Scholarly article on topic 'Response of the soil water content of mobile dunes to precipitation patterns in Inner Mongolia, northern China'

Response of the soil water content of mobile dunes to precipitation patterns in Inner Mongolia, northern China Academic research paper on "Earth and related environmental sciences"

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Journal of Arid Environments
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{"Horqin Sand Land" / "Rainfall characteristics" / "Soil moisture"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — S.X. Yao, C.C. Zhao, T.H. Zhang, X.P. Liu

Abstract We analyzed the relationship between soil water content (SWC) dynamics in mobile dunes to a depth of 100 cm and precipitation patterns from June to July 2010 in the Horqin Sand Land. The precipitation was dominated by small events of 0.1–3.0 mm, which accounted for 52% of the total events. Precipitation >20 mm had the highest intensity, accounting for 50% of the total precipitation. SWC differed significantly among the soil layers: mean SWC was greatest from 80 to 100 cm and lowest from 40 to 60 cm. SWC from 0 to 100 cm was significantly affected by relative humidity, water barometric pressure and minimum temperature, and the SWC of 0–40 cm was obviously influenced by precipitation amount and wind velocity. Precipitation <5 mm did not replenish SWC, precipitation between 5 and 20 mm provided some replenishment to SWC from 0 to 40 cm, and precipitation >20 mm increased significantly SWC from 0 to 100 cm. In addition, precipitation intensity significantly affected the infiltration rate, with higher intensity leading to deeper and faster infiltration. At longer intervals between precipitation events, SWC in each soil layer decreased continuously over time; however, SWC from 0 to 80 cm changed little within the first 3 days, and SWC from 0 to 100 cm started to decrease greatly after 5 days.

Academic research paper on topic "Response of the soil water content of mobile dunes to precipitation patterns in Inner Mongolia, northern China"

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Journal of Arid Environments

journal homepage: www.elsevier.com/locate/jaridenv

Response of the soil water content of mobile dunes to precipitation patterns in Inner Mongolia, northern Chinaq

S.X. Yao a b, C.C. Zhao b*, T.H. Zhang b, X.P. Liub

a Lanzhou City University, Lanzhou 730070, China

b Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, 320 Donggang West Road, Lanzhou 730000, China

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ARTICLE INFO

Article history: Received 20 August 2012 Received in revised form 5 March 2013 Accepted 3 June 2013 Available online 22 June 2013

Keywords: Horqin Sand Land Rainfall characteristics Soil moisture

ABSTRACT

We analyzed the relationship between soil water content (SWC) dynamics in mobile dunes to a depth of 100 cm and precipitation patterns from June to July 2010 in the Horqin Sand Land. The precipitation was dominated by small events of 0.1—3.0 mm, which accounted for 52% of the total events. Precipitation >20 mm had the highest intensity, accounting for 50% of the total precipitation. SWC differed significantly among the soil layers: mean SWC was greatest from 80 to 100 cm and lowest from 40 to 60 cm. SWC from 0 to 100 cm was significantly affected by relative humidity, water barometric pressure and minimum temperature, and the SWC of 0—40 cm was obviously influenced by precipitation amount and wind velocity. Precipitation <5 mm did not replenish SWC, precipitation between 5 and 20 mm provided some replenishment to SWC from 0 to 40 cm, and precipitation >20 mm increased significantly SWC from 0 to 100 cm. In addition, precipitation intensity significantly affected the infiltration rate, with higher intensity leading to deeper and faster infiltration. At longer intervals between precipitation events, SWC in each soil layer decreased continuously over time; however, SWC from 0 to 80 cm changed little within the first 3 days, and SWC from 0 to 100 cm started to decrease greatly after 5 days.

