DP or Diastatic Power of malt is a measure of the starch digesting ability of a group of enzymes found in malted grains. Growing season has some influence, but not dramatic. The potential level of DP is set by the particular barley variety. However, in developing malting varieties, plant breeders select malting barley for the it brewing qualities of which DP is one. Thus, barley malting varieties today all have the potential to develop DP in the 130 - 160 range. When the maltster is making a base malt from a given variety of barley, he develops these enzymes during the germination process. However, when he dries the malt in the kiln, he sacrifices some of the enzymatic activity to get the proper color and flavor in the finished product. High heat at high moisture results in enzyme denaturation. The maltster dries the malt carefully at lower temperatures (90 - 100F) to get the moisture down to 12% or so. He then increases the temperature to about 180F to develop color and flavor. Most of the enzymatic activity is saved by doing this. Obviously, there is some variation in DP batch to batch so each lot is analyzed for its DP level as well as several other parameters.
Now for barley malts of higher colors, like pale ale malt, Vienna malt, and others, the DP levels go down with increasing color. Roasted malts like caramel malts, chocolate malt, etc have no DP. Malted wheat has a DP like barley base malts.
6 row barley once had a significantly higher DP level than 2 row. However, today the difference between these is much smaller. For example, a 6 row barley malt runs 150 - 160 DP and 2 row 130 - 150. For us all-malt home and craft brewers, this is excess DP for brewing. For brewers who make beers containing starch adjuncts ( AB, Coors, etc), they need the higher DP to breakdown the added starch in the mash.
In terms of a correlation of modification and DP, for the level of modification we see in commercial malts, there is not a strong correlation here. The enzymes are produced early in the germination process and then just take time to breakdown the endosperm of the kernel resulting in modification.
I hope this helps.