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.

Interception

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.

(b) Inflow-Outflow Measurement (Water balance principle)

ET = P + R1 – R2 Os - S

(c) Study of Groundwater Fluctuations

Daily rise and fall of GW table give an indication of ET losses.

  (d) ET Equations

Due to the lack of basic data and the difficulties in measurement

required in the field methods lead to the development of ET

equations that relates the ET with readily available climatic data.

Evapotranspiration Equations

Infiltration is the flow of water into the ground through the earth

surface.

   Infiltration is extremely important in hydrologic modeling of

rainfall-runoff process because it can affect not only the timing, but

also the distribution and magnitude of surface runoff.

   Factors Affecting Infiltration Rate:

(a)  Type & extent of vegetal cover;

(b) Condition of surface crust;

(c)  Temperature;

(d) Rainfall intensity;

(e)  Soil properties;

(f) Water quality.

Infiltration

Some Fundamentals of Subsurface Flow

• Soil Properties:

1.     Porosity: = (Vwater + Vgas) / Vtotal

0.25 < < 0.75 (see Table 2.6.1 in Chow et al., 1988)

2.     Soil Moisture Content ():

- By volume: = Vwater / Vtotal;

- By mass: = Mwater / Mdry soil .

3.     Soil Density: soil = Msoil / Vsoil 

4.     Bulk Density of Soil: soil = Msoil / Vtotal

5.     Void Ratio: e = (Vwater + Vgas)/ Vsoil , (0.25 ~ 2.0)

6.     Degree of Saturation: S = Vwater / (Vwater + Vgas)

7.     Field Capacity: Soil water content (by volume) after saturated soil

has drained under gravity to equilibrium.

Soil Porosity

Some Fundamentals of Subsurface Flow

• Movement of water in soil is primarily governed by gravity and surface tension.

• Soil Structure

• Soil Classification

• Soil Water Retention

• Factors Affecting Infiltration Rate

• Soil Water Characteristics Curve (SWCC)

 

• Continuity Equation: 

1-Dimensional:

 

where q = Darcy flux = flow rate / cross sectional area of soil

0z

q

t

Surface Tension/Soil Structure

Soil Classification

Soil-Water Retention

Factors Affecting Infiltration Rate

Soil Water Characteristic Curves

Governing Eq. For Unsaturated Flow• Continuity Equation (1D):

where q = Darcy flux = flow rate / cross sectional area of soil

 

• Momentum Equation (Darcy’s law):

• where –ve sign indicate flow direction is coincident to decreasing in head; K is the hydraulic conductivity of the soil; h is the total head.

• In unconfined saturated flow, the total head is consisted of potential head and friction.

• In unsaturated flow, the suction force () must be included h = + z +V2/2g with = the suction head (depends on ); z = potential head; and V2/2g 0.

• Substitute q into the continuity equation, we have the following 1D equation, called Richard's equation, for unsteady, unsaturated flow

)( Kz

Dzt

0z

q

t

z

hKq

Measurement of Infiltration• Infiltrometer: An artificial application of water to enclosed sample

areas. (see Figure in handout)

• Types:

(a.1) Rainfall Simulator - Infiltration capacity is determined from

rainfall-runoff hydrographs.

(a.2) Flooding Type – Include tubes & concentric rings.

Accumulate Rainfall = Accumulate Infiltration

= Volume of H2O added / area of ring

(b) Hydrograph Analysis: Analyze rainfall-runoff hydrographs that are actually occurring in a watershed.

• The derived estimation of infiltration can ultimately be no more accurate than the precision of the measurements of rainfall & runoff.

Rainfall Simulator/Infiltrometer

Infiltrometer Test

Infiltration Terminologies• Infiltration Capacity (Potential Infiltration Rate) (fp) - The maximum rate at which

soil can absorb water through its surface. • Infiltration Rate, f(t) - Rate of water entering the soil surface. If there is no limit

on the water supply for infiltration, f(t) = fp. Otherwise, 0 f(t) < fp.

•

• fo = initial infiltration rate

fc = ultimate infiltration rate

f(t) - fc = excess infiltration rate  

• Cumulative Infiltration, F(t) - Depth of infiltration from the beginning of rainfall to any time, t.

F(t) = Area under the infiltration curve

• Wetting Front - Change of soil moisture content with depth is so great so as to

give the appearance of a sharp discontinuity between the wet soil above and the dry soil below.

t

0d )(f

f(t)

fo

fc

t