Discussion
Impact of infiltration from precipitation or flood
irrigation on soil water
The soil water is significantly influenced by the precipitation,
evapotranspiration, and infiltration, showing that the SWPs and SWCs
increase during the irrigation /precipitation period and decrease during
the non-irrigation period (Fig. 3 and
4). The SWCs and SWPs before and after the 59.2 mm large rain event
during the period 25-27 June 2019, the 68mm irrigation on 3 August 2019,
the 137mm irrigation on 6 July 2020 and the 122m irrigation on 1 August
2020, were used to evaluate the process of precipitation /irrigation
infiltration and soil water movement (Fig. 4).
At the surface layer (0-50 cm), it was generally found that the SWCs
increased from the range of 0.18-0.33 to the range of 0.25-0.37 after
the large rain. The average SWP at 0-50cm increased from -120 to -56 cm
and the values below a depth of 50 cm were relatively stable. Similarly,
soil water isotopic compositions were depleted in δD and
δ18O in soil layers above 50 cm in response to the
large rainfall event. The average value δD at 0-50cm ranged from −37.8‰
to −53.4‰ and δ18O from −6.17‰ to −7.64‰,
respectively. Main changes in the SWCs, SWPs and isotopic compositions
at 0-50cm depth during precipitation and non-precipitation periods was
greater than those at other depths. Therefore, it can be identified that
the soil at 0-50 depth with bare ground surface is mainly influenced by
precipitation infiltration and evaporation to a greater extent compared
with below. These results correspond to the findings of Wang et al.,
(2012), who found that the depth mainly affected precipitation
infiltration and evaporation was 0-50cm in Yuncheng, Shanxi Province.
The SWCs exhibited a sharp increase when irrigation began and then
decreased rapidly as it was turned off. The increase in SWCs occurred
layer by layer from the upper horizons, suggesting piston flow was the
dominant mode of irrigation-driven soil water flow. The soil water δD
and δ18O in 2019 and 2020 had almost the same range as
irrigation water, which indicated rapid infiltration of soil water
across the entire profile to irrigation (Fig. 4). After 6 days of
irrigation, the SWCs at 20 and 30 cm depth decreased by 0.053 and 0.039
in August, 2019 and 0.058 and 0.032 in August, 2020, respectively. For
the same interval of time the SWCs at 70 and 100 cm depths of soil only
decreased by 0.014 and 0.006 in 2019 and by 0.009 and 0.022 in 2020,
respectively. These differences may be the fact that water at the top
part continued to move down the profile. The middle soil was leaching
water to the soil below it but at the same time it was receiving water
from the soil above it, which resulted in the slower rates of SWCs
decrease at 70 and 100cm depth than those at the top part (20 and 30cm).
The similar patterns of changes in SWCs were observed in irrigated
agricultural field of a desert oasis in Northwest China (Li et al.,
2018).
Owing to the root water uptake in 2020, the SWPs variation mainly
occurred in the upper 50 cm layer during the jointing period because of
evapotranspiration. The SWPs at 20, 30 and 50cm showed a remarkable
decrease with time, ranging from the average value of -204.3cm
H2O on 28 June to -336.5 cm H2O on 6
July. During the tasseling period, the SWPs at 70 and 100 cm were found
to exhibit abruptly lower values than those on the jointing period. The
SWPs of 70 and 100cm soil depths continuously decreased with average
value from -213.4cm H2O on 6 July to -400.3 cm
H2O on 1August. The results indicate that some of the
maize root system has reached the depth of 100cm during the tasseling
periods due to the increased water consumption. The maximum root depth
was similar to that in Beijing described in the related research (Ma and
Song, 2016).
Since the soil water at 0-50 cm for bare ground surface is mainly
affected by precipitation, evaporation, the SWP and SWC at these depths
have relatively major changes with time (Figs. 3 and 4). The isotopic
compositions of the soil water samples collected from 20,30 and 50 cm
depths vary more diverse with the isotopic values of rain than those
from other depths. Those implied that ordinary rainfall events and even
continuous rain with 59.2 mm rain rarely affected the soil water in the
deeper layers.
