	22.5	22.5	22.5	22.5	22.5	135
	W3	30.0	30.0	30.0	30.0	30.0	30.0	180
		8/17	8/24	9/23
	W1	15.0	15.0	15.0				45
Third batch	W2	22.5	22.5	22.5				67.5
	W3	30.0	30.0	30.0				90

Note: (W1: 15.0 mm, W2: 22.5 mm, W3: 30.0 mm).

.

2.3. Measurements

Alfalfa was manually sown on April 10 in strips with a spacing of 0.5 m. To ensure the nutritional value and palatability of alfalfa, it was mowed and harvested at the right time during the first flowering period. Three crops of alfalfa were harvested, with the last crop harvested at the end of September after the greening process starts in early April each year. The experimental area was equipped with an HOBOU30 farm weather station, and observations of temperature, rainfall, wind speed, relative humidity, barometric pressure and wind direction were made during the crop reproduction period. The field water holding capacity of undisturbed soil was measured in laboratory by taking soil samples with ring knives in the experimental area. The ring knife method, also called the indoor determination method, usually involves collecting the predicted plot of in situ soil with a ring knife followed by draining, absorbing and drying the soil in the laboratory and finally conducting soil field water-holding measurements [24]. The soil moisture sensors were installed in each test processing plot to measure the real-time soil moisture, and the volumetric soil water content was also calculated by drying method for comparison.

The growth stage of alfalfa was divided into green returning stage, branching stage, budding stage and flowering stage [25]. The hay yield was determined by sample method after cutting 3 times a year at the beginning of flowering stage. Fresh alfalfa with a sample square area of 1 m × 1 m was cut, weighed and baked first for 30 min under 105 °C, then 48 h under 65 °C.

2.4. Methods

2.4.1. Water Balance Equation

Water consumption of alfalfa was calculated with water balance equation:

$$E{T}_c=P+I-\Delta W-Q$$

(1)

where ET_c is evapotranspiration in each period (mm); P is the effective precipitation in the corresponding period (mm); I is the irrigation amount in the corresponding period (mm); $\Delta W$ is the change of soil water storage in the corresponding period (mm); Q is the lower boundary water flux in the corresponding period (mm).

The change of soil water storage ($\Delta W$) in each growth period was calculated according to the soil water content of each test treatment, and the formula is:

$$\Delta W=\frac{\left({\theta }_{i+1}-{\theta }_i\right)}{100}\times \gamma \times h$$

(2)

where θ_i is the initial soil moisture content (%) in the corresponding period; θ_i+1 is the soil moisture content at the end of the corresponding period (%); γ is soil bulk density (cm³/g); h is the planned wet soil layer depth (mm).

The lower boundary water flux of alfalfa was calculated according to the soil negative pressure measured in the experimental field. The replenishment and leakage of soil water at the lower boundary of the soil-planned wet layer were calculated by the fixed plane flux method using a negative pressure meter [26]. The calculation formula of fixed plane flux method is:

$$q\left(z_{1-2}\right)=-k\left(\overline{h}\right)\left(\frac{h_2-h_1}{\Delta z}+1\right)$$

(3)

where Δz = z₂ − z₁, $\overline{h}=\frac{h_1+h_2}{2}$, z₁ and z₂ are the two positions of the measurement, h₁ and h₂ are the soil negative pressure values at z₁ and z₂ section, respectively, from which the soil water flow per unit area in the period t₁ to t₂ can be obtained, and the flow of any section can also be obtained from:

$$Q\left(z\right)=Q\left(z_{1-2}\right)+{\displaystyle {\int }_z^{z_{1-2}}Q\left(z,t_2\right)dz-}Q\left(z,t_1\right){\displaystyle {\int }_z^{z_{1-2}}dz}$$

(4)

Water productivity refers to the output per unit of water consumption of crops, which is equal to the ratio of crop output to net water consumption of crops. The crop water productivity was calculated with the following formula:

$$WP=\frac{Y}{E{T}_c}\times 0.1$$

(5)

where WP is water productivity (kg/m³), and Y is crop yield (kg/hm²). Other symbols are the same as above.