Crown Copyright © 2013 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Soil moisture is an important parameter of the hydrological cycle of terrestrial ecosystems (Bindlish et al., 2003; Schneider et al., 2008; Song et al., 2007), and plays a critical role in predicting and understanding various hydrologic processes, including weather changes, precipitation pattern, runoff generation, and irrigation scheduling (Puri et al., 2011). Soil moisture also is one of the most important ecological factors in sandy ecosystems, where its shows significant variation (Chen et al., 2011; Das et al., 2008; Entin et al., 2000; Mahmood et al., 2004; Wagner et al., 2003). Thus, decreasing soil water content (SWC) is a main driving force in the development of desertification (Berndtsson and Chen, 1994; Chen et al., 1996; Nash et al., 1991). Many researchers have described the responses of SWC to land use (Fu et al., 2000; Huang et al., 2009; Yao et al., 2012), terrain (Bergkamp, 1998; Berndtsson and Nodomi, 1996; Svetlitchnyi et al., 2003; Tomer and Anderson, 1995), and vegetation types (He and Zhao, 2002; van Rheenen

q This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

* Corresponding author. Tel.: +86 93 1496 7160. E-mail addresses: zhao_chch1978@163.com, zhao_cc@yahoo.cn (C.C. Zhao).

et al., 1995; Wilson and Kleb, 1996). Researchers have also found that rainfall characteristics such as amount, frequency, and intensity affect the temporal and spatial heterogeneity of SWC (Reynolds et al., 2004; Sala et al., 1992; Wilson et al., 2004).

The Horqin Sand Land lies in a semi-arid area of eastern Inner Mongolia, in northern China. Due to the long-term influence of heavy grazing, land reclamation for agriculture, and extensive harvesting of fuelwood, this region has become one of the most severely desertified areas in China (Zuo et al., 2008). In recent years, desertification has produced distinctive mobile dune landscapes in this region. The soil water conditions and their relationship with precipitation dynamics in such dune areas have become one of the most important areas of research on land-surface processes in arid China (Wang and Takahashi, 1999). However, despite the obvious importance of these processes in determining the effectiveness of efforts to slow or reverse desertification, no studies have described the dynamics of SWC and their relationship with the precipitation patterns in dune areas. In addition, SWC data obtained during most previous research was obtained at a daily scale or longer, so little SWC data is available at an hourly scale (Miller et al., 2007).

In the present study, we used hourly SWC data to describe the temporal dynamics of SWC in five soil layers in an area of mobile dunes in the Horqin Sand Land. Using this data, we firstly examined the relationships between SWC and the precipitation patterns,

0140-1963/$ — see front matter Crown Copyright © 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/jjaridenv.2013.06.003

including precipitation amount, intensity and duration. And then we assessed the changes in SWC over time in response to rainfall events with its pattern characteristics.

2. Materials and methods

2.1. Study site description

The study site is located in the southern part of the Horqin Sand Land in eastern Inner Mongolia, China (42°55'N, 120° 42'E, 345 m a.s.l.). The landscape in this area is characterized by sand dunes alternating with gently undulating lowland areas. The soils are sandy, light yellow and loose in structure. The main soil type is classified as an Arenosol in WRB (ISSS, ISRIC and FAO) (Deckers et al., 1998). The climate is temperate, semi-arid and continental, receiving 360 mm annual mean precipitation, with 75% of this falling in the June-September period, versus a mean annual potential evaporation of 1935 mm. The mean annual temperature is 5.8-6.4 ° C, with a minimum mean monthly temperature of -12.6 to -16.8 °C in January and a maximum mean monthly temperature of 20.323.5 ° C inJuly. There was little rain or snow during the late autumn— early spring period at which was coupled generally with frequent strong winds. The period from November to May was therefore considered as the major wind-erosion season (Li et al., 2005). Based on wind data (1997—2002) from the weather station of the Naiman Desertification Research Station, the annual mean wind speed ranges between 3.4 and 4.1 m s-1 at 2 m height, and the prevailing wind directions over the erosive season are S, SSW, SW, NNW, WNW, NW, N and NNE. The winds occur with high speeds in excess of 4 m s-1 at 2 m height (a threshold wind speed to initiate sand movement; Zhang et al., 2004) in most days of the erosive season. As severely wind-erosion, soil particles of mobile dunes are mainly concentrated in 2—0.1 mm and 0.1—0.05 mm (Su and Zhao, 2003).

2.2. Meteorological data

All meteorological data during the study period, including hourly precipitation data, daily maximum temperature, minimum temperature, relative humidity, net radiation, water barometric pressure and wind velocity were obtained from an automatic meteorological station less than 60 m from the study site.

