Soil moisture potential and water content in the unsaturated zone within the arid Ejina Oasis in Northwest China

Transcription

1 Soil moisture potential and water content in the unsaturated zone within the arid Ejina Oasis in Northwest China X. Zhou Æ L. Wan Æ B. Fang Æ W. B. Cao Æ S. J. Wu Æ F. S. Hu Æ W. D. Feng Abstract Three soil profiles were selected in the Ejina Oasis, northwest China, to determine water content profiles and evolution of soil moisture potentials in the unsaturated zone within the arid area. The total soil moisture potentials have been monitored for about 3 months in 2001 at different depths in the soil profiles. The occurrence and movement of water in the unsaturated zone was analyzed using the zero flux plane (ZFP) method. It is shown that convergent ZFPs and divergent ZFPs may occur at depths between 0.5 and 3.0 m, and that the depth of the ZFPs was controlled by the root zone of plants growing on the land surface. Profiles of the total soil moisture potentials were observed to be coincident with those of the water contents at the three experimental sites. The total soil moisture potential showed a slight increasing trend and the ZFPs tend to vanish from summer to winter as the water extraction by roots decreased. Evapotranspiration through vegetation has an important bearing on the water content and the total potential in the unsaturated zone. Keywords Soil moisture potential Æ Water content Æ Zero flux plane Æ Unsaturated zone Æ Oasis Æ Arid environment Æ NW China Received: 12 January 2004 / Accepted: 30 March 2004 Published online: 11 May 2004 ª Springer-Verlag 2004 X. Zhou (&) Æ L. Wan Æ B. Fang Æ W. B. Cao Æ S. J. Wu F. S. Hu Æ W. D. Feng School of Water Resources and Environment, China University of Geosciences, Xueyuan Road 29, Beijing, China zhouxun@cugb.edu.cn Tel.: Fax: Introduction Scattered oases occur in the endorheisms in northwest China. Some of the oases are found to have shrunk in the past decades (Zhang 1994; Chen 1996). The Ejina Oasis is in the lower reach of the Heihe endorheism in northwest China. There is continuing concern about the degradation of the ecologic system in this oasis, which is due mainly to the extensive development of water resources within the Heihe endorheic basin (Zhang 1994). Water contents in the unsaturated zone play an important part in the development of vegetation in an oasis (Feng and others 2001; Li and Islam 2002). Knowledge of the soil moisture in a vertical profile in the unsaturated zone is important for understanding the vulnerability of an arid environment. A number of factors affect the occurrence and movement of soil moisture content. Precipitation, evapotranspiration, soil porosity, hydraulic conductivity and vertical hydraulic gradient are the most important factors (Wellings 1984; Cooper and others 1990). Measurement of water contents and soil moisture potentials in vertical soil profiles under natural conditions is crucial for water conservation in arid oases (Ragab and others 1997; Mdaghri-Alaoui and Eugster 2001). However, measurement or estimation of water flux in a soil profile in the field is difficult. Because of the variability of the hydraulic conductivity in the unsaturated zone, Darcy s equation cannot be directly used to calculate water flux in the profile (Cooper and others 1990). The ZFP method, which is a relatively simple and practical method, has been used by a number of researchers in studies of experiments on soil moisture and soil water budget in the unsaturated zone (Wellings 1984; Dreiss and Anderson 1985; Cooper and others 1990; Shen and others 1992; Jing 1994; Zhang and others 1998). The method measures the flow of water in the soil profile by monitoring the total soil moisture potential, and no knowledge of soil hydraulic properties is required. The total soil moisture potential in the unsaturated zone in arid areas in northwest China has seldom been examined. Experiments were therefore conducted to provide detailed information on the movement and occurrence of water content in the unsaturated zone in the Ejina Oasis. The objectives of this study were to estimate the vertical distribution of water content in the unsaturated zone and to better understand the evolution of soil moisture potentials in the soil profiles beneath this oasis by using the ZFP method. DOI /s Environmental Geology (2004) 46:

