NRES 201 Lectures 18 19 Fall 2014 The Hydrologic Cycle NRES 201 The Hydrologic Cycle 1 The Hydrologic Cycle Today s lecture topics The global hydrologic cycle Fate of precipitation and irrigation water The soil plant atmosphere continuum 2 1 NRES 201 Lectures 18 19 Fall 2014 The Hydrologic Cycle The global hydrologic cycle Fundamental realities The world s water resources are not evenly distributed Annual precipitation varies From in for the Atacama Desert in Chile To 460 in for a rain forest region of northeastern India Seasonal variation can also be extensive When flooding is followed by drought Atacama Desert Source http news softpedia com Mawsynram India Source http theatlantic com 3 Water management has changed From the old way adapt culture to environment Settle near water Harvest the rain Live as nomads To the new way adapt environment to culture Irrigate arid lands to make them productive Build cities in the desert Deplete the aquifers Source http eye4ethnic blogspot com Palm Springs California Source http wellwornroad com 4 2 NRES 201 Lectures 18 19 Fall 2014 The Hydrologic Cycle Global stocks of water 1400 million km3 of water on Earth Most of this water is in the oceans and does not cycle Active cycling involves 0 05 of the Earth s water in The atmosphere The surface layer of oceans Shallow groundwater Lakes and rivers Source Brady and Weil 2008 Soils 5 Global water use Dominated by irrigated agriculture Increases by 10 per decade Highest per capita consumption in the USA Source http www nap edu Per capita water consumption Source http advisoranalyst com Source http www wrsc org 6 3 NRES 201 Lectures 18 19 Fall 2014 The Hydrologic Cycle How water cycles Solar energy drives evaporation Water vapor forms clouds The clouds move and form precipitation Rain and snowfall return water to land and seas And lead to runoff and percolation that resupply streams rivers and the oceans 78 86 Source Brady and Weil 2008 7 Water balance The usual focus is a watershed Land area drained by a single system of streams Precipitation falling on a watershed can be Stored in the soil Returned to the atmosphere by evaporation or transpiration evapotranspiration Discharged by surface or subsurface flow Represented as P SS ET D where P precipitation irrigation SS soil storage ET evapotranspiration D discharge 8 4 NRES 201 Lectures 18 19 Fall 2014 The Hydrologic Cycle Impacted by management Irrigation increases the input of water And calls for strategies to maximize soil storage SS by minimizing discharge D Clear cutting trees in a forest will shift the output of water From evapotranspiration ET To discharge D Source Brady and Weil 2008 9 Fate of precipitation and irrigation water Interception discharge or soil storage Precipitation intercepted directly by plant foliage may not reach the soil Up to 50 in forests Increased by sublimation of snow collected on conifer branches Source Brady and Weil 2008 10 5 NRES 201 Lectures 18 19 Fall 2014 The Hydrologic Cycle Water that reaches the ground may Enter the soil by infiltration More likely with good soil structure Or remain on the surface if rainfall or snowmelt exceeds infiltration The result is ponding or runoff Prairie potholes South Dakota Source http ecointheknow com 11 Water that enters the soil will leave by Evaporation from the soil surface Plant uptake transpiration Subsurface drainage Can account for up to 50 of water input Some availability through capillary rise during dry periods Source http web2 geo msu edu 12 6 NRES 201 Lectures 18 19 Fall 2014 The Hydrologic Cycle Factors affecting infiltration Type of vegetation Runoff can be serious from cultivated fields under row crops Especially early in the growing season Very little runoff from land under Undisturbed forests Turfgrass Source http en wikipedia org Source http josephforestenterprises com 13 Stem flow Many plant canopies direct rainfall toward the stem Which increases water inputs to the surrounding soil While decreasing them elsewhere Corn canopies act like funnels to redirect rainfall into the crop row Enhances saturated soil flow Contours indicate water potential in kPa Source Brady and Weil 2008 14 7 NRES 201 Lectures 18 19 Fall 2014 The Hydrologic Cycle Snowfall Can promote infiltration if it occurs before freezing weather Acts as insulation to keep the soil unfrozen Has the opposite effect if heavy snowfall occurs after freezing Because the Source Brady and Weil 2008 insulating effect keeps the soil frozen 15 Soil management Strategies to maximize infiltration while minimizing runoff Cover cropping Maintain dense vegetative cover between primary crops to Open root channels Promote earthworm activity Protect surface soil structure Timely killing of the Source http limecreekwatershed word cover crop important press com to control soil drying 16 8 NRES 201 Lectures 18 19 Fall 2014 The Hydrologic Cycle Furrow diking Tillage system that creates ridge like barriers alongside row crops The ridges hold irrigation and rain water to allow more time for infiltration Source http www ars usda gov 17 Minimizing compaction From heavy farm or forestry equipment Minimize wheel traffic Stay off wet soils Use wide tires From recreational pressure Restrict foot Note the ponding traffic to existing trails From cattle on But not here Cattle were here rangeland Limit grazing Source Brady and Weil 2008 density 18 9 NRES 201 Lectures 18 19 Fall 2014 The Hydrologic Cycle Urban watersheds Infiltration reduced by Soil compaction by heavy construction equipment Land coverage by impermeable pavement or buildings The increased surface runoff Overwhelms stream flow Causes flooding Available options Permeable pavement Rain gardens Source http wps prenhall com Source http www wolfpaving com 19 Soil properties Greater infiltration for Sands Well granulated soils Greater runoff for Clays Fragipan soils Precipitation was the same for both watersheds Source Brady and Weil 2008 20 10 NRES 201 Lectures 18 19 Fall 2014 The Hydrologic Cycle The soil plant atmosphere continuum Water flows From the soil to the plant From the plant to the atmosphere From higher to lower potential Two critical factors Rate of water supply to roots Rate of water loss from leaves transpiration Source Brady and Weil 2008 21 Evapotranspiration Refers to soil water loss through either evaporation or transpiration The two processes cannot be easily resolved Transpiration preferable to evaporation Because it is essential for plant growth and