Rainfall is a main water source for dr yland farming in north western loess plateau.The aver age total amount of annual precipitation( 443mm) is 2. 757×1011m３

which is about 9. 2 times of annual surface water and ground water used at present.Because of the uneven distr ibution on time and space

and “being easy to lose and changable”of rainfall

soil and water loss and drought are seriously. T he precipitat io n use rate is about 40%

the w ater use ef ficiency of crops is only 3. 0～4. 5 kg / ( hm2·mm) . The collecting and regulating of rainfall by eng ineer ing project s is a link point of soil and water conservat io n and farming production

it is also a way of water-saving agr iculture. Not only can it reduce soil and water loss by collecting rainfall

but it can also make rainfall tog ather to produce great effect on raising yield by irrig at ing at critical growth stage of crop.There is a long histo ry on collecting rainfall in north western region

and there are small reser voirs

dams

pools

wells

terraces separated by slopeland and other ways

water har vesting cellar is more than 1 million. The problem is these ways have not formed a system

and shortage of a good plan. Many problems on rain water collecting

storing and high efficient use need to be solved.