2.3. SWC and precipitation data

All the SWC data were collected from the water cycle research field at the Naiman Desertification Research Station. This site consisted of 23 concrete basins, each 2 x 2 x 2 m3, that were constructed in the summer of2009. Three of the basins were filled with sandy soil from mobile dunes near to the station. In each basin, we installed five soil moisture Minitrase (6050X3K1, ICT, USA) to automatically measure the volumetric soil water content (SWC, %) in five soil layers to a depth of 100 cm, at 20-cm intervals. The SWC data was recorded hourly starting in the spring of 2010. The dataset

used in this paper extended from 24:00 on 4 June 2010 to 07:00 on 24 July 2010. No data was recorded during the maintenance period for the instruments (from 08:00 to 18:00 on 17 July). Because of instrument malfunctions, there was also no data for the soil layer from 40 to 60 cm from 01:00 on 13 July to 18:00 on 17 July. The sample size was 1070 records for the layer from 40 to 60 cm and 1173 for the other soil layers. The SWC data of each soil layer was the average of three basins.

2.4. Soil properties

For each basin, three undisturbed soil samples from 20, 40, 60, 80 and 100 cm were taken using a cylindrical metal core with a volume of 100 cm3. Soil bulk density was firstly measured using the volume—mass relationship, and a same soil sample nearby was then to determine other basic soil properties. Soil organic matter content was determined using the K2Cr2O7—H2SO4 wet oxidation method. Soil particle size distribution was determined by the dry sieving method. As the Horqin Sand Land is a severely wind-eroded region, Soil particles are mainly concentrated in 0.25—0.1 and 0.1 — 0.05 mm (Su and Zhao, 2003). So, soil particle size fraction in this study was divided into three groups: coarse sand (2—0.1 mm), fine-sand (0.1—0.05 mm) and clay and silt (<0.05 mm). Soil saturated hydraulic conductivity (Ksat) of each soil layer was determined in situ using a Guelph Prememeater (2008KI, Santa Barbara, CA93105, USA). The data for bulk density, organic matter content, soil saturated hydraulic conductivity and distribution of particle size fractions were averages of the three replicates for each basin (Table 1).

2.5. Data analysis

The primary statistical analysis was performed using version 11.5 of the SPSS software (SPSS Inc., Chicago, IL, USA), and differences in SWC between the five soil layers was tested for significance using one-way ANOVA. When the ANOVA results revealed significant differences, we used the Post Hoc Tests to identify significant differences between pairs of values, P-values of 0.05 were considered as a significant.

3. Results

3.1. Precipitation patterns

From 24:00 on 4 June 2010 to 07:00 on 24 July 2010, 23 precipitation events were recorded, and amounted to 109.4 mm in total. Among the 23 events, the minimum precipitation was 0.2 mm and the maximum was 34.4 mm. Most of the precipitation comprised small precipitation event, with <3.0 mm of precipitation (Fig. 1), and the events with >6.0 mm were a few. We classified the amount of precipitation into five levels: 0.1 to 3.0, 3.1 to 6.0, 6.1 to 10.0, and 10.1 to 20.0 and >20.0 mm. The number of precipitation events was 12 for event with 0.1—3.0 mm, accounting for 52% of all precipitation events; in addition, there were 7 events with 3.1 —

Table 1

Soil physical and chemical properties at the five soil depths in mobile dune.

Soil depth (cm) Bulk density (g cm 3) Total porosity (%) Organic matter content (%) Soil saturated hydraulic conductivity (mm/min) Soil particle size distribution (%) 2—0.1 mm 0.1—0.05 mm <0.05 mm

0—20 1.51 43.16 0.47 5.32 97.41 2.20 0.06

20—40 1.50 43.22 0.40 6.53 97.45 2.14 0.07

40—60 1.51 43.01 0.47 5.19 97.28 2.24 0.07

60—80 1.51 43.02 0.36 7.52 97.12 2.43 0.08

80—100 1.53 42.37 0.42 4.90 96.91 2.43 0.08

Mean 1.51 42.96 0.42 5.89 97.23 2.29 0.07

60 50 40 30 20 10

I No. of precipitation events ] Total precipitation - Precipitation intensity

25 '-3

0.1~3.0 3.1~6.0 6.1~10.0 10.1~20.0 Category of precipitation (mm)

Fig. 1. Precipitation characteristics during study period.

