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Moisture Dynamic In Yellow Soil And Its Environmental Factors In Karst Mountainous Area Of Guizhou

Posted on:2008-04-13Degree:DoctorType:Dissertation
Country:ChinaCandidate:T M JiangFull Text:PDF
GTID:1103360215965513Subject:Use of agricultural resources
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It has become an international topic how to improve the utilization of rainfall and limited water resources, to reduce the water requirement and consumption by agriculture. Soil moisture is a vital factor for crop growth, vegetation recovery and ecological construction. Recently, with the application of new technologies and methods, researchers in China have made great achievements in the fields of modeling water transport in agro-ecosystem, optimizing and regulating field water in agriculture and so on. However, most of the research was carried out in north China plain and the loess plateau, the semiarid zone of China. They are seldom applied in the areas of southern China with seasonal drought, especially in the karst region of southwestern China. Located in the southwest, Guizhou karst mountain area is one of the areas with most serious soil erosion and water shortage, In this region, seasonal drought is frequent, and seriously limits the sustainable development of agriculture. Yellow soil (Xanthozem), which accounts for 41.9% of the total land area of Guizhou and 25.3% of the total yellow soil of China, is a major soil type for agriculture in Guizhou. In this research, a typical karst mountain area of Jiuchang Town, XiuWen County is selected as an experimental area, where the yellow soil is used to analyze the soil moisture dynamics and their environmental factors. The research provides a scientific basis for demonstrating and spreading of anti-drought technologies.The study focused on providing scientific basis for solving the seasonal drought problem in karst area of Guizhou. According to the spatio-temporal variation of soil moisture in yellow soil, three patterns of land utilization were considered involving corn-wheat intercropping pattern, vegetable-vegetable successive cropping pattern and bared land (check) on three slopes of 6.5°, 9.5°and 16°in the investigation. The experimental plot is 10.0m in length and 4.0 m in width. 594 data for runoff in 66 times and 3420 data for soil moisture in 76 times were collected from June, 2001 to July, 2003. The soil moisture in layers of 0-10cm, 10-20cm, 20-40cm, 40-60cm was measured at an interval of 10 days, and those from layers of 60-80cm, 80-100cm with an interval of 20 days, respectively. All the data were categorized by cropping patterns and gradients and analyzed by methods of the traditional statistics, cross-spectral analysis and wavelet analysis to investigate the spatio-temporal variation of yellow soil moisture. The water holding capacity of yellow soil, precipitation transport on sloping land, soil moisture dynamics and its affecting factors had been discussed in this paper. The main achievements of this study are as follows:1. Yellow soil is easily attacked by seasonal drought due to its heavy texture, high water holding capacity, low specific water capacity, and narrow range of the available soil moisture.In yellow soil, the physical clay particle (<0.01mm) totals up to 691.7-880.5g·kg-1, of which the extremely clayey particle (<0.001mm) accounts for more than 44%. The clay content is the highest in the layer of 20-40 cm of yellow soil profile, which goes against infiltration. The water-holding capacity of yellow soil is related to contents of clay fractions with the size <0.001mm and <0.01mm in yellow soil profile. There is a significant positive correlation between clay content and field capacity, wilting coefficient and available soil moisture, a negative correlation between sand content, silt content, bulk density and field capacity, wilting coefficient, the content of available soil moisture.The specific water capacity decreases to the scale of 10~(-2) from 10~(-1), and to scale of 10~(-3) from 10~(-2) when the water potential decreases from -2.5~-10kPa to -10~-30kPa, and then to -100~-300kPa. The yellow soil water content decreases very quickly and the soil water discharges rapidly with a large amount when the suction is less than 100kPa. As the suction exceeds 100kPa, soil water releases slowly with a less amount. In 0-20cm layer of yellow soil the field water holding capacity is as high as 282.2g·kg-1, with a wilting coefficient more than 250.0 g·kg-1 and the range of the available soil moisture is only 3-5 g·kg-1. This is the reason that yellow soil is frequently influenced by seasonal droughts.The total soil reservoir capacities in 0-100cm layer of yellow soil is 5515.3 m3·hm-2, in which 4094.1 m3·hm-2 is the water-storage capacity and 1421.26 m3·hm-2 is the transmission volumetric capacity, making up 74.23% and 25.77% of the total volumetric capacities, respectively. The available water capacity is only 287.1m3·hm-2, accounting for 9.69% of the water-storage volumetric capacity and therefore it is the reason that yellow soil is easily attacked by seasonal drought. The unavailable water volumetric capacity of yellow soil takes up 90.31% of the water-storage volumetric capacity, which is useless for crops.2. High surface runoff leads to low utilization of rainfall on yellow soil slopes under traditional cropping patterns in karst mountain areas of Guizhou.The surface runoff on yellow soil slopes coincides with the rainfall reception of soil-plant system. Surface runoff appears frequently and changes dramatically from May to August of the runoff peak time; then gently from March to April and also from September to November. The rainfall reception of soil-plant system on yellow soil slopes begins from March, having a low volume from the middle 10 days of March to the middle 10 days of April, and then a great amount of rainfall reception from the last ten days of April to June. The frequency of rain reception is low in July and amount of reception is considerable. It rains frequently in August, and the rainfall reception is limited. It rains infrequently in September. It rains frequently and the rainfall reception is a bit less in October. It seldom rains from November to February of the coming year and there is hardly any rain to infiltrate. It is the key period from April to October to increase deep soil infiltration and improve the rainfall utilization.The relationship of surface runoff (Rs), the rainfall (P) and the rainfall reception (Pe) on yellow soil slopes could be determined as P = Pe + Rs in quantity. The surface runoff increases rapidly with an acceleration as the rainfall increases. Therefore, the formula for surface runoff with rainfall could be simulated as Rs=-a+bP-cP2+dP3. The rainfall reception increases with the increasing rainfall at the very beginning, later it increases with declining speed, and then decline as the rainfall reaches 70-80 mm. Thus, the equation for the rainfall reception with rainfall could be simulated as Pe=a+a.