Optimization of ponds allocation in small hilly watershed in Southwestern China

The annual rainfall in southwestern China is usually abundant in hilly areas. Nevertheless, due to the effects of terrain and disproportional rainfall distribution, the efficiency of rainwater use for farmland irrigation is actually low. Coupled with the weak storage capacity and the lack of water reservation projects, the severe shortage of water appears in some districts. As for ponds widely applied in hilly area, the phenomenon frequently occurs that large idle capacity and severe overflow lead to low irrigation utility. Moreover, gravity irrigation is regarded as a general requirement for newly-built ponds so as to reduce cost of pumping irrigation. This paper discusses an optimized allocation method of ponds under the primary goals of relatively adequate rainwater harvest for pond-irrigating and maximum gravity irrigation rate in a hilly watershed. The method takes the natural advantage of terrain elevation difference of hilly areas into account.Based on the actual situation of study area, the pond-irrigation was set to aim at meeting the water requirement of fruiters, vegetables and dry crops from July to October. With the terrain data, the hilly watershed was divided into 38 catchment areas by hydrological analysis tool in ArsGIS software. The crop irrigation schedule from July to October was determined by referring to technical code for rainwater collection, storage and utilization. Then the actual irrigation requirement in each catchment area could be calculated with the crop cultivated area being computed respectively. Given the local annual average rainfall from July to October, the minimum threshold was calculated to guarantee the rainwater harvest for 200 m~3 pond, with which the catchment path of the small watershed was extracted based on the surface runoff-flowing model. Meanwhile, the appropriate number of 200 m~3 ponds to build in each catchment area was figured out to meet the crop irrigation requirement from July to October with reference to the irrigation schedule. Finally, the allocation of ponds would be optimized by setting rational allocation constraints as follows: firstly, slope of the site should be smaller than 5 degree; secondly, the pond should be built within 500 m from rural residential areas, 100 m from crop areas and 100 m away from land for mining and industry; thirdly, the sites would lie along the first level catchment path to avoid the overlap of the water water-collecting area between the adjacent ponds and the higher position would be given priority to with the two constraints above satisfied. Fourthly, if the number of sites matching the first three constraints was larger than the appropriate number to be built, the higher position site would be preferred. The suitable pond sites are screened in the catchment path with ArcGIS spatial analysis. 65 sustainable sites for 200 m~3 pond are selected using this method. The average water-collecting area of all the pond sites was estimated to be 0.31 ha and prospected to guarantee the relatively adequate rainwater harvest of 200 m~3 pond under the condition of rainfall in study area. Furthermore, the gravity irrigation rate of farmland within the watershed came to nearly 50%, showing that the method contributes to reduce the irrigation cost. Additionally, the method can provide guidance for the allocation of pond in different specifications and reference for other kind of rainwater harvesting engineering in land consolidation project planning in hilly areas.