Hydrology 1
Precipitation and Hydrologic Cycle
- Precipitation Interpretation:
- Average precipitation in an area.
- Simpler method: add all precipitation.
- Consider the area; closer to the form, combination of flow of water.
- Groundwater flow is part of the hydrologic cycle.
- Surface runoff: flow of water.
- Water amount decreases due to:
- Voids.
- Volume of water and air affecting infiltration.
- Infiltration Processes:
- Difference in pressure.
- Difference in temperature.
- Occurs in mountains due to difference in elevation.
Hydrologic Cycle
- Sun:
- Main driver for the hydrologic cycle, which has no start and end.
- Hydrologic Cycle Processes:
- Solid to water.
- Vapor evaporation in plants.
- Sublimation.
- Deposition.
- Free evaporation.
- Condensation.
- States of Matter:
- Plasma (4th state of matter).
- Bose-Einstein Condensate (5th state).
- Reached at absolute zero temperature (approximately -273^\circ C).
- Hydrologic Budget:
- Measure or balance the hydrologic budget.
- Ecosystem and water under the ground.
- Hydrology vs. Hydraulics:
- Hydrology: study of water.
- Hydraulics: foundation for water supply engineering.
System Inflow and Outflow
- Example:
- Projecting the water level of Angat Dam.
- System Dynamics:
- Q{in} = Q{out} \pm \Delta S_s = 0
- Q_{in}: Inflow.
- Q_{out}: Outflow.
- \Delta S_s: Change in storage.
- Combined inflow (Q{in}) and outflow (Q{out}) equation.
- (Q{in} - Q{out}) = 0
- Q{in} = Q{out} \pm \Delta S_s = 0
Water Distribution
- Distribution Types:
- Atmospheric.
- Surface (rivers, lakes).
- Subsurface.
- Biological.
- Volume of Water:
- World water quantity.
- Flowrate:
- Measured in km^3/year.
- Global Annual Water Balance:
- Ocean.
- Land.
Water Movement
- Atmosphere to Ground:
- Water travels from the atmosphere to the ground.
- Phase Changes:
- Water vapor to liquid.
- Water vapor to solid (ice).
- Particle Behavior:
- Tendency of small particles to become large particles.
- Frozen Precipitation (Snow):
- Sponge texture (large voids).
- Symmetrical shape.
- Fast velocity (big or solid, bumabagsak).
- Rainfall Droplets:
- Movement of rainfall from atmosphere to earth's surface.
- For pressure 101.3 kPa and density 1000 kg/m^3.
- As diameter increases, terminal velocity increases.
- Velocity is due to the diameter.
- Exact Computation:
- Diameter calculation using volume of sphere relative to density of water and air.
- Rainfall Measurement:
- Measured in heights.
Rainfall Intensity
- Measurement Intervals:
- Every 5 minutes.
- Example: 6:00, 6:05, 6:10, 6:15, 6:20, 6:25, 6:30.
- Rainfall Intensity Calculation:
- Convert 5-minute rainfall to 30-minute rainfall interval.
- I_{max} = 5.28 \text{ in/hr}
- Maximum depth.
Rainfall Gauges
- Interpretation:
- Interpret data from one rainfall gauge.
- If multiple gauges, simply get the average.
- Consider the area.
- Precipitation:
- Total area.
Areal Rainfall Methods
- Thiessen Method:
- Connect the rain gauges (no intersection to each other).
- Get the midway of each connecting line and draw a perpendicular line.
- Get the area corresponding to a particular gauge.
- Compute areal rainfall (weighted average).
- Formula: \text{Average Rainfall} = \frac{\sum Ai Pi}{\sum A_i}
- Example Data:
- P1 = 10.0 \text{ mm or in}, A1 = 0.22 \text{ km}^2 \text{or mi}^2, A1P1 = 2.2
- P2 = 20.0 \text{ mm or in}, A2 = 2.02 \text{ km}^2 \text{or mi}^2, A2P2 = 40.5
- P3 = 30.0 \text{ mm or in}, A3 = 1.35 \text{ km}^2 \text{or mi}^2, A3P3 = 40.5
- P4 = 40.0 \text{ mm or in}, A4 = 1.60 \text{ km}^2 \text{or mi}^2, A4P4 = 64.0
- P5 = 50.0 \text{ mm or in}, A5 = 1.95 \text{ km}^2 \text{or mi}^2, A5P5 = 97.5
- \sum Ai = 9.14 \text{ km}^2 \text{or mi}^2, \sum AiP_i = 284.6
- Average rainfall = 284.6 / 9.14 \approx 31.1 \text{ mm or in}
- Isohyetal Method:
- Overcomes difficulties by constructing isohyets.
- Uses observed depths at rain gauges and interpolation between adjacent gauges.
