Evapotranspiration partitioning components in an irrigated winter wheat field: A combined isotopic and micrometeorologic approach

With dramatic consequences, the drought, and meteoric characteristic, is one of a principal and structural data for Moroccan agriculture. The arid and semi arid areas take up 27% of Moroccan surface and 87% of arable land, whose 60% is covered by cereal culture [1]. The cereal production remains insufficient to meet the country interior needs in this food product. To increase productivity by the improvement of the farming techniques is not only imperative, but it is the only solution to be considered. Enormous efforts were made to increase the yield. But, the techniques used are limited by drought which affects this crop during all its vegetative cycle. The irrigation of cereals remains the only possible way to improve the production. According to the FAO estimation, until 40% of food products produced in world are cultivated under irrigation, however a great quantities of water used for this purpose are lost by the escapes in the irrigation systems. In addition, the irrational irrigation practices are moreover one of the principal soil salinity causes. Approximately, 1/10 of the irrigated surfaces in the world were degraded by salt. Also the climatic changes make more and more areas in world exposed to the drought and desertification risk. The improved irrigation practices will contribute to preserve water and to protect the vulnerable soil. One widely used approach is conventional deficit irrigation (DI) but it requires crop specific information for its effective use [2]. 

In the arid and semi arid areas, which suffers from a water shortage due to the scarcity of rainfall and to the increasing demands of water under the demography pressure and agricultural activity effect, the evapotranspiration constitutes the most important factor of water loss, whose, its determination is capital for a well control for water resources management.

 Contrary to classic methods traditionally used, which remain insufficient (micro-lysimetry, sap flow) to determine correctly the evapotranspiration partition, the isotopic geochemical studies provides important information’s to conclude this partition and to understand the extraction water processes by roots. Indeed, the heavy stables isotopes content increases in water soil by soil evaporation. On the other hand, the water extraction by the roots is without effect on this concentration. Water from growing-season precipitation is rapidly lost from the rooting zone by the transpiration or soil evaporation depending, in part, on the size of the precipitation event [3] and structural and physiological characteristics of the vegetation [4] the ratio of transpiration (T) to evapotranspiration (ET) is a synthetic parameter that integrates ecophysiological and micro environmental controls on total ecosystem water exchange [5].

Indeed, in an isotopic steady-state condition of leaf water, transpiration introduces into the atmosphere a vapour whose isotopic signature is identical to that of root water [6]. In a δ 2H-δ 18O diagram the signature of water vapour originally from transpiration belongs to the local meteoric water isotopic composition. The evaporation causes a little modification of isotopic composition of rainfall seepage in the surface layers of ground [7, 8]. Moreover, the roots system, which is often widely developed in these surface soil layers, allow, by evaporation the plant alimentation in heavy isotope rich water. At last, the root extraction changes strongly the water distribution in different soil horizons and then the availability towards the vegetable covers. There are obviously a lot of consequences on the agronomic and hydrologic levels. The soil relative humidity, the hydraulic potential, the hydraulic conductivity, the root structure, the chemistry of soil solutions, and the evaporation - transpiration ratio are a parameters whose the determination is essential to understand the water transfer in soilplant-atmosphere continuum. 

In this regard, a better understanding of the water balance is essential for exploring water-saving techniques and to avoid the contamination of ground water. The most important components of water balance in semi-arid areas are the evapotranspiration and the deep percolation. Effective methods, such as lysimetric method, sap flow measurement techniques, and micrometeorological techniques are used to measure or estimate ET. But, there are several limitations in using these methods. Stable isotopic tracer methods offer a new opportunity to study the components of ET at the field-scale, from the leaf level to ecosystem, and can partition the ET from different compartments of the ecosystem incorporating measurement of water vapour.