2.4.2. FAO-56 Crop Coefficient Methods

In FAO-56 crop coefficient method, the ET_c is defined as the product of crop coefficient (K_c) and reference evapotranspiration (ET₀), and K_c is divided into soil evaporation coefficient (K_e) and basal crop coefficient (K_cb) [25,27].

$${\mathit{ET}}_c{=K}_c{\times \mathit{ET}}_0={(K}_{\mathit{cb}}{+K}_e{)\times \mathit{ET}}_0$$

(6)

The ASCE-EWRI (2005) standardized the Penman–Monteith method for grass reference evapotranspiration (ET₀) with a condensed, simplified form from the original Penman–Monteith method [25]:

$${\mathit{ET}}_0=\frac{0.408\Delta \left(R_n-G\right)+\gamma \frac{900}{T_a+273}{u(e}_s-e_a)}{\Delta +\gamma (1+0.34u)}$$

(7)

where ET₀ is reference evapotranspiration (mm d⁻¹) for daily time steps; R_n and G are net radiation at the crop surface and soil heat flux density at the soil surface (MJ m⁻² d⁻¹) for daily time steps; T_a is the daily air temperature at 2.0 m height (°C); Δ is the slope of the saturation vapor pressure curve at T_a (kPa °C⁻¹); u is the wind speed at the reference height (m s⁻¹); e_s and e_a are saturated and actual vapor pressure at T_a, respectively (kPa).

Based on FAO-56 [25], K_cb can be expressed as:

$$K_{\mathit{cb}}=K_{c\mathit{min}}{+(K}_{\mathit{cb}\mathit{full}}-K_{c\mathit{min}})\times \left[1-\exp (-0.7\times \mathrm{LAI})\right]$$

(8)

$$K_{\mathit{cb}\mathit{full}}=\min \left(1.0+0.1h,1.2\right)+\left[0.04\left(u_2-2\right)-0.004\left({\mathit{RH}}_{\mathit{min}}-45\right)\right]{\left(\frac{h}{3}\right)}^{0.3}$$

(9)

where K_{c min} is the minimum value of basal crop coefficient for bare soil (=0.15), K_{cb full} is the basal crop coefficient when crop has nearly full ground cover and h is the plant height.

K_e can be expressed as:

$$K_e{=K}_r\left(K_{c\mathit{max}}-K_{\mathit{cb}}\right)\le f_{\mathit{ew}}{K}_{c\mathit{max}}$$

(10)

$$K_{c\mathit{max}}=\max \left(\left\{1.2+\left[0.04\left(u-2\right)-0.004\left({\mathit{RH}}_{\mathit{min}}-45\right)\right]{\left(\frac{h}{3}\right)}^{0.3}\right\},\left\{K_{\mathit{cb}}+0.05\right\}\right)$$

(11)

where K_{c max} is the maximum value of K_c following rain or irrigation, f_ew is the fraction of the soil that is wetted (=0.5) for irrigation, K_r is the evaporation reduction coefficient dependent on the cumulative depth of water depleted from the soil surface, which is expressed as follows [25,28]:

$$K_r=\frac{\mathit{TEW}-D_e}{\mathit{TEW}-\mathit{REW}}=\frac{1000(\mathsf{\theta }s-0.5{\mathsf{\theta }}_{\mathit{wp}}{)Z}_e}{\mathit{TEW}-\mathit{REW}}$$

(12)

where TEW is total evaporable water (mm), which is the maximum depth of water that can be evaporated from the soil when the soil surface has been initially completely wetted; D_e is the cumulative depth of evaporation (depletion) from the soil surface layer (mm); REW is the readily evaporable water, which is the maximum depth of water that can be evaporated from the soil surface without restriction; θ_s is the actual surface volumetric soil water content and θ_wp is the surface soil water content at wilting point (=0.12 m³ m⁻³ in this study). Z_e is the depth of the surface soil layer that is subject to drying by evaporation (=0.10 m in this study).

