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Movement of water across a watershed varies with season, year, and position within the watershed. The water balance of different positions throughout watersheds will require an understanding of the water use patterns of different cropping or land management systems. To implement the concepts of Total Maximum Daily Load (TMDL) will require an understanding of the water balance across a watershed. Studies have been conducted on the year-around water balance of cropping systems in the subsurface drained areas of central Iowa since 1991. These measurements have been made on a range of different tillage systems in corn and soybean rotations in different soils within Walnut Creek watershed. Seasonal water use is dependent upon the precipitation, soil, and nutrient management of the crop. Variation of water use within a field varies with soil type. Development of water balance for these soils and crop rotations revealed that drainage patterns varied with the soils within the watershed. In the low organic matter soils drainage started earlier in the year and lost more water than higher organic matter soils. These findings have been incorporated into methods for assessing watershed water balance methods. Understanding these dynamics across a watershed allows the development of the potential best management practices to meet TMDL requirements.