Hydrology is the study of how water moves from the atmosphere to the land or ocean and back again – commonly known as the water cycle. This is a VERY simplified description of the hydrologic cycle, the movement of water is influenced by many factors. 

DIAGRAM OF HYDROLOGIC CYCLE

Evaporation 
Liquid water can become water vapor when sunlight (or other energy) heats the water molecules. Water is subject to evaporation whenever it is exposed to the atmosphere, such as on the surface of a lake, river or ocean or on a plant, animal or soil.  

Transpiration  
Plants move water from the soil through their roots to their leaves.  This water is lost to the atmosphere as water vapor in a process called transpiration. Transpiration provides a pathway between water stored deep in the soil and the atmosphere.  Water movement to the atmosphere by evaporation and transpiration is sometimes combined into the single term, evapotranspiration. 

Sublimation 
When water is frozen solid as ice or snow, it can be heated and directly converted to water vapor, without melting into liquid water.

Precipitation 
When water vapor condenses, it forms droplets that fall to Earth as rain, snow, or hail (depending upon the temperature). Precipitation is the ultimate source for all water that flows though streams and rivers and enters lakes and groundwater. 

Interception 
Some precipitation never reaches the ground because it is captured as it falls onto the surface of vegetation, twig and leaf litter, or other above ground structures. In areas with healthy vegetation, interception can store up to 25% of the precipitation that falls. 

Infiltration  
When precipitation falls onto a permeable surface, such as soil, it can enter pores and soak into the soil. Soil pore size depends upon the inorganic particle size (sand, silt or clay) and whether biological organisms, such as plants, worms or moles, have created large holes called macropores.   

Runoff 
Water that cannot be intercepted, stored or infiltrate the soil will run off the soil surface. Runoff follows topography, so the streams and rivers where surface water runoff comes together are usually in valleys and geographic depressions. Urban areas have large amounts of impervious surface, such as concrete and rooftops, so a large percentage of precipitation is lost through runoff. 

Storage 
Water is stored when it isn’t actively being cycled. Typical reservoirs where water is stored include the ocean, lakes, wetlands, groundwater, snow and ice. Some reservoirs of stored water are essential for sustaining stream flow during the dry summer. The length of time water is stored in one of these reservoirs may vary from hours to hundreds of years.   

Groundwater 
Water that infiltrates soil usually reaches an impermeable soil or rock barrier (called an aquitard).  Once a barrier is met, water will fill the soil pore spaces and create an aquifer or an underground reservoir.  The water table is the level below which the ground is saturated with water. Water can flow back and forth between surface and groundwater, depending upon how far below the soil surface the water table is.  

Climate 
Climate is defined as the long-term weather pattern of a particular area, this includes temperature, precipitation, and seasonality. It is important to consider that even in areas with high average precipitation, like western Oregon and Washington, the majority of that precipitation falls during a few months of the year. Large rain events can exceed the soil infiltration rate, and once lakes and wetlands are filled, much of the water runs off to the ocean. In areas with higher elevations, some of this precipitation will fall as snow, where it can be stored until it melts in the spring and summer. 

Physiography 
Physiography is a term derived from "physical" + "geography", and refers to the topography and the parent material it is derived from. Topography, the shape of the land, can influence weather; clouds build up on mountain ranges and precipitation is more likely to fall in the form of snow at higher elevations. Parent material is the mineral rock from which soil is formed. In the Pacific Northwest, the dominant parent material is of volcanic origin, although flood and glacial deposits are also present. Parent material influences the flow of water. Rock may create a barrier, while water will flow through deposited soils. The size of pore space in different rock and soil types further influences whether water will flow on the surface or as groundwater.