HYDROLOGIC ABSTRACTIONS Problems in hydrologic design requires the modeling of precipitation-runoff relationship. - Runoff = Total Precipitation – “ Losses “ - Types of Hydrologic Abstractions (Losses): (A) Interception; (B) Depression Storage; (C) Evaporation; (D) Transpiration; (E) Evapotranspiration; (F) Infiltration.
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HYDROLOGIC ABSTRACTIONS Problems in hydrologic design requires the modeling of precipitation-runoff relationship. - Runoff = Total Precipitation – “ Losses.
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HYDROLOGIC ABSTRACTIONS
Problems in hydrologic design requires the modeling of precipitation-runoff relationship.
Definition: Fraction of the gross precipitation input which wets and adheres to above ground objects until it is returned to the atmosphere through evaporation.
Depression Storage
Definition: Rainwater retained in puddles, ditches, and other depressions on the ground surface. As soon as rainfall intensity exceeds the local infiltration capacity, the rainfall excess begins to fill depression.
Water held in depression at the end of rain either evaporates or contributes to soil moisture and/or subsurface flow by following the infiltration.
Depression storage may be of considerable magnitude and may play an important role in hydrologic analysis. Stock ponds, terraces, and contour farming all tend to moderate flood by increasing depression storage.
- Note:Retention - storage held for a long period of time and depleted by evaporation.Detention - short-term storage depleted by flow away from the storage location.
The 1st abstraction that occurs in hydrologic cycle and it, along with the depression storage, is sometimes considered as the initial loss.
Evaporation
Factors Affecting Evaporation: (a) Solar radiation; (b) Vapor pressure difference between a water surface and the overlaying air; (c) Temperature; (d) Wind; (e) Atmospheric pressure; (f) Quality of water. Some Statistics: · Mean annual evaporation in USA – 53.3 cm (Northeast) · Lake Mead ( Hoover Dam near Las Vegas) 8 x 105 AF/yr = 1x 109 m3/yr 14.5% of 1995 total water supply to HK from Mainland China · HKO’s report : 153cm/yr at King’s Park. (1961 – 1990)
Using Evaporation PansIt is most commonly used method for determining the evaporation.
Etrue = Cp x Epan where Cp is pan coefficient, 0.70~0.95. Value of Cp varies considerable from month to month, but fairly consistent from year to year. Types of Pan : (Chow, 1964, pp.11.6~11.7) a) USWB Class A Land Pan (Cp 0.7)
b) US Bureau of Plant Industry Sunken Pan (Cp 0.95) * by far the best for measuring lake evaporation c) Colorado Square Sunken Pan (Cp 0.75~0.86) d) USGS Floating Pan (Cp 0.8)
Evaporation Determination
EvaporationPans
A process whereby water is taken from the soil moisture storage by roots and passes thru the plant structure and is evaporated from cells in the leaf called stomata.
Factors Affecting Transpirationa) Temperature; b) Solar Radiation; c) Wind; d) Soil Moisture; e) Plant type
Measurement of Transpiration a. Phytometer
Transpiration rate (depth/time)
b. Potometer Comment : Large scale field measurements of transpiration are virtually impossible under prevailing field conditions so it is common to find measures of consumptive use (combination of evaporation and transpiration).
Transpiration
tA t2
_Wt1
_W
Evapotranspiration
The process by which water is evaporated from wet surface and transpired from plants, i.e. sum of evaporation and transpiration.
Evapotranspiration (ET) = Consumptive Use One of the practical applications of estimating ET is in the design of irrigation water supply system. The terms potential evapotranspiration & consumptive use are involved.
Potential Evapotranspiration (PET)The evapotranspiration (ET) that would occur if there was an adequate soil-moisture supply at all time.
This term implies an ideal water supply to the plant. If water supply to the plant is less than PET, the deficit will be drawn from soil moisture storage.
Determination of ET
(a) Lysimeter Measurement
ET = I – S
There exist some difference between lysimeter and natural conditions.