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Surface Water


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  • 1. Hydrological Cycle and Water Resources
  • 2. Overview Intersection of Atmosphere, Hydrosphere, Lithosphere and Biosphere Hydrologic Cycle Components Water Budget The supply of water available for use Groundwater aquifers examples of aquifers as resources Global distribution of water resources Pollution
  • 3. transpiration vegetation Root uptake precipitation for Atmosphere Biosphere photo- evapotranspiration synthesis infilitration soil evaporation evaporation precipitation percolation Hydrosphere Lithosphere runoff oceans groundwater watersheds (drainage basins) aquifers river systems Karst topography
  • 4. Hydrologic Cycle Describes the way that water passes between hydro-, atmo-, litho- , biospheres In general describes the balance between water leaving the atmosphere (precipitation) and water re- entering the atmosphere (evaporation, and transpiration) More specifically details the processes that occur in between the general processes described above (infiltration, percolation, runoff, photosynthesis)
  • 5. Infiltration Water access to subsurface regions of soil moisture storage through penetration of the soil surface Occurs at a constant rate, measured the same way as precipitation e.g., inches per hour, millimeters per hour If precipitation rate exceeds the infiltration rate, runoff will occur. Capillarity Forces that cause water to move upward through the soil Plant root uptake (transpiration is essentially capillary action through the plant, from root to leaf) Evaporation of water at the soil’s surface Common in arid regions
  • 6. Runoff occurs when individual soil particles can no longer hold onto infiltrated water Dynamic tension between attractive forces (holding soil and water particles together) and gravity (pulling water away from soil). Affected by the size of soil particles: smaller particles have a greater summed surface area, and can hold onto water more strongly Sand, silt, clay Hygroscopic water (or unavailable water) held tightly by soil, not available to plants or capillary evaporation (attractive forces much greater than gravity) Capillary water (or available water) soil holds water tightly enough to prevent runoff, but not capillarity (attractive forces balanced with gravity) Saturation Gravitational water (runoff) soil cannot hold onto water (gravity exceeds attractive forces)
  • 7. The ability of soil to retain moisture is a direct consequence of hydrogen bonding between water and soil particles (colloids) Texture of colloids affects retention of water as well Gravel Sand 2mm Silt 50 μm Clay 2 μm Finer particles retain more water
  • 8. Gravitational water – not held by soil, available to plants or Root runoff and percolation Root exerts additional Capillary capillary force water – held by the colloid, but available Colloid to plants Hygroscopic water – held tightly by the colloid
  • 9. Finer particles create smaller pores, so there is less gravitational water, hence better moisture retention
  • 10. Finer particles have more surface area per volume, which allows greater opportunity for water to attach, and causes greater amounts of hygroscopic water as well as capillary water. As particle size decreases, capillary water increases initially, then decreases as hygroscopic water increases 4 2 1 1 4 4 4 4 8 8 4+4+4+4 2 4 4 4 4 4 = 16 4 4 4 4 8 8 4 4 4 4 Surface area = 16 Surface area = 32 Surface area = 64
  • 11. Field Capacity This refers to the maximum amount of capillary/available water a specific soil type can hold. Adding more water to the soil, after field capacity is reached, results in the build up of gravitational water, and runoff and percolation occur Wilting Point The amount of hygroscopic water that a specific soil type can hold When all available water is gone and only hygroscopic water remains, there is no water available to plants (hence the name) Varies with soil particle size mixture smaller particles hold more water clays can hold water too strongly (less available) Loams tend to have highest field capacity
  • 12. % Soil Moisture % Fine Particles % Organic matter
  • 13. Plant-Water Interaction Plant roots exert an attractive force on available soil water (Capillary Force) Photosynthesis (absorbs solar energy) 6CO2 + 6H2O C6H12O6 + 6O2 Respiration (releases solar energy) C6H12O6 + 6O2 6CO2 + 6H2O Storing energy as sugar or starch also stores water; plant growth and life processes release water Transpiration: respiratory water released as water vapor
  • 14. Soil-Water Budget A mathematical way of expressing the difference between input (precipitation) of water into an area against its output (evapotranspiration and runoff). Conceptually very similar to hydrological cycle PRECIP = ACTET + SURPL + ΔSTRG where ACTET = POTET – DEFIC Precipitation = Evapotranspiration + gravitational water + available water Varies over space and time
  • 15. Variables PRECIP: Precipitation ACTET: Actual evapotranspiration. Occurs when plants do not have sufficient water to reach their maximum metabolic potential POTET: Potential evapotranspiration. Represents the maximum metabolic potential of plants, and occurs when there is sufficient water DEFIC: The amount of water short of achieving POTET SURPL: Surplus water, corresponds to gravitational water ΔSTRG: Change in storage. Corresponds to capillary water (available water). Negative if plants are utilizing the available water Positive if available water is being recharged to field capacity
  • 16. Scenarios PRECIP > POTET POTET = ACTET DEFICIT = 0 SURPL + ΔSTRG > 0 Fall: ΔSTRG > 0, SURPL = 0; Available water increases until Field Capacity is achieved (Soil Moisture Recharge) Spring: SURPL > 0, ΔSTRG = 0 (after Field Capacity is reached), gravitational water; runoff and percolation occur PRECIP < POTET DEFIC > 0 ACTET = POTET – DEFIC SURPL = 0, no runoff or percolation ΔSTRG < 0, available water is utilized until the wilting point is met