Discussion of soil degradation mechanism and efficient irrigation schedule in irrigated arid lands

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1.Vegetable cultivation in a greenhouse was performed using the microirrigation under soil-water control. It was shown clearly that the yield and quality of leafy vegetables were produced by suitable soil water range. The soybeans cultivating experiments were compared using three different types of mulching ; recycled paper mulch, black poly-film mulch, and silver film mulch. Growth of soybeans in the experimental field under recycled paper mulch was the best under the same amount of microirrigation, the water use efficiency was also high, and it was useful as water-saving cultivation. The microirrigation schedules with tank systems were recommended using the simulation models with irrigation parameters measured in the Tottori sand fields located in the west part of Japan. The simulation was carried out using daily rainfall, evapotranspiration and water retention curves of sand mixing with artificial zeolite. By applying characteristics of artificial zeolite to the microirrigation sche … More dules, saving amount of irrigation water were estimated for vegetables cultivated in Tottori sand fields.2.An experimental study was made on the effect on the soil of changing rainfall intensity in a short period of time. In the test on constant rainfall intensity. crust was formed at the initial stage. Unlike the constant rainfall intensity test, the soil loss gradually increased with the passage of time in the test on increasing the rainfall intensity. The cause for soil loss increase, however, differs between clayey soil and sandy soil. In the test on decreasing the rainfall intensity, the soil loss decreased with time. A change in the rainfall intensity strongly affected the clayey soil in terms of splash rate. Experiments were carried out by supplying artificial rainfall on the bare slope. In this experiment, rill networks that develop on the bare slopes and the time variations of soil run-off were measured. Particle size of run-off soil was analyzed and the relationship between the soil hardness, clay content of original soil and soil loss was investigated. And confinned that the change of configuration of soil surface was deeply influenced by the clay content of soil.3.The effects of clay minerals, clay contents and salinities of equilibrated solution to evaluate Na-Ca exchange selectivity using Montmorillonite and Illite were studied. The dielectric constant ε and bulk electrical conductivity (EC) of Na- and Ca-bentonites were measured by time-domain reflectometry (TDR). Bentonite samples though prepared by distilled water provided considerable magnitude of EC having a maximum peak value at volumetric water content θ=0.60 m^3 m^<-3>, the lowest limit of θ for saturaion : 3.88 dS m^<-1> for Na-bentonite and 1.45 dS m^<-1> for Ca-bentonite. Due to energy loss, TDR waveform was almost completely attenuated between θ=0.45 and 0.87 m^3 m^- for Na-bentonite and between θ=0.36 and 0.50 m^3 m^<-3> for Ca-bentonite. A two-stage laboratory method which consists of multi-step outflow and evaporation phase was developed for rapid estimation of the soil water retention function and hydraulic conductivity function K from near-saturation to air-dry. The function was determined by curve-fitting of the equilibrium outflow data and psychrometric data. The K function was estimated inversely using evaporation data and final water content profile. Compared with K data from other reliable method, reliability of the presented method was verified. Less