+bP+cP2-dP3. The rainfall, which causes surface runoff, takes up as high as 72% of the annual precipitation. The runoff from bare land, accounts for 42.4% of the annual rainfall, while the other 58.6% is taken up by soil-plant system. The surface runoff accounts for 31.8% of the rainfall and this leads to a rainfall reception coefficient of no more than 68.2% on yellow soil slopes under traditional cropping patterns. Although the runoff on yellow soil slopes under traditional cropping patterns is smaller than that on the bared land, the runoff coefficient is still high and the utilization of rainfall on yellow soil slopes under traditional cropping patterns needs to be improved, In addition, there is a positive correlation between gradient and runoff and negative correlation between gradient and rainfall reception.3. There is large difference of yellow soil moisture within seasonal distribution and vertical profile.The variation of yellow soil moisture with time tends to be an oscillation of wave with a rhythm of 30-60 days. The water content of yellow soil varies from season to season. Spring is the season with high evaporation leading to soil moisture moving downward clearly, and the soil moisture is at the negative phase, which is disadvantageous to shallow root crops. Summer and Autumn are the accumulation stage of soil moisture from infiltration of rainfall and the soil water moving downward, at this stage the soil water content is at positive phase. Winter is the low evaporation and accumulation stage of soil moisture and the soil water content alternates between positive and negative phases. Due to its narrow available soil water, yellow soil is likely disturbed by seasonal drought in spite of a much high value of soil moisture in summer and autumn, however.There is a similar distribution of inflexion and periodical characteristics between the adjacent layers in yellow soil profile. The causality between layers is obvious. The Variance coefficient (cv) of soil moisture, which is related to depth (h) of yellow soil, could be simulated as cv=0.5197h-0.4894. The 0-20cm depth is the water active layer with a water content of 250-400 g·kg-1, which is the channel and interface for water transmission between soil and atmosphere. The 20-60cm depth is the water sub-active layer with a water content of 400-500 g·kg-1, a transition zone between water active layer and the relatively steady layer and also a channel for water transportation. The 60-100cm depth is the relatively steady layer with a water content of more than 550 g·kg-1, where the soil water is relatively stabilized, which is the water storage bank for water transformation between soil and atmosphere.4. Slope and cropping are important factors in utilization of rainfall and maintenance of yellow soil moisture.In karst mountainous areas of Guizhou, the surface runoff on yellow soil slopes increases with the increasing gradient, which reduces the utilization of rainfall by soil-plant system. The gradient is also a key factor to vertical variation water content in the profile of yellow soil on the sloping land. Except of the water active layer from 0 to 20cm in profile of yellow soil, water content of the layers within 20-100cm shows the same trend: 9.5°> 16°> 6.5°. Since the physical clay content of yellow soil on 6.5°slope is 100g·kg-1, much higher than that on 9.5°and 16°slopes, the infiltration and transformation of rainfall by soil-plant system on the 6.5°slope is seriously limited. It is the main reason that the soil water content on 6.5°is less than that on 16°.To the exclusion of the abnormality that occurs on the 6.5°slope, the yellow soil on the 9.5°slope always has a priority in soil moisture maintenance compared with the yellow soil on 16°slope. The variation of yellow soil moisture is up to the rain event in temporal dimension, especially 0-40cm layer.With the seasonal changes, the efficiency for rainfall utilization by soil-plant system alters between corn-wheat and vegetable-vegetable cropping patterns. The crop plantation enhanced the rainfall utilization in comparison with the bare land pattern. The inflexions of yellow soil water variations from 0 to 100 cm depths, especially within the layer of 0-20cm, are nearly in the same location or close to in the temporal dimension. The evapo-transpiration of soil-plant system is enhanced while the infiltration is improved by crops growing. Thus, the frequency and extent of yellow soil moisture variation accelerates considerably by crops growing. However, the influence by crops growing attenuates with the depth, which enables a stable and high content of soil water in the relatively steady layer. When soil contains much water, the moisture variation in 0-20 cm depth of yellow soil under the corn-wheat pattern is much greater than that under the vegetable-vegetable pattern. The bared land holds quite a high soil water content in the 0-100 depth because of limited influence by vegetation evapotranspiration, etc..In summary, it can be seen from all above that the yellow soil in karst mountain areas of Guizhou has an extremely clayey texture, which leads to a strong water-holding capacity, a limited specific water capacity, an extremely narrow range of available soil moisture, an obstacle for water movement upward and downward and a limitation of available soil water in top soil. It is the reason that the yellow soil in karst mountain areas of Guizhou is easily to be attacked by seasonal drought. The runoff coefficient of yellow soil slopes is high under the traditional cropping patterns and the rainfall utilization rate needs to be enhanced. The limited water storage of yellow soil in Spring unbalanced the supply of soil water through a year. Gradient has a positive effect on runoff, and a negative effect on utilization of rainfall and soil moisture content. Crop plantation increases the reception of rainfall during raining period, while increases the soil moisture consumption during the non-raining period. At a certain extent, crop plantation increases the variation of soil water content. Therefore, it is necessary to take an action to start the engineering construction for available rainfall collection and extend the drying farming technologies combined with agricultural, biological measure as well as the measures for soil and water conversation as to maximize the utilization of rainfall and optimize the effect of soil reservoir on the field water supply.
Keywords/Search Tags:Karst mountain area, Yellow soil, Water-holding capacity, Moisture dynamic, Guizhou
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