An analysis on the influence of precipitation infiltration on groundwater under different irrigation conditions in the semi-arid area

The West Liaohe Plain is a typical semi-arid area, where the process of rainfall infiltration to replenish groundwater is a key link in its vertical hydrological cycle. In this paper, we compare and analyze the impact upon soil moisture movement and water infiltration after the shift of irrigation method from flood irrigation to mulched drip irrigation under mulch through setting up in-field in-situ observation points and carrying out groundwater depth dynamic observation. The results show that compared with mulched drip irrigation under mulch, flood irrigation has a stronger response to rainfall infiltration and a quicker response time in the rise of underground depth. With the decrease of groundwater level, the effect of rainfall infiltration to replenish groundwater is significantly weakened. In the flood irrigation area, the groundwater depth at about 8 m already has no obvious response to a small amount of rainfall. However, the groundwater depth at 6 m in the area of mulched drip irrigation under mulch already has no response to rainfall. Therefore, when groundwater extraction is carried out in irrigation areas, reasonable groundwater extraction levels should be designated in light of different irrigation methods to maintain the sustainable utilization of groundwater.


INTRODUCTION
The impact of human activities and of climate change is exacerbating the contradiction between the supply and demand of water resources in various regions (Xia & Shi ). As the contradiction becomes more and more prominent, human beings continue to develop towards high-efficiency water-saving agriculture. At the same time, when water-saving measures are contributing to the increase of crop yield, we should also pay attention to the impact that these water-saving measures will exert on groundwater replenishment in related regions. For instance, with the popularization of refined irrigation measures such as sprinkler irrigation and micro-drip irrigation, measures like concrete lining adopted in various levels of the channel system in the water delivery and distribution system have

Study areas
The West Liaohe Plain is located in the eastern part of the Xiliao River Basin, with its major part in Tongliao City, and lies between 41 05 0 to 45 13 0 N and 116 10 0 to 123 35 0 E. Its area is approximately 65,000 km 2 . Within this region, the annual average precipitation is less than 380 mm, and the annual average water surface evaporation is more than 1,000 mm. Typically characterized with semi-arid features, this area normally has no direct surface runoff after rainfall.
Vertical precipitation infiltration is an obvious phenomenon here, the groundwater is abundant and vertical recharge for groundwater is stable. As a relatively intact watershed within the farming-pastoral zone in the semi-arid area, the West Liao River has hydrological cycle characteristics that typically belong to the farming-pastoral zone in the semiarid area. Its plain area fully exhibits the rainfall-evaporationinfiltration cycle, especially the process of rainfall infiltration to replenish groundwater. In recent years, the irrigation mode in the West Liaohe Plain has shifted from flood irrigation to mulched drip irrigation under mulch. Therefore, this paper takes the West Liaohe Plain as an example to study the impact of changes in irrigation method upon groundwater in a semi-arid farming-pastoral zone ( Figure 1).
With the continuous expansion of the irrigated area, the increase in the number of irrigation wells, and the over-  part of the water is lost and the amount of infiltrated water is thus reduced; second, due to refined irrigation settings, irrigation return water will no longer be generated. This reduces the potential replenishment of groundwater, and thus affects the hydrological cycle process.

Design and measurement
In this paper, South Tallin  The soil parameters are shown in Table 1.
HOBO-U30 is adopted to automatically monitor the dynamic situation of soil water content in the section The data are recorded at an interval of 2 hours. The specific arrangement of the probes for the two irrigation methods is shown in Figure 3. The rainfall data is from the weather station in the monitoring area. As shown in Table 2, a total of 7 rainfalls has been recorded, of which the maximum rainfall was recorded as 72 mm on August 3.

Changes in soil moisture content
As shown in Figure 4, in terms of soil moisture content, the flood irrigation area responds more quickly to rainfall than the area of mulched drip irrigation under mulch in general.
In Under the condition of a rainstorm, for instance a rainfall of 72 mm on August 3: in the flood irrigation area, the soil moisture content at the depth of 50 cm increased by 7% two hours after the rainfall, the soil moisture content at the depth of 100 cm increased by 4.1% four hours after the rainfall, and the soil moisture content at the depth of    It is worth noting that after the rainstorm, a rainfall of 32.5 mm occurred in the study area on August 10. As the interval between the two rainfalls was only one week, the soil moisture content at the depth of 2 m increased by 1.4% 14 days after the rainstorm; in the area of mulched drip irrigation under mulch, however, the soil moisture content at the depth of 2 m increased by 0.9% 19 days after the rainstorm, which proves that under the condition of a rainstorm, both the rainfall infiltration depth and the infiltration replenishment amount in the flood irrigation area are greater than those in the area of mulched drip irrigation under mulch.

Changes in groundwater depth
There shows that under the same rainfall conditions, the shallower the groundwater depth, the more severe the impact of rainfall infiltration on the groundwater. As the groundwater depth deepens, the groundwater level rises only when heavy monthly rainfall occurs, and the greater the rainfall is, the earlier the groundwater level rises.
In comparison, more dramatic response to rainfall infiltration and quicker response time for groundwater depth to rise occur in flood irrigation than in mulched drip irrigation under mulch. In the flood irrigation area, the groundwater It is shown that for the West Liaohe Plain, where flood irrigation is a major means of irrigation, the promotion of mulched drip irrigation under mulch will exert a significant impact on rainfall infiltration and the quantity of irrigation return water, and change the critical depth of rainfall infiltration replenishment. On the one hand, there will be basically no irrigation return water generated, and on the other hand, the shift will affect rainfall infiltration and replenishment of groundwater, a key process of the hydrological cycle, both of which will affect the balance of extraction and replenishment of underground water. Therefore, appropriate groundwater extraction levels should be set for flood irrigation and mulched drip irrigation.

CONCLUSIONS
For the West Liaohe Plain, which is a typical semi-arid area, the process of rainfall infiltration to replenish with groundwater is a key link in its vertical hydrological cycle. With different irrigation methods, the replenishment depth of groundwater through rainfall infiltration is different. In In terms of the effect of rainfall infiltration to replenish with groundwater, flood irrigation has a stronger response to rainfall infiltration and has a quicker response time in the rise of underground depth compared with mulched drip irrigation under mulch. With the decrease of groundwater level, the effect of rainfall infiltration to replenish with groundwater is significantly weakened. It shows that in the flood irrigation area, the groundwater depth at about 8 m has already no obvious response to a small amount of rainfall. Only when the monthly rainfall exceeds 300 mm, the groundwater depth rises by 0.98 m one month after the rainfall. However, the groundwater depth at 6 m in the area of mulched drip irrigation under mulch has already no response to rainfall. Therefore, when groundwater extraction will be carried out in irrigation areas, reasonable groundwater extraction levels should be designated in light of different irrigation methods to maintain the sustainable utilization of groundwater.