|Other Abstract||As a dominant existing form of water, soil water is the biggest factor which limits to the growth of vegetation in desert ecosystem. Not only does it can make an important impact on the occurrence, evolution and productivity of sand soil, but also it can play a significant role in maintaining the structure stabile and functioning properly for desert ecosystem. Ecological engineering protection-forest along the Tarim desert highway runs through the Taklimakan desert, due to a high evaporation intensity of this region, it is very difficult to maintain a state of relative balance for the sandy soil moisture, the normal water demands for most plants can not be met. Consequently, the key problem of ecological engineering of shelterbelt is to solve the problem of water. However, the soil salinization caused by the saline water irrigation, especially the high salinity saline water irrigation, which has been used to irrigate the vegetation of this region for a long-term, has become an important barrier factor for the construction and maintenance of the shelterbelt. Based on this, it is very necessary to carry out the dynamics of soil water and salt and its numerical simulation and so on in this region. On the one hand, in order to reveal the spatial and temporal dynamic laws of the soil water and salt for the artificial shelterbelt in the center of Taklimakan Desert under saline water drip-irrigation, the temporal characteristics and two-dimensional spatial distributions for its soil water and salt among four irrigating cycles of months from April to July had been measured and the related main factors were analyzed. On the other hand, the soil water characteristic curves for different depths of the soil in Tarim desert highway shelterbelt were measured and studied, which were used to provide the soil hydraulic parameters for the numerical simulation of soil water and salt transport and analyzed the soil physical water properties for different depths of sand soil of the shelterbelt. This paper can not only provide theoretical basis for controlling soil salinization and predicting dynamics of soil water and salt in this region, but also can make a good influence on developing and maintaining the shelterbelt and optimizing further its irrigation regulation in later stage. The main results showed as follows:
(1) The dynamic regularity of soil water and salt of shelterbelt was obvious.
①The dynamics of the soil water and salt on space and time for the artificial shelterbelt took obviously cyclical laws. When it referred to the temporal changes of the soil water and salt, on the other hand, in an irrigating cycle, the dynamics of soil water displayed three changeable stages which were divided into the rapidly decreasing period(1－4 d), the slowly changing period(4－10 d) and the almost stable period(10－15 d), respectively, while the dynamics of soil salt content undergone two changeable stages which were divided into the desalting process(1－7 d) and the salt accumulation process(7－15 d). Meanwhile, it took a decreasing law along with days according with Power Model(y＝8.746t-0.270, t＝1, 2, 3…) for the soil water, but the relationship between the soil salt and the time was consistent with Parabolic Function(y＝0.009t2－0.138t＋2.269, t＝1, 2, 3…). ②When it referred to the spatial distributions of the soil water and salt, the average soil moisture content was reduced by One-Linear Regression Function while the average soil salt content was increased by One-Linear Regression Function along with the distance on the horizontal space(0～60 cm). On the vertical space(0～120 cm), it showed an approximate unimodal curve for the distribution of soil water, and its peak was at the depth of 20 centimeters in the vertical space. However, for the soil salt, its characteristics of distribution were conformed with Reciprocal Function, and it presented marked phenomenon of salt accumulation at the surface of soil, especially in the ranges of horizontal distance which was away 45～60 cm from the irrigating drips, where the content of salt was up to 10～20 g/kg.
(2) The effects of measured and fitted soil water characteristic curves of the shelterbelt were good.
①The soil water characteristic curves of different depths of the soil were not all the same. Among them, the topsoil (0～5 cm) curve is very different from the other curves(P＜0.05) which from 0 cm to 150 cm of the soil profile, except the middle depth (40～60 cm), while the other curves were little different from another one(P＞0.05), besides the middle depth (40～60 cm). ②The structural differences among the depths of the soil profile could be reflected by the measured soil water characteristic curves, resulting from the correspondence between the features of the curves and the soil basic physical properties in different depths of soil. Briefly, the characteristics that the capacity of soil water retention was stronger, the saturated moisture content and residual moisture content were higher, which were consistent with the features that the soil bulk was smaller and the proportion of the silts and clays were higher. ③The fitted precision of van Genuchten Model and Brooks-Corey Model were higher in the low suction less than 1.0×105 Pa while the fitted precision of Gardner Model was a little higher in the above suction from 1.0×105 Pa to 15.0×105 Pa. However, on the whole, the formers were superior to the latter on fitting the soil water characteristic curves, because the results of correlation coefficients and root-mean-square errors between simulated values and measured values of soil moisture content by van Genuchten Model and Brooks-Corey Model were better than the values calculated by Gardner Model. Summarily, the three models can be used to fit the soil water characteristic curve of this region.
(3)The soil water physical properties among the soil layers of shelterbelt showed some differences.
①From strong to weak, the capacity of soil water retention for different depths were followed respectively by 0～5 cm, 40～60 cm, 100～150 cm, 60～100 cm, 5～40 cm while the soil water supply were followed respectively by 5～40 cm, 60～100 cm, 0～5 cm, 40～60 cm, 100～150 cm, and it took on a strong water supply under the soil suction on Pressure which was less than 1.5×105 Pa. The available moisture content of soil was decreased gradually from the upper depth to the lower depth and its average was 17.3%. ②Saturated moisture capacity, capillary moisture capacity and field moisture capacity were also showed a synchronized decreasing trend from the upper depth to the lower depth；on the other hand, the relationship was closed between three kinds of soil moisture holding-capacity and soil bulk, which was consistent with one-linear regression function. ③The upper limit of available water was the soil moisture content corresponding to the suction of 0.3×105 Pa and it was suitable to take the value which was 1.5×105 Pa as the boundary point of suction for readily available water and resistant available water. The readily available water content in all the depths were more than the resistant available water content, and the value of former was 13.1%, which was three times of the latter. Compared with the others, the readily available water content in the depths of 5～40 cm and 60～100 cm took the most supreme proportion while the resistant available water content in the depths of 5～40 cm and 60～100 cm took the highest proportion in the range of available soil water content, and both the values of them were the biggest.
(4)The result for the numerical simulation of soil water and salt transport had certain reliability based on the root water absorption model.
①In terms of the soil root density distribution, the feature of distributions of the root length density on the horizontal space(0～60 cm) for Haloxylon ammodendron was according with Exponential Function along with the distance on the horizontal space(0～60 cm), while the feature of distributions of the root length density for Haloxylon ammodendron was according with One-Linear Regression Function along with the distance on the vertical space (0～120 cm), including the reduced distribution along with the distance from 0 cm to 30 cm and the increased distribution along with the distance from 30 cm to 120 cm. ②In terms of the numerical simulation of soil water and salt transport, the result for the numerical simulation of soil water and salt transport of the shelterbelt in an irrigating cycle had certain reliability based on the root water absorption model. Specifically, the precision(the average value of RSME is close to 0.01) for the numerical simulation of soil water transport was a little better while the precision(the average value of RSME is close to 0.17) for the numerical simulation of soil salt transport was a little worse on the whole. Overall, this numerical simulation could be applied to predict the dynamics of soil water and salt for this region.|