In this paper, the hydrologic characteristics of semi desert basin was analyzed, two different water balance equations respectively for desert covered area and non desert covered area in semi desert basin were developed, and hence a calculation method for desert covered area was proposed.
Because of dry climate ,strong sunlight and lacking water resource in the areas such as Ningxia Gansu and Shaanxi, some districts is becoming desert, so that land is washed away crops are infected and the harnessing of desert is hindered, In order to solve these problems the auto-control water saving system in desert area is designed.