Estimation of soil water storage change and recharge to
groundwater
For the observation period, the
soil
water storage, evapotranspiration /soil evaporation, infiltration
(irrigation plus precipitation), deep percolation and groundwater
capillary rise in the different land cover types between May and
September was calculated using the water balance
Eq. (1), and the results are presented
in Table 5.The soil water storage
change differed for bare ground in 2019 and for maize in 2020 about the
amount and temporal development. The
soil water storage (SWS)in 2019 firstly decreased from 7 may to 11 June
(S1 and S2 stage) and then increased from 12 June to 5 September (S3, S4
and S5 stage) and lastly decreased from 6 to 18 September (S6 stage).
The increasing SWS may result from the downward movement of the soil
water from infiltration of irrigation/ precipitation and the depletions
of SWS could be due to the soil evaporation. The infiltration was the
main water input component. The highest (183.3 mm) and lowest (7.8 mm)
volumes of water occurred at S5 and S1 stage, respectively. The
capillary rise from groundwater accounted for only 2.2% of the total
inputs in the balance, which varied temporally according to groundwater
level. In terms of outputs, the
deep
percolation (312.0 mm) was the main component throughout the entire
study period, accounting for 57.4% of the total outputs. The lowest
value was recorded at S1 stage (3.1 mm, 0.6% of outputs), while the
highest occurred at S5 stage (183.3 mm, 24.1% of outputs). As expected,
deep percolation increased with the expansion of infiltration, from
66.1mm at S3 stage to 131.0 mm at S5 stage. The soil evaporation
(231.2mm) corresponded to 42.6% of the outputs in the entire study
period of 2019. Although the highest
soil evaporation (69.3 mm) was recorded at S4 stage, the infiltration
was not largest at this stage. The data shows that the soil evaporation
is not only affected by the SWS, but also by the other factors,
such
as temperature, wind speed etc.
Temporal dynamics of the SWS was
different for maize in 2020 compared with bare ground in 2019. Field
water balance components over the growing season status was calculated
(Table 5). The SWS decreases during the three periods (at the sowing,
tasseling, and filling stage) due to the excess of evapotranspiration
relative to the irrigation/precipitation infiltration. The largest
depletions of the SWS (-58.1mm) could be observed at tasseling stage,
which is likely caused by increasing maize water use during the growing
season. The infiltration and capillary rise were 535.9mm and 8.7mm,
respectively, which were similar with that in 2019. However, the highest
value of infiltration was 150.1 mm recorded at seedling stage which was
smaller than that at S5 stage in 2019. The total deep percolation was
156.6mm, accounting for 29.2% of the total infiltration precipitation
or irrigation. The deep percolation was lowest (3.3mm) at the sowing
stage and highest (41.4mm) at tasseling stage. The ET in the maize field
was larger than deep percolation, which suggests that
the water lost by ET in the
irrigated areas was the greatest output component in the water balance.
The total ET for maize was 431.3mm, which was very similar to that found
by other researchers (Ren et al., 2016; Xu et al., 2013). In addition,
the roots of maize
at tasseling stage were mainly
distributed in the depth range of 0–100 cm, where soil water
consumption intensity was the highest over observation period.
Therefore,
the
maize development and grain yield will consume more soil water.
As already mentioned, the depth of infiltration from precipitation was
approximately 50 cm, indicating that the infiltration of precipitation
cannot be attributed to groundwater recharge. However, the groundwater
level response following surface irrigation was obtained. Combined with
the analysis of the water table fluctuations, it can be reasonably
concluded that the groundwater may be recharged by the lateral
groundwater flow leaked from the ditch and the infiltration of surface
irrigation. The fast
leakage
of ditch resulted in maximal groundwater level response following most
irrigations. For example, the
water table response occurred on 1 July 2019, with a maximum rise of 16
cm to the beginning of irrigation (Fig. 2). The water level before the
start of irrigation was about 360 cm below ground level and it was
observed that a significant water level rise (114cm) started about 22 h
after the end of irrigation. The observed rise rates of the water level
are approximately 9.5cm/h. However, the small deep percolation was only
30 mm during the irrigation which is the smallest recharge into
groundwater. It can be speculated that the recharge from the leakage of
ditch is indeed responsible for the groundwater level rise on this
basis.