| The Weibei Highland is situated in the middle-north part of Shaanxi province and composed of five cities and 30 counties. Agriculture, as its largest industry since ancient times, plays an important role in the economy of Shaanxi province and the west development of China. Compared with the other regions of the loess plateau, the Weibei Highland is relatively flat. Its major landforms are loess mounds, tablelands and gully mounds; it has flat surface, deep soil, porous soil structure, good soil permeability, high water-holding capacity and thus are referred to as "a natural reservoir of soil water". It has favorable climatic conditions, sufficient sunshine, rich warmth resources and high productive potentials of sunlight and warmth; but it is deficient in surface water and underground water and thus its agricultural production mainly relies on natural rainfall so that it is a typical region of rain-fed farming. It has low rainfall that varies year by year and unevenly distributes among different seasons, and most of which occurs in unavailable and poorly available forms, and heavy rainfalls and rainstorms, or does not match with the period of crop growth, and this is its most dominant raining pattern. Therefore, drought and water shortage as well as low soil fertility and severe water and wind erosions become important limiting factors to prevent its agricultural production from quick development. Many approaches of high-efficiency rainwater use have been developed in various regions to make as full use of limited rainfall as possible and the thesis collects and studies these approaches and summarizes that these approaches contain five practices and their corresponding measures. 1. Preparing farmland to retain as much water as possible. Such measures as land leveling and terrace land constructing should be adopted to transform "three-loss"land into "three-retaining"land so as to retain as much rainfall falling on ground surface as possible, increase soil infiltration, reduce runoff and finally effectively control water and soil losses. According to the investigations concerned, terrace land could reduce surface runoff by about 70%, increase soil water-holding capacity by more 30%, had 2-3 times and 3-4 times as much crop yield and economic return as those of slope land respectively. 2. Conducting proper soil tillage to maximize water storage in soil. During summer and winter fallows, deep tillage, zero tillage, rotary (harrowing) tillage should be combined with stubble (straw) mulching and film mulching to form different tillage systems conducive to effectively store rainfall and retain soil moisture so that fallow lands in winter and summer can keep as much rainfall in soil as possible. Where the deep tillage integrated with stubble (straw) mulching and the micro-collecting practice of water integrated with two mulching modes were adopted for fallow lands in summers, the water storages in 0-2m soil was determined immediately before wheat sowing to increased by 70-100mm and the rates of water storage in 0-2m soil to reach 50-70%, compared with those where traditional tillage practices were adopted. Where the zero tillage integrated with bent-stalk mulching and the rotary (harrowing) tillage integrated with chopped stalk mulching were adopted for fallow lands in winter, the water storages in 0-2 m soil was measured immediately before corn sowing to increase by 10-30mm and the water contents in topsoil was measured before corn spring sowing to still reach 15-17%, which was favorable for spring sowing. 3. Improving land fertility so as to increase water use efficiency through fertilization. Scientific fertilization at increased rates is the effective approach to increase soil nutrients, raise land fertility, increase crop yields and greatly improve rainwater use efficiency. Fertilization at increased rates is conducive to not only increasing water use efficiency through fertilization but also increase fertilizer use efficiency through water; fertilizer rates should depend on water supply; scientific fertilization stresses both the combination of organic and inorganic fertilizers and the combination of nitrogen, phosphorous and potassium fertilizers as well as fertilization methods. It was experimentally showed that the yield of winter wheat could increase by 95.0% with the combination of nitrogen and phosphorous fertilizers compared with that compared with the fertilization without any combinations and by 31.7% with the fertilization of only nitrogen fertilizers compared with that without any fertilizations. The yield of winter wheat with the fertilization of only phosphorous fertilizers insignificantly changed, even slightly decreased, compared with that without any fertilizations; The yield of winter rapeseed with the combination fertilization of nitrogen and phosphorous fertilizers increased by 205.4% compared with that without any fertilizations, the yield of winter rapeseed with the fertilization of only nitrogen fertilizers increased by 116.2% compared with that without any fertilization, and the yield of winter rapeseed with the fertilization of only phosphorous fertilizers did changed compared with that without any fertilizations. The yield of winter pea with the fertilization of only phosphorous fertilizers significantly increased by 29.9% on average compared with that of the control, and the yield of winter pea with the fertilization of nitrogen and phosphorous fertilizers showed the second high increase by 21.2% comparedwith that of winter pea without any fertilizations; the yield of winter pea with the fertilization of only nitrogen fertilization insignificantly increased by only 7.5% compared with that of winter pea without any fertilizations. 4. Optimized farming practices and efficiently and wisely using water. A great number of researches have demonstrated that all proper crop arrangements, proper crop rotations and optimized farming models and standardized key techniques play a role in raising rainwater use efficiency while optimized farming models show the most significant effect. Of many farming models, the faming models with crops planted at the sides of ridge-mulching film show much more prominent effects on high-efficiency rainwater use. According to researches, the yield and water use efficiency of wheat planted at the side of ridge-covering film increased by 25-30% and 3.5-4.1kg/(mm·hm2), respectively, compared with those of furrow-planted and traditionally-fertilized wheat; the yield and water use efficiency of corn planted at the side of ridge-covering film increased by 49.0% and 7.3kg/(mm ·hm2), respectively, compared with those of furrow-planted and traditionally-fertilized corn. 5. Collecting rainwater for supplementary irrigation to realize temporal and spatial control of water. The Weibei highland is well known for its high rainfall fluctuation, and so the rainwater-collecting system in rainy seasons can effectively collect and store rainwater and runoff and be capable of the temporal and spatial control of them thereby transforming passive fight against drought into active fight against drought and realizing the temporal and spatial utilization of rainwater. Researches demonstrate that although the Weibei highland has a low rainfall, its limited rainwater, if completely collected, amounts to a considerable amount of water. In the crucial periods of crop growth, especially when drought occurs, the facilities and methods of water-saving irrigation can be utilized for supplementary irrigation so that the yield-and profit-increasing effects can be obtained. According to experiments, the yield of film-mulched corn complementarily irrigated with 60 mm water during fifteen days before heading (called the big-speaker-shape period) increased by 32.4%, compared with that without the supplementary irrigation during fifteen days before heading (called the big-speaker-shape period); the yield of film-mulched wheat complementarily irrigated with 22.5mm water at the jointing stage increased by 43.5% compared with that without the supplementary irrigation at the jointing stage.
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