2 Soil moisture potential and zero flux planes Water occurring in an unsaturated zone has soil moisture potential. It is the difference in the soil moisture potential that makes water move from places where the soil moisture potential is high to places where the soil moisture potential is low. The total potential in the unsaturated zone includes: (1) gravitational potential, (2) matrix potential, (3) pressure potential, (4) solute potential and (5) temperature potential (Jing 1994). Gravitational and matrix potentials generally dominate and the effect of the latter three types of potential can be ignored. Thus, the total soil moisture potential (/) at any given point in the unsaturated zone can be expressed as / ¼ / g þ / m ð1þ where / g, / m are the gravitational potential and the matrix potential at that point, respectively. The vertical flux, q, at any given depth in the unsaturated zone can be expressed according to Darcy s law: q ¼ KðÞ where K (h) is the hydraulic conductivity, h is the water content, // z is the gradient of total soil moisture potential, z is the vertical coordinate. The total soil moisture potential may increase or decrease with depth owing to evaporation on the soil surface, water extraction by roots and water draining towards the water table. A plane dividing the zone of upward moving water from downward moving water is called a zero flux plane (ZFP), where the gradient of total soil moisture potential is equal to zero. If a given point in a profile of total soil moisture potential meets the condition of / / z =0, i.e., an extreme point, it is the location of a ZFP. There are two types of ZFPs as two types of extreme points may appear along a potential profile. A maximum value point indicates a divergent zero flux plane (DZFP). Above the DZFP, / / z < 0, and water moves upward. Below the DZFP, / / z > 0, and the water moves downward. A minimum value point indicates a convergent zero flux plane (CZFP). Above the CZFP, / / z > 0, and water moves downward. Below the DZFP, / / z < 0, and the water moves upward. Thus, the movement of water in an unsaturated zone can be examined by investigating the occurrence, type and evolution of ZFPs existing in a total soil moisture potential profile. Site and field measurement The Ejina Oasis lies in the northeastern part of the Heihe endorheic basin in west Inner Mongolia, northwest China (Fig. 1). The east branch of the intermittent Heihe River flows through the oasis from the south and terminates to the north in the terminal East Juyan Lake. The area, surrounded by deserts and gobis, is an extremely arid area with a precipitation of less than 50 mm. Surface runoff is not normally observed during the year. The Heihe River, which stems from the Qilian Mountain in Qinghai Province and flows from the south to the north through the Hexi Corridor in Gansu Province, is the only source of water to recharge the groundwater in the Ejina Oasis. The Ejina region is an alluvial and lacustrine plain underlain by unconsolidated sediments of Quaternary age. The three experimental soil profiles (He3, He4 and He5) were 10, 8, 6 km to the north of the town of Ejina (Fig. 1). The vegetation conditions and the lithology revealed by the profiles are summarized in Table 1. The soil in the unsaturated zone is composed mainly of clayey sand, sandy clay and fine sand with occasional gravels. The vertical profiles of the soil moisture potential of the three soil profiles were measured in the field with mercury manometer tensiometers which consist of 12 probes (11 probes for soil profile He4). The tensiometer probes were installed in the unsaturated zone at depths ranging between the land surface and water table with a 0.2 or 0.3 m interval as listed in Table 1. Soil samples were taken at depths listed in Table 1 for determining soil water contents and water soluble salt contents. The soil moisture potentials were gauged twice a day, at approximately 9:00 am and 7:00 pm for the first week after the installation of the tensiometers in mid-august, 2001, and once a week since then until late October, Most of the tensiometer probes worked well, though some erroneous data were collected. Measurements of these ceramic cups needed to be calibrated according to systematic analyses of gauging errors. Some of them were not considered. Measurement of soil moisture potentials could not be carried out in soil profile He4 beyond the first week of installation of the tensiometer because of the failure of the tensiometer. Results and discussion Changes in soil moisture potential and water content versus depth are shown in Figure 2 for soil profiles He3, He4, and He5, respectively. In the soil profile He3, the total soil moisture potentials range from )500 to )200 cm water. There are three zero flux planes occurring at depths of about 1.0, 1.7 and 2.3 m. Of the three ZFPs the upper and lower ones are CZFPs, while the middle one is a DZFP. The positions of these ZFPs remained virtually constant during the first week of measurement in mid-august, The soil moisture potentials at the two CZFPs were the lowest, therefore, water above and below the CZFPs moved toward the CZFPs from above and below. The main vegetation at He3 includes tamarisk and grass. The root systems of the grass and the tamarisk developed well at depths of about 1.0 and 2.3 m (Wu 2000). The coincidence of the CZFPs with the root systems suggests that a contribution to evapotranspiration accounts for the majority of water lost from the unsaturated zone within the Ejina Oasis. The water content 832 Environmental Geology (2004) 46:

