The Köppen designation for a semi-arid climate is BS.
In the Köppen climate classification, B identifies dry climates and S identifies steppe, or semi-arid, conditions. The BS category sits between desert climates, coded BW, and more humid climate groups. It describes places where precipitation is below potential evapotranspiration but not low enough to qualify as a desert.
The code can be refined with a third letter: BSh means hot semi-arid, while BSk means cold semi-arid. The distinction depends mainly on mean annual temperature. A common mix-up is reversing BS and BW: BW is desert, whereas BS is the somewhat wetter steppe category. The Sahel, parts of Central Asia, and portions of the western United States include BS-type climates.