Crop Water Demands

Env-contamination, source : wikimedia commons

Crop water demand is the amount of water that is required to meet the evapotranspiration rate so that crops may grow. The evapotranspiration rate is the amount of water that is lost to the atmosphere through the leaves of the plant, as well as the soil surface. Water is important in agricultural productions and it is supplied to the soil via rainfall or irrigation processes. The supply of water through rainfall is not adequate in most of the climatic or agricultural zones. The climatic variations do not allow rain to occur when it may be needed by the farmers. Rainfall usually occurs when there is meeting point between the North easterly winds (dry winds) and the South westerly winds (wet winds). The resulting rain supply water required by the crops for their metabolic activities. The climatic variations provide different depths of rainfall. The inadequacy of irrigation system particularly in the northern parts.

Rainfall determines the growing seasons of the crops. The depth of water available to crops is most important to a farmer and the quantity of water the soils can hold at any time depends on the soils organic matter content and the fineness of the particle. Consequently, the higher the organic matter in the soil, the greater the depth of water the soil can store.

Water is important to the soil because it helps to cool the soil, dilutes the soil deposits, softens tillage pans(hard soil surface),provides medium for bacterial and chemical activities in the soil, nourishes the soil, provides water required for all the metabolic activities including photosynthetic process, increases the yields of crops and dissolves all available nutrients that can be absorbed and assimilated by the growing crop.
Water can be supplied to the soil and the crop but they may not be available to the crops. Available water is therefore, that portion of water in the soil that can be readily absorbed by plants roots. It is also the amount of water released by the soil for the crops when the equilibrium soil -water matric potential is decreased from field capacity to 15bars. Available nutrients are those elements in the soil that can be readily absorbed and assimilated by the growing crops particularly when the water required to dissolve the elements are available too.

Peak Period and Seasonal Consumptive Uses

Peak period consumptive use is the average daily water use rate of a crop during the few days of the highest consumptive use of the season. Peak use periods of various crops occur at different times in a cropping season hence when ever two or more crops are involved their various peak periods of consumption use rate must be considered. This is required in order to meet the various crops requirements. They are used in the design of irrigation systems.

Seasonal consumptive use is the total quantity of water used in evapotranspiration by a cropped area during the growing season. It is also the depth of water in CM or hectare -cm per hectare used in evaluating the seasonal irrigation water supplied.

Potential and Actual Evapotranspiration

Potential evapotranspiration is the sum of the evapotranspiration(water consumption) from large vegetation covered land surface with adequate supply of moisture at all times. It is also the evapotranspiration from growing short green vegetation completely shading the ground surface and never short of moisture availability. Potential evapotranspiration depends on the climatic conditions f a particular place or zone.

Actual evapotranspiration is the amount of evapotranspiration occurring under an existing field condition. Actual evapotranspiration depends on the climate, available moisture in the soil and particularly at the crop root zone. Water needs to be applied at the effective root zone of a crop. The effective root zone of a crop is defined as the soil depth at which the root of an average matured plants are capable of reducing soil moisture to the extent that it should be replaced by rain or irrigation.

Estimation and Calculation of Crop Water Demands

To estimate the water requirement of a crop we need to firstly measure the evapotranspiration rate. Once we know the ETO, the crop water demand can be calculated. There are two methods used to measure the evapotrasnspiration rate of crops. Namely the evaporation pan and using equations that predicts evapotranspiration rate based on climatic parameters.

Using Penman Equation, which takes into consideration,the reference rate, the climatic parameters of temperature, solar radiation, wind speed and humidity.

Where

ETo= reference evapotranspiration [mm day-1].
Rn= net radiation at the crop surface [MJ m-2 day-1].
G= soil heat flux density [MJ m-2 day-1].
T= air temperature at 2 m height [°C].
u2= wind speed at 2 m height [m s-1].
es= saturation vapour pressure [kPa].
ea= actual vapour pressure [kPa].
es - ea=saturation vapour pressure deficit [kPa].
D= slope vapour pressure curve [kPa °C-1].
g= psychrometric constant [kPa °C-1].

ET0 represents the potential, evapotranspiration rate that can occur. Notwithstanding, the crop water demand is usually less than ET0, including the growth stage of the crop, the leaf that provides shade to the ground, and other features of the crops that differentiate them from each other. When we take this factors into considerations, ET0 is then converted into ETc, through the crop-specific coefficient, Kc.
ETc will represent the evapotranspiration rate of the crop under standard conditions.
When calculating ETc, one must know the growth stages of the crop, their duration and select the proper Kc coefficient that need to be used.

Therefore,

ETc = Kc*ET0.

Let us use the potato crop for example.

Growth stage: Initial growth
Kc for initial stage: 0.45
ET0 (measured by a local meteorological station): 9 mm/day

ETc = Kc*ET0 = 0.45 X 9 = 4.05 mm/day

Conclusion

Crop water demands is the amount of water needed by various crops to grow optimally
The water demands of crops is totally dependent on the climate, crop type and growth stage.
When the climate is sunny and hot the crops demands for more water daily than during cool and cloudy climate. The maize or sugarcane crops demands more water than the millet or sorghum crops. fully grown crops demands more water than nursery crops.

Reference

[1] Crop water needs
[2] Crop water use
[3] Crop water demand
[4] Crop water use and growth
[5] Water Requirements of Crops