3 profile at He3 shows a similar trend as the soil moisture potential. The highest values of water content correspond very closely to the CZFPs, while the lowest values of water content are in correspondence with the DZFPs. The soil profile He4 was beneath a poplar forest. The total soil moisture potentials were in the range between )550 and )80 cm water. A DZFP was found to appear at a depth of 1.2 m. Below the DZFP, a CZFP was observed at a depth of 1.5 m. The depth of the CZFP was coincident with the depth of the root zone of the poplar. The root system of the poplar ranges in depth from 1.0 to 1.88 m (Wu 2000). Water moved to this zone owing to water extraction by roots. It can also be noticed that the water content profile has a similar trend as the total soil moisture potential. Two minimum value points existed in the water content profile, which correspond to the two CZFPs in the potential profile. Unfortunately, the tensiometer probe at the depth of 0.2 m was ruined, and the CZFP at the depth of about 0.5 m was not observed clearly. Fig. 1 Location of the study area. a The Heihe Endorheic Basin (only shown are the lower reach, the Ejina Basin, and the middle reach, the Hexi Corrider). 1 bedrock; 2 desert; 3 river; 4 intermittent river; 5 terminal lake; 6 provincial boundary. b The Ejina Oasis (only shown is the central part of the oasis). 1 highway; 2 intermittent river; 3 soil profile; 4 borehole sampled Table 1 General description of the three soil profiles Profile Main vegetation Lithology Number of tensiometer probes Depth of tensiometer probes (m) Depth of soil sampled (m) Depth to water table (m) He3 Tamarisk, grass Clay sand and fine sand with gravel He4 Poplar Clayey sand, clay, fine sand , 0.4, 0.6, 0.8, 1.0, 1.2, 1.5, 1.8, 2.1, 2.4, , 0.6, 0.8, 1.0, 1.2, 1.4,1.6, 1.8, 2.0, 2.2, 2.4 He5 Tamarisk, grass Clay, fine sand , 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 2.2, , 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, , 0.5, 1.0, 1.5, 2.0, , 0.5, 1.0, 1.5, 2.0, 2.5, Environmental Geology (2004) 46:

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5 b Fig. 2 Soil profile (a), water content profile (b) and total soil moisture potential profiles (c, measured at about 9:00 am, and d, at about 7:00 pm) at sites He3 ( upper), He4 ( middle) and He5 ( lower). 1 clay; 2 sandy clay; 3 clayey sand; 4 fine sand The soil profile He5, where tamarisk and another grass grow, is 6 km to the north of the capital of Ejina County. The depth to the water table was 3.32 m. The total soil moisture potentials varied between )750 and )50 cm water. Three ZFPs were found at depths of about 0.6, 1.2 and 1.6 m, respectively. Of the three ZFPs, the upper and lower ZFPs were DZFPs, while the middle one was a CZFP. Tamarisk is the main vegetation at this site. The root system of the tamarisk grows at a depth ranging from 1.0 to 2.5 m. The position of the CZFP was Fig. 3 Changes in total soil moisture potentials with time in soil profile He3. a At depth from 0.4 to 1.5 m. b At depth from 1.8 to 3.0 m within the root zone of the tamarisk. In addition, the total soil moisture potential profile is quite similar to the water content profile, indicating that water uptake by roots was the primary mechanism for the occurrence of the ZFPs. Figure 2 also shows the total soil moisture potentials vs. depth in the soil profiles He3, He4 and He5 for the first week after the installation of the tensiometers in mid- August, Profiles of the total soil moisture potential changed very little in each day of the first week and changes in the potentials were not significant in the morning and in the evening in each of the days. The positions of the ZFPs in the three profiles remained almost the same during this period. As no rainfall occurred during this period of time, it seems that evapotranspiration through vegetation was the principal discharge of water in the unsaturated zone in the Ejina Oasis. The lithology of each of the three soil profiles changes very little. It is likely that the lithology is not a major factor affecting the occurrence of the ZFPs. Environmental Geology (2004) 46:

