A DHC-8 returned safely to the departure airport following the failure of the #2 Hydraulic System shortly after takeoff.

Narrative

Once a positive rate was established I called for 'gear up.' When the gear up selection was made the master caution light illuminated. The First Officer stated that it was a '#2 ISO HYD VLV' caution light. I noted that the # 2 hydraulic contents had dropped to just under one quart and that landing gear advisory lights indicated a discrepancy - three amber gear door advisory lights and one red nose gear unsafe light. We continued to climb and asked ATC for a level off at four thousand feet. I told them that we had a gear problem; that we were running some checklists and would get back with them. The after takeoff checklist was completed and I asked for the '#2 ISO HYD VLV' checklist. This was completed; however; the unsafe gear indication required an airspeed of 140 KIAS which I complied with. The First Officer talked to the Flight Attendant who confirmed the main landing gear were up but the doors were still open. Upon further discussion with the First Officer we decided a return to the departure airport was most prudent. Given the very poor weather conditions; strong gusty winds; heavy rain; wet runways and the loss of some important hydraulic systems we opted to declare an emergency and have all the equipment in place should anything occur during the landing rollout. The company; Flight Attendant; and passengers were notified and we were vectored around to land. The approach was normal; a manual gear extension was performed with a 3 green down and locked indication. The landing was uneventful. We taxied to the gate; deplaned the passengers; pinned the gear and called maintenance.