Study On Runoff And Water Balance Model Under Mechanized Conservation Tillage For Dryland Farming

Unreliable and limited rainfall, soil deterioration, and water erosion are the most important limitations to the sustainability of dry(and farming in Nortern China. Conservation tillage has received great attentions to solve these problems. And many researches have been done with great achievements on conservation tillage systems and machinery in China. However, further study was still needed to investigate the effects of conservation tillage factors such as surface cover and tillage on runoff and erosion, to quantify and model the effects of conservation tillage practices on runoff and soil water balance, and to further explore the effecting mechanism of residue cover, tillage and compaction on soil physical properties and crop production. According to the above questions, the following aspects of researches are conducted in this dissertation. I. A Field experimental plot, equipped with automatic weather station, tipping bucket and data logger system that can measure rates of runoff and rainfall as a function of time, was set up on low- slope hess farmland to test different conservation tillage systems, and to compare them with conventional moldboard plough practice (CK). The effects of surface cover, tillage and compaction on rLinoff and erosion were studied as well. Besides, Rainfall simulator experiments were carried out to further investigate and quantify the effects of those conservation practices on runoff and infiltration. This systematical experimental study on runoff and erosion under conservation tillage, providing valuable data and advanced methodology. The results of two years showed that conservation tillage, which features more residue cover and less soil disturbance, could remarkably reduce runoff by 52.5% and reduce erosion by 80.2% compared to CK. Among the three factors, surface cover is more efficient which could reduce runoff by 47.3% and reduce erosion by 77.6%. Wheel traffic also has a considerable impact on runoff and infiltration especially under wet antecedent conditions when traffic increases soil bulk density, reduces pore size and continuity, accordingly reduces infiltration and increases runoff under zero and minimum tillage. II. Based on the above experiments and the existing models, a runoff and soil water balance model was set up to assess the impacts of conservation tillage practices in China. According to the characteristics of conservation tillage, the procedure of runoff prediction using USDA Curve Numbers was modified in this model by incorporating the effects of residue cover, til!age, field slope and rainfall intensity etc.. In the model, Curve Number changed dynamically with daily residue cover, surface roughness, soil water and rainfall intensity. Runoff was calculated daily as a function of rainfall, soil water, cover, slope and rainfall energy. A simplified Priestley-Taylor equation was employed in the water balance model to calculate potential evapotranspiratiori and the effect of residue cover on evapotranspiration was also considered in the model. Preliminary results of model calibration and validation showed that the model explained 80~?0% of the variations in daily runoff and 82?4% of the variations in soil water content. The model was designed to predict runoff, soil water balance for different management options and different conservation tillage practices in dryland areas of Northern China. Supplied with local soil and crop parameters and daily climate data, the model can simulate soil water balance on daily basis toevaluate different conservation tillage systems and to assess their adaptability to different climate or soil...