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.