- With a dense network of rain gauges, isohyetal maps can be constructed using computer programs for automated contouring.
- Example Data:
- Average Rainfall (mm or in), enclosed Area (km^2 or mi^2) and Rainfall Volume (mm or in).
- Between isohyets 0-10, Area = 0.88, Average Rainfall = 5, Rainfall Volume = 4.4
- Between isohyets 10-20, Area = 1.59, Average Rainfall = 15, Rainfall Volume = 23.9
- Between isohyets 20-30, Area = 2.24, Average Rainfall = 25, Rainfall Volume = 56.0
- Between isohyets 30-40, Area = 3.01, Average Rainfall = 35, Rainfall Volume = 105.4
- Between isohyets 40-50, Area = 1.22, Average Rainfall = 45, Rainfall Volume = 54.9
- Between isohyets 50-60, Area = 0.20, Average Rainfall = 53, Rainfall Volume = 10.6
- Estimated Total Area: 9.14, Total Rainfall Volume: 255.2
- Average rainfall = 255.2 / 9.14 \approx 27.9 \text{ mm or in}
- Example Data:
- Reciprocal Squared Distance Method
Sample Problem: Thiessen Method
- Problem: Compute the mean aerial rainfall using Theissen's Method for a small urban watershed with four rainfall gauges.
- Given:
- Rainfall recorded at each gauge during a storm event.
- Gage A: 81.50 mm
- Gage B: 73.00 mm
- Gage C: 75.25 mm
- Gage D: 76.25 mm
- Solution:
- Areas:
- A_1 = (800 \times 300) / 2 = 120,000 m^2
- A_2 = ((800 + 200) / 2) \times 400 = 200,000 m^2
- A3 = A2 = 200,000 m^2
- A_4 = (700 \times 800) - (400 \times 300) / 2 - (400 \times 400) / 2 = 280,000 m^2
- Mean Aerial Rainfall:
- P = \frac{81.5A1 + 73A2 + 75.25A3 + 76.25A4}{A1 + A2 + A3 + A4} = 75.98 \text{ mm}
- Areas:
Return Period
- Determine the number of years of data, n.
- Set rainfall duration for analysis (5 minutely, hourly, daily, etc.).
- Find the maximum depth for the duration in each year.
- Rank the depths from highest to lowest for all years (greatest amount at top of list, rank = m = 1).
- Compute return period:
- T = \frac{n+1}{m}
- where n is the number of years of data, m is the rank of data from highest (m=1) to lowest (m=n).
- Corresponding probability = 1/T
- Example: For T = 100 year event, the probability = 0.01.
- T = \frac{n+1}{m}
- Partial Duration Series:
- Algorithm swaps out maximum for during n years with n maximum in n years (e.g., more than 1 value per year allowed).
Additional Notes on Return Period
- n = 20 years.
- Ocanerank.
- Reciprocal of return period.
- Chance na marit.
- Probability is inversely proportional to area.
- n = 26 years.
- 1/20.
- Dependent on historical data.
- Change of state from water to water vapor.
Comparing Water Vapor to Moist Air
- Specific Humidity:
- Mass of water vapor per unit mass of moist air.
- Vapor Pressure:
- Dalton's law of partial pressures states that the pressure exerted by a gas (its vapor pressure) is independent of the presence of other gases.
- Based on ideal gas law:
- If the total pressure exerted by the moist air is p, then p - e is the partial pressure due to the dry air:
- p - e = \rhoa Ra T
- e = \rhov Rv T
- T: Absolute Temperature in K
- e: Vapor pressure
- R_v: Gas constant of water vapor
- \rho_v: Density of water vapor
- If the total pressure exerted by the moist air is p, then p - e is the partial pressure due to the dry air:
- Specific Humidity Approximation:
- q_v = 0.622 \frac{e}{p}
- R_a: Gas constant for dry air (287 J/kg\cdot K)
- \rho_a: Density of dry air
- \rho = \rhoa + \rhov
- R_v: Gas constant for water vapor
- p = [\rhoa + (0.622)\rhov] R_a T
- Gas Constants Relationship:
- Ra = Ra (1 + 0.608qv) \approx 287 (1 + 0.608qv)
- Saturated Vapor Pressure:
- For a given air temperature, there is a maximum moisture content the air can hold, and the corresponding vapor pressure is called saturation vapor pressure.
- e_s = 611 e^{\frac{17.27T}{237.3 + T}}
- where e_s is Saturated vapor pressure (Pa = N/m^2) and T is Temperature (\degree C)
- Example Values:
- T = -20, e_s = 125
- T = -10, e_s = 286
- T = 0, e_s = 611
- T = 5, e_s = 872
- T = 10, e_s = 1227
- T = 15, e_s = 1704
- T = 20, e_s = 2337
- T = 25, e_s = 3167
- T = 30, e_s = 4243
- T = 35, e_s = 5624
- T = 40, e_s = 7378
- Relative Humidity:
- Ratio of the actual vapor pressure to its saturation value at a given air temperature:
- Rh = \frac{e}{es}
- Ratio of the actual vapor pressure to its saturation value at a given air temperature:
- Dew Point Temperature:
- The temperature at which air would just become saturated at a given specific humidity.
