Research On Soil Moisture Variation In Shallow Groundwater Area Of Arid Regions

Shihezi reclamation district is a typical irrigated agriculture area with fragile ecology inXinjiang, which is mainly characterized by drought and shallow groundwater. The total waterresources of this area is scarce, the contradiction between water supply and demand andecological environment is serious day and day with the development of the society. Therefore,analyzing and discussing soil moisture variation law under shallow groundwater condition，can provide security of water resources for continuous development of agricultural in aridareas, and have a great significance for reasonable development and utilization of waterresources and protection of ecological environment.The author took121crops of Shihezi reclamation district as a study area. carried outfield experiment, constantly monitored the dynamics of groundwater, soil water of the studyarea, systematically analyzed the characteristics of soil water dynamic under shallowgroundwater conditions and its influencing factors, analyzed the regular pattern of distributionof soil moisture profile, established empirical formula for change of soil water content withsoil depth; established a coupled transport model of water, vapor, heat, dry air, quantitativelysimulated the conversion process of precipitation-soil water-groundwater, revealedconversion process and mechanism of water and energy of each interface and between eachother. The results can provide theoretical reference for prediction of soil moisture andirrigation management of agriculture, and provide a scientific basis for sustainabledevelopment and utilization of groundwater resources. This study mainly obtained thefollowing conclusions:(1) Though the analysis of measured air temperature and soil temperature of variousdepths during June2013-May2014, the author found that there was a close relationshipbetween soil temperature and air temperature, shallow soil temperature fluctuated with airtemperature, change trends of the two temperatures were basically same, as soil depthincreased, soil temperature fluctuations was weaken. Changes in soil temperature laggedbehind the air temperature changes, and the hysteresis time increased with increasing soildepth. Using Fourier series to establish the soil temperature fluctuation equation can wellreflect the actual variation of soil temperature.(2)Measured data of the study area during June to October showed that soil water content of each depth had a decreasing trend along with the time extension. Taking80cm depth as thedividing line, the lower soil water content was significantly higher than the upper part of thesoil. Precipitation mainly affected soil water content of the shallow20cm depth soil,evaporation mainly affected soil water content of the upper60cm depth soil.Soil watercontent of0~60cm soil change greatly before and after freezing and thawing, which increasedsignificantly at the beginning of melting when compared with the value at the initial freezingstage; soil water content of80~200cm was stable, slightly reduced before and after freezingand thawing.(3) Measured groundwater depth data showed that groundwater levels of the study areawas in the rising stage from late February or early March to late July or early August of eachyear, and was in a decline stage from August to next February. For the same depth of soil, soilwater content decreased with groundwater depth increasing. The closer the soil got to thewater table, more significant the dynamic correlation between soil water content andgroundwater depth was. During June2013to May2014, R2for correlation between soil watercontent at200cm depth and groundwater depth was0.9986.(4) Though the analysis of measured soil water content of the study area, and the contrastwith distribution of soil water content of Shaanxi Jinghuiqu Experiment Station wheregroundwater depth greater than20m, the author found that the distribution of soil watercontent in the study area had strong ties to the groundwater depth. In this regard, the authorestablished empirical formula of soil water content changes with depth. Variation of the soilwater content profile was analyzed in two stages: the first was0~80cm, the second was80~200cm. The calculations of the formula and the measurements matched.(5) On the basis of the existing unsaturated soil water and gas two-phase flow model, theauthor established a mass and heat transfer model of water, air two-phase flow in unsaturatedzone–saturated zone for the study area, to simulate the coupled process of soil water, vapor,heat, dry air transfer under shallow groundwater conditions. Use relative root mean squareerror RRMSE and average relative error AVRE to evaluate the accuracy of the model. Thesimulated soil water content of the bottom soil which was close to the groundwater surfacewas better than that of the upper soil, but for the simulated soil temperature, the bottom wasworse than the upper. As a whole, RRMSE of the simulated soil temperature and soil watercontent were all less than10%, AVRE were all close to1, which suggested that the model canwell simulate the law of the measurement data variation.(6) Under the condition that day rainfall was9.7mm, infiltration can affected soilmoisture of about20cm depth for coupling model that considering soil air pressure, for thesingle-phase model up to about30cm depth. After precipitation, increment of shallow soil water content of of the coupling model was less than that of single-phase model. Whenprecipitation occured and after a short time, without considering soil air pressure willunderestimate soil moisture evaporation rate. The results showed that the groundwater fluxmodel for soil water recharge51.1%of the total amount of evaporation. The flux result of themodel showed that the groundwater recharge for soil water accounted for51.1%of totalevaporation, which indicated that groundwater had a very important role of supply for soilwater. Analysis of0~200cm soil water balance showed difference of the simulated andcalculated evaporation was1.73%, which indicated that simulation results of the modelestablished in this paper were reliable.