From 9News.com (Dave Delozier):
Because of the dry winter and spring the South Platte River is not flowing at a level to allow farmers, like Fritzler, to continue to use it as an irrigation source. Unless conditions change Fritzler will not receive any more above surface water for their crops. “We have no surface water,” [Glenn Fritzler] said. “We have an abundance of groundwater, but we do not have the permission to use it.”
Fritzler and other farmers in Weld County want to use their wells to pump groundwater from the aquifers beneath their farms. They are being prevented from doing that by a 2006 Colorado Water Court ruling. In that ruling, the court said farmers, like Fritzler, are only allowed to pump a limited amount of groundwater. If farmers exceed their allotment of groundwater, they are charged $3,000 a day for each pump used.
When the surface water ran out, Fritzler started pumping his allotment of groundwater. It will last for one week. The current drought conditions are expected to extend well beyond that one week supply.
More South Platte River basin coverage here.