Prior to a morning hot air balloon flight from a park that the pilot always launched from, the pilot observed on his way to the launch site that there was no movement in the trees or movement of flags, indicating that there was little or no wind. The winds were also reported as being variable at 3 knots at a weather reporting station near the launch site. Once at the park, the pilot also launched three pilot balloons (PIBALS) to confirm the wind. All three PIBALS indicated that safe, flyable, conditions existed. After liftoff, the pilot's GPS indicated that the winds were at 2 miles per hour (mph). Approximately halfway through the flight, the surface winds picked up to 10 mph. Based on local knowledge the pilot believed that they did not have many opportunities for landing sites. The pilot therefore decided to land in a field that they were approaching. After positioning for landing in the field, he advised the passengers they would be landing, and scrubbed off some speed by allowing the basket to contact the top of a tree as they approached the field. As they cleared the tree, the pilot began to vent air from the balloons envelope. The basket then touched down "very hard," tipped over, and then dragged across the ground for 30 to 40 feet. Upon exiting the basket, one of the passengers advised that his leg was sore but he was able to walk. A few days later, the pilot received an email from the passenger's wife in which she advised the pilot that her husband had gone to see a doctor and had x-rays, and that he had a broken right tibia.