Compare several years of weather data. Temperature and Growing Degree Day data on a year to year basis are also valuable. Many times yield variability can be related to drainage and water management. The two main reasons for improving the drainage on agricultural land are for soil conservation and enhancing crop production. Research conducted in Ohio and throughout the Midwest has documented many benefits of agricultural drainage improvements.
Drainage probably causes more variability in yield than any other factor.
Environmental conditions control a significant amount of the crop growth potential compared to what we can control. In Ohio, a good drainage system is our best economic practice for managing soil moisture. Soil survey maps, topography, tile location, and drainage patterns are very important pieces of diagnostic information. Note compaction related problems from farming in wet years which can affect drainage patterns. Pests Maps or even general recorded information on weed, insect, and disease patterns in fields can be very valuable in yield map interpretation.