Material and Methods

Study site

The study site lies in the Yellow River irrigation area of Yongning County which is located in the middle of the Yinchuan Plain (Fig. 1). The climatic characterization was carried out using the 1951–2020 monitoring data series from National Meteorological Information Center at Yinchuan Station (20km from the study site). According to such data, the climate in this region is semi-arid. The annual average temperature is 9.2°C, with a wide both daily and seasonal fluctuation. The precipitation is unevenly distributed throughout the year. The least precipitation occurs during winter and the most rain falls from June to September which accounts for 70.6% of the mean annual precipitation (196 mm). The max monthly precipitation is 148.7 mm, mainly as light rain and moderate rain. The mean annual potential evaporation is 1595 mm with a strong seasonal dynamic ranging from about 32 mm per month in winter to up to 238 mm per month in summer. Mean relative humidity during these seasons varied between 31.6 and 62.7 percent. There are 144-159 frost-free days and 2888 h of sunshine per year.
The Yellow River runs through the area from south to north, so water used for irrigation is diverted from the Yellow River. The Hanyan Ditch and Huinong Ditch are the main ditches in the irrigation region. The traditional surface irrigation has been widely implemented for more than 60 years. In recent years, the demand for irrigated agriculture is increasing due to climate change and population growth. Maize, wheat, soybean and some vegetables are the main crops. Owing to a large amount of water from surface irrigation, groundwater is shallow and the depth to water table annually varies from 2.3 to 4.3 m.

Experimental design

The experiments were conducted at plot 2 of Guanqiao farm from April 2019 to December 2020. In order to assess the different land covers on the soil water movement under irrigation or precipitation, the plot 2 was with bare ground surface in 2019 and planted with maize between 7 May and 20 September, 2020. The plot 1 was planted with maize both in 2019 and 2020. The irrigation schedule in plot 1 was implemented same as that in plot 2. The irrigation was carried out during the maize growing season (May to September) and post-harvest autumn (8 November) to maintain the soil water. The surface irrigation date was 1 May,25 May, 2 July, 14 July, 3 August, 16August, 30 August , 8 November in 2019 and 25 May , 5 June, 6 July, 1 August in 2020 (Fig. 2b). The field was flood-irrigated with water diverted from the Hanyan Ditch into a 50cm wide open channel (Fig. 1). The maize growing season can be divided into sowing (from 7 to 18 May), seedling (from 19 May to 3 June), jointing (from 4 to 10 July), tasseling (from 11July to 10 August), filling (from 11August to 5 September) and maturity (from 6 to 18 September) stages.
A test pit (1.0 m wide by 1.6 m long with 2.7m depth) was excavated in the plot 2 adjacent to the north edge for soil characterization and installation of soil water sensors, automatic tensiometers and suction lysimeters. In addition, the experimental well and rain gauge were also installed inside of the plot 2. The schematic of plot layout is presented in Fig. 1.

Field monitoring and sampling

To measure soil water variations in the soil layer, a set of measurement systems was installed in the pit west, east and north wall in September 2018 (Fig. 1). To ensure stabilization between the probes and the surrounding soil, only data between April 2019 and October 2020 were analyzed. The SWC was measured by means of the Time Domain Reflectometry (TDR) technique (TDR-310S, Acclima, USA). The SMP was measured using water-filled hydraulic tensiometer (Yang et al., 2018; Yang et al., 2014). The electronic pressure sensor near the top was installed to measure the force required to remove water from the soil. A set of eight probes of each parameter were horizontally installed at depths of 20, 30, 50, 70, 100, 150, 200 and 270cm. The groundwater level sensor was fixed to measure the distance between the ground and the water table. The rain gauge was the tipping bucket rain gauge type (SL3-1, Shanghai Meteorological Instrument Factory Co., Ltd., China) to monitor the rainfall. All the data were automatically recorded at 5-min intervals and averaged every 30 min by data logger (CR10X, Campbell Scientific, Inc., USA).
Rain water was collected by a rain collector near the rain gauges. After each rainfall event, rainwater was collected and immediately transferred to a bottle and then sealed and stored. Soil water was sampled after each rainfall or irrigation event at the depth same as that of sensors (Yang et al., 2018), using a suction lysimeter designed by Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences (IGSNRR, CAS), which was composed of a Teflon pipe and porous ceramic tube. A vacuum pump of about −0.8 MPa was applied to the suction lysimeter for 12 h of equilibrium to collect soil water. Irrigation water and ground water was sampled directly from ditch at irrigation event and pump discharge very month, respectively.
Soil samples, obtained at an interval of 20 cm from the 0–270 cm depth in the test pit, were measured for particle size analysis using a Mastersizer 2000 (Malvern Instruments, Malvern, UK) in the laboratory. According to the International Classification System, the soil texture at 0~130 cm depth is loam and silty loam at 150~270 cm depth. The bulk density ranged from 1.63 to 1.72 g cm-3 for the soil (Table 1).

