Hydro

• Precipitation


  Part of the atmosphere water and derived from water vapour
• Atmosphere water


  • Mostly exists as vapour, but briefly and locally it becomes a liquid (rainfall and cloud water droplets) or a solid (snowfall, cloud ice crystal and hails)
• The sun is the driving force for the hydrological cycle. Precipitation comes from water vapour generated by the solar radiation from land and ocean.
• Water vapour


  Lighter than air (low air pressure is linked with high moisture, hence more likely to rain)
• The energy required to vaporise water is 2.5x10^6 j/kg (specific latent heat for water vaporization)
• Precipitation


  • Atmospheric moisture is a necessary but not sufficient condition for precipitation. Other factors such as wind, temperature, atmospheric pressure and local landscape can influence precipitation.
• Ice Crystal Process


  Aerosols act as freezing nuclei. Ice crystals grow in size and fall into the ground. They tend to melt before hitting the ground surface.
• Coalescence Process


  Small cloud droplets increase their sizes due to contact with other droplets through collision.
• Three major categories of precipitation

  • Convective/Convectional Precipitation
  • Orographic/Relief Precipitation
  • Cyclonic/Frontal Precipitation
• Convective/Convectional Precipitation


  Heated air near the ground expands and absorbs more water moisture. The warm moisture-laden air moves up and gets condensed due to lower temperature, thus producing precipitation.
• Orographic/Relief Precipitation


  The uplifting of air is caused by natural barriers such as mountain ranges.
• Cyclonic/Frontal Precipitation


  The uneven heating of the earth's surface by the sun results high and low pressure regions, and air masses move from high pressure regions to low pressure regions. If warm air replaces colder air, the front is called a warm front. If cold air displaces warm air, its front is called a cold front.
• Precipitation data


  Point precipitation data are used collectively to estimate areal variability of rain and snow. Rainfall data are usually represented as mm/hour, mm/day, etc.
• Gauge


  A device that measures the amount or quantity of something and shows the amount measured.
• Examples of gauges


  • Rain Gauges
  • Temperature Gauges
  • Pressure Gauges
• Rain gauge


  • Used to study the present as well as the future condition of water resources
  • Used to determine the amount of rainfall with respect to the time at a particular place or area
  • Used to predict the water precipitation at a particular place to make a stable structure like a dam, road, etc
  • Used to design the drainage by showing the discharge of precipitation at given interval of time
• Recording gauge


  An instrument that automatically records the amount of precipitation collected as a function of time
• Non-recording gauge


  Only displays the amount of rain that has fallen. Cannot tell when the rain started, when it stopped, what the intensity of the rain was, or how much the intensity of the rain varied over the course of the storm.
• Importance of rain gauges

  • Keeping plants and grass well-watered
  • Identifying weather patterns and trends for your neighborhood
  • Knowing the best times for planting
  • Identifying potential conditions for flooding
  • Getting better rainfall data
• Areal rainfall


  Precipitation in a specific area expressed as an average depth of liquid water over the area
• Methods to compute rainfall

  • Arithmetic Mean Method
  • Thiessen Polygon Method
  • Isohyetal Method
• Infiltration


  The process by which water on the ground surface enters the soil. Governed by gravity and capillary action.
• Infiltration vs Percolation


  Infiltration occurs in the topmost zone of soil whereas percolation occurs when water passes through soil and rock. Infiltration occurs fast while percolation occurs slowly.
• Porosity


  The open spaces between grains or trapped in grains in a microstructure. Porous materials can absorb fluids or moisture.
• Soil moisture content


  The amount of water present in the soil
• Vadose zone


  The zone between the land surface and the water table. The word Vadose means "shallow" in Latin.
• Field capacity


  The amount of soil liquid moisture held in soil after excess water has drained away and serves as a measure of soil water-holding capacity.
• Field capacity vs Saturation


  At field capacity, spaces between soil particles contain both air and water. Saturation happens when all the pores of a soil is completely filled by water.
• Soil moisture deficit (SMD)

  The difference between the amount of water actually in the soil and the amount of water that the soil can hold.
• Causes of soil moisture deficit

  Agricultural drought can lead to a soil moisture deficit that limits water availability for terrestrial ecosystems.
• Darcy's law


  Equation that describes the capability of the liquid to flow via any porous media like a soil.
• Darcy's law for saturated soil

  Q = KSf, where q = Darcy Flux, K = Hydraulic Conductivity, Sf = Friction Slope
• Pore velocity in soil

  The flux (q) divided by porosity (n) to account for the fact that only a fraction of the total soil volume is available for flow
• Infiltration process


  • Saturated Zone - Pore space filled with water
  • Transmission Zone - Small change in water content with depth
  • Wetting Zone - Water content sharply decreases with depth
• Effect of soil types on infiltration

  • Sand
  • Silt
  • Clay
• Other factors affecting infiltration

  • Precipitation
  • Vegetation Cover
  • Ground Slope
  • Land Use
• Infiltration rate


  The velocity or speed at which water enters into the soil
• Transmission zone


  • Lengthening unsaturated zone with uniform water content
• Wetting zone


  • Water content sharply decreases with depth from the water content of the transmission zone to near the initial water content of the soil
• Soil types


  • Sand
  • Silt
  • Clay