NRES 201 Lecture 17 Outline of Last Lecture I Measurement of soil water II Movement of water in soils III Factors affecting soil water availability Outline of Current Lecture I The global hydrological cycle II Fate of precipitation and irrigation water III The soil plant atmosphere continuum Current Lecture The global hydrological cycle Fundamental realities o The world s water resources are not evenly distributed Annual precipitation varies Seasonal variation can also be extensive when flooding is followed by drought o 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 Global Water Use o Dominated by irrigated agriculture o Increases by 10 per decade o Highest per capita consumption in the USA Water Cycles o Solar energy drives evaporation o Water vapor forms clouds o The clouds move and form precipitation o Rain and Snowfall return water to land and seas These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Lead to runoff and percolation that resupply streams rivers and the oceans Water balance o The usual focus is a watershed Land area is drained by a single system of streams o Precipitation falling on a watershed can be Stored in the soil Returned to the atmosphere by evaporation or transpiration Discharged by surface or subsurface flow o Represented as P SS ET D where P precipitation irrigation SS Soil Storage ET Evapotranspiration D Discharge o Impact by management Irrigation increases the input of water and calls for strategies to maximize soil storage by minimizing discharge Fate of precipitation and irrigation water Water that reaches the ground may o Enter the soil by infiltration More likely with good soil structure o Or remain on the surface if rainfall exceeds infiltration The result is ponding or runoff Water that enters the soil may leave by o Evaporation o Plant uptake transpiration o Subsurface drainage Can account for up to 50 of water input Some availability through capillary rise during dry periods Factors affecting infiltration o Type of vegetation o Stem flow o Snow Fall o Soil Management o Furrow diking o Minimizing compaction o Urban watersheds o Soil properties The soil plant atmosphere continuum Water flows from the soil to the plant from the plant to the atmosphere from higher to lower potentials Two critical factors o Rate of water supply to roots o Rate of water loss from leaves transpiration
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