| Mulched drip irrigation is a combination of drip irrigation and mulching. Drip irrigation can regulate irrigation amount and reduce deep percolation; plastic film mulch is always used to prevent evaporation and regulate soil temperature. Mulched drip irrigation has been used widely in Xinjiang for cotton since the early1990s. Shallow brackish water which could be used for agriculture is distributed widely throughout north and northwest China, where is generally a shortage of freshwater. However, there is a serious risk of salinization for the field under mulched drip irrigation with brackish water, because of the strong evaporation and lack of leaching. This paper aims to study the special soil water flow system (SWFS), which controls the soil environments under mulched drip irrigation. The results could be suggested as a theoretical and technological support for sustainable using of brackish water.The soil water and salt transport, irrigation regimes and utilization of brackish water under drip irrigation have been widely studied in these years. However, for the SWFS of mulched drip irrigation, the patterns which control the transport of soil water and solute are still unclear; the further study should be done to define the soil water and solute balance components which is the key part of the groundwater-soil-plant-atmosphere continuum (GSPAC) under mulched drip irrigation; the accurate3-d numerical simulation model also should be built and calibrated to understand the SWFS in detail; the influence of SWFS on spatial variability of field salinity is unclear and the optimized amount of flood irrigation for leaching soil salts accumulated in the root zoon during the growth season is need for the arid region. Thus the following works were carried out based on summary and analysis of the previous achievements.1. Based on the theory of GSPAC and SWFS and pattern analysis of SWFS, field experiments and specific measurements were designed and conduced. The SWFS of mulched drip irrigation with brackish water and its controlling factors were recognized. 2. The groundwater level and quality, soil water quality and potential, soil moisture and evapotranspiration were monitored. According to the monitoring results and patterns of the SWFS under mulched drip irrigation, the soil environments i.e. soil water, salt and temperature dynamic and their relationships with physiology of cotton were analysed. Water balance components of the special SWFS were accurately defined and then the salt balance were calculated. In final, the3-d numerical simulation model of soil water and solute transport was built and calibrated, and was used to reveal the evolution of soil water and solute distribution.3. Based on the status of field salinity influenced by the pattern of SWFS under mulched drip irrigation. Electromagnetic induction EM38-MK2was applied to measure apparent soil electrical conductivity (ECa) before and after flood irrigation in March2013. The spatial distribution and variability of cotton field salinity under mulched drip irrigation with brackish and fresh water were analysed. To evaluate the efficiency of flood irrigation, HYDRUS-1D model with the calibrated parameters of soil water and salute transport was used to optimize the flood irrigation regime.Main conclusions of this research are as follows:1. SWFS of mulched drip irrigation with brackish water and the soil environments(1) Based on the distribution of the measured total soil water potential, patterns of the SWFS under mulched drip irrigation from the soil surface to groundwater table of a relatively homogenous medium may be characterized. The SWFS integrates soil water flow field, soil medium, chemical field, temperature and microorganism fields together as a whole dynamic system changing with time and space. The upper boundary includes the bare soil strip of strong line evaporation, the impermeable-sheet, the very weak evaporative point sinks at plant hole, and the periodic strong drip irrigation point sources. The lower boundary is groundwater table. The water table fluctuates with periodic irrigation could be the sink or source of the SWFS. A divergent curved surface of zero flux (CSZF) results from periodic irrigation. The downward water flow below the CSZF before irrigation and the high water table indicated that excessive irrigation occurred.(2) The SWFS of mulched drip irrigation determines the distribution of soil moisture and salinity. The limited irrigation depth and evaporation of bare soil would result in a high salinity strip along the wetting front and near the bare soil surface. The SWFS is controlled by five components:hydrogeology, irrigation, climate, plant and plastic film mulch.