Study On The Models Of Optimal Rainwater Utilization In Terraced Field System In Loess Hill-gully Region

The loess hill-gully region located in the Loess Plateau, covering the western Shanxi province, the northern Shaanxi province, southern Yingxia Hui autonomous region and central Gansu province. The total area is 358.5 thousand km2, among which, most serious area of soil and water loss is about 156 thousand km2, its annual sediment discharge into Yellow River accounting for 80% of the total sum, is up to 1.4 trillion tons. The loess hill-gully region belongs to continental monsoon climate, lower rainfall, higher relative rainfall variability; and its annual rainfall mainly focusing on June to September, much cloudburst, strong evaporation, low vegetation coverage. Drought, barren soil and soil and water loss are the main limited factors of ecological environment, agricultural production and regional economic development. There is much large area of fundamental farmland created during several decades of management in the loess hill-gully region, among which, terraced field is 3.8 millions hm2, accounting for 74%. Because of disharmony between the periods of rainfall and crop requiring water, high silt content in loess, poor water conservation ability and high rate of evaporation, it will be difficult to achieve high and steady production in this region even the terraced field could conserve water, soil and fertilizer. Thus, we combine 4 measures as soil and water conservation, cultivation to fight drought and water conservation by agricultural methods and auxiliary irrigation with collected rainwater into the technology system of optimal rainwater utilization in terraced field. Based on the analysis of rainfall in test field and available rainwater amount in terraced field system, 10 different treatments according to 2 cropping patterns and 5 models of auxiliary irrigation were designed and conducted in Wangjiagou watershed in western Shanxi province. The results show that: (1) The mean annual rainfall amount is 479.3 mm in test field, and the yearly relative variability is 0.6%~57.8%, average 23.1%, which demonstrates an obvious divergence on annual rainfall. The monthly relative variability is 6.1%~186.7%, average 78.4%, which is comparatively higher than that of yearly relative variability. This reflects a great disparity in monthly rainfall of a year. The uneven distribution of rainfall within years and among years frequently leads to spring drought, summer drought or waterlog of summer and autumn in average rainfall years or even full rainfall years. (2) The roads, sloping cultivated land and low-density forest land at terraced field periphery in Wangjiagou watershed are ideal water collection ground, and is up to50.7 thousand m~3 a year. If the rainwater use rate is 70%, the available rainwater in terraced field system reaches to 35.5 thousand m3. And 352.2mm amount of rainwater can be collected through the surface of terraced field in Wangjiagou watershed from May to September. If 1/9 surface of terraced field is used to collect rainwater, existing 230.8 hm2 terraced field will collect 101.6 thousand m3 amount of rainwater. (3) Applying auxiliary irrigation with collected rainwater will improve rainwater use rate, ameliorate soil water content and effectively resist drought. Being serious drought in July, soil water content in 0cm~40cm layer by ridge and film covering treatment in 2001and 2002 is 8.23%, 7.77%, and that of high-yield ditch and film covering treatment is 8.67%, 8.66%, but that of conventional cultivation is only 6.12%, 7.31%. Obviously, the soil water content of auxiliary irrigation treatment is higher than that of conventional cultivation, which shows better drought resistance. As for the whole growing period of maize, soil water variation of conventional cultivation is acute, following by ridge and film covering treatment, high-yield ditch and film covering treatment. (4) Variance analysis of yield under 2 factors shows that there exits a significant difference in yield among cropping pattern, irrigation regimes and their mutual effects. The yield of ridge and film covering and high-yield ditch and film covering treatment is much higher than that of conventional cultivation. The optimal models are ridge and film covering (1: 7) and high-yield ditch and film covering (periphery), and the model of high-yield ditch and film covering is commonly better than that of ridge and film covering.