6.0 mm, 2 events with >20.0 mm, and 1 event each with 6.1 — 10.0 mm and 10.1—20.0 mm. The precipitation amounts in each category (from smallest to largest) were 7.6, 29.0, 6.4, 11.2, and 55.2 mm, accounting for 7%, 27%, 6%, 10% and 50% of the total precipitation, respectively. Among these precipitation events, the intensity was highest for precipitation >20.0 mm (27.6 mm h-1),

') for the 0.1-3.0 mm

and the intensity was lowest (0.6 mm h category.

3.2. Dynamics of SWC

3.2.1 Distribution of SWC in the soil profile

ANOVA revealed statistically significant differences (P < 0.05) in SWC among the soil layers (Table 2). In addition, SWC varied greatly in the mobile dunes, with a coefficient of variation (CV) ranging from 9 to 15%. SWC was highest from 80 to 100 cm and lowest from

40 to 60 cm. The 95% confidence interval for SWC indicated that when SWC was at a low level, the range of its values became narrower and stabilized. The maximum skewness and kurtosis of SWC all occurred in the 40—60 cm layer, whereas the minimum skew-ness and kurtosis occurred in the 60—80 cm layer. These results indicate that the SWC distribution had a sharper peak in the 40— 60 cm layer than that in a normal distribution, whereas the shape of the peak for the 60—80 cm layer was similar to that in a normal distribution. The maximum standard deviation and CV for SWC appeared in the 80—100 cm layer, followed by the 0—20 cm layer, with the minimum values of both parameters in the 60—80 cm layer.

3.2.2. Temporal dynamics of SWC

As precipitation increased, SWC also increased, particularly after heavy precipitation, and especially in the top soil layers (0—20 cm) (Fig. 2). The first two bigger precipitation amounts were 34.4 mm and 20.8 mm, recorded from 13:00 to 19:00 on 12 July and from 16:00 to 17:00 on 27 June respectively. And the SWC increased by 39.7% and 30.8%, respectively, after these precipitation events (Fig. 2). This indicated that the SWC of the study site's sandy soil depended strongly on the precipitation magnitude. The rate of change of SWC was consistent with the magnitude of the precipitation event, with a dramatic increase in SWC after heavy precipitation, but SWC gradually decreased thereafter due to evaporation until the next precipitation event occurred (Fig. 2).

3.3. Response of SWC to the precipitation patterns

3.3.1. Correlation between SWC and meteorological data

We calculated Pearson's correlation coefficient (r) between the SWC in the each soil layer with some meteorological factors (Table 3), including precipitation amount (P), maximum temperature (Tmax), minimum temperature (Tmin), relative humidity (RH), net radiation (£*), water barometric pressure (HB) and averaged

Table 2

The statistical characteristics of the SWC (%) and the data distribution for the five soil layers.

Soil depth (cm) Mean Max. Min. 95%Confidence interval S.D. CV(%) Skewness Kurtosis

Lower Upper

0-20 7.38b 14.90 6.20 7.32 7.45 1.05 14 1.86 8.13

20-40 7.28c 13.30 6.50 7.23 7.33 0.79 11 2.35 11.02

40-60 5.24e 8.90 4.60 5.20 5.27 0.59 11 2.84 11.04

60-80 5.95d 7.80 5.30 5.92 5.99 0.55 9 0.87 1.03

80-100 8.49a 13.00 4.80 8.42 8.56 1.24 15 0.88 1.79

Mean 6.87 11.60 5.48 6.82 6.92 0.84 12 1.76 6.40

Note: Means labeled with different letters differ significantly (the difference followed by the Post Hoc Tests, P < 0.05).

Time (Hour Day Month)

Fig. 2. Hourly dynamics of SWC (%) at mobile dune as a response to precipitation in three soil layers.

Table 3

Correlation coefficient between SWC in each soil layer and meteorological factors.

