Background Small watershed is the basic unit in the comprehensive control of soil erosion and water loss. It is of great significance to carry out the division of small watersheds and determine the control direction for soil and water conservation layout at county scale. Yiyuan county, located in the national key management area, one of the key counties for the comprehensive control of soil and water conservation at the state and provincial level, was taken as the research object in the paper.

Methods The basic data were remote sensing ZY-3 with resolution 2.1 m and 1:50 000 maps of topography, soil, administrative division and forest resources. The characteristics of soil erosion in the Yiyuan county were obtained by CSLE model, and its main influencing factors vegetation coverage was calculated by NDVI (normalized difference vegetation index) and field survey. The elevation, topographic fluctuation and slope were extracted and the landform types were classified based on the DEM. The land use was determined by human-computer interaction interpretation and field verification. The division of a small watershed was determined based on the ArcSWAT and 25 m DEM combined with rivers and administration division. The layout of soil conservation and the key area of prevention and control were determined by:1) firstly investigating the needs of soil and water conservation via field investigation, and 2) then by spatial overlay and statistical analysis based on the basic information such as land use, slope, forest and grass vegetation coverage, soil erosion and geomorphology according to the combined method of "top-down" and "bottom-up" and the demand of soil and water conservation while taking a small watershed as an unit.

Results 1) The total area of soil erosion in the county was 765.99 km², accounting for 46.83%, mainly mild erosion, distributed in the southeast, southwest and northeast of the county. 2) There were 68 small watersheds in the county. The average area of a small watershed was 24.06 km², and channel density was from 1.87 to 2.99 km/km². Soil erosion area percentage of 45% small watersheds was more than 50%. 3) Four soil and water conservation districts and "3 Belts and 3 Areas" key layouts were taken out according to the needs of soil and water conservation and characteristics of small watersheds. 4) The 21 small watersheds would be developed into ecologic and clean-type small watersheds, with enclosure mainly (55%). The 47 small watersheds would be developed into ecological economy watershed, of which mainly with enclosure (45%) in the northeast, with economic fruit forest (60%) in the southeast, and with enclosure (35%) and bench terrace (30%) in the southwest.

Conclusions Soil and water conservation layout and measures configuration at county level, based on the small watersheds and polygon, may provide technical and scientific support for county soil erosion control and soil and water conservation management.