crop water req

7/30/2019 Crop Water Req

1/35

It is essential to know the water requirement of acrop which is the total quantity of water requiredfrom its sowing time up to harvest. Naturallydifferent crops may have different water

requirements at different places of the same country,depending upon the climate, type of soil, method ofcultivation, effective rain etc.

The total water required for crop growth is notuniformly distributed over its entire life span whichis also called crop period.


2/35

Variation in requirement of water for paddy


3/35

Irrigation water need


4/35

Variation of crop water requirement

The effect of the major climatic factors on cropwater needs may be summarized as follows:

Sunshine

Temperature

Humidity Wind speed


5/35


6/35

Relation between ref EVPT and Kc Since the same crop grown in different climatic

variations have different water needs, it has beenaccepted to evaluate the evapotranspiration rate

for a standard or reference crop and find out thatof all other crops in terms of this reference.

Grass has been chosen as standard reference forthis purpose. The evapotranspiration rate of this

standard grass is, therefore, called the referencecrop evapotranspiration and is denoted as ETO,the function of the climatic variables.


7/35

E i i f f ETO


8/35

Estimation of reference crop ETO

Experimental methods, using the experimentationdata from evaporation pan.

Theoretical methods using empirical formulae,that take into account, climatic parameters.


9/35

Experimental method

Estimation of ET0 can be made using the formula

ETO = Kpan x Epan

Where ETO is the reference cropevapotranspiration in mm/day, Kpan is acoefficient called pan coefficient and Epan is theevaporation in mm/day from the pan.


10/35

Theoretical methods

Blanney-Criddle formula:

This formula gives an estimate of the meanmonthly values of ETO, which is stated as

ETO = p ( 0.46 Tmean + 8.13)

Where p is the mean daily percentage of annualday time hours and has been estimated accordingto latitude; Tmean is the mean monthlytemperature in degrees Centigrade and may betaken as x (Tmax + Tmin) for a particularmonth. Thus using the above Equation, ETC canbe evaluated for each month of the growingseason, from which the total water need for the fullgrowing season of the crop may be found out.


11/35

Penman-Monteith method:

This method suggests that the value of ETO may beevaluated. Refer Hydrology by Jayarami reddy Refer irrigaton principles by G.L.Asawa


12/35

Application interval of irrigation water


13/35

Total water requirement in growing a crop

The water that is required to irrigate a field or plotof land growing the particular crop not only has tosatisfy the evapotranspiration needs for growingthe crop, but would also include the following:

Losses in the form of deep percolation whileconveying water from the inlet of the field uptoits last or tail end as the water gets distributed

within the field

Water requirement for special operations likeland preparation, transplanting, leaching ofsalts, etc.


14/35

The net irrigation requirement (NIR) is definedas the amount of irrigation water required to be

delivered in the field to meet the consumptiverequirement of crop as well as other needs such asleaching, pre-sowing and nursery waterrequirement (if any). Thus,

NIR = CIR + LR + PSR + NWRWhere

LR = Leaching requirement

PSR = Pre-sowing requirement NWR = Nursery water requirement


15/35

Field Irrigation Requirement (FIR) is definedas the amount of water required to meet the net

irrigation requirements plus the amount of waterlost as surface runoff and through deeppercolation. Considering a factor a called the

water application efficiency or the field

application efficiency which accounts for the lossof irrigation water during its application over thefield, we have

FIR =NIR/ a


16/35


17/35

The capacity of the water supply source may betermed as the gross irrigation requirement

(GIR), defined as:GIR = FIR/c

In the above equation, c is thewater conveyanceefficiency.


18/35


19/35

Canal Irrigation


20/35

it is desirable that a canal off taking from a river should beable to irrigate as much an area as possible.

The general layout concept can be explained when studiedin respect of the off take point of the canal and thesurrounding contours, is shown in Figure below


21/35


22/35


23/35

Consider the possible alignments of canals as shown inFigure 3. A shown in the figure, the right bank canal

(generally termed RBC) and off taking from apoint R on the river bank may be alignedsomewhere in the region bounded by R-R, thecontour line at the elevation of R or the right

riverbank R-R. It is not possible to align the canalalong R-R, as there would not be any slope,whereas an alignment along R-R would meanzero command area for the canal. Hence, a

suitable slope of the canal that is neither too flatnor too steep would be the most appropriate.Based on the above logic, possible alignments of theright and left bank canals (RBC and LBC) have been

shown in Figure 4.


24/35


25/35

In order to demonstrate the effect of the adjacent

valley slopes on the canal layout, the right and leftbank contours have been chosen in such a way that theslope of the right bank valley is flatter than that of theleft bank. Hence, as may be observed from Figure 5, for

the same canal slope on both the banks, the right bankcanal covers a larger command area (the area betweenthe canal and the river).


26/35


27/35

The alignment of a canal can be done in such a way

that it is laid up to a ridge line between two valleys,which would allow a larger command area for the samecanal, as shown in the figure, which shows possiblecontour lines between two rivers that the canal off

taking from one river is able to irrigate areas betweenthe river of the adjacent valley, too.

As far as possible, curves should be avoided in the

alignment of canals because the curves lead todisturbance of flow and a tendency to silt on the innerbend and scour the toe of the outer (concave) bend.However, if curves have to be provided; they should be

as gentle as possible.


28/35

the permissible minimum radius of curvature for achannel curve is shorter for lined canals than unlinedones and is shorter for small cross sections than forlarge cross sections of canals.


29/35

Design of lined channels

The Bureau of Indian Standards code IS: 10430 -1982Criteria for design of lined canals and guidelines forselection of type of lining (Reaffirmed in 1991)

recommend trapezoidal sections with rounded cornersfor all channels-small or large. However, in India, theearlier practice had been to provide triangular channelsections with rounded bottom for smaller discharges.

The geometric elements of these two types of channelsare given below:


30/35


31/35

Select a suitable slope for the channel banks. Theseshould be nearly equal to the angle of repose of the

natural soil in the subgrade so that no earth pressure isexerted from behind on the lining. For example, forcanals passing through sandy soil, the slope may bekept as 2H: 1V whereas canals in firm clay may have

bank slopes as 1.5H: 1V canals cut in rock may havealmost vertical slopes, but slopes like 0.25 to 0.75H: 1Vis preferred from practical considerations.

Decide on the freeboard, which is the depth allowance

by which the banks are raised above the full supplylevel (FSL) of a canal. For channels of differentdischarge carrying capacities, the valuesrecommended for freeboard are given in the following

table:


32/35

Berms or horizontal strips of land provided at canal

banks in deep cutting, have to be incorporated inthe section, as shown in Fig

The berms serve as a road for inspection vehicles and also help to

absorb any soil or rock that may drop from the cut-face of soil or rock

of the excavations. Berm width may be kept at least 2m. If vehicles are

required to move, then a width of at least 5m may be provided.


33/35

For canal sections in filling, banks on either side haveto be provided with sufficient top width formovement of men or vehicles, as shown in Fig

Next, the cross section is to be determined for the channel section.


34/35


35/35

The longitudinal slope (S) of the canal may vary from

reach to reach, depending upon the alignment. Theslope of each reach has to be evaluated from thealignment of the canal drawn on the map of theregion.

For the given discharge Q, permissible velocity V,longitudinal slope S, given side slope , and Manningroughness coefficient, n, for the given canal section,find out the cross section parameters of the canal,

that is, bed width (B) and depth of flow (D). Use two equations

Continuity eqn

Mannings Eqn

crop water req

Documents