(3) The diurnal dynamic of soil water and salinity at the depths of15,25and40cm in the first day of after a drip irrigation is controlled by the SWFS, which is flatten out with the increasing depth. The wide row (covered with plastic mulch) zoon has the highest soil water content and the inter-mulch zone with no mulch has the lowest soil water content due to the strong root water uptake and evaporation. For the wide row zoon, the highest and lowest soil salinity are at the depth of15cm40cm respectively. For the narrow row with the drip line, the lowest soil salinity is at the depth of15cm under strong leaching. For the inter-mulch zone, soil salts accumulate at the depth of15cm. The soil temperature increases earlier under mulch than the inter-mulch zone with no mulch. Transpiration and photosynthesis are mainly occurred in day time, and the photosynthesis is more sensitive to the changes of environment than the transpiration.For a typical irrigation period of blooming stage, the dynamic of soil water content and temperature become gradual with the increasing depth. At the depths of15,25and40cm, the soil water dynamic of narrow row zoon is greater than the inter-mulch zone due to the root water uptake and evaporation, and the soil water content is higher than the narrow row zoon and the inter-mulch zone. For the inter-mulch zone, water content at the depth of25cm is lower than that of15and40cm. The soil temperature at the depths of15,25and40cm is affected by the redistribution of soil water and the temperature of irrigation water. Besides the diurnal cyclical fluctuations, the amplitude of soil temperature series increases gradually over time in the period. The transpiration and photosynthesis rate have diurnal cyclical fluctuations, and the characteristics of lower rate before and higher rate after irrigation are affected by dynamic of soil water, temperature and salinity during the irrigation period.The soil moisture and temperature at the depths of15and25cm at different growth stages were studied. Dynamic intensity of soil moisture at the irrigation stages is greater than the non-irrigation stages. The wide row zoon has the maximum mean soil moisture, followed by the narrow row, and the inter-mulch zone has the minimum mean soil moisture. The increasing of irrigation amount would increase the soil moisture and the narrow row zoon has the greater amplitude of fluctuation than the wide row and inter-mulch zone. Soil salts at depths of0-60cm accumulated during the growth season and reached the maximum at the later flowering stage. Soil temperature of the surface layer is mainly controlled by air temperature. However, SWFS of mulched drip irrigation also has some effects on the soil temperature. The narrow row zoon has the lower soil temperature than that of the wide row and inter-mulch zone. Plastic film can increase the soil temperature and prevent evaporation, thus the ratio of photosynthetic to transpiration is significantly higher than other growth stages which have the large leaf area index.2. Soil water and salt balance calculation and the numerical simulation of SWFS under mulched drip irrigation with brackish water(1) Based on the scale transformation and conversion methods, soil water balance of the unsaturated zone was calculated from August15to August24,2012. The total evapotranspiration is32.27mm and the soil surface evaporation at inter-mulch zone is only1.61mm.95%of the evapotranspiration is transpiration. The water balance components are calculated respectively. The statistical balance error is2.19mm which indicates a high accuracy.53.2%of the drip irrigation returns into groundwater and severe excessive irrigation occurred. Salt balance calculation shows that, the salts out of the calculated zone is115g/m2, which is1.56times of that into the zone. Thus, even under the condition of irrigation with brackish water, the large irrigation amount may also have the effects of leaching salt into the groundwater.(2) Based on the pattern analysis of SWFS, soil salt and water balance calculations and dynamic of detailed spatio-temporal monitoring, the3-d numerical simulation of soil water and salt transport model under mulched drip irrigation is recognized. The reliability of the model is also verified. This study further improves the accuracy of numerical simulation and model structure, and gets the optimum soil water and solute transport parameters.