- Sample Problem:
- Given:
- Air pressure = 100 kPa
- Air temperature = 20 ^\circ C
- Dew-point temperature = 16 ^\circ C
- Calculate:
- Specific humidity
- Relative Humidity: \frac{e}{e_s}
- e_s = 611 e^{\frac{17.27T}{237.3 + T}} \approx 2339 Pa
- e = 611 e^{\frac{17.27T}{237.3 + T}} \approx 1819 Pa
- R_h = \frac{1819}{2339} \approx 78\%%
- q_v = 0.622 \frac{e}{p} \approx 0.0113 \frac{\text{kg of water}}{\text{kg of moist air}}
- Given:
Terminologies
- Evaporation:
- Process by which liquid water passes directly to the vapor phase.
- Transpiration:
- Process by which liquid water passes from liquid to vapor through plant metabolism.
- Sublimation:
- Process by which water passes directly from the solid phase to the vapor phase.
- Vapor Pressure:
- Water vapor normally behaves as an ideal gas.
- Partial pressure of water (vapor pressure) adds to partial pressures of the other gaseous constituents.
- Water vapor is about 1-2% of the total pressure.
- Humidity:
- Quantity of water vapor present in air (absolute, specific, or a relative value).
- Specific Humidity:
- Ratio of the mass of water vapor in moist air to the mass of air.
- Dew Point Temperature:
- Temperature at which air becomes saturated at a given specific humidity.
Factors Influencing Evaporation
- Energy supply for vaporization (latent heat):
- Solar radiation.
- Transport of vapor away from evaporative surface:
- Wind velocity over surface.
- Specific humidity gradient above surface.
- Vegetated surfaces.
- Supply of moisture to the surface:
- Evapotranspiration (ET).
- Potential Evapotranspiration (PET): moisture supply is not limited.
- Evapotranspiration (ET).
- Evaporation from Pan:
- National Weather Service Class A type.
- Installed on a wooden platform in a grassy location.
- Filled with water to within 2.5 inches of the top.
- Evaporation rate is measured by manual readings or with an analog output evaporation gauge.
Methods for Estimating Evaporation
- Energy Balance Method:
- Parameter is energy from on.
- Rn - Hs - G = Lv \rhow E
- R_n = net radiation, W/m^2
- L_v = latent heat of vaporization, kJ/kg
- \rhow = density of water, kg/m^3
HS = Sensible heat to air
G = Heat conducted to the ground
- Example:
- For a particular location, the average net radiation is 185 W/m^2, air temperature is 28.5°C, relative humidity is 55%, and wind speed is 2.7 m/s at a height of 2m. Determine the open water evaporation rate in mm/d using the energy method.
- L_v = 2.501 \times 10^6 - 2370T = 2501 - 2.37 \times 28.5 = 2433 kJ/kg
- E = \frac{Rn}{\rho Lv} = \frac{185}{2433 \times 10^3 \times 996.3} = 7.63 \times 10^{-8} m/s = 6.6 mm/d
- For a particular location, the average net radiation is 185 W/m^2, air temperature is 28.5°C, relative humidity is 55%, and wind speed is 2.7 m/s at a height of 2m. Determine the open water evaporation rate in mm/d using the energy method.
- Aerodynamic Method:
- Ea = B(es - e)
- B is the vapor transfer coefficient with units of mm/day
- B = \frac{0.1022}{[ln(z/z_0)]}
- where uz is the wind velocity (m/s) measured at height z (cm) and z0 is the roughness height (0.01-0.06 cm) of the water surface.
- Ea = B(es - e)
- Combined Method:
- E = \frac{\Delta}{\Delta + \gamma} Er + \frac{\gamma}{\Delta + \gamma} Ea
- Er is the vapor transport term and Ea is the aerodynamic term.
- \gamma is the psychrometric constant (approximately 66.8 Pa/°C).
- \Delta is the gradient of the saturated vapor pressure curve.
- \Delta = \frac{4098e_s}{(237.3 + T)^2}
- E = \frac{\Delta}{\Delta + \gamma} Er + \frac{\gamma}{\Delta + \gamma} Ea
- Priestly Taylor Method:
- Similar to the combined method, with constant.
- E = 1.3 \frac{\Delta}{\Delta + \gamma} E_r
Vapor Flow Rate
- mv = \rhow A E
- V = A \frac{dh}{dt}
- \rho_w \approx 1000 kg/m^3