Cover Large background photo is summer air-mass thunderstom over Walnut Gulch Experimental watershed; top left is schematic map of the USDA-Agricultural Research Service Walnut Gulch Experimental Watershed and its 10 primary gaged subwatersheds. Center is Walnut Gulch Flume 121, and bottom shows Walnut Gulch Flume 1. Photos courtesy of Roger Simanton.