Factor affecting crop water requirement

                                                                                   

     

Crop water requirement is affected  by climate (weather), soil conditions, crop factors and others such as the cultural practices, the amount of water available for growing crops and the size of the field.

Climate (weather).

• Temperature, As the air temperature increases, much water near or from the soil and  water surfaces, and in the plants, is heated , transformed into vapour and escapes to the atmosphere for example higher temperatures increase evapotranspiration rate, and vice versa.                        
• Relative humidity: As relative humidity( amount of moisture in the atmosphere) decreases, much water is changed into vapour and escapes to the atmosphere to balance the vapour pressure i.e., low relative humidity increases the evapotransipiration rate.                                     
• Intensisty and duration of sunlight: Intense and long  duration sunlight indirectly increases the temperature of soil, water and air with a consequent increase in evaporation. Moreover, plants growing in areas where sunlight is intense for a long time tend to manufacture large amounts of carbohydrates  through the process of photosynthesis. Since water is one of the inputs in the process of  photosynthesis, the higher  the process the higher the  amounts of water used, and vice versa.                                                                                                                                         
• Wind: Wind is the air movement in the atmosphere . Strong movement of the air clears the atmosphere of moisture, leaving the atmosphere relatively dry with a very low vapour pressure. Water in the soil, water bodies and plants vaporizes and escapes to the atmosphere to fill the created deficit in moisture. Therefore, the stronger the winds the higher the evapotranspiration, and vice versa.

            Soil facrors

• Permeability: A permeable( porous)soil will allow easy movement of water, air( vapour) and plant roots. Increased permeability of soil will facilitate and increase the rate of movement and amount of air  out of the soil through evaporation. Moreover, larger amounts of water will percolate faster in porous soil. The plant roots will also penetrate deeper in porous soils, absorbing larger amounts of water that are liable to be used in transpiration increase, and vice versa.                                                                                                                                                
• Elevation of water table: Higher water table will reduce percolation and expose much water  to evaporation, especially with higher soil and air temperatures.                                                          
• Depth of soil to impervious layer: With shallow depth less amounts of water will be lost in percolation,moreover, plant roots will not penetrate deeper enough to absorb and transpire much amounts of water.The shallower the soil the reduced percolation and consumptive use.      
• Salt content, The higher the salt concentration  the lower the soil permeability, hence, the lower the percolation and evapotrnspiration.                                                                                                          
• Soil fertility: Crops will grow faster and develop larger canopies in fertile soils.The rate of photosynthesis will also increase to meet the food demand of the faster growing cells. All these will increase the consumptive use.

          Crop factors

• 1  Crop intensity: Closely planted plants will absorb, consume and transpire increased amounts of water. Moreover, if the crop doesn’t adequately cover  the soil surface the amount of water evaporated will also be high. All these may lead to the increased water need by the crop.                 
• Stage of development of the crop, Generally, the rate of evaporation of water immediately after  planting and before the plants develop much foliage to cover the soil surface is high. As the crop grows and develops more foliage, transpiration also increases. The consumptive use is normally the highest at the end of the vegetative growth stage. As the crop  matures, the consumptive use decreases substantially and generally stops at harvesting time.                           
• Length of the growing period: The longer the crop takes to reach maturity, the larger the amount of water  required  to keep the crop in the field, and vice versa.                                          
• Type of crop: Some crops grow better in the presence of standing water or in over-saturated soils. They require large amounts of water from land preparation to just a few days  before harvesting. This will increase the amounts of water  lost through evapotranspiration and deep percolation  especially in deeper soils. Crop with large canopies will transpire in their tissues may need less water.                                                                                                                         
• Health of the crop: The growth and development of diseased and insect-damaged plants are  normally inhibited. If they develop less canopy and poor rooting systems, they will  absorb and transpire less amount of water. On the other hand, if they grow slowly they may take longer in the field with a consequent increased demand of water.                                                                                                                                                                                                                 Cultural practices                                                                                                                 
• Fertilising  the soil: Crops will grow and develop large canopies faster with increased consumptive use.                                                                                                                              
• Pest control: If  weeds are not removed, they will consume some of the water that was meant for the crop, necessitating the application of large amounts of water to make for the amount consumed weeds. Moreover, controlling diseases  and  insect pests will make plants healthy, grow faster and develop larger canopies with increased  demand for consumptive use.                     
• Tillage operations: Excessive pulverization and  compaction of the soil may render it  impervious  or create plough  pans. Plant root penetration  and effective soil  depth may also be reduced. All these may reduce water percolation and evapotranspiration .                                           
• Cropping systems: Mixing of cover crops  and other crops  may reduce evaporation and deep percolation.                                                                                                                                                    
• Irrigation practices, Different methods of irrigation need different amounts of water. For instance  , subsurface irrigation needs less amounts of water  than surface irrigation due to the reduced evaporation and application losses with the former.                                                                                                                                                                    
• Size of the field: The size of the field will influence such parameters as the size of each plot( border or basin), size of irrigation stream to apply, time  of water application, the extent of soil  properties variation and cultural   practices . All these will have varying influence  on the amount of water required to growth the crop.