Evaluating performance of soil water movement and groundwater recharge in an irrigated agricultural area of Yinchuan Plain, China

Surface irrigation has been predominantly used for field crops in agriculture area to boost agricultural yields and outputs, however, this may also raise groundwater tables, salinize soils and reduce water quality due to poor irrigation management. Therefore, it is essential for requiring a better understanding of the hydrologic mechanisms related to soil water fluxes (e.g., evaporation, transpiration, infiltration, deep percolation and groundwater capillary rise) by surface irrigation. This study investigated the impact of surface irrigation on soil water movement and recharge to groundwater in the Yellow River irrigation area of Yinchuan Plain, China. Combining comprehensive filed observation and stable isotopic techniques, we described the soil water mechanism under two land covers (bare ground or maize) in 2019 and 2020. The soil depths affected by precipitation infiltration and evaporation were mainly 0-50 cm, while the soil influenced by irrigation was the entire profile in the mode of piston flow. According to soil water potential variation from 70 to 100 cm, we conclude that the maize root took up the soil water up to the depth of 100 cm during the tasseling period. The infiltration and capillary rise in 2020 were similar with those in 2019. However, the total deep percolation was 156.6 mm in 2020 which was smaller than that in 2019 because of the maize root water uptake. The leakage of ditch was the major recharge resource of groundwater for the fast water table rise. This study is critical for agricultural water management to improve irrigation efficiency and water use efficiency in arid regions.

Development and Validation of a Model for Soil Wetting Geometry Under Moistube Irrigation

We developed an empirical soil wetting geometry model for silty clay loam and coarse sand soils under a semi-permeable porous wall line source Moistube Irrigation (MTI) lateral irrigation. The model was developed to simulate vertical and lateral soil water movement using the Buckingham pi (π) theorem. This study was premised on a hypothesis that soil hydraulic properties influence soil water movement under MTI. Two independent, but similar experiments, were conducted to calibrate and validate the model using MTI lateral placed at a depth of 0.2 m below the soil surface in a soil bin with a continuous water supply (150 kPa). Soil water content was measured every 5 minutes for 100 h using MPS-2 sensors. Model calibration showed that soil texture influenced water movement (\(p\) < 0.05) and showed a good fit for wetted widths and depths for both soils ( \(nRMSE\) = 0.5% − 10%; \(NSE \ge\) 0.50; and d-index \(\ge\) 0.50. The percentage bias \(\left(PBIAS\right)\) statistic revealed that the models’ under-estimated wetted depth after 24 h by 21.9% and 3.9% for silty clay loam and sandy soil respectively. Sensitivity analysis revealed an agreeable models’ performance values. This implies the applicability of the model for estimating wetted distances for an MTI lateral placed at 0.2 m. However, further experimentation under varying scenarios for which MTI would be used, including field conditions, is needed to further validate the model and establish robustness. MTI wetting geometry informs placement depth for optimal irrigation water usage.

Soil Freeze-Thaw and Water Transport Characteristics Under Different Vegetation Types in Seasonal Freeze-Thaw Areas of the Loess Plateau

In the arid and semi-arid regions of the Loess Plateau, seasonal freezing and thawing influence soil water movement, and water movement directly influences vegetation growth. However, currently, research with regard to freezing and thawing processes under various vegetation types and the mechanism of soil water movement is lacking. Therefore, the present study explored soil water migration characteristics of two typical vegetation types [arbor land (AL) and shrub land (SL)] on the Loess Plateau during seasonal freezing and thawing processes using bare land (BL) as a control. We used field measured data for hourly soil temperature (ST) and soil water content (SWC) at a depth of 100 cm below the soil surface from November 2017 to March 2018. Freezing and thawing process was divided into three stages based on ST change (initial freezing period, stable freezing period, and thawing period). Compared with previous studies in this area, ST is lower than expected, and SWC migration characteristics are also different. The results revealed that: 1) the maximum freezing depth of AL and SL was 60 cm, which was 30 cm less than that of BL. The freezing date of each soil layer in BL was the earliest and average ST value was the lowest. BL had the highest degree of freezing. The freezing of all soil layers in AL occurred at a later date than that of SL. ST and the minimum soil freezing temperatures were higher than those of SL, and the capacity of AL to resist freezing was higher; 2) the SWCs in AL and BL at depths of 0–10 cm and 10–30 cm decreased, whereas SWCs of AL and BL at a depth of 60 cm increased by 152 and 146%, respectively. The SWCs of SL at soil depths of 0–10 cm, 10–30 cm, and 30–60 cm increased by 46.3, 78.4 and 205%, respectively. The amount and distribution of soil moisture in SL were optimum when compared to those of AL and BL. The results of the present study could provide a scientific basis for vegetation restoration in arid and semi-arid areas of the Loess Plateau.

