Dry farming is the raising of crops in semiarid areas without making use of irrigation. The essential principle of dry farming is the encouragement of efficient retention of moisture by the soil, coupled with selection of crops that can make best use of that moisture. Among the techniques employed are tilling the land and eradicating weeds; wide spacing of crops; leaving the stubble after harvest to act as a snow-trap over winter; letting clods of earth or dry vegetable matter lie on the land so as to reduce water runoff and soil erosion; contouring of fields; and, occasionally, allowing fields to lie fallow in alternate years. Crops may be of two types: those whose growing seasons permit them to evade the summer drought; and those which are able to survive the drought by reduction of their own moisture loss.