Soil depth (cm) Precipitation Maximum Minimum Relative Net radiation (MJ/m2) Water barometric Averaged wind

amount (mm) temperature (°C) temperature (°C) humidity (%) pressure (kPa) velocity (m/s)

0-20 0.29** -0.19 0.37** 0.73** -0.09 0.70** -0.53**

20-40 0.16** -0.15 0.39** 0.66** -0.06 0.66** -0.50**

40-60 0.03 -0.03 0.53** 0.52** -0.18 0.58** -0.15

60-80 0.01 -0.17 0.49** 0.59** -0.13 0.61** -0.27

80-100 0.01 -0.23 0.21 0.44** -0.05 0.38** -0.23

Note: *p < 0.05; **p < 0.01.

Table 4

Pattern characteristics of four precipitation events in precipitation duration, amount, and intensity.

Precipitation patterns

Rain time

01:00 to 03:00 17 June (event 1)

16:00 To 17:00 27 June (event 2)

13:00 to 19:00 12 July (event 3)

04:00 to 07:00 20 July (event 4)

Duration of precipitation (hr) Precipitation amount (mm) Precipitation intensity (mm hr1)

3.0 3.4

2.0 20.8 10.4

8.0 34.4 4.3

4.0 11.2 2.8

wind velocity (V). We found that there was a significant positive correlation between SWC and RH, HB (P < 0.01), and between Tmin and SWC except 80—100 cm. In addition, the SWC for layers from 0 to 20 and 20—40 cm each has a significant positive correlation with precipitation amount, and a significant negative correlation with wind velocity. These results showed that relative humidity, water barometric pressure and minimum temperature were the main factors to affect SWC, and precipitation amount and wind velocity also took some influence to SWC.

3.3.2. Variation of SWC during a rainfall event

We selected four precipitation events with different duration, amount, and intensity to analyze the details of the SWC changes after a rain. Table 4 summarizes the rainfall characteristics of these events. In order to analyze the variation of SWC during precipitation, we calculated the incremental of SWC at five soil layers in the four precipitation events. Let a0 be SWC at 1 h before precipitation, and a1 denoted the SWC at first hour after precipitation, a2 denoted the SWC at second hours after precipitation, and so on. And let n denote the hours of precipitation sustained. Hence, the incremental of SWC in a precipitation event was n= 1(ai - ai_1). The result indicated that SWC increased a few when the precipitation was 3.4 mm, and the precipitation with 11.2 mm provided some replenishment to the SWC only at 0—20 cm, but the SWC at 0— 60 cm has noticeable increasing when a precipitation was more than 20.0 mm (Table 5).

3.3.3. Response of SWC to the precipitation patterns

After analyzed the change of SWC after four precipitation events mentioned above, we found that the changes in SWC at profile with

Table 5

Variation of SWC (%) at each hour during precipitation. Positive sign represent an increase in SWC (%) compared with to the previous hour.

Soil Precipitation event

depth (cm) Event 1 Event 2 Event 3 Event 4

(P = 3.4 mm) (P = 20.8 mm) (P = 34.4 mm) (P = 11.2 mm)

0-20 0.0 5.3 4.7 1.7

20-40 -0.1 4.5 4.9 0.0

40-60 0.0 0.9 3.5 0.0

60-80 0.1 -0.1 0.0 0.0

80-100 0.0 0.0 0.0 0.0

a depth of 100 cm are noticeable for the highest precipitation amount and intensity (i.e. 34.4 mm h-1 in Fig. 3a). This suggested that at least 20 mm of precipitation would be required for rainwater to reach a depth of 100 cm at dunes and replenish the soil water (i.e., 20.8 mm h-1 in Fig. 3b). However, when the precipitation was 5—20 mm, its maximum infiltration depth was 40 cm only (11.2 mm in Fig. 3c), and precipitation of <5 mm exerted hardly to an influence on SWC at any depth (3.4 mm in Fig. 3d). This suggested that precipitation <5 mm may play a little role for replenishing soil moisture in the Horqin Sand Land. The precipitation events between 5 and 20 mm provided some recharge to a depth of up to 40 cm, but only the precipitation with >20 mm increased the SWC in deeper soil layers (>40 cm). This result was consistent with previous results in the Kubuqi desert (Wei et al., 2008).

