| In recent times, water shortages have become major factors restricting agricultural development. To ensure China's food security, it is necessary to promote water-saving agriculture. Research on this aspect has been focusing on the agricultural water balance based on precipitation, crop water requirement, soil reservoir, and rain harvesting for supplemental irrigation using technique of 3S. However, few researches have been taken into account of water balance, potential of collectable rainwater resource with small hydraulic engineering. And most researches have had been focusing in Northwest China rather than seasonal-drought region in Southern China. Consequently, in this study, small hydraulic engineerings are collocated in order to enhance the coefficient of rainwater harvesting based on analysis of climate, evaportranspiration and soil reservoir. And the study area is in Daba Village of Rongchang County with the water-saving agriculture demonstration site there. The scale for research is local scale because of the terrain fragmentation. The results include that:(1)Seasonal drought in the study area occurs in three periods of March and April, May and early June, late June to early September. And soil drought is more serious than meteorology drought. For meteorology drought, the occurrence frequency of spring drought from March to April is 45.0%, while summer drought from May to early June and hot summer drought from late June to September 22.6% and 35.5%, respectively. However, the occurrence frequency of spring drought, summer drought, and hot summer drought is respectively lower than the probability of wet (dry) years. For soil drought, probability of spring drought is 61.2%, while summer drought 52.6%, hot summer drought 64.5%. But probability sight drought, mid-drought and serious drought in spring drought, summer drought, and hot summer drought is not same with stationary distribution probability. For spring drought, probability of mid-drought and serious drought is little lower than stationary distribution probability. For summer drought, occurrence frequency of it is approximately equal to the stationary distribution probability. For hot summer drought, probability of slight drought and mid-drought is lower than stationary distribution probability, while probability of serious drought is higher than stationary distribution probability. For spring drought and hot summer drought, probability of three degrees of spring drought both higher than stationary distribution probability.(2) The land use patterns have significant influence on the evapotranspiration, and are correlated with terrain grades. Cultivated land is the major land use pattern with area of dry land 81.8105hm~2 and paddy field 33.5616hm~2. Dry land is mainly distributed on the terrain grade of 8°～15°and 15°～25°, while paddy field on＜2°. The evapotranspiration of dry land is 460mm, while paddy field 592mm with rice-rape patterns. However, the evapotranspiration is only 456mm if the paddy land use pattern only is rice-cultivated. The evapotranspiration decreases along the larger of terrain grade if the land use pattern for paddy is rice-rape cultivated, while the evapotranspiration increase along the larger of terrain grade if the land use pattern for paddy only is rice-uncultivated.(3) The soil water-holding capacity of dry land is small, but the ratio of available water-storage is bigger, and the major influencing factors are the thickness of soil, slope azimuth, and land use type. The variable degree of soil water properties is saturation>field capacity> wilting point. On the soil profile, available water content is the steadiest in the level of 0~20cm, though saturation and field capacity vary most. The bulk density ranges from 1.33 to 1.58, and is correlated with the soil water characteristic. Especially the saturation, the relativity is-0.982. Soil water-holding capacity, water-storage capacity, and available water-storage is 1365m～3/hm~2. 1365 m～3/hm~2, and 804 m～3/hm～2, respectively, and available water-storage contributes 58.9% of soil water-holding capacity. The proportion of water-storage capacity and available water-storage in the soil water-holding capacity becomes respectively bigger as the level ranges from surface layer to bottom, from top to foot of the hill, and the land use pattern from woodland to dry land to paddy land.(4) There is serious shortage of water resource for agriculture because of the disaccord of time between pericepitation and water requirement, which is resolved by collocation of hydraulic engineering in local level. There is obvious deficient of water resource between requirement and supply for dry land and paddy field. For dry land, the deficient happens in January, February, March, April, May, July, August and December, with the largest deficient of 21mm, 28 mm, 32 mm, 15 mm, 30 mm, 42 mm, 58 mm, 53 mm, respectively, and the smallest deficient of 11 mm, 18 mm, 21 mm, 1 mm, 0 mm, 0 mm, 0 mm and 0 mm, separatedly. For paddy field, the shortage mainly appears in March and April, and the deficient is 1.57×10~4m~3. In order to meet the water requirement, 8 hydraulic accumulators with 100 m~3 cubage and 19 hydraulic accumulators with 50 m~3 cubage and 5.05hm~2 paddy field that used to store water are collated based on the analysis of capacity of precipitation and runoff. However, the water-storage of all the hydraulic accumulators can not completely meet the requirement of dry land so that it must obtain water from paddy field, especially in July and August in which sweet potato require much water.All in all, the paddy field that used to store water is desigened for meeting water requirement of paddy field in March and April. The conclusion of this study could be taken as a scientific base, in which water resource is optimally collocated for agriculture in terrain fragments region in Sichuan basin. And future work will be focused on the measure of irrigation on the basis of Purple Hilly area of Sichuan Basin.
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