(3) Based on the results of numerical model, soil water-salt dynamic under micro spatial and temporal scales are analyzed. Numerical model is the essential complement to conventional monitoring and would deepen the recognition of SWFS under mulched drip irrigation. Soil moisture and salinity of the narrow zone with the drip line have the most quickly response to the irrigation event and have the strongest dynamic. Simulation results show that soil moisture at the dripper is higher than other positions, and the intersection intensity of infiltration along the drip line is greater than the transverse intersection. The wet and desalted pocket formed after the irrigation adapts to the distribution of roots and planting pattern and benefits to cotton growth. There would be a process of soil water and salt redistribution after a irrigation event. The soil salinity at the depths of15,25and40cm decreases rapidly responded to the drip irrigation and increases again due to the evapotranspiration. For the narrow row zone, the strongest salinity dynamic is at the depth of25cm under the influence of root water uptake.3. Spatial variability of field salinity and flood irrigation(1) EM38-MK2would be suggested to rapidly investigate the field salinity distribution of root zone (0-60cm). There is a significant linear regression between ECa measured by EM38-MK2and soil salinity. The correlation coefficients of the regression model before and after flood irrigation are0.616and0.778respectively. The interpretation model should be calibrated under different moisture conditions.(2) Statistical characteristics of soil salinity from the interpretation models of soil salinity indicate that the scenario of brackish water irrigation has the higher salinity level and the lower variation coefficient than the scenario of fresh water irrigation. The optimal semi-variance models of soil salinity under brackish water and fresh water irrigation are exponential and Gaussian models respectively. Both models are characterized by strong spatial correlation. Compared to fresh water irrigation, the scenario of brackish water irrigation with the higher salinity level has the lager range variation increasing the spatial dependency of soil salinity. Spatial variability of the field salinity is mainly characterized by the overlay influences of background and drip irrigation events. The extreme points of high salinity distributed on the sides of drip lines. The scenario of fresh water has the stronger spatial variability than the scenario of brackish water.(3) The optimal semi-variance models for before and after irrigation scenarios with the approximate range variations are exponential and spherical models, respectively. Still (Co+C) of after irrigation scenario is much larger than that of the before irrigation scenario. Thus, spacial variation of soil salinity was enhanced after flood irrigation. The maps of spatial distribution of soil salinity were predicted by Kriging interpolation, which showed that inappropriate flood irrigation regime resulted in accumulating soil salts rather than leaching salts for the depth of0-60cm. Soil salts increased4×104kg/hm2after flood irrigation. The optimum irrigation regime calculated by the HYDRUS-1D software could save255mm of fresh water and leach41.9%of the soil salts. Recommendations for further work are listed below:(1) Precise observation of soil chemical and microorganism fields is helpful for further understanding the SWFS of mulched drip irrigation. Computing speed of3-d numerical model of soil water and solute transport under mulched drip irrigation should be improved in future. To reduce percolation into groundwater, the higher productivity should be reached for increasing the irrigation frequency and decreasing the applied water of an irrigation event.(2) Based on the detail survey of filed salinity measured by EM38-MK2, soil salinity degrees can be defined. The numerical simulations are useful for calculating the rational amounts of flood irrigation for variable field salinity. Sustainable irrigation with brackish water requires that suitable drainage canals and places are made available.(3) The following questions of soil properties monitoring need further study in future. The in-situ monitoring equipment of soil water, salt and temperature is developed quickly nowadays. The calculations, however, are intensively affected by soil moisture. In addition, soil water extractor has the problem of sampling delay or too little volume for measuring.The major innovations are as follows:1. Combined with multiple fields, SWFS of mulched drip irrigation with brackish water is introduced, which would be used to analyse the dynamic of soil environments, including soil water, salinity, temperature, etc.2. This study carried out the precise monitoring and calculation of soil water and salt balance. Combined with the measured dynamic of soil water and solute, an optimized accuracy model of3-d numerical simulation with rational structure and optimum soil water and solute transport parameters is built to reveal the evolution of soil water and solute distribution.3. Electromagnetic induction EM38-MK2was applied to quickly survey the field salinity. Combined with the characters of soil salinity distribution of SWFS under mulched drip irrigation with brackish water, the spatial variability of field salinity is analyzed and numerical simulation is used to optimize the scheme of food irrigation for leaching accumulated soil salts. |
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