Using Soil Water Stable Isotopes to Investigate Soil Water Movement in a Water Conservation Forest in Hani Terrace

Water conservation forests significantly contribute to the stability of mountain agricultural ecosystems in Hani Terrace. In this study, we analyzed the relationship between the stable isotopic composition of soil water and precipitation to determine the mechanisms of soil water movement in the small watershed of Quanfuzhuang. We observed significant seasonal variations in soil water sources: antecedent precipitation was the dominant supply during the dry season, and current precipitation dominated during the rainy season. The recharge ratio of precipitation to soil water in the grassland was significantly higher than that in the arbor land and shrubland. The influence of water infiltration, old and new soil water mixing, and soil evaporation on the soil water stable isotopes gradually decreased from the surface (0–20 cm) to the deep (60–80 cm) soil. We observed significant seasonal variability in average soil water δ18O in the upper 0–60 cm and lower variability at 60–100 cm. The average soil water δ18O was generally higher in the dry season than in the rainy season. The mixing of old and new water is a continuous and cumulative process that is impacted by soil structure, soil texture, and precipitation events. We therefore identified a significant time delay in soil water supply with increasing soil depth. Moreover, the piston flow of soil water co-occurred with preferential flow, and the latter was the dominant supply during the rainy season.

A bench-scale assessment of the effect of soil temperature on bare soil evaporation in winter

To quantitatively evaluate in the laboratory the effect of soil temperature on bare soil evaporation, this study uses two indoor soil columns and homogenized sand as an example to carry out the experimental study of soil temperature on bare soil evaporation in winter. The results show that the soil temperature directly affects the change in bare soil evaporation and that the effect decreases as the soil temperature decreases. Because of the influence of soil temperature, the soil water movement accelerates, and the soil water content increases. At a depth of 50 cm, the average difference in soil water content between groups A and B was 7.61%. The soil evaporation when considering the soil temperature was obviously greater than that without considering the soil temperature. This shows that in a laboratory environment where the soil temperature is higher than the room temperature in winter, the effect of the soil temperature on bare soil evaporation is significant. Soil temperature directly affects soil water movement and distribution, which is one of the important influencing factors affecting bare soil evaporation.

Ecohydrological Separation Hypothesis: Review and Prospect

The ecohydrological-separation (ES) hypothesis is that the water used for plant transpiration and the water used for streams and groundwater recharge comes from distinct subsurface compartmentalized pools. The ES hypothesis was first proposed in a study conducted in the Mediterranean climate region, based on the stable isotope method in 2010. To date, the ES hypothesis has proven to be widespread around the world. The ES hypothesis is a new understanding of the soil water movement process, which is different from the assumption that only one soil reservoir in the traditional hydrology. It is helpful to clear the water sources of plants and establish a new model of the ecohydrological process. However, the theoretical basis and mechanism of the ES hypothesis are still unclear. Therefore, we analyzed the characteristics of ES phenomenon in different climatic regions, summarized the research methods used for the ES hypothesis, concluded the definitions of tightly bound water and mobile water, discussed the mechanism of isotopic differences of different reservoirs and their impacts on ES evaluation and pointed out the existing problems of the ES hypothesis. Future research should focus on the following three aspects: (a) detailed analysis of ES phenomenon characteristics of different plant species in different climatic regions; (b) further understanding of the ES phenomenon mechanism; (c) improvement of the experimental methods.

