Design Method of Rainwater Harvesting System for Afforestation in Loess Plateau

On the basis of natural conditions of lack of rainfall on the Loess Plateau, this paper presents a method of design and construction in rainwater harvesting system for afforestation, which is suitable to the regions of 300-600 mm rainfall. The system consists of micro-catchments of various forms and sizes that include planting areas, where trees are planted and trapped runoff, and contributing areas, where overland flow are produced by rainfall. The design is based on rainfall, runoff coefficients of contributing areas, evapotranspiration of trees and soil surface, water deficiency of soil, and environmental capacity of precipitation in the region, and so on. Runoff coefficient of contributing areas with YJG (organic silicon chemical treatment), compacted surface soil slope and natural slope micro-catchments, are 0.8-0.89, 0.23-0.36, and 0.08-0.10 respectively. According to the Penman method, the soil water deficiency varies from 50 mm to 300 mm. In the region of 400 mm precipitation, the contributing area treated with YJG is 3-4 m² for timber forests, 8-10 m² for cash trees respectively; the contributing area treated with compacted soil surface is 6-8 m² for timber forests, 10-12 m² for cash trees respectively; the contributing area of natural slope is 8-10 m² for timber forests, 12-15 m² for cash trees respectively. Transpiration from trees of micro-catchment in YJG, compacted surface soil and natural slope treatment is by 47.65%-53.31%, 24.10%-36.93%, and 18.65%-29.55% of total rainfall (rainfall and harvested rainwater)respectively after the system was applied in the region. This system, which has been widely practising on the Loess Plateau, is now known as runoff forestry.