terms related to SWBM

##Terms related to Soil Water Balance Model (SWBM)

1. Soil Moisture Depletion
2. Evapotranspiration
3. Reference Evapotranspiration
4. Crop Coefficient
5. Actual Crop Evapotranspiration
6. Crop Water Requirement
7. Effective Rain
8. Irrigation Requirement

####Soil Moisture Depletion (SMD):-

• SMD is the percentage of the maximum amount of water the soil can hold that has already been used by the plant. The percent available water is the percentage of that same maximum amount that remains.

• So, "65% depletion" means that 65% of the soil's total water capacity has been used. In other words, 65% of the water that was present in the soil when it was saturated is now gone. At 65% depletion, there is 35% available water remaining.

• These numbers are important because different plants start to suffer stress at different soil moisture levels. Recommended allowable depletions (Doorenbos and Pruitt 1977) are expressed as a percentage of the available water (Table 1). For most crops, an allowable depletion of 50% is used.

#####Table 1:- Recommended allowable depletions

• Soil moisture depletion can help to manage irrigation scheduling that might affect crop yield or water use.

####Evapotranspiration

• Evapotranspiration (ET) is a term used to describe the amount of evaporation and plant transpiration from the Earth's land surface to atmosphere.

• Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and water bodies.

• Transpiration accounts for the movement of water within a plant and the subsequent loss of water as vapor through its leaves. Evapotranspiration is an important part of the water cycle.

####Reference Evapotranspiration: An indicator of atmospheric demand for water

• The evapotranspiration rate from a reference surface is called the reference crop evapotranspiration or reference evapotranspiration and is denoted as ETo.
• The reference surface is a hypothetical reference crop with an assumed crop height of 0.12 m (4.72 in), a fixed surface resistance of 70 sec m-1 (70 sec 3.2ft-1) and an albedo of 0.23, closely resembling the evapotranspiration from an extensive surface of green grass of uniform height, actively growing, well-watered, and completely shading the ground.
• Reference evapotranspiration was introduced to study the evaporative demand of the atmosphere independently of crop type, crop development and management practices.
• The only factors affecting ETo are climatic parameters. The FAO Penman-Monteith method is widely used to calculate reference evapotranspiration.
• For more details

####Crop Coefficient

• Crop coefficients are properties of plants used in predicting evapotranspiration (ET). it is denoted by Kc.
• Kc values account for the difference between reference evapotranspiration and actual crop evapotranspiration and link them.
• Crop coefficient for a given crop and growth stage is usually determined experimentally.
• Each agronomic crop has a set of specific crop coefficients used to predict water use rates at different growth stages.
• An example of a Kc curve as a function of days or weeks after planting is shown in Figure 1

#####Figure1. Crop coefficients (Kc) vary according to plant growth stage [(Kc)ini: crop coefficient for initial growth stage; (Kc)dev: coefficient for plant development stage; (Kc)mid: coefficient for mid season; and (Kc)end: coefficient toward the end of the season].

####Actual Crop Evapotranspiration

• The crop evapotranspiration differs distinctly from the reference evapotranspiration (ETo) as the ground cover, canopy properties and aerodynamic resistance of the crop are different from grass.
• In the crop coefficient approach, crop evapotranspiration is calculated by multiplying ETo by Kc.

####Crop Water Requirement *It is amount of water required to compensate the evapotranspiration loss from the cropped field and metabolic activities of crop.

• Theoretically, crop water requirement and crop evapotranspiration are considered as similar terms. But there is approximately 3 to 4 % of variation between them.

####Effective Rain: Amount of rainfall available for plant use

• Effective rainfall refers to the percentage of rainfall which becomes available to plants and crops.
• It considers losses due to runoff, evaporation and deep percolation.
• It helps to estimate irrigation requirement.

####Irrigation requirement

• Irrigation requirement is the total quantity of water applied to the land surface in supplement to the water supplied through rainfall and soil profile to meet the water needs of crops for optimum growth.
• It is determined by subtracting effective rainfall from crop water requirement.