| Transitional zone between the semiarid and arid regions, with a mean annual rainfall of 250-350 mm, is a climatically sensitive and ecologically fragile belt. This zone lies in the Monsoon Edge Area in the northern China. The highly fluctuation of the climate and population pressure mainly led to the vulnerability of the regional ecology. The boundary line between arid and semiarid region shift in the space at a maximum distance of 110-130 km during the extreme dry and wet year. Water is the key limiting factor for the environmental construction and agriculture production. Rainfall is the major water resource, and poorly distributed in time and space. Rainfall harvesting and water saving system would be an alternative for the improvement of the environment and the establishment of agricultural ecosystem.The paper, with this fragile belt as an interesting study area, was based on the experimental study on in-situ rainfall harvesting and field investigation. The experiment was conducted at the ecological agriculture experiment station (Gaolan County, Gansu Province), Chinese Academy of Sciences, in 1998-1999. The paper was designed to explore runoff characteristics for various artificial catchments and evaluate the catchments based on the runoff efficiency, analyze water quality of harvested water from cistern and catchments, and investigate the effect of on-field rainfall harvesting and mulching combination on crop production. Based on the above studies. a microcatcliment model for ecosystem construction was proposed to rehabilitate the environment of the Loess Plateau. The results of the experiments are listed as follows: (1) More water would be collected from catchments by means of the change of surface soil structure and the management of vegetation cover, or by increase of natural catchment area and use artificial catchments. (2) The purpose of the management of the catchment is to decrease rainfall loss due to vegetation interception and infiltration. Runoff from natural loess slope occurred as infiltration-excess process, and it was mainly dependent upon the rainfall intensity, rain type and antecedent soil moisture. The minimum runoff-producing rainfall of the natural loess slope is 6-8.5 mm, antecedent soil moisture decreased the threshold rainfall by 2.5-3.0 mm. The amount of the rainfall and rain intensity had little effect on the water yield from plastic film, asphalt felt, and concrete treated catchments, which had a threshold rainfall of 0.1-1.5 mm. (3) Many water-harvesting systems have no filters before runoff entering cisterns except silt trap in the Loess Plateau. Usually, these systems were poorly maintained and prone to fecal contamination. High content of coliforms was detected in water samples from 7-month storage cellar, so the rainwater catchment should be often cleaned and some simple filters are needed in most cases if the water is for human consumption. (4) Incorporation of supplemental irrigation and in-situ rainfall harvesting with mulching can better utilize small rainfall events and store water near the plant roots, it can significantly increase rain use efficiency.Based on the current economic and ecological conditions, natural loess slope, compacted clay soil and plastic film treated catchments are easy to be spread due to low cost, easy replication and high adaptability. The cost of concrete catchment is high, and it should be combined with yard and road construction for multi-purpose use. Temporarily moveable plastic film catchment can be recommended to use in rainy period in the field or yard for storing rainwater in the cistern for supplementary irrigation. The trend of high efficient use of harvested water is to combine the water harvesting with installation agriculture, and the high level artificial catchments can be used. The ecological environment construction is very important in the loess Plateau. The microcatchment model for ecosystem construction, with small watershed as a unit, is characterized by incorporation of rainfall harvesting with reversion of tilled slopeland to woodland and grassland. >25°slope is protected as water source area by prevent human and livestock interference, Tree and grass can be planted in the gentle slope by means of microcatchment water harvesting techniques. Also the natural slope can be treated as catchment for collecting runoff into the cistern for supplemental irrigation for crop of the farmland, which is the main part of the system for supply food to farmer by various water-saving agriculture techniques. The village and yard is the center of human activity, which can be regarded as an economic zone by using harvested water to develop greenhouse and stock raising.
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