Hydrometeorology & Hydrologic Cycle

• Hydrometeorology is the study of the hydrologic cycle, focusing on the interrelationship between the atmosphere and terrestrial phases.
• Meteorology studies the atmosphere and the movement of water in vapor or liquid form.
• Hydrology focuses on the distribution and occurrence of water on and under the Earth's surface.

Hydrologic Cycle

• The hydrologic cycle involves the vertical and horizontal movement of water in vapor, liquid, or solid form.
• This movement occurs between the Earth's surface, subsurface, atmosphere, and oceans continuously.
• Processes involved include:
• Evaporation occurs when liquid water converts to vapor and moves away from the evaporating surface.
• Transpiration is liquid water vaporization/removal from plant tissues through stomata.
• Condensation is the process by which water vapor becomes a liquid.
• Precipitation occurs when condensation forms clouds, resulting in moisture falling from the atmosphere.
• Runoff occurs when water flows over land towards rivers, lakes, and oceans as either surface or subsurface flow.
• Infiltration is the filtration of water into the soil's inner layers, depending on the soil's structure and nature.
• Percolation is the vertical movement of water through the soil profile.
• Seepage is water flow under gravitational force through permeable layers, indicating lateral water movement in the soil.
• Water's residence time in the hydrologic cycle can be approximately 9 days in the atmosphere:
• Water consumed by crops evaporates over weeks.
• Water stays in rivers and streams for 2-6 months.
• Water stays in lakes and glaciers for 20-100 years.
• Water stays in the ocean for ~3,200 years..
• Ground water can stay up to 10,000 years.
• The Earth's water distribution is approximately 3% freshwater and 97% saline water.
• Of the freshwater, 69% is stored in glaciers and ice caps.
• About 30% is stored as groundwater with only 0.3% in surface waters.
• Surface water mostly stored in lakes comprises of 87% of surface water.

Hydrologic Budget Equation

• The Hydrologic Budget Equation, also known as the "water budget equation," expresses the law of conservation of mass and is given by: inflow = outflow
• Can also be expressed as: inflow – outflow = ±change in storage
• A hydrologic system is a structure or volume in space with a boundary that accepts water & various other inputs to produce outputs.
• Expanded Equation: ±ΔS = P + (Rin – Rout) + (Gin – Gout) – ET
  • ΔS = Change in storage
  • P = Precipitation
  • Rin = Runoff inflow
  • Rout = Runoff outflow
  • Gin = Groundwater inflow
  • Gout = Groundwater outflow -ET = Evapotranspiration

Watershed

• A watershed, also known as a "catchment area" or "basin," is an area that topographically contributes water to a specific downstream point.
• It's an area of land that drains water, sediment, and dissolved materials to a common receiving body or outlet.
• A watershed divide is the boundary line along topographic ridges that separates two adjacent watersheds.
• Divides are defined by the highest elevation draining to a stream.
• Watershed delineation is the process of identifying and delineating the boundary or divide of a watershed.
• Watershed Area Classifications
• Small Watersheds have an area < 250 km².
• Medium Watersheds range from 250 km² - 2500 km².
• Large Watersheds are those exceeding 2500 km².
• Watershed Characteristics include factors influence the ability of a watershed to store and discharge water.
• Runoff and hydrologic processes are affected by:
• Drainage Area describes the volume of water generated from rainfall. Larger areas produce larger runoff.
• Drainage Basin Length describes the distance along the main channel from a watershed outlet to the basin divide.
  • The basin length is needed to measure the travel time of water in the watershed.
  • For small watersheds, length (L in km) can be estimated using its area (A in km2) according to the formula L = 1.312A0.568
  • Basin slope is the rate of change of elevation to the distance along the principal flow path.
  • The slope is determined using the formula: S = (EL1 - EL2) / L
    • EL1, EL2 are the elevations at points 1 and 2
    • L is the length of the stream.
  • Basin Shape describes the runoff collection at the watershed outlet.
  • Circular watersheds produce peak flows in a short time.
  • Elongated watersheds have a longer time for water to reach the outlet.

Streamflow & Runoff

• Runoff is the portion of precipitation that makes its way towards stream channels, lakes, or oceans either as surface or subsurface flow.
• Runoff occurs when precipitation exceeds the demands of evaporation, interception, infiltration, surface storage/detention and channel detention.
• Runoff is considered the "excess precipitation" that remains after storages are full and saturated.
• Streamflow is surface water flowing through rivers towards an outlet and it is the quantity of that water over fixed period of time.
• Sources of streamflow includes:
• Direct precipitation of stream surface and tributaries.
• Overland flows (surface runoff), a thin sheet of water over the land surface.
• Subsurface flow (interflow), infiltrated water moving laterally in the vadose zone.
• Baseflow (groundwater flow), water that contributes to streamflow from groundwater.

Streamflow Measurement

• Stage or river stage refers to the height of the water surface stream level measured from an arbitrary datum.
• Stage has a discharge value, and a rating curve describes the relationship between stage and discharge.
• River stage can be measured using staff gauges, wire gauges, and float-gauges.
• Staff gauges are graduated staff made of durable materials fixed into a cross section of a stream.
• Wire-weight gauges are gauges that measures water level from structures and wires suspended from bridges and measure height from surface.
• Float-Gauge Recorders use automatic stage recorders with float movements fluctuating and recorded on charts.
• Streamflow data also uses the relation Q = AV, where Q is discharge, A is area, & V is velocity.
• Water velocity can be measured using:
  • Manning's Velocity Equation, : V = (k / n) * R^(2/3) * S^(1/2); where V is velocity, k is 1 for SI units or 1.486 for English units, n is roughness coefficient, R is hydraulic radius, and S is slope.
  • Float Method requires timer & floating material, measuring the surface velocity: Vs = D/t; calculate average velocity at V = 0.8Vs
  • Current Meter Method uses a device to measure the velocity of water; applicable to larger streams/rivers.
  • The average velocity in each subsection is measured by:
    • V = V0.6d if d < 0.6m
    • V = (V0.2d + V0.8d)/2 if 0.6m ≤ d ≤ 2m
    • V = (V0.2d + 2V0.6d + V0.8d)/4 if d ≥ 2m -Total discharge is the summation of all the discharge calculated which is shown as Q = ∑AiVi