Precipitation intensity controlled the infiltration rate more strongly than precipitation amount did do (Fig. 3a and b). When the precipitation intensity was 4.3 mm h-1, it took at least 5 h for the wetting front to reach the layer 20—40 cm (Fig. 3a). In contrasting, when the precipitation intensity was 10.4 mm h-1, it took only 1 h for the wetting front to reach the same layer (Fig. 3b), even though the former's amount was higher than that in the latter (34.4 mm vs. 20.8 mm). This indicated that stronger precipitation intensity leading to a shorter infiltration time. We also noted that SWC in the top layer (i.e. 0—20 cm) increased rapidly with an increasing precipitation amount, which showed a similar trend for precipitation intensity. The peak value of SWC in deeper layers (i.e. below 40 cm) always occurred later than the time of peak precipitation. Therefore, the response of SWC to precipitation may occur later at greater depths.

These results clearly indicate that soil moisture conditions in sand dunes depended strongly on the precipitation patterns. In other words, its change was not only just relying on the precipitation amount, but also on precipitation intensity and frequency. The recharge of precipitation into the various soil layers depended on both the precipitation amount and its intensity. For a given precipitation duration, a stronger precipitation intensity appears to a shorter the infiltration time. For a given amount of precipitation, a shorter duration results in a higher intensity, and this provides a greater increase in soil moisture (Wei et al., 2008).

3.3.4. Change of SWC as a function of the precipitation interval

Precipitation interval represents the time between two consecutive precipitation events. A longer interval usually means a

(a) Event 3 SWC (%)

0 3 6 9 12 15 —•—Before

..........precipitatio

¿3*40 &

Q60 80 100

-After 1 hr

—X—After 5 hr

After 8 hr

After 12 hr

After 24 hr

(b) Event 2 SWC (%)

0 3 6 9 12 15-

0 20 40 J60 80 100

-Before precipitatio n

-After 1 hr

—X—After 5 hr

After 8 hr

After 12 hr

After 24 hr

(c) Event 4 SWC (%)

0 3 6 9 12 15

Q60 80 100

-Before precipitatio n

-After 1 hr

—X—After 5 hr

After 8 hr

-After 12 hr

After 24 hr

(d) Event 1 SWC (%)

0 3 6 9

0 20 40 ^60 80 100

■ Before precipitatio n

- After 1 hr

—X— After 5 hr

-0— After 8 hr

■After 12 hr

After 24 hr

Fig. 3. Changes in soil water content (SWC) as a result of rainwater infiltration into the soil after some precipitation events, such as (a) event 3 (34.4 mm), (b) event 2 (20.8 mm), (c) event 4 (11.2 mm), and (d) event 1 (3.4 mm). Data are plotted for the depth at the bottom for each layer (e.g., at 20 cm for the 0—20 cm layer).

period of drought. We analyzed the changes in SWC as a function of precipitation interval to explore the changes in SWC after a precipitation event. During this study period, there were five intervals lasting at least 72 h (Table 6). Generally, the mean change rate in SWC was low, for example, its maximum was 0.52% per h with the most values less than 0.2% per h. SWC changed little when the precipitation interval was during the first 72 h, and its change rate increased with an increasing duration, and reached a maximum value at 120 h and decreased thereafter, probably because soil matric tension held the remaining water more strongly due to SWC was below a threshold level. SWC changed most rapidly in the 020 cm and 80-100 cm layers, and decreased least in the 40-60 cm layer.

Table 6

Mean hourly change in SWC (%) between precipitation events. Negative sign represent a decrease in SWC (%) compared with to the previous hour.

Time period

Duration Soil depth (cm) (hours)

0-20 20-40 40-60 60-80 80-100

5 June—12 June 192 -0.13 -0.01 -0.03 0.00 -0.04

18 June—20 June 72 0.00 0.00 0.00 0.00 -0.03

23 June—26 June 96 -0.05 0.00 0.00 -0.03 -0.03

2 July—6 July 120 -0.50 -0.18 -0.04 -0.08 -0.20

15 July—19 July 120 -0.14 -0.18 0.00 -0.18 -0.52

Mean 120 -0.16 -0.08 -0.01 -0.06 -0.16

4. Discussion and conclusions

The precipitation pattern was dominated by small precipitation events in our study area. Precipitation with 0.1—3.0 mm accounted for 52% of the total numbers of rainfall events, but only accounted for 7% of the total precipitation amounts during this study period. Precipitation events >20 mm were a little, but accounted for 50% of the total precipitation. This confirms previous findings that heavy precipitation events in drylands have a greater impact on total precipitation amount (Loik et al., 2004).