Using HYDRUS 2D/3D to Evaluate Soil Water Movement in Drip-irrigated Young Populus tomentosa Plantations on Sandy Loam Soil

&lt;p&gt;&amp;#160; &amp;#160; &amp;#160;Understanding the rules of soil water movement under drip irrigation can provide data support and theoretical basis for developing precise drip irrigation strategies. In this study, a two-years-old &lt;em&gt;Populus tomentosa &lt;/em&gt;plantation under surface drip irrigation on sandy loam soil was selected to measure the dynamics of soil water potential (&lt;span&gt;&lt;em&gt;&amp;#968;&lt;/em&gt;&lt;/span&gt;&lt;em&gt;&lt;sub&gt;s&lt;/sub&gt;&lt;/em&gt;), wetting front and soil water content (&lt;span&gt;&lt;em&gt;&amp;#952;&lt;/em&gt;&lt;/span&gt;) during irrigation and water redistribution periods were investigated in field experiments. Then, the observed data in the field were used to evaluate the accuracy and feasibility of the HYDRUS-2D/3D model for simulating the short-term soil water movement. Besides, the validated model was used to simulate the dynamics of wetting front under different initial soil water content (&lt;span&gt;&lt;em&gt;&amp;#952;&lt;/em&gt;&lt;/span&gt;&lt;em&gt;&lt;sub&gt;i&lt;/sub&gt;&lt;/em&gt;). During irrigation, the variation of &lt;span&gt;&lt;em&gt;&amp;#968;&lt;/em&gt;&lt;/span&gt;&lt;em&gt;&lt;sub&gt;s&lt;/sub&gt;&lt;/em&gt;, horizontal and vertical movement distances of the wetting front, and &lt;span&gt;&lt;em&gt;&amp;#952;&lt;/em&gt;&lt;/span&gt; within the wetting volume with irrigation duration could be described by the logistic function (&lt;em&gt;R&lt;sup&gt;2&lt;/sup&gt;&lt;/em&gt; = 0.99), the logarithm function (&lt;em&gt;R&lt;sup&gt;2&lt;/sup&gt;&lt;/em&gt; = 0.99), the power function (&lt;em&gt;R&lt;sup&gt;2&lt;/sup&gt;&lt;/em&gt; = 0.82), and the polynomial function (&lt;em&gt;R&lt;sup&gt;2&lt;/sup&gt;&lt;/em&gt; = 0.99), respectively. At the end of irrigation, the horizontal and vertical movement distances of the wetting front reached 22.9 cm and 37.3 cm, respectively. The &lt;span&gt;&lt;em&gt;&amp;#968;&lt;/em&gt;&lt;/span&gt;&lt;em&gt;&lt;sub&gt;s&lt;/sub&gt;&lt;/em&gt; and &lt;span&gt;&lt;em&gt;&amp;#952;&lt;/em&gt;&lt;/span&gt; within the soil wetting volume were 61.6% and 30.9% higher than those at the start of the irrigation, respectively, but the &lt;span&gt;&lt;em&gt;&amp;#968;&lt;/em&gt;&lt;/span&gt;&lt;em&gt;&lt;sub&gt;s &lt;/sub&gt;&lt;/em&gt;decreased to its initial level about 120 hours later after the stop of irrigation. The average deviations of the horizontal and vertical wetting radius between the simulated and measured values were 1.3 and 4.5 cm, respectively. The mean RMSE and RMAE of HYDRUS-2D/3D for simulating &lt;span&gt;&lt;em&gt;&amp;#952;&lt;/em&gt;&lt;/span&gt; at the end of irrigation and during water redistribution were 0.021 cm&lt;sup&gt;3&lt;/sup&gt;&amp;#8729;cm&lt;sup&gt;-3&lt;/sup&gt; and 9.7%, respectively. The movement distances of wetting front in the experimental plantation under various soil drought degrees (soil water availabilities were 40%, 60%, 73% and 80%) were obtained through scenarios simulations using HYDRUS-2D/3D. And it was found that the wetting front could move further under higher &lt;span&gt;&lt;em&gt;&amp;#952;&lt;/em&gt;&lt;/span&gt;&lt;em&gt;&lt;sub&gt;i&lt;/sub&gt;&lt;/em&gt;, and the movement distance of the wetting front was always smaller in the horizontal direction than in the vertical direction under different &lt;span&gt;&lt;em&gt;&amp;#952;&lt;/em&gt;&lt;/span&gt;&lt;em&gt;&lt;sub&gt;i &lt;/sub&gt;&lt;/em&gt;conditions. Consequently, HYDRUS-2D/3D can be used to well simulate the short-term soil water movement in drip-irrigated young &lt;em&gt;P. tomentosa&lt;/em&gt; plantations on sandy loam soil. In addition, the constructed figure (describes the variations of the horizontal and vertical soil wetting distances with the irrigation duration) can be used to determine the reasonable irrigation duration for the plantations of &lt;em&gt;P. tomentosa&lt;/em&gt; and other tree species on sandy loam soil.&lt;/p&gt;