HYDROLOGIC CYCLE     I. INTRODUCTION A. Hydrologic cycle--the continuous recycling of water from oceans, through the atmosphere to the continents and back to the oceans B. Water is the most active erosive medium II. DEFINITIONS A. Transpiration--water vapor released by plants 1. 54-97% of water is involved in evaporation and transpiration B. Run off--water that flows over the land surface 1. 2-27% of water is involved in run off C. Infiltration--flowage of water into pores, (groundwater) 1. 1-20% of water is involved win infiltration D. Running water 1. Stream gradient--the slope over which streams flow 2. Slope--the steepness of a river's course a. Steeper the slope, greater the river velocity b. Greater the velocity, more erosive 3. Determining the slope--rise/run equals slope or gradient (ft/mile) E. Pores--open spaces in soil, sediment, or rocks F. Porosity--the capacity for sediments and rocks to contain water 1. Expressed as the % of the sediment or rock volume that is open space 2. Ranges for different rock types a. Metamorphic, igneous, limestone, and dolostone=low porosity unless fractured b. Unconsolidated sediment, soil, gravel, etc. = high porosity G. Permeability--the capacity of the sediment or rocks to transmit water. The degree determines the ease with which water flows through sediments or rocks 1. Permeability indicates the occurance of porosity, but porosity doesn't always mean its permeable. Because a. If open spaces are not connected b. Or open spaces are so small the flow is inhibited by friction 2. Example sediment that is vary porous, but pores are so small that there is no movement a. Swells with wetting b. Shrinks with drying H. Aquifer--a permeable layer transporting groundwater. Best aquifers are deposits of well-sorted, well-rounded sand and gravel e.g. sandstone or limestone: fractures & solution cavities I. Aquicludes/aquitards--permeability is so slight that they are effective barriers to groundwater movement e.g. evaporites, clays and shales J. Water table--directly above zone of saturation, normally mimics topography K. Zone of Saturation--where surface water soaks into pores L. Drawdown or cone of depression--lowering of watertable due to pumping, "well can go dry" M. Perched water tables--where descending surface water encounters an aquitard. A trapped, local saturated zone N. Margins of water bodies--watertable III. AQUIFERS A. Unconfined--merely the saturated zone, able to rise and fall with changes in climate (wells drilled here require pumping) B. Confined--aquifer overlain by an aquitard (impermeable unit) retarding wupard movement of water 1. Recharge Area--where confined aquifers intersect the land surface, and are recharged with water 2. Artesian--where water rises towards the elevation of the water table (high fluid pressures confined above and below) 3. Potentiometric surface--the imaginary level to which water, in a group of artesian wells, tends to rise IV. EROSION BY GROUNDWATER A. Carbonate rocks--water moves through factures in limestone or marble, dissolves and enlarges openings      H2O + CO2 -- > H2CO3      (rain)               Carbonic acid B. Karst topography--that which results from groundwater dissolution of carbonates e.g. sinkholes, disappearing streams, solution valleys C. Cave formation--combined effects of weathering and erosion by groundwater 1. Caves--natuallly formed subsurface opening, generally connected to the surface, large enough for a person to enter 2. Cavern--a very large cave or system of interconnected caves e.g. Carlsbad, N.M. 3. Cave deposits--dripstones, water seeps through cave, some CO2 in water escapes, depositing a small amount of calcite a. Stalactites--icicle-shaped structures hanging from ceiling b. Stalagmites--water that drips from ceiling, precipitates on the ground c. Column--the joining of stalactites and stalagmites