Surface water resources are not distributed in a homogeneous way in many regions of the world, irrigation is applied in these areas in order to ensure vegetation sustainability due to irregular and inadequate rainfall. Irrigation water requirement is calculated with the help of the following equation.
IR= Net Irrigation requirement, mm ETc= Evapotranspiration, mm Peff= Effective rainfall, mm
The irrigation requirement occurred as a result of evapotranspiration can be meet by precipitation. Precipitation usually occurs in the form of rain in plant growing season. Therefore, in determining of irrigation water requirement, the rainfall used by plants must be taken into account. However, a certain amount of rainfall is used by plants through surface runoff and deep infiltration. The rainfall that is stored and used in the root zone of the plants is called effective rainfall.
2.3.1 Net irrigation requirement for each irrigation
In the irrigation scheme, in the event that evapotranspiration is not met with natural rainfall, the deficit should be met with irrigation. In a operation unit, the quantity of irrigation water is determined with the help of the following formula.
dn: Net irrigation requirement for each irrigation, mm FC= Field Capacity, %
WP= Wilting point, %
Ry= Allowable soil water deficit % (0.3-0.4 can be taken for turf)
Yt= bulk density, g/cm3 D= the root depth, mm
As shown in the equation 3, there are two basic features affecting quantity of irrigation water. These are soil and plant features. In the irrigation area, due to the fact that the plants in each operation unit have different evapotranspiration, the quantity of the bounden irrigation water is also changing. At the same time, depending on soil characteristics, the water-holding capacity representing the rest quantity of water between field capacity and wilting point, also changes while water-holding capacity is low in a sandy soil, it is high in a clayed soil. In contrast to the sandy soils, due to the high water-holding capacity of clayed soils, more irrigation water will be applicated. Therefore, in the preparatory stages of recreation areas, when creating operation units, ensuring the collection in the same area of the same kind of plants and the fields with the soil structure will provide great convenience to users in the operation of irrigation system. This planning is the only way to ensure the uniform water distribution in the system without causing the overuse of excess water. In case of ignoring this situation, in the operation units that exits different plants or they are nested, some plants will be overwatered, and also some plants will be watered insufficiently. In addition, depending on soil characteristics, while the pondings may be in some areas, there will be losses of deep-seepage in some areas.