Water requirement is defined as the quantity of water required by a crop or a diversified pattern of crops in a given period of time for its normal growth at a place under field conditions. The source of water may be anything like wells, tanks, artisan wells of canals of rivers.
Water requirement – Crop water requirement is the water required by the plants for its survival, growth, development and to produce economic parts.
This requirement is applied either naturally by precipitation or artificially by irrigation. Hence the crop water requirement includes all losses like:
• Transpiration loss through leaves (T).
• Evaporation loss through soil surface in cropped area (E).
• Amount of water used by plants (WP) for its metabolic activities, which is, estimated as less than 1% of the total water absorption. These three components cannot be separated so easily. Hence, the ET loss is taken as crop water use or crop water consumptive use.
• Other application losses are conveyance loss, percolation loss, runoff loss etc., (WL).
• The water required for special purpose (WSP) like puddling operation, ploughing operation, land preparation, leaching requirement, for the purpose of weeding for dissolving fertilizers and chemicals etc. Hence, the water requirement is symbolically represented as:
WR = T + E + WP + WL + WSP
The other application losses and special purposes are mostly indented for wetland cultivation. Hence, for garden land crop, the ET loss alone is accounted for crop water requirement. The estimation of the water requirement of crop is one of the basic needs for crop planning in the farm and for the planning of any irrigation project. Water requirement includes the losses due to ET or CU and losses during the application of irrigation water and the quantity of water required for special purposes or operations such as land preparation, transplanting, leaching etc. Hence, it may be formulated as follows for demand point of view as;
WR = ET or Cu + application loss + water for special needs.
It can also be stated based on supply source as follows:
WR = IR + ER + S
IR – Irrigation requirement
ER – Effective rainfall
S – Contribution from groundwater table.
Hence, the idea about crop water requirement is essential for farm planning with respect to total quantity of water needed and its efficient use of various cropping schemes of the farm or project area. This crop water requirement is also needed to decide the stream size and design the canal capacity. The combined loss of evaporation and transpiration from a cropped field is termed as evapotranspiration which is otherwise known as consumptive use and denoted as ET and this is a part of water requirement.
CU = E + T + WP
Therefore, WR = CU + WL + WSP
The crop water requirement can also be defined as water required to meet the evapotranspiration demand of the crop and special needs in case of wet land crop and which also includes other application losses both in the case of wet land and garden land crops. This is also known as crop water demand.
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