International Commission on Irrigation & Drainage Commission Interationale des Irrigation et du Drainage

Multilingual Technical Dictionary (MTD)
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Term No.Term Name/Term DescriptionAction
245
Normal equations
The set of simultaneous equations required to be solved in the method of least squares for obtaining the regression line. The normal equations are :.....
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256
Error in equation
A departure from the relationship expressed by an equation either because the equation is not a complete representation of the situation or because it is disturbed by extraneous sources of variation; as distinct from effects such as observational errors in the variables themselves......
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682
Hydrologic equation
The water balance equation (inflow = outflow + font Face=Symbol>Dstorage) which expresses the basic principle that during a given time interval the total inflow to an area must equal the total outflow plus the net change in storage......
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1112
Groundwater equation
A mathematical derivation from the disposition of rain : R = E + S + I, where R is rainfall, E is evaporation and transpiration, S is stream flow, and I is groundwater increment......
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3079
Chezy`s equation
A formula expressing the relation between the velocity of water, hydraulic radius, and friction slope; thus V=C√RS  , where V = velocity (m/sec), R = hydraulic radius (m), S = sine of the slope angle due to friction (friction slope or gradient of the total head line) (m/m), and C = Chezy’s roughness coefficient (m1/2/sec)......
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3080
Manning`s equation
1- A formula giving relationship between velocity of flow V (m/sec in SI or ft/sec in FPS), hydraulic radius R (m or ft), and friction slope S (m/m or ft/ft) as follows:

V = k/N . R2/3.S1/2
Here, k is a constant (1.00 m1/3/m1/3 in SI units, 1.486 ft1/3/ft1/3 in FPS) and N is Manning’s coefficient of roughness (unitless).

2-An empirical equatio.....
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3081
Kutter`s equation
An empirical formula expressing the value of coefficient C in the Chezy`s formula, in terms of a coefficient of roughness, the hydraulic radius and the friction slope, expressed as :

         k1 + k2/S + k3/N    
C = -----------------------
        1 + N/√R . {k1 + k2/S}

where C = Chezy’s roughness coefficient (m1/2/sec, ft1/2/sec), S = fric.....
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3082
Lacey`s equation
An empirical equation for alluvial channel. Equation propounded by Lacey for design of channels designs that relates regime velocity, slope and sediment characteristics as :
        1  
V = --- . R3/4 S1/2
        Na
wherein the absolute roughness coefficient Na = 0.0225f1/4 in which Lacey’s silt factor    f = 1.76d1/2, d being mean diameter of.....
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3116
Bed load equation
The general relationship between bed load rate, flow condition, and composition of the bed material.
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3143
Kennedy`s equation
An empirical equation that applies to channels with erodible boundaries in alluvial soils carrying small sediment loads expressed as Vo = 0.55 db0.64 where Vo is non-silting, non-scouring, mean velocity (termed as critical velocity) (in m/sec) and db is the average depth (in m) over the bed including the side-slopes. The coefficient and exponent de.....
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3156
Darcy-Weisbach equation
Equation used for calculating head loss due to friction in a conduit and expressed as-

LV2
hf   = f .  ------
             D 2g

where,  hf is head loss due to friction (m, ft), f = Darcy-Weisbach friction factor (unitless), D = conduit diameter (m, ft), L = conduit length (m, ft), V = flow velocity (m/sec, ft/sec), g = acceleration due to gravity.....
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3157
Hazen-Williams equation
Equation used in the analysis of pressure pipe systems for calculation of velocity of flow or discharge  and expressed as-
V = k.C.R0.63.S0.54
where, V = flow velocity (m/sec, ft/sec), k = constant (0.85 in SI, 1.32 in FPS), C = Hazen-Williams roughness coefficient (unitless), R = hydraulic radius (m, ft), S = friction slope (m/m, ft/ft).
.....
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3158
Colebrook-White equation
Equation used to calculate Darcy-Weisbach friction factor iteratively from the expression-
1/√f = -2 log { k/14.8R + 2.51/Re√f }     for free surface
1/√f = -2 log { k/12.0R + 2.51/ Re√f }    for full flow
where, f is friction factor (unitless), k = Darcy-Weisbach roughness height (m, ft), R = hydraulic radius (m, ft), Re = Reynold’s n.....
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3170
Navier-Stokes equation
Momentum equation applied to a small control volume of incompressible fluid; usually written in vector notation.
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4942
Combination equation
A mathematical combination of energy balance and vapour flow relationship to estimate potential evaporation.
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4943
Penman Equation
One form of the combination of equation (See 4942) used to estimate potential evaporation.
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4944
Penman-Monteith Equation
A mathematical combination of energy balance and vapour flow relationships, plus aerodynamic resistance to sensible head and power transfer and crop canopy resistance to vapour movement, to estimate potential evaporation......
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6485
Soil loss equation
Equation used to predict the amount of soil that will erode from a unit area over a year`s time under varying conditions of rainfall, slope,cropping system and management practices......
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8742
Bertrand qualifying equation
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