The following points highlight the four main factors that influence the water requirement of crops. The factors are: 1. Influence of Climate 2. Influence of Crop Characteristics 3. Influence of Soil Factors 4. Influence of Crop Management Practices.
Factor # 1. Influence of Climate:
Principal climatic factors influencing crop water requirement are:
1. Precipitation
2. Solar radiation
3. Temperature
4. Wind velocity
5. Relative humidity.
Since, the crops are not irrigated under dryland conditions; the question of irrigation needs does not arise. In irrigated areas, well-distributed rainfall during the crop season minimises the irrigation needs of crops.
A crop grown in sunny and hot climate needs more water per day than the same crop grown in a cloudy and cooler climate. When it is dry, crop water needs are higher than when it is humid. High wind velocity increases the crop water requirement relative to calm climate.
Highest crop water needs are thus found in areas which are hot, dry, windy and sunny. The lowest values are found when the climate is cool, humid and cloudy with little or no wind. Effect of major climatic factors on crop water needs has been indicated in Table 6.2.
Factor # 2. Influence of Crop Characteristics:
Crop water requirements (Table 6.3) vary depending on their:
1. Growth habit
2. Canopy development
3. Leaf area
4. Sensitivity to drought
5. Duration etc.
Tall crops and varieties intercept more solar radiation and have more daily water requirement than short crops and varieties. Crops with deep root system, in general, have higher water requirement relative to shallow rooted crops. High leaf area increases crop water needs. Longer the duration, higher crop water requirement.
Crops differ in their response to moisture deficit (drought resistance). When crop water requirements are not met, crop with high drought sensitivity suffer greater reduction in yield than crops with a low sensitivity (Table 6.4).
Factor # 3. Influence of Soil Factors:
Coarse textured and well-aggregated soils retain less water and have low hydraulic conductivity at relatively higher tensions. Hence, they support less ET compared to fine textured soils, unless too frequent irrigations are provided. Ridges and furrows minimise evaporation loss and increase water availability to the crop. Dark coloured soils absorb more heat leading to higher rate of ET from soil surface.
Factor # 4. Influence of Crop Management Practices:
In general, any crop management practice aimed at improving crop growth, development and final yield increases crop water requirement:
1. Keeping the soil surface relatively moist through frequent irrigations increases the crop water requirements
2. Sprinkler, border and check basin irrigations result in wetting larger surface area relative to furrow, corrugation and drip irrigations leading to higher water loss through evaporation and hence high crop water requirements
3. Stirring the surface soil to create soil mulch minimises water loss through evaporation
4. Weed management practices reduces loss of water through transpiration by weeds
5. Fertilizer application may increases crop water needs by producing greater biomass leading to higher transpiration losses
6. Plant population and row spacing also influence evapotranspiration leading to variation in crop water requirements.