**Zhigong Peng 1,2, Baozhong Zhang 1,2,\*, Jiabing Cai 1,2, Zheng Wei 1,2,\*, He Chen 1,2 and Yu Liu 1,2**


Received: 28 September 2019; Accepted: 10 December 2019; Published: 13 December 2019

**Abstract:** Due to the large spatial variation of groundwater depth, it is very difficult to determine suitable irrigation schedules for crops in shallow groundwater area. A zoning optimization method of irrigation schedule is proposed here, which can solve the problem of the connection between suitable irrigation schedules and different groundwater depths in shallow groundwater areas. The main results include: (1) Taking the annual mean groundwater depth 2.5 m as the dividing line, the shallow groundwater areas were categorized into two irrigation schedule zones. (2) On the principle of maximizing the yield, the optimized irrigation schedule for spring wheat in each zone was obtained. When the groundwater depth was greater than 2.5 m, two rounds of irrigation were chosen at the tillering–shooting stage and the shooting–heading stage with the irrigation quota at 300 mm. When the groundwater depth was less than 2.5 m, two rounds of irrigation were chosen at the tillering–shooting stage, and one round at the shooting–heading stage, with the irrigation quota at 240 mm. The main water-saving effect of the optimized irrigation schedule is that the yield, the soil water use rate, and the water use productivity increased, while the irrigation amount and the ineffective seepage decreased.

**Keywords:** crop model; water consumption; yield; water production function; irrigation schedule optimization

#### **1. Introduction**

In a shallow groundwater area, the groundwater is supplied to the aeration zone through capillary rise becoming soil water available to the crops. The interaction between soil water and groundwater varies due to the depth of groundwater [1–4]. For the sake of greater water economy, crop yield, and seeking the greatest advantage from the regulating effect of groundwater in soil water, scholars are particularly interested in the impact of different groundwater depths on crop growth. Kong et al. [5] studied the effect of different groundwater depths on crop growth using a lysimeter, finding that a depth of 1.5–2.5 m was conducive to crop growth, and when this depth was more than 2.0 m, the existent irrigation schedule was unable to meet the normal growth of crops. Kruse et al. [6] pointed out that in the areas with shallow groundwater depth, the groundwater recharge affected the water and the biological and chemical processes of the soil–plant–atmosphere continuum, and if no irrigation was provided, the optimal groundwater depth for winter wheat was about 1.5 m. Wang et al. [7] studied the effect of different groundwater depths on crop growth, showing that different groundwater depths led to differences in crop root distribution, which in turn affected the crops' water-yield

response mechanism. Zhang [8] using a lysimeter, studied the drought crops' groundwater utilization, suggesting that in the suitable groundwater depth, the groundwater used by drought crops accounted for 50% to 70% of the evapotranspiration. Yang et al. [9] using the HYDRUS software, simulated the influence of different groundwater depths on the irrigation quota of mulched, drip-irrigated cotton, finding that the drip irrigation quota was 330 mm, 450 mm, or 550 mm with the groundwater depth respectively at 1.5 m, 2.0 m, and 3.0 m. Zhuang et al. [10] studied the recharge effect of different groundwater depths on the cotton root layer, pointing out that when the groundwater depth did not exceed 2 m, the cotton irrigation schedule should be developed with the consideration of the groundwater recharge. Wang et al. [11] studied the spring wheat recharge modes under different groundwater depths, showing that the recharge was the largest at the groundwater depth of 1.0 m, and there was basically no replenishment with the groundwater depth at 3.0 m or greater. Liu et al. [12] technically supported by a lysimeter with controlled groundwater depth, determined the deficit irrigation schedule for crops under different groundwater depths. Relevant researchers pointed out that groundwater action was particularly critical in the analysis of the soil–crop–atmosphere system water balance in an arid oasis [13–18]. The deficit irrigation schedule in shallow groundwater areas could improve groundwater utilization but limited the influence on yield [19–21]. Karimov et al. [22] pointed out that a shallower groundwater depth promoted phreatic evaporation.