Laboratory experiment

All the water samples were filtered through 0.2μm filters before isotope analysis and analyzed in IGSNRR, CAS. The stable isotopes of water samples (δD, δ18O) were analyzed using off axis integrated-cavity output laser spectroscopy (Model DLT-100; Los Gatos Research Inc.). All samples were normalized to internal laboratory water standards that were previously calibrated relative to the Vienna Standard Mean Ocean Water (VSMOW, 0 ‰). Results were expressed as parts per thousand deviations from the VSMOW with analytical precisions of ±1.0‰ (δD) and ±0.1‰ (δ18O).

Calculation methods

To investigate the soil moisture depletion and incomplete recharge, we calculate the daily evapotranspiration rates using a simplified water balance equation (Jipp et al., 1998):
\(ET=W+P+I-D_{p}+D_{r}\) (1)
Where ET is evapotranspiration, \(W\) is the change in soil moisture storage in the soil profile (to 270cm) between successive field measurements, P is precipitation, \(I\) is irrigation, \(D_{p}\)is deep percolation below the root zone,\(D_{r}\)is capillary rise from shallow water table over the same period. All variables are expressed in mm/d.
Vertical water flux is calculated using Darcy’s equation with estimates of the soil water potential (SWP) gradient and unsaturated hydraulic conductivity.
Darcy’s law (downward positive) is:
\(D=-K\left(h\right)\left(\frac{\partial\psi}{\partial z}\right)\)(2)
Where D is the soil water flux (mm/d), for which a negative value represents soil moisture upward moving (\(D_{r}\)) and a positive value represents soil moisture downward moving (\(D_{p}\)), K (h ) is the unsaturated hydraulic conductivity (mm/d) at the depth ofh (cm) and ψ is the soil water potential (cm H2O) which is calculated by soil matric potential plus gravity potential. The zero point for gravity potential is defined at the ground surface.
To calculate deep percolation (\(D_{p}\)) or capillary rise (\(D_{r}\)) between 200 and 270 cm, equation (2) can be written in a differential scheme as follows:
\(D_{p}\text{\ or\ }D_{r}=D=-\sqrt{K_{200}K_{270}}(\frac{\psi_{200}-\psi_{270}}{70})\)(3)
Where \(D_{p}\) is the positive value of D and \(D_{r}\) is the negative value of D , K200 andK270 are the unsaturated hydraulic conductivity at 200 and 270 cm depth (mm/d), respectively. ψ200 and ψ270 are the SWP at 200 and 270 cm depth (cm H2O), respectively.
The van Genuchten model (Van Genuchten, 1980) is used to calculate the unsaturated hydraulic conductivity in the vadose zone.
The VG model for moisture retention curve is:
\(\theta(S)=\theta_{r}+(\theta_{s}-\theta_{r})\left[1+\left|\partial S\right|^{n}\right]^{-m}\)(4)
Where \(\theta(S)\) is SWC (cm3cm-3), θ sis the saturated water content (cm3cm-3), θ ris the soil residual content (cm3cm-3), n and m are the shape parameters of soil water characteristic, m  = 1–1/n , 0<m <1. According to the measurement of the water retention curve, the VG model was fitted by Retention Curve (RETC) Computer Program.