The last couple of summers a crucial water-retaining slough at Trudy Dawson’s Boulder Valley ranch has run dry early, forcing her cattle to return from grazing before she’d finished the seasonal hay baling.  “That was awful,” Dawson said on a recent afternoon, recalling watching her herd lap up stale water. “I just knew something’s got to change.”  Jefferson County has been in a drought since the early 2000s, and in each of the last six years has at some point been...