In summary, the proportion of phreatic evaporation varies notably with groundwater depth. To be rational, an irrigation schedule should fully consider the groundwater recharge under different groundwater depths. Previous studies on the effect of different groundwater depths on crop growth were mainly based on the controlled groundwater table by a lysimeter, with the groundwater table remaining unchanged during the whole crop growth period, which is not in line with the actual situation, because there is significantly daily variation in the groundwater table throughout the crop growth period. Therefore, the studies based on controlled groundwater table can hardly represent the actual change of groundwater depth throughout the crop growth period. The studies on the effect of different groundwater depths on crop growth, irrigation amount, and irrigation schedule optimization are mostly based on experiment stations; however, in shallow groundwater depth areas the groundwater table varies greatly from place to place. Therefore, the problem of how to apply the experimental results to a large expanse of areas in urgent need of a solution.

Hetao Irrigation District is the largest gravity irrigation district by water diverted from the Yellow River. According to the overall water allocation plan of the Yellow River watershed, the quota of water diversion to Hetao Irrigation District has decreased from 5.18 <sup>×</sup> <sup>10</sup><sup>9</sup> <sup>m</sup><sup>3</sup> to 4.00 <sup>×</sup> <sup>10</sup><sup>9</sup> m<sup>3</sup> . This ever-decreasing diversion will gravely affect the grain production in the irrigation district, making the conflict between supply and demand even more serious [23,24]. After the implementation of water-saving projects in the Hetao Irrigation District, the amount of water diversion for agricultural purposes has been cut notably. The result is that the groundwater table has been falling year on year [25]. Li et al. [26] pointed out that the spatial variation of groundwater depth was great in the Jiefangzha Region, and in the well irrigation area, the groundwater table was of a funnel shape with a groundwater depth more than 2.5 m, and in some localities, the groundwater table exceeded 4.5 m. It can be seen that with dwindling water diversion from the Yellow River and the growing well irrigation area, the spatial difference in groundwater depth is increasing.

The findings of previous studies on the crop irrigation schedule in Hetao Irrigation District were based on groundwater table at experiment stations in specific years. The results from experiment stations can hardly reflect the great difference in groundwater depth throughout the irrigation district, and so the application of related findings to a larger area has great limitations. Spring wheat is one of the main grain crops in Hetao Irrigation District, and wheat production plays an important role in grain production in this district. As spring wheat in Hetao Irrigation District grows in the dry season, irrigation is the key to its high yield. In Hetao Irrigation District, the net irrigation quota of spring wheat has been cut to about 300 mm. In shallow groundwater depth areas, it is difficult to maximize the use of the soil water in the soil and thus the water use efficiency is low. In areas with greater

groundwater depth, the groundwater recharge is reduced, resulting in the water deficit during certain growth stages.

Compared with the field experiments, studying crop water consumption characteristics based on models has benefits such as the freedom from geographical restrictions, time and financial efficiency, and additional system observables. In addition to the above, it is also possible to remove some interference factors, thus helpful to expose some behaviors among variables. Therefore, technically built on a verified crop growth simulation model, this study investigates the water-yield response mechanism of spring wheat for different groundwater depths, and constructs a spring wheat water production function for each zone. From the above information, an optimization method of zoning irrigation schedule is developed, which solves the problem of groundwater spatial variability in shallow groundwater areas. It is hoped that this study may provide some useful reference for the optimization of irrigation schedules in shallow groundwater areas.

## **2. Materials and Methods**

## *2.1. Study Area*

The experiment was carried out from March 2015 to July 2016 in the Jiefangzha Region of Hetao Irrigation District, Inner Mongolia. The Jiefangzha Region is at N 40◦320–N 41◦110 , E 106◦510–107◦230 , and its elevation varies between 1030–1046 m. Most of the irrigation area is located within the jurisdiction of Hangjinhouqi of Inner Mongolia Autonomous Region. The Jiefangzha Region, with a controlled area of 21.57 <sup>×</sup> <sup>10</sup><sup>4</sup> hm<sup>2</sup> and an irrigated area of 14.21 <sup>×</sup> <sup>10</sup><sup>4</sup> hm<sup>2</sup> , is the second largest in Hetao Irrigation District. This irrigation area has a comparatively flat terrain, and the overall terrain, high in the southwest and low in the northeast, has an average slope of about 0.02%. The Jiefangzha Region is featured by the arid or semiarid climate. The average annual precipitation and evaporation from a free water surface are 140 mm and 2096 mm respectively, and the annual average temperature is 9 ◦C. The average annual sunshine hours are 3181 h, the frost-free period is 130–150 d, and the annual average groundwater depth is 1.86 m. According to the American soil classification system, the soil of this irrigation area is dominated by silt loam. Table 1 summarizes the soil's physical properties in the study area.


**Table 1.** Soil physical properties in the study area.