The single crop coefficient of a certain crop under standard conditions can be found in FAO-56. The so-called standard conditions refer to the coefficient of large area crops without diseases and pests, with optimal water and soil conditions, appropriate fertilization amount, and full harvest under given climatic conditions. FAO-56 suggests that the growth cycle of crops can be divided into four stages: early growth, development, middle growth and late growth. According to FAO-56, the single crop coefficients of alfalfa in different growth periods are K_{c ini} = 0.40, K_{c mid} = 1.20 and K_{c end} = 1.15.

Since the climate, water and soil conditions in the study area are different from those under standard conditions, the recommended single crop coefficient needs to be revised. At the initial growth stage, ET₀ = 3.09 mm/d, the crop coefficient after correction can be obtained from FAO as K_{c ini} = 0.42, and K_{c mid} and K_{c end} need to be corrected according to wind speed and minimum relative humidity with the formula presented by FAO-56 [25].

3. Results and Analysis

3.1. Evapotranspiration of Alfalfa Calculated by the Crop Coefficient Methods

The crop coefficient methods recommended by the FAO-56 are methods to calculate the actual evapotranspiration of crops, which can simply and accurately reflect the evapotranspiration patterns and characteristics of different agroecosystems during different growth stages [28].

Figure 1 shows the daily change process of the field-measured evapotranspiration (ET_c) of alfalfa under subsurface drip irrigation and calculated by the single and dual crop coefficient methods during the whole growth period (P + I in the figure represents rainfall and irrigation). It can be seen from the figure that the crop water demand during the whole growth period of alfalfa was very uneven. The water demand of the second batch of alfalfa was the largest (=167.4 mm), while the water demand of the first and the third batch was relatively small (=142.3 mm and 148.58 mm), but there was little difference between them. The comparison between the calculated ET_c and the measured values showed that the ET_c of alfalfa calculated by the dual crop coefficient method was close to the measured values in most cases during the whole growth period, and the variation trend was the same, while the ET_c of alfalfa calculated by the single crop coefficient method had a certain deviation from the measured values. At the early growth stage, the ET_c calculated by the single and dual crop coefficient methods was slightly lower than the measured values; in the other three growth stages, the calculated ET_c was very close to the measured values, especially in the development stage and the middle growth stage. The calculated ET_c using the dual crop coefficient method was more consistent with the measured values.

Table 3. The comparison of measured and calculated evapotranspiration by the single and dual crop coefficient methods.

Growth Stage	Single Crop Coefficient				Dual Crop Coefficient
	Calculated (mm)	Measured (mm)	ΔETc (mm)	R (%)	Calculated (mm)	Measured (mm)	ΔETc (mm)	R (%)
First batch	153.47 Aa	142.25	11.2	7.89	148.96 Bb	142.25	6.71	4.72
Second batch	161.29 Bb	167.43	−6.14	−3.67	164.38 Aa	167.43	−3.05	−1.82
Third batch	158.82 Aa	148.58	10.24	6.89	152.31 Bb	148.58	3.73	2.51
Whole stage	473.58 Aa	458.26	15.3	3.34	465.65 Bb	458.26	7.39	1.61

Notes: The capital letter in the table denotes significant differences between variables at p < 0.01 level, while the small letter denotes significant differences at p < 0.05 level.

