The utilization of water for crop production has been recognized as essential since agricultural practices began to be developed. Irrigation was practiced in old civilizations, mainly in the Orient and the Mediterranean basin.
A few centuries ago, agricultural production changed, mainly because new crops began to be cultivated and agricultural production became progressively oriented toward the market. Farmers soon recognized that yields could be increased when water was applied in larger amounts. The concept of crop water requirements (CWR) probably did not exist when irrigation was practiced for subsistence purposes but surely developed as evidence began to accumulate on the dependency of yields on the water applications. These concepts became important when large engineering works began to be built to provide for water supply to newly irrigated areas and it became necessary to estimate the water volumes to be supplied.
The empirical concept of CWR became progressively better described through the combination of scientific progress in physics and physiology. The following definitions are utilized in describing crop water requirements [1, 2]:
• Evaporation is the physical process by which a liquid is transferred to the gaseous state.
• Transpiration is the evaporation process of liquid water within a plant through the stomata and plant surfaces into the air.
• Evapotranspiration (ET) is the combined process by which water is transferred from a vegetative cover and soil into the atmosphere through both transpiration from plants and evaporation from the soil, from dew, and from intercepted water on the plant surfaces.
The concept of CWR is still diverse according to the discipline that approaches the question. However, an agreement on this concept is essential because it is in the basis of irrigation planning, irrigation scheduling, and water delivery scheduling, as well as water resources planning and management. A well-accepted concept was introduced by Doorenbos and Pruitt , who defined CWR as the depth of water (mm) needed to meet the water loss through évapotranspiration from a disease-free crop, growing in large fields under nonrestricting soil conditions including soil water and fertility, and achieving full production potential under the given growing environment.
This concept accommodates all processes affecting the water use by a crop but excludes the influences of local advection, water stress, poor soil and poor fertility management, or inappropriate farming conditions. Thus, a complementary concept is used when actual crop and field conditions are considered. This is the consumptive crop water use (CWU) defined as the depth of water (mm) utilized by a crop through ET and cultivated under given farming conditions in a given growing environment. Therefore, when optimal cropping conditions are met, CWU = CWR. Both CWR and CWU apply to irrigated and rainfed crops. However, the first corresponds to potential yield production whereas the second relates to the real crop condition.
The concept of crop evapotranspiration (ETC) is intimately connected with CWR and CWU. It is defined as the rate of ET (mm day-1) of a given crop as influenced by its growth stages, environmental conditions, and crop management. To determine the crop ET, the reference evapotranspiration (ET0) is used and it refers to a reference crop cultivated in reference conditions such that its rate of ET (mm day-1) reflects the climatic conditions characterizing the local environment. The transfer from ET0 to ETc is done by adopting the crop coefficients (Kc), which represent the ratio between the rates of ET of the cultivated crop and of the reference crop, that is, Kc = ETc/ET0.
For irrigated crops, another main concept is the irrigation water requirement (IWR), defined as the net depth of water (mm) that is required to be applied to a crop to fully satisfy its specific CWR. The IWR is the fraction of CWR not satisfied by rainfall, soil water storage, and groundwater contribution.
When irrigated crops are grown under suboptimal conditions, particularly deficit irrigation—that is, they do not achieve full production potential—the net depth of water to be applied then is termed deficit irrigation water requirement (DIWR), which corresponds to the adoption of a given allowable deficit in the water supply to the crop.
Depending on the irrigation supply conditions, irrigation method, equipment utilized, irrigation scheduling, and the ability of the irrigator, the amounts of water to be applied have to compensate for all system losses and lack of efficiency. These include losses through percolation, seepage or leakage in the conveyance and distribution systems, management spills of excess water in canal systems, and runoff and deep percolation in the fields. Therefore, the gross irrigation water requirement (GIWR) corresponds to the gross depth of water (mm) to be applied to a crop to fully satisfy its CWR. When it is necessary to add a leaching fraction to ensure appropriate leaching of salts from the soil profile, this depth of water is also included in GIWR.
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