The size of the reservoir can vary widely depending on brand and the purpose of the hydration pack. Some hydration packs are extremely small and minimalist, designed to add as little load as possible while running or cycling, while others are more suited for backpacking and extended hikes, equipped with huge bladders. Bladder sizes can range from as little as 50 ounces all the way up to 400+ ounce packs. Some hydration packs include additional storage space in the form of zipper pockets or mesh pouches, while others are simply a means to affix the hydration bladder to the back for easy carrying. Pressurized hydration bladders are included on some models which allow the hose to spray water or even move water through an in-line filter before drinking. Hydration packs are commonly used for outdoor recreational activities, such as hiking, bicycling, and kayaking.