Of the several phases in the hydrological cycle, that of evaporation is one of the most difficult to quantify. Certainly, it is difficult to define the unseen amounts of water stored or moving underground, but above the ground surface, the great complexities of evaporation make it an even more elusive quantity to define; yet evaporation can account for the large differences that occur between incoming precipitation and water available in the rivers. In the UK, if annual totals are considered, evaporation would appear to deprive parts of south-east England of all of its rainfall; in actual practice, evaporation amounts vary seasonally and surplus surface water feeds the rivers in winter. In hotter climates with seasonal rainfall, evaporation losses cause rivers to dry up and river flows are dependent on excessive, heavy rainfall in the wet season.