The change of irrigation system from surface to located provides the irrigated farm with an important potential for improving the efficiency in the use of irrigation water, but this quality may not be expressed if the irrigation system change is not accompanied, also, of a change in irrigation habits by end users.
The technical improvement that involves drip irrigation allows an adequate adjustment of water quantities to the needs of crops, but for that these needs must be known by technicians and farmers.
The calculation for the determination of crop water requirements proposed by the Food and Agriculture Organization of the United Nations is the most widespread method. The procedure, in summary, estimates the water needs from:
1. The climatic variables that determine the evaporative demand or reference evapotranspiration (ETo) and a factor linked to the crop, called the crop coefficient (Kc).
2. In this way, the water needs or crop evapotranspiration (ETc) are calculated as:
ETc = ETo * Kc
Naturally, in the previous expression, we must consider the effect of rain, in the event that it occurs.
The amount of rain that is effectively used by a crop is a very difficult parameterized value. Therefore, simple estimation models of Effective Precipitation (Pef) have been taken into account for the calculation of Net Irrigation Needs.
NRN = ETc – Pef
From here, the Gross Irrigation Needs are obtained taking into account the Efficiency of the Installation and, where appropriate, the Fraction of Washing when it is necessary to compensate with the management of the irrigation, the negative consequences of the use of saline waters.