shows the absolute deviation (ΔET_c) and relative deviation (R) of the measured and calculated ET_c by the single and double crop coefficient methods, respectively, at different growth stages of alfalfa with buried drip irrigation. It can be seen that the ET_c obtained by the dual crop coefficient method was closer to the measured values than that obtained by the single crop coefficient method. The absolute deviations of the dual crop coefficient for the first, second and third batches of alfalfa were 6.71 mm, −3.05 mm and 3.73 mm, respectively, and the relative deviations were 4.72%, −1.82% and 2.51%, respectively. In terms of the whole growth period, the ET_c calculated by the dual crop coefficient method was closer to the field-measured values, which showed that the dual crop coefficient method can better describe the change of alfalfa ET_c after rainfall or irrigation. This confirms that in reality, the dual crop coefficient method is better than the single crop coefficient method in estimating ET_c, and the findings in this study are quite close to the recommendations by [29]. According to the results of the single crop and dual crop coefficient methods for calculating the ET_c of alfalfa under subsurface drip irrigation, it was determined that the crop water demand of alfalfa under subsurface drip irrigation in the whole growth period in the study area was about 460 mm, of which the second batch had the largest water demand, while the first and the third batches had a small water demand.

3.2. Evapotranspiration and Yield of Alfalfa under Different Irrigation Amounts

Water is the main factor affecting the yield and nutrient utilization of alfalfa, so reasonable irrigation is an important measure to ensure the high quality and high yield of alfalfa [30]. Figure 2 shows the changes in plant height of the second batch of alfalfa under different irrigation water treatments during the growth period. It can be seen that the plant height of low-water treatment (W1) was significantly lower than that of other treatments (W2 and W3). For example, the plant heights under high-, medium- and low-water treatment were 52 cm, 46 cm and 36 cm, respectively, on the second harvest on 30 July, and the plant heights under high-water treatment were 13.04% and 44.44% higher than that under medium- and low-water treatments, respectively. Therefore, the increase in irrigation amount from the medium-water treatment (22.5 mm) to the high-water treatment (30 mm) had little effect on the plant height, but when the irrigation amount decreased from the medium-water treatment (22.5 mm) to the low-water treatment (15 mm), the plant height of the alfalfa decreased significantly. The plant height of the first and third batches of alfalfa also had the same change trend during the growth period.

The hay yield of alfalfa under different treatments is shown in Figure 3. It can be seen that the hay yield of alfalfa in the third batch increased with the increase of irrigation water. The maximum hay yield was found in the W3D2 treatment with a value of 11,349 kg/hm², and the minimum hay yield was found in the W1D1 treatment with a value of 7527 kg/hm². Compared with the minimum value, the maximum hay yield increased by 50.78%. The total hay yield of alfalfa increased by 37.76% and 7.06%, respectively, from the W1D1 to W2D1 treatment and from the W2D1 to W3D1 treatment. Sha et al. [31] conducted an experiment on the effects of drip irrigation water and fertilizer coupling on the yield of alfalfa in the Yellow River irrigation district and presented that appropriate irrigation can significantly increase the hay yield of alfalfa by 13%. The total hay yield of alfalfa increased by 35.90% and 5.30%, respectively, from the W1D2 to W2D2 treatment and from the W2D2 to W3D2 treatment. The total yield of alfalfa hay increased by 35.68% and 5.84%, respectively, from the W1D3 to W2D3 treatment and from the W2D3 to W3D3 treatment. It can be seen that the effect of the irrigation amount on the alfalfa yield was significant when the irrigation amount increased from the low-water treatment (15.0 mm) to the medium-water treatment (22.5 mm), then it increased from the medium-water treatment (22.5 mm) to the high-water treatment (30.0 mm) at the same buried depth of drip irrigation tape.

Table 4. Effect of irrigation amount on hay yield, water consumption (ET_c) and water productivity of alfalfa.