6 Fig. 4 Changes in total soil moisture potentials with time in soil profile He4. a At depth from 0.6 to 1.4 m. b At depth from 1.6 to 2.4 m Figures 3, 4, and 5 show the changes in soil moisture potential at different depths in soil profiles He3, He4 and He5 during the period from mid-august to late October, 2001 (only mid-august, 2001 for soil profile He4). In soil profile He3, the total potentials at different depths increased slightly before September 15, 2001 and changed very little after that. In soil profile He5, the total potentials showed an increasing trend before September 15, 2001 and changed very little after that, except at depths of 0.6 and 2.4 m, where the total potentials remained almost unchanged. The increase in total potentials from summer to winter could be due to seasonal differences in evaporation. In soil profile He4, measurements of total potentials were available only in the first several days after installation of the tensiometer in mid-august, 2001 (Data of potentials measured at 7:00 pm on August 18, 2001 were not true due to the repair of the tensiometer at that time). The total potentials at depths of 0.6 and 0.8 increased slightly, while potentials at deeper depths did not change during this period of time. Figures 6 and 7 show the evolution of soil moisture potential profiles in soil profiles He3 and He5 from mid- August to late October, Two periods of evolution of the total potentials can be observed. From August 16 to September 9, 2001, ZFPs in the profiles developed well and the total potentials were relatively low (as low as )700 cm water in He5). The total potentials fluctuated significantly with depth and increased with time. From September 15 to October 28, 2001, the values of total potentials were relatively high, with maximum total potentials exceeding )50 cm water in He3, and the ZFPs almost did not exist. The total potentials changed very little with depth and with time. Although ZFPs still occurred in soil profile He3, the type of the ZFPs has changed. For instance, the CZFP at a depth of 0.7 m became a DZFP, while the DZFP at a depth of 1 m became a CZFP. The soil moisture potential reflects an attraction force between the water content and the soil matrix in the 836 Environmental Geology (2004) 46:

7 Fig. 5 Changes in total soil moisture potentials with time in soil profile He5. a At depth from 0.2 to 1.6 m. b At depth from 1.8 to 2.2 m unsaturated zone. From summer to winter, vegetation withers gradually and the number of daylight hours decreases. As a result, the evaporation weakens and the water content in the unsaturated zone increases. This may lead to an increase in the attraction force between the water content and the soil matrix. Consequently, the total potential may have an increasing tendency from summer to winter. Summary Field measurements of total soil moisture potential in the unsaturated zone in the Ejina Oasis were conducted over a period of three months. The principal conclusions of this study are summarized as follows. 1. Zero flux planes were observed at depths between 0.5 and 3.0 m in the soil profiles. Total soil moisture potentials range from )750 to )50 cm water within the Ejina Oasis in the arid endorheism in northwest China. Both convergent and the divergent ZFPs were observed. 2. The water content profile at each of the three soil profiles was similar to the total soil moisture potential profile of the corresponding profile. Field data indicated that the depths with low/high water content are of low/high total soil moisture potential. The positions where the ZFPs occurred were determined by water extraction by root system. The CZFPs were usually within the root zone. Water in the unsaturated zone moved towards the CZFPs from above and below. 3. From summer to winter in 2001, a slight increase in total potential was found in the soil profiles and the ZFPs became indistinct. The reason for these was that water extraction by roots decreased as leaves of plants Environmental Geology (2004) 46:

8 Fig. 6 Evolution of soil moisture potential profiles in soil profile He3. a From August 16 to September 9, b From September 15 to October 28, 2001 withered and daylight hours decreased, as evapotranspiration was the chief discharge of water in the unsaturated zone in the Ejina Oasis. 4. The lithology of the soil profile can aect the total potential to some extent. Under the same conditions of Fig. 7 Evolution of soil moisture potential profiles in soil profile He5. a From August 16 to September 9, b From September 15 to October 28, 2001 other factors, the total potential was higher in clayey soil than in sandy soil. Acknowledgements This research was supported financially by the fund from the Ministry of Land and Resources of China for the Special Research Plan of Science and Technology in the Year 2000 (No ). The authors wish to thank Academician Zhang Zonghu, Professor Shen Zhaoli, Professor Ha Chengyou, Professor Qiu Xinfei and Dr. Shan Weidong for their suggestions in the course of the work. Dr. J. M. Zuppi s critical reviewing of an early draft of the article is very much appreciated. 838 Environmental Geology (2004) 46:

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