Lecture 8 Irrigation efficiencies - conveyance - application - storage - water use – distribution

CONVEYANCE EFFICIENCY (Ec) Water is conveyed through conveyance structures like main canal, branch canal, major distributory, minor distributory, field channels and watercourses. These structures may be lined or unlined. During conveyance of irrigation water loss occurs due to seepage, evaporation and percolation.

Rate of flow at field gate (QF) Ec,(%) = ----------------------------------------- x 100 Rate of flow at regulator (QR) Volume of water let into the field(VF) = ----------------------------------------------------x 100 Volume of water diverted from source(VD)

APPLICATION EFFICIENCY In a canal system, all the water let into the field are not retained due to the terrace system of irrigated field. When a block of fields is considered, some of the water let in is drained as runoff in to field drains. This runoff may be useful for further irrigation of subsequent next block of fields or may not be useful for irrigation. In case of well irrigation, application efficiency is generally 100%. If water is lost due to deep percolation resulting from over irrigation, water drained below the root zone and not available for crops is to be accounted as run off.

Volume of water let into the field – Runoff Ea, (%)= -------------------------------------------------------- x 100 Volume of water let into the field =

STORAGE EFFICIENCY Irrigation water applied is stored in the root zone of the cropped field to fill the moisture depleted in the root zone prior to irrigation. When there is difference between the irrigation water retained in the field (VR) and irrigation water needed prior to irrigation (VN), the storage efficiency occurs. The volume retained (VR)is arrived at by deducting the runoff from the volume applied into the field.

Irrigation water needed prior to irrigation (VN) is computed from the soil properties of field capacity (FC), wilting point (WP), Bulk density (BD), Root zone depth (D) and percentage of Soil Moisture Available (SMA) in the field prior to irrigation and area of the field (A).

(FC – WP) x BD x D x (1 – SMA) x A VN = ------------------------------------------------------ 100 where, VN = Irrigation water needed prior to irrigation in m3 FC = Field capacity in % WP = Wilting point in % BD = Bulk density in g/cc D = Root zone depth in m ASM = Available Soil Moisture prior to irrigation in fraction A = Area of the field in m2

Volume of irrigation water retained in the field (VR) Es, (%)= ------------------------------------------------------------------- x 100 Volume of irrigation water in the field(VN)

DISTRIBUTION EFFICIENCY The measure of how effectively the irrigation water is distributed in any method of application is given by distribution efficiency (Ed). It is also referred to as Christiansen’s Uniformity co-efficient (Cu). Ed = [1-(Ŷ/đ)] x 100 Where, Ed = Distribution efficiency in % Ŷ = Mean deviation from the mean depth of application in cm đ = Mean depth of application in cm

CROP WATER USE EFFICIENCY It is the yield of the crop per unit of water consumed for its evapo-transpirative requirements. Experimentally it can be found out by conducting lysimeter studies and expressed as kg/m3. FIELD WATER USE EFFICIENCY This is the yield of the crop per unit of water consumed for its total requirements. Experimentally, it can be found out by measuring the volume of water let into any cropped field over one season and expressed as kg/m3.

PROJECT EFFICIENCY The efficiency with which the water available in any project/ dam/ tank/ any other water resource is used. It is the ratio of the volume of water used for the purpose of crop production to volume of water delivered from the project for this purpose. If the conveyance and distribution losses are low than this efficiency will be good. Most of the Indian projects operate with an efficiency of about 40 to 60 % only.