Treatments	Hay Yield (kg/hm2)	Water Consumption (mm)	Water Productivity (kg/m3)
High moisture (W3)	11,226.5 Aa	462.75 Aa	2.11 Aa
Medium moisture (W2)	10,585.5 Bb	444.47 Bb	2.14 Bb
Low moisture (W1)	7759.0 Cc	405.35 Cc	1.73 Cc

Notes: The capital letter in the table denotes significant differences between variables at p < 0.01 level, while the small letter denotes significant differences at p < 0.05 level.

shows the yield, crop water consumption (ET_c) and water productivity of alfalfa under different irrigation amount treatments. It can be seen that the total yield of the alfalfa increased by 33.75% and 37.58%, respectively, from the low-water treatment (W1) to the medium-water treatment (W2) and then to the high-water treatment (W3). The water productivity of the alfalfa under low-, medium- and high-water treatment was 1.73, 2.14 and 2.11 kg/m³, respectively, which showed that the water productivity of the alfalfa under the medium-water treatment was the largest. It can be seen that the impact of irrigation amount on alfalfa yield, water consumption and water productivity was extremely significant (p < 0.01). Zhao et al. [20] conducted a preliminary study on the response of water consumption law and the yield of alfalfa in shallow-embedded drip irrigation to different irrigation quotas, pointing out that the alfalfa water consumption and irrigation quota showed a linear relationship. From the perspective of water conservation, it was recommended to adopt a 22.5 mm irrigation quota, while a 30.0 mm irrigation quota was suggested to adopt from the perspective of increasing production. This confirms the relationship between irrigation amount and alfalfa yield and water use efficiency, and the findings in this study are quite close to the recommendations by Li et al. [32] that the yield of alfalfa increased with increased irrigation amount when the irrigation amount was between 60–100% of the ET_c.

3.3. Effect of Burial Depth of Drip Irrigation Belt on Alfalfa Yield, Water Consumption and Water Productivity

It was shown in Figure 3 that the hay yield of alfalfa in the third batch increased first and then decreased with the increase of burial depth of laterals under the same irrigation amount. When the irrigation amount was 15.0 mm, the highest yield was found in W1D2 treatment with a value of 7930.5 kg/hm², the lowest was W1D1 with a value of 7527 kg/hm² and the highest yield was 5.36% higher than the lowest. The highest yield was found in the W2D2 treatment with a value of 10,777.5 kg/hm² when 22.5 mm of irrigation water was applied, the lowest was found in W2D1 with a value of 10,369.5 kg/hm² and the highest yield was 3.93% higher than the lowest. The highest yield was found in the W3D2 treatment with a value of 11,349 kg/hm² when 30.0 mm of irrigation water was applied, and the lowest was 11,101.5 kg/hm² in W3D1. Compared with the lowest, the highest yield increased by 2.23%. The yield of alfalfa increased by 5.36% and −1.40% from W1D1 to W1D2 and W1D2 to W1D3, respectively. The yield of alfalfa increased by 3.93% and −1.56% from W2D1 to W2D2 and W2D2 to W2D3, respectively. The yield of alfalfa increased by 2.23% and −1.06% from W3D1 to W3D2 and W3D2 to W3D3, respectively.

As pointed out by Dou et al. [22], there is no reference for the suitable buried depth of laterals in subsurface drip irrigation for different types of soil, and it needs to be determined through experiments in the application of subsurface drip irrigation technology. The influence of the burial depth of drip irrigation laterals on hay yield, water consumption and water productivity of alfalfa conducted in this study was shown in

Table 5. Effect of buried depth of irrigation belt on yield, water consumption and water productivity of alfalfa.

Burial Depth	Hay Yield (kg/hm2)	Water Consumption (mm)	Water Productivity (kg/m3)
10 cm	9666.0 Cc	434.78 Cc	2.2089 Cc
20 cm	10,019.0 Aa	439.81 Aa	2.2662 Aa
30 cm	9886.0 ABb	437.77 ABb	2.2462 ABb

Notes: The capital letter in the table denotes significant differences between variables at p < 0.01 level, while the small letter denotes significant differences at p < 0.05 level.

