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.

A Grey Seasonal Index Model for Forecasting Groundwater Depth of Ningxia Plain

Forecasting the depth of groundwater in arid and semiarid areas is a great challenge because these areas are complex hydrogeological environments and the observational data are limited. To deal with this problem, the grey seasonal index model is proposed. The seasonal characteristics of time series were represented by indicators, and the grey model with fractional-order accumulation was employed to fit and forecast different periodic indicators and long-term trends, respectively. Then, the prediction results of the two were combined together to obtain the prediction results. To verify the model performance, the proposed model is applied to groundwater prediction in Yinchuan Plain. The results show that the fitting error of the proposed model is 2.08%, while for comparison, the fitting error of the grey model of data grouping and Holt–Winters model is 3.94% and 5%, respectively. In the same way, it is concluded that the fitting error of groundwater in Weining Plain by the proposed model is 2.26%. On the whole, the groundwater depth in Ningxia Plain including Yinchuan Plain and Weining Plain will increase further.

Using geochemistry and environmental tracers to study shallow unconfined aquifer recharge and mineralization processes in the Yinchuan Plain, arid Northwest China

Irrigation water extracted from the Yellow River plays a key role in water resource management in the Yinchuan Plain (YCP), arid Northwest China. Investigating the soluble matters (ion and gas) of groundwater provides information to explain the unconfined shallow aquifer recharge and groundwater mineralization processes after long-term flood irrigation activity. Environmental tracing with the elements, 2H, 18O, 3H, and CFCs, combining geochemistry using major ions and selected trace elements, was conducted for 43 water samples from September to October 2019 in the YCP. Evaporite and silicate weathering dominate the shallow unconfined groundwater geochemical compositions. Water–rock interactions control the mineralization characteristics regularly along the groundwater flow paths from the southwest toward the northeast. Stable isotopes suggest that Yellow River water and precipitation in winder and/or from Helan Mountainous area are the main recharge sources. The shallow unconfined aquifer mixed young (post-1940) and old (pre-1940) water with young water ratios from 53.1 to 73.5% inferred from the CFC concentrations and 3H activities. Water reinfiltrations extracted from the Yellow River and from the old groundwater are confirmed. Lateral flow recharge for the shallow unconfined aquifer is less indistinctive than that from the water re-infiltration in the plain areas.

Interaction between Surface Water and Groundwater in Yinchuan Plain

The interaction of surface water (SW) and groundwater (GW) is becoming more and more complex under the effects of climate change and human activity. It is of great significance to fully understand the characteristics of regional SW–GW circulation to reveal the water circulation system and the effect of its evolution mechanism to improve the rational allocation of water resources, especially in arid and semi-arid areas. In this paper, Yinchuan Plain is selected as the study area, where the SW–GW interaction is intensive. Three typical profiles are selected to build two-dimensional hydrogeological structure models, using an integrated approach involving field investigation, numerical simulation, hydrogeochemistry and isotope analysis. The SW–GW transformation characteristics are analyzed with these models, showing that geological structure controls the SW–GW interaction in Yinchuan Plain. The SW–GW flow system presents a multi-level nested system including local, intermediate and regional flow systems. The runoff intensity and renewal rate of different flow systems are evidently different, motivating evolution of the hydro-chemical field; human activities (well mining, agricultural irrigation, ditch drainage, etc.) change the local water flow system with a certain impacting width and depth, resulting in a variation of the hydrological and hydro-chemical fields. This study presents the efficacy of an integrated approach combining numerical simulation, hydrogeochemistry and isotope data, as well as an analysis for the determination of GW-SW interactions in Yinchuan Plain.