The phrase “so as to” means “in order to.” Now, the sentence says that land graders can flatten uneven farmlands “so as not to” = in order not to waste rainwater. So, the sentence suggests that the previous models of land graders used to waste rainwater. But this advanced model can flatten uneven land not to waste water. This meaning is illogical because land graders do not waste water any which way. Land graders have got nothing to do with rainwaters. This sentence presents an illogical meaning and hence is incorrect.