. It can be seen that the influence of the burial depth of laterals on water consumption, yield and water productivity of alfalfa was significant at the level of p < 0.05. The influence of lateral burial depth at 20 and 30 cm on water consumption, yield and water productivity of alfalfa was not significant at the level of p < 0.01. It is suggested that drip irrigation laterals should not be buried too deep (>30 cm). Otherwise, the soil water is difficult to be absorbed by crop roots and is easy to cause deep leakage, so it is recommended to adopt a burial depth of 20 cm for drip irrigation laterals. Fu et al. [33] reported that although water-conserving irrigation methods increased the average soil water content during growing seasons compared to flood irrigation, only the subsurface drip irrigation laterals buried at depths of 20 and 30 cm can significantly increase the alfalfa yield. As described elsewhere in the literature [34], with the shallowness of installation depth, the availability of soil water increased, and the soil water stress was minimized because most roots distributed near the shallow soil, e.g., the lateral root of alfalfa is mainly distributed at a depth of 20–30 cm.

The calculation results of water consumption and water productivity of alfalfa in the first, second and third batches and total growth seasons under different irrigation amounts and lateral burial depths are shown in Figure 4 and Figure 5, respectively. The water consumption during the first batch of alfalfa was 132.40~147.96 mm, and the water productivity was 1.77~2.44 kg/m³. During the second batch of alfalfa, the water consumption was 139.02~166.36 mm, and the water productivity was 2.07~2.48 kg/m³. The water consumption during the third batch of alfalfa was 130.01~150.91 mm, and the water productivity was 1.77~2.43 kg/m³. For the second batch, the water consumption of W1D1, W1D2 and W1D3 treatments was 139.02, 140.58 and 139.37 mm, respectively, and the water productivity was 2.07, 2.13 and 2.12 kg/m³, respectively. The water consumption of W2D1, W2D2 and W2D3 treatments was 158.93, 161.32 and 160.20 mm, respectively, and the water productivity was 2.41, 2.45 and 2.43 kg/m³, respectively. The water consumption of W3D1, W3D2 and W3D3 treatment was 163.87 mm, 166.36 mm and 165.28 mm, respectively, and the water productivity was 2.43, 2.48 and 2.46 kg/m³, respectively.

It can be seen from the above results that the water consumption of alfalfa had little difference at the same growth period, and the water productivity first increased and then decreased with the increase of the burial depth of the drip irrigation laterals, but the decrease was small, indicating that the water productivity was reduced when the drip irrigation laterals were buried too deep. It is recommended to use a burial depth of 20 cm. Jiang et al. [35] conducted a preliminary study on the response of tomato yield and irrigation water use efficiency under subsurface drip irrigation at different lateral depths and pointed out that when the drip irrigation laterals were buried shallow, higher water consumption was caused by transpiration, and when the drip irrigation laterals were buried deeper, more water needed to flow out from the drip head to form a thicker layer of water to be transported to the root system. The water consumption and water productivity of alfalfa increased with an increase of irrigation amount when the burial depth of the drip irrigation laterals was the same. For the second batch of alfalfa under W1D2, W2D2 and W3D2 treatments, the water consumption was 140.58 mm, 161.32 mm and 166.36 mm, respectively, and the water productivity was 2.13, 2.45 and 2.48 kg/m³, respectively. With the increase in irrigation amount, the water consumption increased by 14.75% and 3.12%, respectively, and the water productivity increased by 14.95% and 1.21%, respectively. The water consumption and water productivity of alfalfa in the first and third batch had the same variation trend as that in the second batch.

It was shown in Figure 4 and Figure 5 that the total water consumption of alfalfa during the whole growing season for different treatments ranged from 401.43 to 463.45 mm, and the water productivity ranged from 1.88 to 2.45 kg/m³. Under the same burial depth of laterals, the water consumption and water productivity increased with the increase in irrigation amount. The water consumption of alfalfa increased by 9.90% and 4.79%, and the water productivity increased by 25.35% and 2.16%, respectively, from W1D1 to W2D1 treatment and from W2D1 to W3D1 treatment. The water consumption of alfalfa increased by 9.65% and 3.52%, respectively, from W1D2 to W2D2 and from W2D to W3D2 treatment, and the water productivity increased by 23.94% and 1.72%, respectively. The water consumption of alfalfa increased by 9.40% and 4.04%, respectively, from W1D3 to W2D3 and from W2D to W3D3 treatment, and the water productivity increased by 24.02% and 1.73%, respectively.

