Water Balance

The water balance is dealt with in Section 5.1.6, where information on computing the soil water-threshold values also is provided.

The water balance for irrigation scheduling requires information on soil hydraulic properties (soil water content (9) at field capacity and at wilting point for the different soil layers, or the water retention and hydraulic conductivity curves in case of mechanistic models); weather data on rainfall and reference evapotranspiration (see Section 5.1.4);

and crop characteristics relative to crop growth stages, root depths, plant heights, and crop coefficients (see Section 5.1.5). In contrast, some models use other evapotranspiration models, requiring specific inputs.

The water balance can be performed using only weather data to compute the soil water depletion. The majority of models use this approach. Models can be applied in real time to plan the seasonal irrigation calendar, or to study alternative strategies for irrigation scheduling [7, 9, 10]. When models are applied in real time, they often use information on actual soil water observations to adjust the forecast of the irrigation date and depth.

The threshold for water stress can be defined by a crop-specific factor of soil water depletion, namely thepfactor [26, 27]. This approach is described in Section 5.1.6, where this factor is related to the management-allowed deficit (MAD) [11]. This approach is utilized in more common models [7, 10, 28]. Mechanistic models usually incorporate a plant-growth submodel. This allows an appropriate response of the model to water deficits, not requiring the definition of a soil water depletion [10, 29].

The yield impacts of irrigation scheduling strategies are considered through a simple yield-evapotranspiration function [26] in less-sophisticated models. On the contrary, yield is an output of most mechanistic models, namely the CERES-type models [10]. Some models also incorporate any function of crop responses to salinity, resulting from parameters and information provided in the literature [30, 31].

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