Introduction

Surface irrigation has been predominantly used for field crops in agriculture area in China, and other arid and semi-arid regions of world (Khan et al., 2014; Lecina et al., 2011; Li et al., 2020; Merchán et al., 2013; Nagaraj, 1999; Wu et al., 2016). According to the estimations, at least 300 million hectares are irrigated worldwide today, about more than five times in the early twentieth century (FAO, 2011). Although irrigation has helped boost agricultural yields and outputs, in many arid and semi-arid regions, surface water from the river diversion has markedly raised groundwater tables, salinized soils and reduced water quality due to poor irrigation management (Ren et al., 2016; Scanlon et al., 2007). Therefore, it is essential for better understanding the hydrologic mechanisms related to soil water fluxes (e.g., evaporation, transpiration, infiltration, and deep percolation) by surface irrigation.
Water infiltration into the soil is the principal means for replenishment of moisture and recharge to groundwater. Previous studies suggested that moisture movement in the unsaturated zone was controlled by the rate and duration of irrigation, the antecedent soil moisture conditions, the water table depth, the vegetation cover and the soil type (Dahan et al., 2007; Kumar, 2003; Min et al., 2018; Ochoa et al., 2009; Teshome et al., 2018; Wang et al., 2012; Zheng et al., 2019). Therefore, surface irrigation has a strong influence on soil water movement and groundwater recharge. This kind of scenario is typically found in the Yellow River irrigation area of Yinchuan Plain, China. In this agricultural area, surface irrigation often exceeds plant consumptive demand and excess irrigation percolates below the root zone and ultimately joins the shallow aquifer (Qian et al., 2012). There are increasing water problems in this region, including the change in the water table (Cui and Li, 2012; Wei, 2013), soil and groundwater salinization (Zhang et al., 2009; Zhang et al., 2015), the contamination of water by nitrate derived from nitrogen fertilization (Jing et al., 2015; Ke et al., 2014). However, little interest has been paid to the movement of the percolating water and seasonal recharge to groundwater in different land covers. Thus, there is still a need to investigate the dynamics of soil water regime under surface irrigation.
When the importance of the soil water movement in an agriculture ecosystem is considered, it is obvious that one measurement is not appropriate to reflect the vertical profiles. Therefore, a variety of methods to be taken are of great consequence for knowing how soil water changes through the profile and recharges to groundwater in time. Some of these methods involve measurements of soil matric potential (SMP) and soil water content (SWC), changes in the groundwater level, and the use of isotopes to reveal water flux. The SMP drives liquid water movement in the soil. It has been a useful way to describe water movement (Scanlon and Goldsmith, 1997; Scanlon et al., 2005), to calculate evaporation (Liu and Zhan, 2017; Wang et al., 2012), and to study the effects of irrigation on salt leaching (Wang et al., 2019) in Groundwater-Soil-Plant-Atmosphere-Continuum system. The SWC measurement in the unsaturated zone is a common method for quantifying soil water recharge (Ochoa et al., 2009), providing useful information on effects of land use changes (Scanlon et al., 2007) and estimating infiltration process and deep percolation (Dahan et al., 2009; Dahan et al., 2007; Gutiérrez-Jurado et al., 2017; Ochoa et al., 2007). Additionally, isotopes tracers can extend our understanding of evaporation rates (Allison and Barnes, 1983; Barnes and Allison, 1988; Mahendrappa et al., 1966; Robertson and Gazis, 2006), the extent of soil water recharge (Garvelmann et al., 2012; Sprenger et al., 2018), residence times of water along a soil profile (Gazis and Feng, 2004), the infiltration and groundwater recharge (Bengtsson et al., 1987; Song et al., 2009; Tan et al., 2017; Wang et al., 2012), root water uptake patterns (Dawson and Ehleringer, 1991; Ma and Song, 2016; Rothfuss and Javaux, 2017; Wang et al., 2010; Zhao et al., 2018). In summary, the combined physical and isotopic techniques have been proven useful in investigating vertical water movement in the unsaturated zone and identifying the recharge mechanism to groundwater.
In this study, a time series of precipitation, SMP, SWC, water table depth with stable isotope tracers (δ18O and δD) was intensively collected in the Yellow River irrigation area of Yinchuan plain, China (Fig. 1). The objectives of this work are therefore to: (1) describe the different characteristics of soil water movement in the vegetation season under two land covers (bare ground or maize), (2) assess the impacts of infiltration from precipitation or surface irrigation on soil water, (3) estimate the soil water storage change and recharge to groundwater. These analyses are also important to policy makers, particularly in water scarcity contexts when changes in water management are planned to reduce water use in farmland and restrain the rise of groundwater level in the irrigation area.