Previous researchers mostly measured SWC at a longer time-scale (e.g., daily or monthly), whereas the present study provided much higher resolution (i.e., hourly). The resulting large amount of data permitted a more reliable and precise assessment of the soil water changes over time. Though our observation of a period longer than 1000 h, we found that SWC changes differed greatly among the soil layers, following a complicated pattern.

With increasing depth in the soil profile, SWC firstly tended to decrease, and then increased again. This contrasts with previous research results in the study area, in which SWC first increased, then decreased, and then finally increased with increasing depth in the soil (He and Zhao, 2002; Yao et al., 2012). This difference may be caused by the soil characteristics of the mobile dunes in the present study. These dunes are strongly affected by the strong local winds, the bare and loose soil surface, and the low vegetation cover. As a result of wind erosion that depleted the surface soil in fine particles, and greatly changed soil physical and chemical properties of the

mobile dunes of different layers (Table 1). These properties directly influence soil moisture conditions (Berndtsson and Nodomi, 1996). In addition, the strong winds in our study area would increase evaporation from the surface layers, leading to depletion of moisture in these shallow layers compared with deeper layers.

Some researchers have suggested that precipitation interacts significantly with soil moisture (Salve et al., 2011; Southgate and Master, 1996; Yang et al., 2008). We found that precipitation had a great impact on the upper soil layers (0—40 cm), but a less on the deeper soil layers (below 40 cm). This was consistent with the results of He and Zhao (2002) and Li et al. (2004). Based on the observed variation in SWC in mobile dunes after precipitation, He and Zhao (2002) also reported that soil moisture in sandy land depended strongly on the temporal and spatial variation of precipitation. The time required for SWC to respond to precipitation and the infiltration depth of the precipitation was closely related to the precipitation patterns. The infiltration depth increased with increasing precipitation amount, but the effect of precipitation intensity was stronger. It took less time for precipitation to infiltrate to a given depth at higher precipitation intensity than at lower intensity. The response time of soil moisture to precipitation increased with increasing depth in the soil, which was consistent with the results of Yang et al. (2008). In summary, the impact of precipitation on soil moisture in the mobile dunes was a complex and dynamic process that was affected by the precipitation amount, intensity, and duration.

In addition to precipitation, SWC was also greatly influenced by other meteorological factors, such as relative humidity, water barometric pressure and minimum temperature (Table 3), and SWC has a significant positive correlation with these factors. The further relationship between SWC and these factors should be considered and studied in the future to obtain more comprehensive cognition on SWC. Finally, the hydraulic conductivity value has largely affected the rate of infiltration, and a higher hydraulic conductivity caused a larger infiltration rate (Yao et al., 2013).

Mobile dunes play an important landscape-level role in the redistribution of precipitation (Berndtsson and Nodomi, 1996). In previous research on mobile dunes in the Horqin Sand Land, soil moisture variation was mainly caused by variations in precipitation and evapotranspiration, and the study site was mainly covered by an aeolian sandy soil with a strong infiltration capacity and a low water-holding capacity (Alamusa et al., 2005). SWC in dunes was not influenced by groundwater because the water table was deeper than 6 m at their site. However, due to low vegetation cover and rapid formation of a dry sand layer at the bare surface after precipitation, evapotranspiration was less than that in vegetated dunes (Alamusa et al., 2005). Therefore, a larger proportion of the precipitation that falls on mobile dunes is transformed into soil moisture through infiltration, and some may even enter the groundwater through deep percolation despite the losses to evaporation.

Based on this experiment results and previous research, we should conclude that the presence of mobile dunes exerts an important role for a replenishment of groundwater through precipitation infiltration in this region. Even our results suggest that this replenishment was significant at higher precipitation intensity or amount only. Furthermore, this preliminary conclusion needs to be confirmed by further study.

Acknowledgments

We thank Dr. Randy Kutcher (Plant Pathology of Cereal and Flax Crops) and Dr. Xueyong Zhao, Yuqiang Li (Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences) for their helpful comments for an early version of this manuscript. This study was supported by the National Natural Science Foundation of China (No. 40871004).

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