this lecture discusses the hydrologic cycle the majority of water is held in the ocean basins and isn't available for human consumption the greatest amount of freshwater is found in the cryosphere that storage location where water is in solid form such as glaciers sea ice and snow water in the cryosphere is generally not considered accessible for human consumption only a very small percentage of the Earth's total water is found as freshwater at the surface in lakes and streams it's much less than half a percent there's more water below the ground as groundwater than there is above the ground in lakes and rivers the hydrologic cycle simply shows the storage locations and transfer mechanisms of the waters in the Earth's atmosphere system storage locations include the atmosphere ocean basins and glaciers transfer processes include evaporation and precipitation other transfer processes include transpiration and evapotranspiration transpiration is the loss of water in vapour form from plants evapo transpiration is the combined loss from plants and land surfaces these terms describe a loss because the water is no longer available for consumption interception is an important part of the hydrologic cycle interception is when precipitation is interrupted before it hits the bare soil in a natural environment trees are very effective at interception interception allows precipitation to move gently into the soil as opposed to hitting the surface directly when precipitation hits the surface directly soil is disrupted particles of soil are dislodged these can plug the soil and ultimately prevent infiltration interception also helps prevent sheet erosion sheet erosion is when runoff moves a very thin layer of topsoil downslope water in the zone of aeration is called soil moisture the zone of aeration is usually the uppermost portion of the soil profile where air pockets still exist as the water continues its downward movement under the force of gravity the water can collect in amounts that saturate the soil this is the zone of saturation and water in the zone of saturation is called groundwater the dividing line between the two zones is the water table the water table is not fixed meaning that it can move up and down depending upon precipitation or groundwater use when plants intercept rainfall water travels down the branches and trunk that water is called stem flow water that drips off the canopy is called canopy drip through fall is water that falls between the leaves and branches of the canopy different plants will have different ratios of stem flow to through fall however stem flow is typically the preferred pathway for soil moisture infiltration occurs when precipitation initially moves into the soil each soil type has different infiltration rates dependent upon the ratio of clay to silt to sand some surfaces like sand allow greater infiltration urban surfaces and deserts both have low infiltration as such these surfaces are more susceptible to flooding with locations that have low infiltration rates like deserts and urban locations precipitation intensity is often larger than the infiltration rate that means that even small amounts of precipitation can't infiltrate into the soil and instead run off as flooding once the water infiltrates the surface it starts a slow downward movement into the substrate this is percolation percolation is controlled by the availability of pore space for water to move into and the ability for the water to move through the soil porosity describes the available pore space which is a function of existing saturation levels permeability describes the rate at which water moves through the soil and is a function of the different soil types