Measuring or Estimating Soil Water

Observation of the soil water content and the soil water potential (definitions and measurement techniques are described in Section 5.2) can be used to schedule irrigations.

The main difficulties consist in the spatial variability of the soil water properties and in appropriately exploring the soil water-measurement cadences by the irrigating farmer. Spatial variability requires careful selection of measurement locations along the cropped field. Difficulties in using these techniques by the farmers can be overcome by specialized support services.

Soil water measurements currently are used for irrigation scheduling by establishing the soil water balance of the cropped field when measuring the soil water by use of a neutron probe [13, 14]. Time-domain reflectometry also is becoming popular, given improvements in measurement techniques [15]. A threshold value for the soil water content is selected and observations are made periodically that are used to forecast the day when the threshold will be attained. The soil water-depletion rate utilized for this forecast is established from two or three observations spaced several days apart. A simple graphical procedure often is used to forecast the irrigation date. The application depth is calculated from the difference between the soil water at field capacity and at the threshold [1].

Tensiometers are also very popular to schedule irrigations. Tensiometers measure the soil water potential, thus providing only information on the irrigation date. Also, threshold soil water potential is defined with this objective, and successive tensiometer readings allow forecasting of when the threshold will be attained. The application depths are either previously fixed or are computed from the soil water-retention curve (see Section 5.2). Applications are discussed elsewhere [1].

The soil water conditions and the evapotranspiration can be estimated using remote-sensing techniques [16-20]. These techniques overcome problems of spatial variability, especially when used with a geographic information system. The use of remote-sensing techniques requires an irrigation scheduling support service to provide information and advice to farmers. Irrigation scheduling using soil water measurements can be performed at the farm scale on large commercial farms [21], but other solutions are required for small farms.

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