Comparison of drainage estimation methods in irrigated citrus orchards

Three different methods of measuring drainage were selected and applied to two irrigated citrus orchards for 3 years. The methods were: (1) a simple soil water capacity model, (2) a chloride balance, and (3) the LEACHM model. In the first method, the soil is assumed to have a given water holding capacity, plants cannot extract water from soil below a certain level, and evapotranspiration varies with soil water content in a well-defined pattern. In the second method, drainage in a given period of time is estimated from the chloride input to soil (mainly with the irrigation and rain water), the changes in soil chloride, and the average chloride concentration of soil solution at the depth where drainage is estimated. In the LEACHM model, water transport in soil is assumed to follow Richards equation, and evapotranspiration depends on soil water content, soil hydraulic properties, the evaporative conditions of the air, and root properties and distribution. Two citrus orchards planted with mature trees, under flood irrigation, were used for the comparison of methods. The three methods provided drainage estimates that differed in most cases by less than 13%, although in some particular season and plot, a given method deviated from the other two by up to 56%. The soil water capacity model is appealing because it only needs a few parameters for calibration, and can be easily programmed in a spreadsheet. The main advantage of the chloride balance approach is that it requires neither calibration nor an estimate of evapotranspiration: this latter fact converts the chloride balance into a good alternative method of measuring evapotranspiration. The precision of the drainage estimates by chloride balance is mainly determined by the spatial variability of soil chloride relative to the chloride input to the soil in the measurement period; in most cases, to obtain a reasonable precision, this period should be longer than 1 or 2 months. The LEACHM model requires more data for calibration, but it can provide additional information on water and solute distribution in the soil profile with time.