It can also be seen from Table 5 that the effect of irrigation amount on water consumption and water productivity was extremely significant (p < 0.01). Under the same irrigation amount, the water consumption of alfalfa increased by 1.71% and −0.48%, respectively, and the water productivity increased by 3.59% and −0.92%, respectively, from W1D1 to W1D2, and from W1D2 to W1D3 treatment. The water consumption of alfalfa increased by 1.48% and −0.71%, and the water productivity increased by 2.42% and −0.86%, respectively, from W2D1 to W2D2 and from W2D2 to W2D3. The water consumption of alfalfa increased by 0.24% and −0.21%, and the water productivity increased by 1.98% and −0.85%, respectively, from W3D1 to W3D2 and W3D2 to W3D3 treatment. Results similar to this study were reported by Wang et al. [36] on the effect of coupling water and fertilizer in subsurface drip irrigation on alfalfa yield which stated that the irrigation amount was 900~2250 m³/hm², and the alfalfa yield was the highest (=1438.13 kg/hm²). It can be seen from Table 4 that the burial depth of laterals at 20 cm and 30 cm had a significant impact on water consumption at p < 0.05, and the burial depth at 20 cm and 10 cm had a significant impact on water consumption at p < 0.01. The burial depth had a significant effect on water productivity at p < 0.01, and the burial depth of 20 cm and 30 cm had a significant effect on water productivity at p < 0.05. It can be concluded that moderately increasing the irrigation amount and burial depth of laterals can increase water productivity, but the drip irrigation laterals should not be buried too deep to avoid deep leakage, which is not conducive to the absorption and utilization of water by crops, thus reducing the water productivity. The influence of buried depth of subsurface drip irrigation laterals on water consumption and water productivity in northwest China is roughly consistent with the study Guo et al. [21] conducted in the North China Plain.

4. Conclusions

Soil water distribution directly affects the growth of a plant’s root system, determines its ability to absorb nutrients from the soil, causes the change of nutrient content in the plants and then affects the gray yield. In this experiment, batch-type drip irrigation laterals were used to investigate the changes in alfalfa growth and yield under different irrigation levels and lateral burial depths, and the evapotranspiration of alfalfa under subsurface drip irrigation was estimated by the FAO-56 crop coefficient methods. The results showed that:

(1)

The evapotranspiration calculated by the dual crop coefficient method was closer to the observation values. The water consumption, yield and water productivity of alfalfa under subsurface drip irrigation increased significantly with the increase of irrigation amount (p < 0.01); when the irrigation amount increased from 260.4 to 506.9 mm (an increase of 94.7%), the crop water consumption increased from 418.4 mm to 477.2 mm (an increase of 14.1%).

(2)

The influence of the burial depth of the drip irrigation laterals on water consumption, yield and water productivity of alfalfa was significantly different (p < 0.05), and both increased first and then decreased with the increase of the burial depth of the laterals, but the decrease was not significant. It was suggested to use a 20 cm burial depth for laterals.

(3)

The effect of increasing the irrigation amount from low- to medium-water treatment was obviously better than that of increasing the irrigation amount from medium- to high-water treatment. The water consumption of alfalfa in the total growth season was 400~500 mm, the total output was 7500~12,000 kg/hm² and the water productivity was 1.80~2.50 kg/m³. It was suggested to adopt a 22.5 mm irrigation quota for water-saving irrigation, while 30.0 mm was proposed from the perspective of increasing production. The irrigation period of 5–7 days was suggested.