Farmland area and grain cropping index are the main factors that determine grain planting area.
where PI is the cropping index, [A.sub.g] is the grain planting area (ha), and [A.sub.f] is the area of farmland in a certain area (ha).
We further analyzed the main factors causing changes in unit grain yield in Yuxian and Juxian counties and determined that the cropping index and irrigated areas decreased in Juxian, indicating that the increased unit grain yield was mainly caused by chemical fertilizer application and increased machine power.