Possibilité de déduction des besoins en eau d'irrigation à partir de la loi de dessèchement du sol

Abstract

Since the notion of evapotranspiration of a soil and plant system and methods for the determination of both potential and 'effective' evapotranspiration (Ep and E respectively) have been gaining popularity, criteria have been suggested for the calculation of theoretical water applications for farmland requiring irrigation from these values. Practical water applications are then calculated from the theoretical values by adding the water distribution losses. With one of these criteria, the theoretical water applications are taken as proportional to the difference between potential evapotranspiration and rainfall supplies (ΔEp - ΔP) ; a more 'up to date' criterion allows for the contribution of soil moisture retention to the plants' Water consumption by reducing this difference by a conventionally defined part of the grounds readily available moisture capacity. Although the latter criterion is no doubt adequately representative of the physical mechanism of the hydrological relationships within the soil and plant system in a given climate, it does not necessarily follow that it is expressed properly by a numerical method enabling rational quantitative determinations to be made. These can be decluced from a Water balance of land under irrigation which has been established by consideration of a relationship expressing the actual soil drying process. A method of establishing this individual balance on a weekly basis is suggested, which is an extension of Thornthwaite's well-know method of determining the monthly water balance for land supplied solely rainfall. Among other things, this method of Thornthwaite's also gives the 'water deficit' which is merely an index for the soil's irrigation requirements. The considered Calculation mechanism can be confirmed mathematically with the aid of Thornthwaite's soil drying relationship, which enables the quantity of moisture retained by the soil after set periods of time to be evaluated, for any succession of 'dry' and "wet" periods (ΔP < ΔEp) and (Δ > ΔEP) respectively, and allowing for irrigation wider applications. Once suitable crops have been chosen, the quantities of water to apply and when to do so will depend on how the soil moisture content varies. The water balance should preferally be established on a weekly turns, for weekly or several-weekly intervals are in keeping with usual irrigation turn practices. Initial data are easily obtained. Rainfall data are available from National Hydrographical services and evapotranspiration data are found by calculating daily values by Thornthwaite's formula. The method is easier to use for certain definite properly classified climate-soil-crop combinations, i.e. those in which the ground moisture builds up to the 'available moisture' content during at least one 'wet 'period. For the other combinations, the method requires repeated tests and therefore becomes unwieldy. The first combinations are the most frequent, however, especially in a Mediterranean environment. An example is described of an application of the method to land in the Lentini agricultural region of Sicily in 1963, for which no repeated tests were necessarly. A readily available moisture capacity of 100 mm was assumed maximum and minimum amounts of water to be retained in the soil were to be 90 and 40 mm respectively. A theoretical overall water application of 3,100 cu. m/ha was calculated from these assumptions for the May to September irrigation season, where as Σ (ΔEp - ΔP) gave, 5,900 cu. m/ha. This overall water application was divided into eight applications of about 400 cu .m/ha, to be made in alternating fortnightly adf three-weekly turns. A final point to note is that the proposed determinations can be made with reference to either an average hydrological year in a cycle of several years, or to one particular year (as in the example) ; in the former case, indirect results are obtained on which overall technical forecasts can be based, and the second case yields data for specific climate effects. which are useful for fore more detailed prediction of operational requirements for an irrigation undertaking.